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Purpose: The transverse relaxation time T 2$$ {}_2 $$ holds significant relevance in clinical applications and research studies. Conventional T 2$$ {}_2 $$ mapping approaches rely on spin‐echo sequences, which require lengthy acquisition times and involve high radiofrequency (RF) power deposition. An alternative gradient echo (GRE) phase‐based T 2$$ {}_2 $$ mapping method, utilizing steady‐state acquisitions at one small RF spoil phase increment, was recently demonstrated. Here, a modified magnitude‐ and phase‐based T 2$$ {}_2 $$ mapping approach is proposed, which improves T2$$ {\mathrm{T}}_2 $$ estimations by simultaneous fitting of T1$$ {\mathrm{T}}_1 $$ and signal amplitude (A∝PD$$ A\propto PD $$) at three or more RF spoiling phase increments, instead of assuming a fixed T1$$ {\mathrm{T}}_1 $$ value. Methods: The feasibility of the magnitude‐phase‐based method was assessed by simulations, in phantom and in vivo experiments using skipped‐CAIPI three‐dimensional‐echo‐planar imaging (3D‐EPI) for rapid GRE imaging. T2$$ {\mathrm{T}}_2 $$, T1$$ {\mathrm{T}}_1 $$ and PD estimations obtained by our method were compared to T2$$ {\mathrm{T}}_2 $$ of the phase‐based method and T1$$ {\mathrm{T}}_1 $$ and PD of spoiled GRE‐based multi‐parameter mapping using a multi‐echo version of the same sequence. Results: The agreement of the proposed T2$$ {\mathrm{T}}_2 $$ with ground truth and reference T2$$ {\mathrm{T}}_2 $$ values was higher than that of phase‐based T2$$ {\mathrm{T}}_2 $$ in simulations and in phantom data. While phase‐based T2$$ {\mathrm{T}}_2 $$ overestimation increases with actual T2$$ {\mathrm{T}}_2 $$ and T1$$ {\mathrm{T}}_1 $$, the proposed method is accurate over a large range of physiologically meaningful T2$$ {\mathrm{T}}_2 $$ and T1$$ {\mathrm{T}}_1 $$ values. At the same time, precision is improved. In vivo results were in line with these observations. Conclusion: Accurate magnitude‐phase‐based T 2$$ {}_2 $$ mapping is feasible in less than 5 min scan time for 1 mm nominal isotropic whole‐head coverage at 3T and 7T. [ABSTRACT FROM AUTHOR]

Purpose: To compare MR axon radius estimation in human white matter using a multiband spiral sequence combined with field monitoring to the current state‐of‐the‐art echo‐planar imaging (EPI)‐based approach. Methods: A custom multiband spiral sequence was used for diffusion‐weighted imaging at ultra‐high b$$ b $$‐values. Field monitoring and higher order image reconstruction were employed to greatly reduce artifacts in spiral images. Diffusion weighting parameters were chosen to match a state‐of‐the art EPI‐based axon radius mapping protocol. The spiral approach was compared to the EPI approach by comparing the image signal‐to‐noise ratio (SNR) and performing a test–retest study to assess the respective variability and repeatability of axon radius mapping. Effective axon radius estimates were compared over white matter voxels and along the left corticospinal tract. Results: Increased SNR and reduced artifacts in spiral images led to reduced variability in resulting axon radius maps, especially in low‐SNR regions. Test–retest variability was reduced by a factor of approximately 1.5 using the spiral approach. Reduced repeatability due to significant bias was found for some subjects in both spiral and EPI approaches, and attributed to scanner instability, pointing to a previously unknown limitation of the state‐of‐the‐art approach. Conclusion: Combining spiral readouts with field monitoring improved mapping of the effective axon radius compared to the conventional EPI approach. [ABSTRACT FROM AUTHOR]

Purpose: To develop a method for dynamic ∆B0$$ \Delta {B}_0 $$ mapping and distortion correction. Methods: A blip‐rewound EPI trajectory was developed to acquire multiple 2D EPI images in a single readout with an interleaved order, which allows a short TE difference. A joint multi‐echo reconstruction was utilized to exploit the shared information between EPI images. The reconstructed images from each readout are combined to produce a final magnitude image. A ∆B0$$ \Delta {B}_0 $$ map is calculated from the phase of these images for distortion correction. The efficacy of the proposed method is assessed with phantom and in vivo experiments. The performance of the proposed method in the presence of subject motion is also investigated. Results: Compared to conventional multi‐echo EPI, the proposed method allows dynamic ∆B0$$ \Delta {B}_0 $$ mapping at matched resolution with a much shorter TR. Phantom and in vivo results show that the proposed method can provide a comparable magnitude image as conventional single‐shot EPI. The ∆B0$$ \Delta {B}_0 $$ maps calculated from the proposed method are consistent with conventional multi‐echo EPI in the phantom experiment. For in vivo experiments, the proposed method provides a more accurate estimation of ∆B0$$ \Delta {B}_0 $$ than conventional multi‐echo EPI, which is prone to phase wrapping problems due to the long TE difference. In‐vivo scan with subject motion shows the proposed dynamic field mapping method can improve the temporal stability of EPI time series compared to gradient echo (GRE) based static field mapping. Conclusion: The proposed method allows accurate dynamic ∆B0$$ \Delta {B}_0 $$ mapping for robust distortion correction without compromising spatial or temporal resolution. [ABSTRACT FROM AUTHOR]

Purpose: To demonstrate slowly varying, erroneous magnetic field gradients for oscillating readouts due to the mechanically resonant behavior of gradient systems. Methods: Projections of a static phantom were acquired using a one‐dimensional (1D) EPI sequence with varying EPI frequencies ranging from 1121 to 1580 Hz on clinical 3T systems (30 mT/m, 200 T/m/s). Phase due to static B0 inhomogeneities was eliminated by a complex division of two separate scans with different polarities of the EPI readout. The temporal evolution of phase was evaluated and related to the mechanical resonances of the gradient systems derived from the gradient modulation transfer function. Additionally, the impact of temporally varying mechanical resonance effects on EPI was evaluated using an echo‐planar spectroscopic imaging sequence. Results: A beat phenomenon resulting in a slowly varying phase was observed. Its temporal frequency was given by the difference between the EPI frequency and the mechanical resonance frequency of the activated gradient axis. The maximum erroneous, oscillating phase during phase encoding was ±0.5 rad for an EPI frequency of 1281 Hz. Echo‐planar spectroscopic imaging images showed the resulting time‐dependent stretching/compression of the FOV. Conclusion: Oscillating readouts such as those used in EPI can result in low‐frequency, erroneous phase contributions, which are explained by the beat phenomenon. Therefore, EPI phase‐correction approaches may need to include beat effects for accurate image reconstruction. [ABSTRACT FROM AUTHOR]

Purpose: To design an unsupervised deep neural model for correcting susceptibility artifacts in single‐shot Echo Planar Imaging (EPI) and evaluate the model for preclinical and clinical applications. Methods: This work proposes an unsupervised cycle‐consistent model based on the restricted subspace field map to take advantage of both the deep learning (DL) and the reverse polarity‐gradient (RPG) method for single‐shot EPI. The proposed model consists of three main components: (1) DLRPG neural network (DLRPG‐net) to obtain field maps based on a pair of images acquired with reversed phase encoding; (2) spin physical model–based modules to obtain the corrected undistorted images based on the learned field map; and (3) cycle‐consistency loss between the input images and back‐calculated images from each cycle is explored for network training. In addition, the field maps generated by DLRPG‐net belong to a restricted subspace, which is a span of predefined cubic splines to ensure the smoothness of the field maps and avoid blurring in the corrected images. This new method is trained and validated on both preclinical and clinical datasets for diffusion MRI. Results: The proposed network could effectively generate smooth field maps and correct susceptibility artifacts in single‐shot EPI. Simulated and in vivo preclinical/clinical experiments demonstrated that our method outperforms the state‐of‐the‐art susceptibility artifact correction methods. Furthermore, the ablation experiments of the cycle‐consistent network and the restricted subspace in generating field maps did show the advantages of DLRPG‐net. Conclusion: The proposed method (DLRPG‐net) can effectively correct susceptibility artifacts for preclinical and clinical single‐shot EPI sequences. [ABSTRACT FROM AUTHOR]

Conventional echo‐planar imaging (EPI) uses a radiofrequency pulse for excitation and a prolonged echo train to sample k space, while off‐resonance and T2* decay effects caused by magnetic susceptibility variation accumulate within each echo, leading to geometric distortion. Multishot EPI methods, which divide k space into segments, can shorten the effective echo spacing and reduce the distortion on EPI images. But multiple shots cost longer scan time and render susceptibility to motion. In this study, we propose a new "multishot" EPI method termed pseudo multishot EPI (pmsEPI), in which phase‐encoding lines are segmented as in multishot EPI but are collected within a single shot. With the magnetization divided into different pathways via interleaved excitation instead of refocusing in a single long echo train, the total phase error accumulation is reduced in each segmented acquisition, thereby improving distortion of the resultant EPI image. The performance of the pmsEPI method is demonstrated by phantom and in vivo brain experiments on a 3‐T scanner. The experimental results show that the distortion displacements of pmsEPI acquisition compared with conventional EPI decrease by 50% with two pseudo shots and 66% with three pseudo shots, validating the ability of the method to obtain images with reduced distortion in a single shot, although magnetization splitting may induce more than 40% SNR loss and minor artifacts. Specifically, the ability of pmsEPI in diffusion‐weighted imaging with different trajectory options is highlighted, and the flexibility is demonstrated in a single‐shot blip up and down acquisition. [ABSTRACT FROM AUTHOR]

Aim: The aim is to provide an overview of recent research on genetic factors that influence preterm birth in the context of neonatal phenotypic assessment. Methods: This is a nonsystematic review of the recent scientific literature. Results: Maternal and fetal genetic diversity and rare genome variants are linked with crucial immune response sites. In addition, more frequent in preterm neonates, de novo variants may lead to attention deficits, hyperactivity, autism spectrum disorders, and infertility of both sexes later in life. Environmental factors may also greatly burden fetal, and consequently, neonatal development and neurodevelopment through a failure in the fetal epigenome reprogramming process and even influence the initiation of spontaneous preterm pregnancy termination. Minimally invasive analysis of the transcription factors associated with preterm birth helps elucidate labor mechanisms and predict its timing. We also provide valuable summaries of genomic and transcriptomic factors that contribute to preterm birth. Conclusions: Investigation of the human genome, epigenome, and transcriptome helps to identify molecular mechanisms linked with preterm delivery and premature newborn clinical appearance in early and late neonatal life and even predict developmental outcomes. Further studies are needed to fully understand the implications of genetic changes in preterm births. These data could be used to develop targeted interventions aimed at selecting the most effective individual treatment and rehabilitation plan. [ABSTRACT FROM AUTHOR]

Diffusion magnetic resonance imaging (dMRI) of whole ex vivo human brain specimens enables three-dimensional (3D) mapping of structural connectivity at the mesoscopic scale, providing detailed evaluation of fiber architecture and tissue microstructure at a spatial resolution that is difficult to access in vivo. To account for the short T2 and low diffusivity of fixed tissue, ex vivo dMRI is often acquired using strong diffusion-sensitizing gradients and multishot/segmented 3D echo-planar imaging (EPI) sequences to achieve high spatial resolution. However, the combination of strong diffusion-sensitizing gradients and multishot/segmented EPI readout can result in pronounced ghosting artifacts incurred by nonlinear spatiotemporal variations in the magnetic field produced by eddy currents. Such ghosting artifacts cannot be corrected with conventional correction solutions and pose a significant roadblock to leveraging human MRI scanners with ultrahigh gradients for ex vivo whole-brain dMRI. Here, we show that ghosting-correction approaches that correct for either polarity-related ghosting or shot-to-shot variations in a separate manner are suboptimal for 3D multishot diffusion-weighted EPI experiments in fixed human brain specimens using strong diffusion-sensitizing gradients on the 3-T Connectom MRI scanner, resulting in orientationally biased dMRI estimates. We apply a recently developed advanced k-space reconstruction method based on structured low-rank matrix (SLM) modeling that handles both polarity-related ghosting and shot-to-shot variation simultaneously, to mitigate artifacts in high-angular resolution multishot dMRI data acquired in several fixed human brain specimens at 0.7-0.8-mm isotropic spatial resolution using b-values up to 10,000 s/mm² and gradient strengths up to 280 mT/m. We demonstrate the improved mapping of diffusion tensor imaging and fiber orientation distribution functions in key neuroanatomical areas distributed across the whole brain using SLM-based EPI ghost correction compared with alternative techniques. [ABSTRACT FROM AUTHOR]

Purpose: Very high gradient amplitudes played out over extended time intervals as required for second‐order motion‐compensated cardiac DTI may violate the assumption of a linear time‐invariant gradient system model. The aim of this work was to characterize diffusion gradient‐related system nonlinearity and propose a correction approach for echo‐planar and spiral spin‐echo motion‐compensated cardiac DTI. Methods: Diffusion gradient‐induced eddy currents of 9 diffusion directions were characterized at b values of 150 s/mm2 and 450 s/mm2 for a 1.5 Tesla system and used to correct phantom, ex vivo, and in vivo motion‐compensated cardiac DTI data acquired with echo‐planar and spiral trajectories. Predicted trajectories were calculated using gradient impulse response function and diffusion gradient strength‐ and direction‐dependent zeroth‐ and first‐order eddy current responses. A reconstruction method was implemented using the predicted k$$ k $$‐space trajectories to additionally include off‐resonances and concomitant fields. Resulting images were compared to a reference reconstruction omitting diffusion gradient‐induced eddy current correction. Results: Diffusion gradient‐induced eddy currents exhibited nonlinear effects when scaling up the gradient amplitude and could not be described by a 3D basis alone. This indicates that a gradient impulse response function does not suffice to describe diffusion gradient‐induced eddy currents. Zeroth‐ and first‐order diffusion gradient‐induced eddy current effects of up to −1.7 rad and −16 to +12 rad/m, respectively, were identified. Zeroth‐ and first‐order diffusion gradient‐induced eddy current correction yielded improved image quality upon image reconstruction. Conclusion: The proposed approach offers correction of diffusion gradient‐induced zeroth‐ and first‐order eddy currents, reducing image distortions to promote improvements of second‐order motion‐compensated spin‐echo cardiac DTI. [ABSTRACT FROM AUTHOR]

Purpose: Quantitative multi‐parameter mapping (MPM) has been shown to provide good longitudinal and cross‐sectional reproducibility for clinical research. Unfortunately, acquisition times (TAs) are typically infeasible for routine scanning at high resolutions. Methods: A fast whole‐brain MPM protocol based on interleaved multi‐shot 3D‐EPI with controlled aliasing (SC‐EPI) at 3T and 7T is proposed and compared with MPM using a standard spoiled gradient echo (FLASH) sequence. Four parameters (R1, PD, R2*$$ {R}_2^{\ast } $$, and MTsat) were measured in less than 3 min at 1 mm isotropic resolution. Five subjects went through the same scanning sessions twice at each scanner. The intra‐subject coefficient of variation (scan–rescan) (CoV) was estimated for each protocol and scanner to assess the longitudinal reproducibility. Results: At 3T, the CoV of SC‐EPI ranged between 1.2%–4.8% for PD and R1, 2.8%–10.6% for R2*$$ {R}_2^{\ast } $$ and MTsat, which was comparable with FLASH (0.6%–4.9% for PD and R1, 2.6%–11.3% for R2*$$ {R}_2^{\ast } $$ and MTsat). At 7T, where the SC‐EPI TA was reduced to ∼2 min, the CoV of SC‐EPI (1.4%–10.6% for PD, R1, and R2*$$ {R}_2^{\ast } $$) was 1.2–2.4 times larger than the CoV of FLASH (1.0%–15%) and MTsat showed much higher variability across subjects. The SC‐EPI‐MPM protocol at 3T showed high reproducibility and yielded stable quantitative maps at a clinically feasible resolution and scan time, whereas at 7T, MT saturation homogeneity needs to be improved. Conclusion: SC‐EPI‐based MPM is feasible as an additional MRI modality in clinical or population studies where the parameters offer great potential as biomarkers. [ABSTRACT FROM AUTHOR]

Purpose: Severe geometric distortions induced by tissue susceptibility, water–fat chemical shift, and eddy currents pose a substantial obstacle in single‐shot EPI, especially for high‐resolution imaging at ultrahigh field. View angle tilting (VAT)‐EPI can mitigate in‐plane distortion. However, the accompanied strong image blurring prevented its widespread applications. On the other hand, point‐spread function mapping (PSF)‐EPI can correct distortion and blurring accurately but requires prolonged scan time. We present fused VAT‐PSF‐EPI and possibilities for acceleration. Methods: MR signal equations were explicitly derived to quantify image blurring in VAT‐EPI and the maximum acceleration capacity in VAT‐PSF‐EPI. To validate the theoretical prediction, phantom measurements with varying in‐plane parallel imaging factors, slice thicknesses, and RF pulses were conducted at 7 Tesla. In addition, in vivo human brain scans were acquired with T2 and diffusion weighting to assess distortion and blurring correction. Results: VAT can effectively suppress distortion, and the introduced image blurring is corrected through PSF encoding. Up to fourfold acceleration (only 5 shots) in VAT‐PSF‐EPI was achieved compared with standard PSF‐EPI without VAT. VAT‐induced signal loss was mitigated by adjusting the sequence parameters and EPI resolution. In vivo T2‐weighted EPI data with 1.4 mm3 resolution demonstrate immunity to water–fat chemical shift‐induced distortion. Very high‐spatial resolution diffusion‐weighted EPI (0.7 × 0.7 × 2.8 mm3 and 1.2 mm3) demonstrates the immunity to eddy current‐induced distortion. Conclusion: VAT‐PSF‐EPI is a novel spin‐echo EPI‐based sequence for fast high‐resolution diffusion imaging at ultrahigh field. [ABSTRACT FROM AUTHOR]

Purpose: To develop a robust reconstruction pipeline for EPI data that enables 2D Nyquist phase error correction using sensitivity encoding without incurring major noise artifacts in low SNR data. Methods: SENSE with 2D phase error correction (PEC‐SENSE) was combined with channel‐wise noise removal using Marcenko–Pastur principal component analysis (MPPCA) to simultaneously eliminate Nyquist ghost artifacts in EPI data and mitigate the noise amplification associated with phase correction using parallel imaging. The proposed pipeline (coined SPECTRE) was validated in phantom DW‐EPI data using the accuracy and precision of diffusion metrics; ground truth values were obtained from data acquired with a spin echo readout. Results from the SPECTRE pipeline were compared against PEC‐SENSE reconstructions with three alternate denoising strategies: (i) no denoising; (ii) denoising of magnitude data after image formation; (iii) denoising of complex data after image formation. SPECTRE was then tested using high b$$ b $$‐value (i.e., low SNR) diffusion data (up to b=3000$$ b=3000 $$ s/mm2$$ {}^2 $$) in four healthy subjects. Results: Noise amplification associated with phase error correction incurred a 23% bias in phantom mean diffusivity (MD) measurements. Phantom MD estimates using the SPECTRE pipeline were within 8% of the ground truth value. In healthy volunteers, the SPECTRE pipeline visibly corrected Nyquist ghost artifacts and reduced associated noise amplification in high b$$ b $$‐value data. Conclusion: The proposed reconstruction pipeline is effective in correcting low SNR data, and improves the accuracy and precision of derived diffusion metrics. [ABSTRACT FROM AUTHOR]

Purpose: The phase mismatch between odd and even echoes in EPI causes Nyquist ghost artifacts. Existing ghost correction methods often suffer from severe residual artifacts and are ineffective with k‐space undersampling data. This study proposed a deep learning–based method (PEC‐DL) to correct phase errors for DWI at 7 Tesla. Methods: The acquired k‐space data were divided into 2 independent undersampled datasets according to their readout polarities. Then the proposed PEC‐DL network reconstructed 2 ghost‐free images using the undersampled data without calibration and navigator data. The network was trained with fully sampled images and applied to two‐ and fourfold accelerated data. Healthy volunteers and patients with Moyamoya disease were recruited to validate the efficacy of the PEC‐DL method. Results: The PEC‐DL method was capable to mitigate the ghost artifacts in DWI in healthy volunteers as well as patients with Moyamoya disease. The fourfold accelerated results showed much less distortion in the lesions of the Moyamoya patient using high b‐value DWI and the corresponding ADC maps. The ghost‐to‐signal ratios were significantly lower in PEC‐DL images compared to conventional linear phase corrections, mini‐entropy, and PEC‐GRAPPA algorithms. Conclusion: The proposed method can effectively eliminate ghost artifacts for full sampled and up to fourfold accelerated EPI data without calibration and navigator data. [ABSTRACT FROM AUTHOR]

Emulsion polymer isocyanate (EPI) adhesive is one of the most widely used structural adhesives in the woodworking industry. However, there has been a lack of knowledge on how the EPI‐adhesive interacts with the chemical constituents of wood, in particular, with the wood extractives that are known to influence the bonding quality. In this study, the interactions of the EPI‐adhesive with the water extracts and selected extractives from European wood species were systematically investigated using different analytical techniques. While the alterations in the curing properties of the pure EPI‐adhesive and EPI‐adhesive‐extract mixtures were revealed by in situ rheology and Fourier‐transform infrared spectroscopy, evolved gas analysis, and pyrolysis‐gas chromatography/mass spectrometry, the solid‐state 13C nuclear magnetic resonance were performed for analyzing the final chemical composition of the cured adhesive. Moreover, the influence of the extraction on the mechanical bonding performance was tested by tensile‐shear tests. The study revealed significant interactions between the EPI‐adhesive and tannin‐rich, acidic wood extracts. The acidic chestnut and oak extracts catalyzed the curing reactions and led to a huge increase in the adhesive viscosity. These interactions might affect the bonding quality, for example, adhesive penetration depth and formation of bonding strength, therefore, careful attention is required when bonding acidic wood surfaces. [ABSTRACT FROM AUTHOR]

Purpose: To estimate dynamic off‐resonance due to vigorous body motion in accelerated fMRI of awake behaving nonhuman primates (NHPs) using the echo‐planar imaging reference navigator, in order to attenuate the effects of time‐varying off‐resonance on the reconstruction. Methods: In NHP fMRI, the animal's head is usually head‐posted, and the dynamic off‐resonance is mainly caused by motion in body parts that are distant from the brain and have low spatial frequency. Hence, off‐resonance at each frame can be approximated as a spatially linear perturbation of the off‐resonance at a reference frame, and is manifested as a relative linear shift in k‐space. Using GRAPPA operators, we estimated these shifts by comparing the navigator at each time frame with that at the reference frame. Estimated shifts were then used to correct the data at each frame. The proposed method was evaluated in phantom scans, simulations, and in vivo data. Results: The proposed method is shown to successfully estimate spatially low‐order dynamic off‐resonance perturbations, including induced linear off‐resonance perturbations in phantoms, and is able to correct retrospectively corrupted data in simulations. Finally, it is shown to reduce ghosting artifacts and geometric distortions by up to 20% in simultaneous multislice in vivo acquisitions in awake‐behaving NHPs. Conclusion: A method is proposed that does not need sequence modification or extra acquisitions and makes accelerated awake behaving NHP imaging more robust and reliable, reducing the gap between what is possible with NHP protocols and state‐of‐the‐art human imaging. [ABSTRACT FROM AUTHOR]

Purpose: To accelerate the acquisition of relaxation‐diffusion imaging by integrating time‐division multiplexing (TDM) with simultaneous multi‐slice (SMS) for EPI and evaluate imaging quality and diffusion measures. Methods: The time‐division multiplexing (TDM) technique and SMS method were integrated to achieve a high slice‐acceleration (e.g., 6×) factor for acquiring relaxation‐diffusion MRI. Two variants of the sequence, referred to as TDM3e‐SMS and TDM2s‐SMS, were developed to simultaneously acquire slice groups with three distinct TEs and two slice groups with the same TE, respectively. Both sequences were evaluated on a 3T scanner with in vivo human brains and compared with standard single‐band (SB) ‐EPI and SMS‐EPI using diffusion measures and tractography results. Results: Experimental results showed that the TDM3e‐SMS sequence with total slice acceleration of 6 (multiplexing factor (MP) = 3 × multi‐band factor (MB) = 2) provided similar image intensity and microstructure measures compared to standard SMS‐EPI with MB = 2, and yielded less bias in intensity compared to standard SMS‐EPI with MB = 4. The three sequences showed a similar positive correlation between TE and mean kurtosis (MK) and a negative correlation between TE and mean diffusivity (MD) in white matter. Multi‐fiber tractography also shows consistency of results in TE‐dependent measures between different sequences. The TDM2s‐SMS sequence (MP = 2, MB = 2) also provided imaging measures similar to standard SMS‐EPI sequences (MB = 2) for single‐TE diffusion imaging. Conclusions: The TDM‐SMS sequence can provide additional 2× to 3× acceleration to SMS without degrading imaging quality. With the significant reduction in scan time, TDM‐SMS makes joint relaxation‐diffusion MRI a feasible technique in neuroimaging research to investigate new markers of brain disorders. [ABSTRACT FROM AUTHOR]

The purpose of this study was to investigate the feasibility of two‐dimensional (2D) navigated, interleaved multishot echo‐planar imaging (EPI) to enhance kidney diffusion‐weighted imaging (DWI) in rats at 7.0 T. Fully sampled interleaved four‐shot EPI with 2D navigators was tailored for kidney DWI (Sprague–Dawley rats, n = 7) on a 7.0‐T small bore preclinical scanner. The image quality of four‐shot EPI was compared with T2‐weighted rapid acquisition with relaxation enhancement (RARE) (reference) and single‐shot EPI (ss‐EPI) without and with parallel imaging (PI). The contrast‐to‐noise ratio (CNR) was examined to assess the image quality for the EPI approaches. The Dice similarity coefficient and the Hausdorff distance were used for evaluation of image distortion. Mean diffusivity (MD) and fractional anisotropy (FA) were calculated for renal cortex and medulla for all DWI approaches. The corticomedullary difference of MD and FA were assessed by Wilcoxon signed‐rank test. Four‐shot EPI showed the highest CNR among the three EPI variants and lowest geometric distortion versus T2‐weighted RARE (mean Dice: 0.77 for ss‐EPI without PI, 0.88 for ss‐EPI with twofold undersampling, and 0.92 for four‐shot EPI). The FA map derived from four‐shot EPI clearly identified a highly anisotropic region corresponding to the inner stripe of the outer medulla. Four‐shot EPI successfully discerned differences in both MD and FA between renal cortex and medulla. In conclusion, 2D navigated, interleaved multishot EPI facilitates high‐quality rat kidney DWI with clearly depicted intralayer and interlayer structure and substantially reduced image distortion. This approach enables the anatomic integrity of DWI‐MRI in small rodents and has the potential to benefit the characterization of renal microstructure in preclinical studies. [ABSTRACT FROM AUTHOR]

Purpose: This study aimed at developing a 3D reduced field‐of‐view imaging (3D‐rFOVI) technique using a 2D radiofrequency (RF) pulse, and demonstrating its ability to achieve isotropic high spatial resolution and reduced image distortion in echo planar imaging (EPI). Methods: The proposed 3D‐rFOVI technique takes advantage of a 2D RF pulse to excite a slab along the conventional slice‐selection direction (i.e., z‐direction) while limiting the spatial extent along the phase‐encoded direction (i.e., y‐direction) within the slab. The slab is phase‐encoded in both through‐slab and in‐slab phase‐encoded directions. The 3D‐rFOVI technique was implemented at 3T in gradient‐echo and spin‐echo EPI pulse sequences for functional MRI (fMRI) and diffusion‐weighted imaging (DWI), respectively. 3D‐rFOVI experiments were performed on a phantom and human brain to illustrate image distortion reduction, as well as isotropic high spatial resolution, in comparison with 3D full‐FOV imaging. Results: In both the phantom and the human brain, image voxel dislocation was substantially reduced by 3D‐rFOVI when compared with full‐FOV imaging. In the fMRI experiment with visual stimulation, 3D isotropic spatial resolution of (2 × 2 × 2 mm3) was achieved with an adequate signal‐to‐noise ratio (81.5) and blood oxygen level‐dependent (BOLD) contrast (2.5%). In the DWI experiment, diffusion‐weighted brain images with an isotropic resolution of (1 × 1 × 1 mm3) was obtained without appreciable image distortion. Conclusion: This study indicates that 3D‐rFOVI is a viable approach to 3D neuroimaging over a zoomed region. [ABSTRACT FROM AUTHOR]

Background: Awareness of exocrine pancreatic insufficiency (EPI) in cats has increased since the development of an assay for feline trypsin‐like immunoreactivity (fTLI). Ultrasound findings in cats with EPI have only been reported rarely and described as nonspecific. Hypothesis/Objectives: To describe the ultrasonographic findings, clinical signs, and concurrent diseases in cats with EPI. Animals Twenty‐two client‐owned cats with EPI. Methods: Multicenter retrospective descriptive study including cats with serum fTLI concentration ≤8 μg/L and an abdominal ultrasound examination performed within 6 weeks of fTLI measurement. Sonographic measurements of maximal pancreatic thickness and maximal pancreatic duct diameter as well as ratios of pancreatic duct diameter to pancreatic thickness were obtained. Additional sonographic findings, concurrent conditions, and clinical signs were recorded. Results: The most common clinical sign was weight loss (15/22 cats). Chronic enteropathy was the most common concurrent disease (13/22 cats). In 39% of cats, the pancreas had minimal or no sonographic alterations. Pancreatic duct dilatation (>2.5 mm), pancreatic duct tortuosity with variable diameter, or both were seen in 6/13 cats. The pancreatic parenchyma was subjectively thin in 6 cats. A significant relationship was found between subjectively thin pancreatic parenchyma and increased pancreatic duct size : pancreatic thickness ratio (P =.004). Diffuse gastrointestinal dilatation with echogenic content was observed in 8/22 cats. Conclusion: Exocrine pancreatic insufficiency often causes minimal to no sonographic pancreatic changes. Nonetheless, the findings of thin pancreatic parenchyma, pancreatic duct dilatation, or diffuse small intestinal dilatation with echogenic contents in cats with unexplained weight loss or unformed feces should raise clinical suspicion for EPI. [ABSTRACT FROM AUTHOR]

Purpose: To develop a time‐division multiplexing echo‐planar imaging (TDM‐EPI) sequence for approximately two‐ to threefold acceleration when acquiring joint relaxation‐diffusion MRI data with multiple TEs. Methods: The proposed TDM‐EPI sequence interleaves excitation and data collection for up to 3 separate slices at different TEs and uses echo‐shifting gradients to disentangle the overlapping echo signals during the readout period. By properly arranging the sequence event blocks for each slice and adjusting the echo‐shifting gradients, diffusion‐weighted images from separate slices can be acquired. Therefore, we present 2 variants of the sequence. A single‐TE TDM‐EPI is presented to demonstrate the concept. Next, a multi‐TE TDM‐EPI is presented to highlight the advantages of the TDM approach for relaxation‐diffusion imaging. These sequences were evaluated on a 3 Tesla scanner with a water phantom and in vivo human brain data. Results: The single‐TE TDM‐EPI sequence can simultaneously acquire 2 slices with a maximum b value of 3000 s/mm2 and 2.5 mm isotropic resolution using interleaved readout windows with TE ≈ 78 ms. With the same b value and resolution, the multi‐TE TDM‐EPI sequence can simultaneously acquire 2 or 3 separate slices using interleaved readout sections with shortest TE ≈ 70 ms and ΔTE ≈ 30 ms. Phantom and in vivo experiments have shown that the proposed TDM‐EPI sequences can provide similar image quality and diffusion measures as conventional EPI readouts with multiple echoes but can reduce the overall relaxation‐diffusion protocol scan time by approximately two‐ to threefold. Conclusion: TDM‐EPI is a novel approach to acquire diffusion imaging data at multiple TEs. This enables a significant reduction in acquisition time for relaxation‐diffusion MRI experiments but without compromising image quality and diffusion measurements, thus removing a significant barrier to the adoption of relaxation‐diffusion MRI in clinical research studies of neurological and mental disorders. [ABSTRACT FROM AUTHOR]

Purpose: To improve the robustness of diffusion‐weighted imaging (DWI) data acquired with segmented simultaneous multi‐slice (SMS) echo‐planar imaging (EPI) against in‐plane and through‐plane rigid motion. Theory and Methods: The proposed algorithm incorporates a 3D rigid motion correction and wavelet denoising into the image reconstruction of segmented SMS‐EPI diffusion data. Low‐resolution navigators are used to estimate shot‐specific diffusion phase corruptions and 3D rigid motion parameters through SMS‐to‐volume registration. The shot‐wise rigid motion and phase parameters are integrated into a SENSE‐based full‐volume reconstruction for each diffusion direction. The algorithm is compared to a navigated SMS reconstruction without gross motion correction in simulations and in vivo studies with four‐fold interleaved 3‐SMS diffusion tensor acquisitions. Results: Simulations demonstrate high fidelity was achieved in the SMS‐to‐volume registration, with submillimeter registration errors and improved image reconstruction quality. In vivo experiments validate successful artifact reduction in 3D motion‐compromised in vivo scans with a temporal motion resolution of approximately 0.3 s. Conclusion: This work demonstrates the feasibility of retrospective 3D rigid motion correction from shot navigators for segmented SMS DWI. [ABSTRACT FROM AUTHOR]

Purpose: The characteristic MRI features of multiple sclerosis (MS) lesions make it conceptually appealing to pursue parametric mapping techniques that support simultaneous generation of quantitative maps of 2 or more MR contrast mechanisms. We present a modular rapid acquisition with relaxation enhancement (RARE)‐EPI hybrid that facilitates simultaneous T2 and T2∗ mapping (2in1‐RARE‐EPI). Methods: In 2in1‐RARE‐EPI the first echoes in the echo train are acquired with a RARE module, later echoes are acquired with an EPI module. To define the fraction of echoes covered by the RARE and EPI module, an error analysis of T2 and T2∗ was conducted with Monte Carlo simulations. Radial k‐space (under)sampling was implemented for acceleration (R = 2). The feasibility of 2in1‐RARE‐EPI for simultaneous T2 and T2∗ mapping was examined in a phantom study mimicking T2 and T2∗ relaxation times of the brain. For validation, 2in1‐RARE‐EPI was benchmarked versus multi spin‐echo (MSE) and multi gradient‐echo (MGRE) techniques. The clinical applicability of 2in1‐RARE‐EPI was demonstrated in healthy subjects and MS patients. Results: There was a good agreement between T2/T2∗ values derived from 2in1‐RARE‐EPI and T2/T2∗ reference values obtained from MSE and MGRE in both phantoms and healthy subjects. In patients, MS lesions in T2 and T2∗ maps deduced from 2in1‐RARE‐EPI could be just as clearly delineated as in reference maps calculated from MSE/MGRE. Conclusion: This work demonstrates the feasibility of radially (under)sampled 2in1‐RARE‐EPI for simultaneous T2 and T2∗ mapping in MS patients. [ABSTRACT FROM AUTHOR]

Purpose: Numerous studies report motion as the most detrimental source of noise and artifacts in fMRI. Current motion correction methods fail to completely address the motion problem. Retrospective techniques such as spatial realignment can correct for between‐volume misalignment but fail to address within volume contamination and spin‐history artifacts. Prospective motion correction can prevent spin‐history artifacts but currently cannot update the gradients fast enough to remove k‐space filling artifacts, calling for a hybrid approach to fully address these problems. Theory and methods: Motion can be mathematically formulated into the MR signal equation to describe the motion artifacts at their origin in k‐space. From these equations, it is demonstrated that different motions have different effects on the signal. A novel motion correction algorithm is designed from these equations to remove motion‐induced artifacts directly in k‐space, discrete reconstruction of irregular fMRI trajectory (DRIFT). This method is evaluated rigorously using fMRI simulations and data from a rotating phantom inside the scanner. Results: The results indicate that although some motion types have negligible effects on the MR signal, others produce catastrophic and lasting artifacts even after motion cessation. In simulation, DRIFT is able to remove motion artifacts in the absence of spin history. In a phantom scan, DRIFT significantly attenuates the motion artifacts in the fMRI data. Conclusion: Neither prospective nor retrospective motion correction methods could completely remove the motion artifacts from the fMRI data. However, DRIFT, as a retrospective technique, when combined with prospective motion correction, can eliminate a significant portion of motion artifacts. [ABSTRACT FROM AUTHOR]

Purpose: Low‐field (<1 tesla) MRI scanners allow more widespread diagnostic use for a range of cardiac, musculoskeletal, and neurological applications. However, the feasibility of performing robust fMRI at low field has yet to be fully demonstrated. To address this gap, we investigated task‐based fMRI using a highly sensitive transition‐band balanced steady‐state free precession approach and standard EPI on a 0.55 tesla scanner equipped with modern high‐performance gradient coils and a receive array. Methods: TR and flip‐angle of transition‐band steady‐state free precession were optimized for 0.55 tesla by simulations. Static shimming was employed to compensate for concomitant field effects. Visual task‐based fMRI data were acquired from 8 healthy volunteers. For comparison, standard EPI data were also acquired with TE = T2∗. Retrospective image‐based correction for physiological effects (RETROICOR) was used to quantify physiological noise effects. Results: Activation was robustly detected using both methods in a 4‐min scan time. Transition‐band steady‐state free precession was found to be sensitive to interference from subtle spatial and temporal (field drift, respiration) variations in the magnetic field, counteracting potential advantages of the reduced magnetic susceptibility effects compared to its utilization at high field. These adverse effects could be partially remedied with static shimming and postprocessing approaches. Standard EPI proved more robust against the sources of interference. Conclusion: BOLD contrast is sufficiently large at 0.55 tesla for robust detection of brain activation and may be employed to broaden the spectrum of applications of low‐field MRI. Standard EPI outperforms transition‐band steady‐state free precession in terms of signal stability. [ABSTRACT FROM AUTHOR]

Purpose: To assess the effects of blip‐up and ‐down echo planar imaging (EPI) acquisition designs, with different choices of phase‐encoding directions (PEDs) on the reproducibility of diffusion MRI (dMRI)‐derived metrics in the human brain. Methods: Diffusion MRI data in seven subjects were acquired five times, each with five different protocols. The base design included 64 diffusion directions acquired with anterior‐posterior (AP) PED, the first and second protocols added reverse phase‐encoded b=0s/mm2 posterior‐anterior (PA) PED images. The third one included 32 directions all with PED acquisitions with opposite polarity (AP and PA). The fourth protocol, also with 32 unique directions used four PEDs (AP, PA, right‐left (RL), and left‐right (LR)). The scan time was virtually identical for all protocols. The variability of diffusion MRI metrics for each subject and each protocol was computed across the different sessions. Results: The highest reproducibility for all dMRI metrics was obtained with protocol four (AP/PA‐RL/LR, ie, four‐way PED). Protocols that used only b=0s/mm2 for distortion correction, which are the most widely used designs, had the lowest reproducibility. Conclusions: An acquisition design with four PEDs, including all DWIs in addition to b=0s/mm2 images should be used to achieve high reproducibility in diffusion MRI studies. [ABSTRACT FROM AUTHOR]

Purpose: A segmented k‐space blipped‐controlled aliasing in parallel imaging (skipped‐CAIPI) sampling strategy for EPI is proposed, which allows for a flexible choice of EPI factor and phase encode bandwidth independent of the controlled aliasing in parallel imaging (CAIPI) sampling pattern. Theory and Methods: With previously proposed approaches, exactly two EPI trajectories were possible given a specific CAIPI pattern, either with slice gradient blips (blipped‐CAIPI) or following a shot‐selective CAIPI approach (higher resolution). Recently, interleaved multi‐shot segmentation along shot‐selective CAIPI trajectories has been applied for high‐resolution anatomical imaging. For more flexibility and a broader range of applications, we propose segmentation along any blipped‐CAIPI trajectory. Thus, all EPI factors and phase encode bandwidths available with traditional segmented EPI can be combined with controlled aliasing. Results: Temporal SNR maps of moderate‐to‐high‐resolution time series acquisitions at varying undersampling factors demonstrate beneficial sampling alternatives to blipped‐CAIPI or shot‐selective CAIPI. Rapid high‐resolution scans furthermore demonstrate SNR‐efficient and motion‐robust structural imaging with almost arbitrary EPI factor and minimal noise penalty. Conclusion: Skipped‐CAIPI sampling increases protocol flexibility for high spatiotemporal resolution EPI. In terms of SNR and efficiency, high‐resolution functional or structural scans benefit vastly from a free choice of the CAIPI pattern. Even at moderate resolutions, the independence of sampling pattern, TE, and image matrix size is valuable for optimized functional protocol design. Although demonstrated with 3D‐EPI, skipped‐CAIPI is also applicable with simultaneous multislice EPI. [ABSTRACT FROM AUTHOR]

This paper presents an innovative smart TCON design using Field Programmable Gate Array (FPGA) devices. Instead of facing a massive challenge to deal with a large number of ASICs or ASSPs, a single FPGA is chosen based upon a resolution and refresh rate determined performance need. Leveraging its expanding logic programmability and performance, vendor and panel specific color volume conversion can be achieved with high efficiency. With its IO programmability, various video interfaces can be accommodated. The FPGA TCON is very adaptable to different panels and vendors. [ABSTRACT FROM AUTHOR]

Multi‐shot techniques offer improved resolution and signal‐to‐noise ratio for diffusion‐ weighted imaging, but make the acquisition vulnerable to shot‐specific phase variations and inter‐shot macroscopic motion. Several model‐based reconstruction approaches with iterative phase correction have been proposed, but robust macroscopic motion estimation is still challenging. Segmented diffusion imaging with iterative motion‐corrected reconstruction (SEDIMENT) uses iteratively refined data‐driven shot navigators based on sensitivity encoding to cure phase and rigid in‐plane motion artifacts. The iterative scheme is compared in simulations and in vivo with a non‐iterative reference algorithm for echo‐planar imaging with up to sixfold segmentation. The SEDIMENT framework supports partial Fourier acquisitions and furthermore includes options for data rejection and learning‐based modules to improve robustness and convergence. [ABSTRACT FROM AUTHOR]

This paper develops a model showing how the environmental liability regime and the precision of the disclosed environmental performance indicator affect managers' incentives (1) to reduce actual pollution and (2) to manipulate the reported pollution. I assume a company with a separation of ownership and control which can be held liable for environmental damages and distinguish between a negligence regime and strict liability. The results suggest that if there is no manipulation but only a lack of precision of the disclosed environmental performance indicator, a negligence rule induces lower actual pollution levels than strict liability even though a negligence rule is considered to be more lenient. If managers are able to manipulate the disclosed environmental performance indicator, they will do so and actual pollution levels will generally increase. While manipulation makes it easier for shareholders to escape liability under a negligence regime, shareholders suffer from manipulation under strict liability due to higher actual pollution and higher expected damage compensation payments. Therefore, the manipulation level is higher under a negligence regime. My analysis contributes to the environmental performance and disclosure literature by showing that the liability regime is an important determinant affecting environmental reporting and actual pollution decisions. [ABSTRACT FROM AUTHOR]

Purpose: To ameliorate tradeoffs between a fixed spatial resolution and signal‐to‐noise ratio (SNR) for hyperpolarized 13C MRI. Methods: In MRI, SNR is proportional to voxel volume but retrospective downsampling or voxel averaging only improves SNR by the square root of voxel size. This can be exploited with a metabolite‐selective imaging approach that independently encodes each compound, yielding high‐resolution images for the injected substrate and coarser resolution images for downstream metabolites, while maintaining adequate SNR for each. To assess the efficacy of this approach, hyperpolarized [1‐13C]pyruvate data were acquired in healthy Sprague‐Dawley rats (n = 4) and in two healthy human subjects. Results: Compared with a constant resolution acquisition, variable‐resolution data sets showed improved detectability of metabolites in pre‐clinical renal studies with a 3.5‐fold, 8.7‐fold, and 6.0‐fold increase in SNR for lactate, alanine, and bicarbonate data, respectively. Variable‐resolution data sets from healthy human subjects showed cardiac structure and neuro‐vasculature in the higher resolution pyruvate images (6.0 × 6.0 mm2 for cardiac and 7.5 × 7.5 mm2 for brain) that would otherwise be missed due to partial‐volume effects and illustrates the level of detail that can be achieved with hyperpolarized substrates in a clinical setting. Conclusion: We developed a variable‐resolution strategy for hyperpolarized 13C MRI using metabolite‐selective imaging and demonstrated that it mitigates tradeoffs between a fixed spatial resolution and SNR for hyperpolarized substrates, providing both high resolution pyruvate and coarse resolution metabolite data sets in a single exam. This technique shows promise to improve future studies by maximizing metabolite SNR while minimizing partial‐volume effects from the injected substrate. [ABSTRACT FROM AUTHOR]

Purpose: To develop new encoding and reconstruction techniques for fast multi‐contrast/quantitative imaging. Methods: The recently proposed Echo Planar Time‐resolved Imaging (EPTI) technique can achieve fast distortion‐ and blurring‐free multi‐contrast/quantitative imaging. In this work, a subspace reconstruction framework is developed to improve the reconstruction accuracy of EPTI at high encoding accelerations. The number of unknowns in the reconstruction is significantly reduced by modeling the temporal signal evolutions using low‐rank subspace. As part of the proposed reconstruction approach, a B0‐update algorithm and a shot‐to‐shot B0 variation correction method are developed to enable the reconstruction of high‐resolution tissue phase images and to mitigate artifacts from shot‐to‐shot phase variations. Moreover, the EPTI concept is extended to 3D k‐space for 3D GE‐EPTI, where a new "temporal‐variant" of CAIPI encoding is proposed to further improve performance. Results: The effectiveness of the proposed subspace reconstruction was demonstrated first in 2D GESE EPTI, where the reconstruction achieved higher accuracy when compared to conventional B0‐informed GRAPPA. For 3D GE‐EPTI, a retrospective undersampling experiment demonstrates that the new temporal‐variant CAIPI encoding can achieve up to 72× acceleration with close to 2× reduction in reconstruction error when compared to conventional spatiotemporal‐CAIPI encoding. In a prospective undersampling experiment, high‐quality whole‐brain T2∗ and tissue phase maps at 1 mm isotropic resolution were acquired in 52 seconds at 3T using 3D GE‐EPTI with temporal‐variant CAIPI encoding. Conclusion: The proposed subspace reconstruction and optimized temporal‐variant CAIPI encoding can further improve the performance of EPTI for fast quantitative mapping. [ABSTRACT FROM AUTHOR]

Purpose: Low‐field MRI offers favorable physical properties for SNR‐efficient long readout acquisitions such as spiral and EPI. We used a 0.55 tesla (T) MRI system equipped with high‐performance hardware to increase the sampling duty cycle and extend the TR of balanced steady‐state free precession (bSSFP) cardiac cine acquisitions, which typically are limited by banding artifacts. Methods: We developed a high‐efficiency spiral in‐out bSSFP acquisition, with zeroth‐ and first‐gradient moment nulling, and an EPI bSSFP acquisition for cardiac cine imaging using a contemporary MRI system modified to operate at 0.55T. Spiral in‐out and EPI bSSFP cine protocols, with TR = 8 ms, were designed to maintain both spatiotemporal resolution and breath‐hold length. Simulations, phantom imaging, and healthy volunteer imaging studies (n = 12) were performed to assess SNR and image quality using these high sampling duty‐cycle bSSFP sequences. Results: Spiral in‐out bSSFP performed favorably at 0.55T and generated good image quality, whereas EPI bSSFP suffered motion and flow artifacts. There was no difference in ejection fraction comparing spiral in‐out with standard Cartesian imaging. Moreover, human images demonstrated a 79% ± 21% increase in myocardial SNR using spiral in‐out bSSFP and 50% ± 14% increase in SNR using EPI bSSFP as compared with the reference Cartesian acquisition. Spiral in‐out acquisitions at 0.55T recovered 69% ± 14% of the myocardial SNR at 1.5T. Conclusion: Efficient bSSFP spiral in‐out provided high‐quality cardiac cine imaging and SNR recovery on a high‐performance 0.55T MRI system. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the use of 3D EPI for rapid T1‐weighted brain imaging, focusing on the RF pulse's influence on the contrast between gray and white matter. Methods: An interleaved 3D EPI sequence use partial Fourier and CAIPIRINHA sampling was used to acquire T1‐weighted brain volumes with isotropic resolution, low echo times, and low geometric distortions. Five different RF pulses were evaluated in terms of fat suppression performance and gray–white matter contrast. Two binomial RF pulses were compared to a single rectangular (WE‐rect) RF pulse exciting only water, and two new RF pulses developed in this work, where one was an extension of the WE‐rect, and the other was an SLR pulse. The technique was demonstrated in three clinical cases, where brain tumor patients were imaged before and after gadolinium administration. Results: A fat‐suppressed 3D EPI sequence with a phase encoding bandwidth of around 100 Hz was found to exhibit a good trade‐off between geometrical distortions and scan duration. Whole‐brain T1‐weighted 3D EPI images with 1.2 mm isotropic voxel size could be acquired in 24 seconds. The WE‐rect, its extension, and the SLR RF pulses resulted in reduced magnetization transfer effects and provided a 20% mean increase in gray–white matter contrast. Conclusion: Using a high phase encoding bandwidth and RF pulses that reduce magnetization transfer effects, a fat‐suppressed multi‐shot 3D EPI sequence can be used to rapidly acquire isotropic T1‐weighted volumes. [ABSTRACT FROM AUTHOR]

Purpose: Echo planar imaging is used widely for its imaging speed. However, its applications often suffer from ghost artifacts. In the community, an approach using three navigator echoes is used commonly for the artifact correction. Although this scheme is effective, as the matrix size increases for high‐resolution imaging, the navigator echoes can contribute significantly to increasing the "required minimum TE." To overcome this issue, this work proposes the use of an alternative navigator echo scheme called the "TR‐external" scheme. Methods: The TR‐external scheme reduces the required minimum TE by allocating an additional excitation loop for the navigator echoes before every main excitation loop. In this work, a detailed analysis on the TR‐external scheme was performed to assess its performance in comparison to the standard scheme. Visual fMRI was performed to check the feasibility of using the TR‐external scheme for detecting functional signals. Results: The performance of the TR‐external scheme was comparable with that of the standard scheme in terms of the SNR, elimination of ghost artifacts, and the BOLD detection. For a given matrix size (288 × 288), the TR‐external scheme allowed a substantially shorter TE (5.94 ms) compared with the standard scheme, which resulted in a higher SNR. Furthermore, this feature enabled the submillimeter‐resolution (0.73 × 0.73 mm2) fMRI measurement with a favorable TE (35 ms) at 3 T. The fMRI results revealed that activated voxels are well localized along the cortical ribbon. Conclusion: A TR‐external scheme for EPI phase correction was implemented at 3 T. Its feasibility for submillimeter‐resolution fMRI was successfully demonstrated. [ABSTRACT FROM AUTHOR]

Purpose: To demonstrate the utility of a high‐performance gradient insert for ultrafast MRI of the human head. Methods: EPI was used for the first time with a readout gradient amplitude of 100 mT/m, 1200 T/m/s slew rate, and nearly 1 MHz signal bandwidth for human head scanning. To avoid artefacts due to eddy currents, the magnetic field was dynamically monitored with NMR probes at multiple points, modeled by solid harmonics up to fifth order, and included in the image reconstruction. An approximation of a negligible intra‐echo effect of the eddy currents was made to accelerate the high‐order reconstruction. The field monitoring‐based approach was compared with a recently proposed phase error estimation from separate reconstructions of even and odd echoes. Results: Images obtained with the gradient insert have significantly lower distortions than it is the case with the whole body 30 mT/m, 200 T/m/s gradients of the same system. However, eddy currents of high spatial order must be properly characterized and corrected for in order to avoid a persistent 2D Nyquist ghost. Multi‐position monitoring proves to be a robust method to measure the eddy currents and allows higher undersampling rates than the image‐based approach. The proposed approximation of the eddy currents effect allows a significant acceleration of the high‐order reconstruction by a separate processing of each spatial dimension. Conclusion: Strong gradients with adequate switching rates are highly beneficial for the quality of EPI provided that robust measures are taken to include the contribution of eddy currents to the image encoding. [ABSTRACT FROM AUTHOR]

Purpose: Asymmetric in‐plane k‐space sampling of EPI can reduce the minimum achievable TE in hyperpolarized 13C with spectral‐spatial radio frequency pulses, thereby reducing T2* weighting and signal‐losses. Partial Fourier image reconstruction exploits the approximate Hermitian symmetry of k‐space data and can be applied to asymmetric data sets to synthesize unmeasured data. Here we tested whether the application of partial Fourier image reconstruction would improve spatial resolution from hyperpolarized [1‐13C]pyruvate scans in the human brain. Methods: Fifteen healthy control subjects were imaged using a volumetric dual‐echo echo‐planar imaging sequence with spectral‐spatial radio frequency excitation. Images were reconstructed by zero‐filling as well as with the partial Fourier reconstruction algorithm projection‐on‐convex‐sets. Resulting images were quantitatively evaluated with a no‐reference image quality assessment. Results: The no‐reference image sharpness metric agreed with perceived improvements in image resolution and contrast. The [1‐13C]lactate images benefitted most, followed by the [1‐13C]pyruvate images. The 13C‐bicarbonate images were improved by the smallest degree, likely owing to relatively lower SNR. Conclusions: Partial Fourier imaging and reconstruction were shown to improve the sharpness and contrast of human HP 13Cbrain data and is a viable method for enhancing resolution. [ABSTRACT FROM AUTHOR]

Purpose: To develop a motion‐robust extension to the recently developed echo‐planar time‐resolved imaging (EPTI) approach, referred to as PROPELLER EPTI with dynamic encoding (PEPTIDE), by incorporating rotations into the rapid, multishot acquisition to enable shot‐to‐shot motion correction. Methods: Echo‐planar time‐resolved imaging is a multishot EPI‐based approach that allows extremely rapid acquisition of distortion‐free and blurring‐free multicontrast imaging and quantitative mapping. By combining k‐space encoding rotations into the EPTI sampling strategy to repeatedly sample the low‐resolution k‐space center, PEPTIDE enables significant tolerance to shot‐to‐shot motion and B0 phase variations. Retrospective PEPTIDE data sets are created through a combination of in vivo EPTI data sets with rotationally acquired protocols, to enable direct comparison of the 2 methods and their robustness to identical motion. The PEPTIDE data sets are also prospectively acquired and again compared with EPTI, in the presence of true subject motion. Results: The PEPTIDE approach is shown to be motion‐robust to even severe subject motion (demonstrated > 30° in‐plane rotation, alongside translational and through‐plane motion), while maintaining the rapid encoding benefits of the EPTI technique. The technique enables accurate quantitative maps to be calculated from even severe motion data sets. While the performance of the motion correction depends on the type and severity of motion encountered, in all cases PEPTIDE significantly increases image quality in the presence of motion comparative to conventional EPTI. Conclusion: The newly developed PEPTIDE technique combines a high degree of motion tolerance into the EPTI framework, enabling highly rapid acquisition of distortion‐free and blurring‐free images at multiple TEs in the presence of motion. [ABSTRACT FROM AUTHOR]

Purpose: Nyquist ghost artifacts in echo planar imaging (EPI) are originated from phase mismatch between the even and odd echoes. However, conventional correction methods using reference scans often produce erroneous results especially in high‐field MRI due to the nonlinear and time‐varying local magnetic field changes. Recently, it was shown that the problem of ghost correction can be reformulated as k‐space interpolation problem that can be solved using structured low‐rank Hankel matrix approaches. Another recent work showed that data driven Hankel matrix decomposition can be reformulated to exhibit similar structures as deep convolutional neural network. By synergistically combining these findings, we propose a k‐space deep learning approach that immediately corrects the phase mismatch without a reference scan in both accelerated and non‐accelerated EPI acquisitions. Theory and Methods: To take advantage of the even and odd‐phase directional redundancy, the k‐space data are divided into 2 channels configured with even and odd phase encodings. The redundancies between coils are also exploited by stacking the multi‐coil k‐space data into additional input channels. Then, our k‐space ghost correction network is trained to learn the interpolation kernel to estimate the missing virtual k‐space data. For the accelerated EPI data, the same neural network is trained to directly estimate the interpolation kernels for missing k‐space data from both ghost and subsampling. Results: Reconstruction results using 3T and 7T in vivo data showed that the proposed method outperformed the image quality compared to the existing methods, and the computing time is much faster. Conclusions: The proposed k‐space deep learning for EPI ghost correction is highly robust and fast, and can be combined with acceleration, so that it can be used as a promising correction tool for high‐field MRI without changing the current acquisition protocol. [ABSTRACT FROM AUTHOR]

Purpose: To enable clinical applications of quantitative magnetization transfer (qMT) imaging by developing a fast method to map one of its fundamental model parameters, the bound pool fraction (BPF), in the human brain. Theory and Methods: The theory of steady‐state MT in the fast‐exchange approximation is used to provide measurements of BPF, and bound pool transverse relaxation time (T2B). A sequence that allows sampling of the signal during steady‐state MT saturation is used to perform BPF mapping with a 10‐min‐long fully echo planar imaging‐based MRI protocol, including inversion recovery T1 mapping and B1 error mapping. The approach is applied in 6 healthy subjects and 1 multiple sclerosis patient, and validated against a single‐slice full qMT reference acquisition. Results: BPF measurements are in agreement with literature values using off‐resonance MT, with average BPF of 0.114(0.100‐0.128) in white matter and 0.068(0.054‐0.085) in gray matter. Median voxel‐wise percentage error compared with standard single slice qMT is 4.6%. Slope and intercept of linear regression between new and reference BPF are 0.83(0.81‐0.85) and 0.013(0.11‐0.16). Bland‐Altman plot mean bias is 0.005. In the multiple sclerosis case, the BPF is sensitive to pathological changes in lesions. Conclusion: The method developed provides accurate BPF estimates and enables shorter scan time compared with currently available approaches, demonstrating the potential of bringing myelin sensitive measurement closer to the clinic. [ABSTRACT FROM AUTHOR]

Purpose: To propose a novel 3D ultrafast gradient echo‐based MRI method, dubbed RASE, using quadratic‐phase encoding. Theory and Methods: Several characteristics of RASE, including spin behaviors, spatial resolution, SNR, and reduction of susceptibility‐induced signal loss, were analytically described. A way of compensating for TE variation was suggested in the quadratic phase‐encoding direction. Lemon, in vivo rat and mouse images were demonstrated at 9.4T, including a feasibility study for DCE‐MRI as one of promising applications. Results: RASE was successfully demonstrated by lemon and in vivo rat brain imaging, showing a good robustness to field inhomogeneity. Contribution of the quadratic phase to signal enhancement in a range of magnetic susceptibilities was also evaluated by simulation. Taking a geometric mean of 2 phantom data acquired with opposite gradient polarities effectively compensated for the effect of TE variation. Preliminary DCE‐MRI results were also presented, showing that RASE could more accurately estimate Gd concentration than FLASH. Conclusion: RASE offers a shorter effective TE, having less sensitivity to field inhomogeneity and T2* effects, much less Nyquist ghosting or chemical‐shift artifacts than gradient echo EPI (GE‐EPI). We highly anticipate that RASE can be an alternative to GE‐EPI in many applications, particularly those requiring high spatial and temporal resolutions in a broad volume coverage. [ABSTRACT FROM AUTHOR]

Purpose: To develop an efficient distortion‐ and blurring‐free multi‐shot EPI technique for time‐resolved multiple‐contrast and/or quantitative imaging. Methods: EPI is a commonly used sequence but suffers from geometric distortions and blurring. Here, we introduce a new multi‐shot EPI technique termed echo planar time‐resolved imaging (EPTI), which has the ability to rapidly acquire distortion‐ and blurring‐free multi‐contrast data set. The EPTI approach performs encoding in ky‐t space and uses a new highly accelerated spatio–temporal CAIPI sampling trajectory to take advantage of signal correlation along these dimensions. Through this acquisition and a B0‐informed parallel imaging reconstruction, hundreds of "time‐resolved" distortion‐ and blurring‐free images at different TEs across the EPI readout window can be created at sub‐millisecond temporal increments using a small number of EPTI shots. Moreover, a method for self‐estimation and correction of shot‐to‐shot B0 variations was developed. Simultaneous multi‐slice acquisition was also incorporated to further improve the acquisition efficiency. Results: We evaluated EPTI under varying simulated acceleration factors, B0‐inhomogeneity, and shot‐to‐shot B0 variations to demonstrate its ability to provide distortion‐ and blurring‐free images at multiple TEs. Two variants of EPTI were demonstrated in vivo at 3T: (1) a combined gradient‐ and spin‐echo EPTI for quantitative mapping of T2, T2*, proton density, and susceptibility at 1.1 × 1.1 × 3 mm3 whole‐brain in 28 s (0.8 s/slice), and (2) a gradient‐echo EPTI, for multi‐echo and quantitative T2* fMRI at 2 × 2 × 3 mm3 whole‐brain at a 3.3 s temporal resolution. Conclusion: EPTI is a new approach for multi‐contrast and/or quantitative imaging that can provide fast acquisition of distortion‐ and blurring‐free images at multiple TEs. [ABSTRACT FROM AUTHOR]

Purpose : To propose a methodology for assessment of algorithms that correct distortions due to motion, eddy‐currents, and echo planar imaging in diffusion weighted images (DWIs). Methods : The proposed method evaluates correction performance by measuring variability across datasets of the same object acquired with images having distortions in different directions, thereby overcoming the unavailability of ground‐truth, undistorted DWIs. A comprehensive diffusion MRI dataset, collected using a suitable experimental design, is made available to the scientific community, consisting of three DWI shells (Bmax = 5000 s/mm2), 30 gradient directions, a replicate set of antipodal gradient directions, four phase‐encoding directions, and three different head orientations. The proposed methodology was tested using the TORTOISE diffusion MRI processing pipeline. Results : The median variability of the original distorted data was 123% higher for DWIs, 100–168% higher for tensor‐derived metrics and 28–111% higher for MAPMRI metrics, than in the corrected versions. EPI distortions induced substantial variability, nearly comparable to the contribution of eddy‐current distortions. Conclusions : The dataset and the evaluation strategy proposed herein enable quantitative comparison of different methods for correction of distortions due to motion, eddy‐currents, and other EPI distortions, and can be useful in benchmarking newly developed algorithms. [ABSTRACT FROM AUTHOR]

Purpose: To develop and translate a metabolite‐specific imaging sequence using a symmetric echo planar readout for clinical hyperpolarized (HP) Carbon‐13 (13C) applications. Methods: Initial data were acquired from patients with prostate cancer (N = 3) and high‐grade brain tumors (N = 3) on a 3T scanner. Samples of [1‐13C]pyruvate were polarized for at least 2 h using a 5T SPINlab system operating at 0.8 K. Following injection of the HP substrate, pyruvate, lactate, and bicarbonate (for brain studies) were sequentially excited with a singleband spectral‐spatial RF pulse and signal was rapidly encoded with a single‐shot echo planar readout on a slice‐by‐slice basis. Data were acquired dynamically with a temporal resolution of 2 s for prostate studies and 3 s for brain studies. Results: High pyruvate signal was seen throughout the prostate and brain, with conversion to lactate being shown across studies, whereas bicarbonate production was also detected in the brain. No Nyquist ghost artifacts or obvious geometric distortion from the echo planar readout were observed. The average error in center frequency was 1.2 ± 17.0 and 4.5 ± 1.4 Hz for prostate and brain studies, respectively, below the threshold for spatial shift because of bulk off‐resonance. Conclusion: This study demonstrated the feasibility of symmetric EPI to acquire HP 13C metabolite maps in a clinical setting. As an advance over prior single‐slice dynamic or single time point volumetric spectroscopic imaging approaches, this metabolite‐specific EPI acquisition provided robust whole‐organ coverage for brain and prostate studies while retaining high SNR, spatial resolution, and dynamic temporal resolution. [ABSTRACT FROM AUTHOR]

Purpose: To provide simultaneous multislice (SMS) EPI reconstruction with k‐space implementation and robust Nyquist ghost correction. Methods: 2D phase error correction SENSE (PEC‐SENSE) was recently developed for Nyquist ghost correction in SMS EPI reconstruction for which virtual coil simultaneous autocalibration and k‐space estimation (VC‐SAKE) was used to remove slice‐dependent Nyquist ghosts and intershot 2D phase variations in multi‐shot EPI reference scan. However, masking coil sensitivity maps to exclude background region in PEC‐SENSE and manually selecting slice‐wise target ranks in VC‐SAKE are cumbersome procedures in practice. To avoid masking, the concept of PEC‐SENSE is extended to k‐space implementation and termed as PEC‐GRAPPA. Furthermore, a singular value shrinkage scheme is incorporated in VC‐SAKE to circumvent the empirical slice‐wise target rank selection. PEC‐GRAPPA was evaluated and compared to PEC‐SENSE with/without masking and 1D linear phase correction GRAPPA. Results: PEC‐GRAPPA robustly reconstructed SMS EPI images from 7T phantom and human brain data, effectively removing the phase error‐induced artifacts. The resulting residual artifact level and temporal SNR were comparable to those by PEC‐SENSE with careful tuning. PEC‐GRAPPA outperformed PEC‐SENSE without masking and 1D linear phase correction GRAPPA. Conclusion: Our proposed PEC‐GRAPPA approach effectively removes the artifacts caused by Nyquist ghosts in SMS EPI without cumbersome tuning. This approach provides a robust and practical implementation of SMS EPI reconstruction in k‐space with slice‐dependent 2D Nyquist ghost correction. [ABSTRACT FROM AUTHOR]

Purpose: Spatial encoding and shimming in MRI have traditionally been performed using dedicated coils that generate orthogonal spherical harmonic fields. The recently introduced multi‐coil hardware has proven that MRI‐relevant magnetic fields can also be created by a generic set of localized coils producing non‐orthogonal fields. As a step towards establishing a purely multi‐coil‐based MRI field generation system, the feasibility of performing conventional Cartesian k‐space encoding and echo‐planar imaging (EPI), as well as concurrent encoding and shimming is demonstrated in this study. Methods: We report the use of Dynamic Multi‐Coil Technique (DYNAMITE) for combined Cartesian encoding and shimming, and EPI using a 48‐channel multi‐coil system. Experiments were performed on phantom objects and biological specimens in a 9.4 T pre‐clinical scanner. Cartesian Fourier‐encoded MRI and EPI were implemented whereby the magnetic fields required for encoding of the three orthogonal spatial dimensions were entirely based on linear combinations of multi‐coil fields. Furthermore, DYNAMITE imaging was augmented by concurrent DYNAMITE shimming with the same hardware. Results: DYNAMITE‐based MR and echo‐planar images were indistinguishable from those acquired with the conventional linear imaging gradients provided by the scanner. In experiments with concurrent DYNAMITE shimming and imaging, shim challenges that would result in extreme spatial distortion and signal loss were corrected very effectively with more than 92% signal recovery in case of extreme Z2 shim challenge that resulted in complete signal dephasing in most slices. Conclusions: We demonstrate the first successful implementation of combined DYNAMITE imaging and shimming and show the feasibility of performing EPI with DYNAMITE hardware. Our results substantiate the potential of multi‐coil hardware as a full‐fledged imaging and shimming system, with additional potential benefits of reduced echo‐time and risk of peripheral nerve stimulation while performing EPI. [ABSTRACT FROM AUTHOR]

Purpose: Echo planar imaging (EPI) is the primary sequence for functional and diffusion MRI. In fetal applications, the large field of view needed to encode the maternal abdomen leads to prolonged EPI readouts, which may be further extended due to safety considerations that limit gradient performance. The resulting images become very sensitive to water‐fat shift and susceptibility artefacts. The purpose of this study was to reduce artefacts and increase stability of EPI in fetal brain imaging, balancing local field homogeneity across the fetal brain with longer range variations to ensure compatibility with fat suppression of the maternal abdomen. Methods: Spectral Pre‐saturation with Inversion‐Recovery (SPIR) fat suppression was optimized by investigating SPIR pulse frequency offsets. Subsequently, fetal brain EPI data were acquired using image‐based (IB) shimming on 6 pregnant women by (1) minimizing B0 field variations within the fetal brain (localized IB shimming) and (2) with added constraint to limit B0 variation in maternal fat (fat constrained IB shimming). Results: The optimal offset for the SPIR pulse at 3 Tesla was 550 Hz. Both shimming approaches had similar performances in terms of B0 homogeneity within the brain, but constrained IB shimming enabled higher fat suppression efficiency. Conclusion: Optimized SPIR in combination with constrained IB shimming can improve maternal fat suppression while minimizing EPI distortions in the fetal brain. [ABSTRACT FROM AUTHOR]

Purpose: To reconstruct artifact‐free images from measured k‐space data, when the actual k‐space trajectory deviates from the nominal trajectory due to gradient imperfections. Methods: Trajectory errors arising from eddy currents and gradient delays introduce phase inconsistencies in several fast scanning MR pulse sequences, resulting in image artifacts. The proposed algorithm provides a novel framework to compensate for this phase distortion. The algorithm relies on the construction of a multi‐block Hankel matrix, where each block is constructed from k‐space segments with the same phase distortion. In the presence of spatially smooth phase distortions between the segments, the complete block‐Hankel matrix is known to be highly low‐rank. Since each k‐space segment is only acquiring part of the k‐space data, the reconstruction of the phase compensated image from their partially parallel measurements is posed as a structured low‐rank matrix optimization problem, assuming the coil sensitivities to be known. Results: The proposed formulation is tested on radial acquisitions in several settings including partial Fourier and golden‐angle acquisitions. The experiments demonstrate the ability of the algorithm to successfully remove the artifacts arising from the trajectory errors, without the need for trajectory or phase calibration. The quality of the reconstruction was comparable to corrections achieved using the Trajectory Auto‐Corrected Image Reconstruction (TrACR) for radial acquisitions. Conclusion: The proposed method provides a general framework for the recovery of artifact‐free images from radial trajectories without the need for trajectory calibration. [ABSTRACT FROM AUTHOR]

Purpose: To improve simultaneous multislice (SMS) EPI by robust Nyquist ghost correction in both coil sensitivity calibration and SMS reconstruction. Methods: To derive coil sensitivity and slice‐dependent phase difference map between positive‐ and negative‐echo images, single‐band EPI reference data are fully sampled with EPI parameters matched to SMS acquisition. First, the reference data are organized into positive‐ and negative‐echo virtual channels where missing data are estimated using low‐rank‐based simultaneous autocalibrating and k‐space estimation (SAKE) at small matrix size. The resulting ghost‐free positive‐ and negative‐echo images are combined to generate coil sensitivity maps. Second, full‐matrix positive‐ and negative‐echo images are SENSE reconstructed from the reference data. Their phase difference or error map is then calculated. Last, SMS EPI is reconstructed using phase error correction SENSE (PEC‐SENSE) that incorporates phase error map into coil sensitivity maps for negative‐echo data. The proposed method was evaluated using both experimental data from 7T systems and simulations. Results: Virtual coil SAKE eliminated Nyquist ghosts in the single‐band EPI, yielding high‐quality coil sensitivity maps and phase error maps. The subsequent PEC‐SENSE robustly reconstructed SMS EPI under various conditions, including presence of in‐plane acceleration, with lesser artifacts and higher temporal SNR than slice‐dependent 1D linear correction method. Conclusion: The proposed procedure of virtual coil SAKE calibration and PEC‐SENSE reconstruction substantially reduces all ghost‐related artifacts originating either directly from SMS EPI data or indirectly from EPI‐based coil sensitivity maps. It is computationally efficient, and generally applicable to all SMS EPI‐based applications. [ABSTRACT FROM AUTHOR]

Background: Pancreatic enzyme supplements for the treatment of exocrine pancreatic insufficiency (EPI) in dogs can be uncoated or enteric coated. Enteric coated supplements might be advantageous. Hypothesis/Objectives: Enteric coated enzyme supplements are superior to uncoated supplements in dogs with clinical EPI. Animals: Eleven dogs with naturally occurring EPI that were apparently free from other diseases. Methods: Randomized, blinded, controlled cross‐over clinical trial comparing a novel micro‐encapsulated enteric coated enzyme supplement to a commercially available uncoated product in dogs with clinical EPI. Search of serum canine serum trypsin‐like immunoreactivity concentration ≤ 2.5 µg/L in the Gastrointestinal Laboratory database was used to identify dogs with EPI. Results: There was no difference −4.46% (95% CI: −7.97%‐–0.96%; P = .15) in the % acid hydrolysis fecal fat (primary outcome) between the enteric coated formulation (median: 11.8%; range 6.4%‐17.0%) and the uncoated pancreatic enzyme replacement product (median: 17.5%; range: 5.2%‐24.9%) in the 11 dogs that completed the study. Other variables did not differ between treatments. Conclusions and Clinical Importance: This study, which had low statistical power, did not detect a difference between formulations. [ABSTRACT FROM AUTHOR]

Purpose: Demonstrate a novel fast method for reconstruction of multi‐dimensional MR fingerprinting (MRF) data using deep learning methods. Methods: A neural network (NN) is defined using the TensorFlow framework and trained on simulated MRF data computed with the extended phase graph formalism. The NN reconstruction accuracy for noiseless and noisy data is compared to conventional MRF template matching as a function of training data size and is quantified in simulated numerical brain phantom data and International Society for Magnetic Resonance in Medicine/National Institute of Standards and Technology phantom data measured on 1.5T and 3T scanners with an optimized MRF EPI and MRF fast imaging with steady state precession (FISP) sequences with spiral readout. The utility of the method is demonstrated in a healthy subject in vivo at 1.5T. Results: Network training required 10 to 74 minutes; once trained, data reconstruction required approximately 10 ms for the MRF EPI and 76 ms for the MRF FISP sequence. Reconstruction of simulated, noiseless brain data using the NN resulted in a RMS error (RMSE) of 2.6 ms for T1 and 1.9 ms for T2. The reconstruction error in the presence of noise was less than 10% for both T1 and T2 for SNR greater than 25 dB. Phantom measurements yielded good agreement (R2 = 0.99/0.99 for MRF EPI T1/T2 and 0.94/0.98 for MRF FISP T1/T2) between the T1 and T2 estimated by the NN and reference values from the International Society for Magnetic Resonance in Medicine/National Institute of Standards and Technology phantom. Conclusion: Reconstruction of MRF data with a NN is accurate, 300‐ to 5000‐fold faster, and more robust to noise and dictionary undersampling than conventional MRF dictionary‐matching. [ABSTRACT FROM AUTHOR]