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Respiratory motion causes misalignments between positron emission tomography (PET) and magnetic resonance (MR)-derived attenuation maps (µ-maps) in addition to artifacts on both PET and MR images in simultaneous PET/MRI for organs such as liver that can experience motion of several centimeters. To address this problem, we developed an efficient MR-based attenuation correction (MRAC) method to generate phase-matched µ-maps for quiescent period PET (PETQ) in abdominal PET/MRI. MRAC data was acquired with CIRcular Cartesian UnderSampling (CIRCUS) sampling during 100 s in free-breathing as an accelerated data acquisition strategy for phase-matched MRAC (MRACPM-CIRCUS). For comparison, MRAC data with raster (Default) k-space sampling was also acquired during 100 s in free-breathing (MRACPM-Default), and used to evaluate MRACPM-CIRCUS as well as un-matched MRAC (MRACUM) that was un-gated. We purposefully oversampled the MRACPM data to ensure we had enough information to capture all respiratory phases to make this comparison as robust as possible. The proposed MRACPM-CIRCUS was evaluated in 17 patients with 68Ga-DOTA-TOC PET/MRI exams, suspected of having neuroendocrine tumors or liver metastases. Effects of CIRCUS sampling for accelerating a data acquisition were evaluated by simulating the data acquisition time retrospectively in increments of 5 s. Effects of MRACPM-CIRCUS on PETQ were evaluated using uptake differences in the liver lesions (n  =  35), compared to PETQ with MRACPM-Default and MRACUM. A Wilcoxon signed-rank test was performed to compare lesion uptakes between the MRAC methods. MRACPM-CIRCUS showed higher image quality compared to MRACPM-Default for the same acquisition times, demonstrating that a data acquisition time of 30 s was reasonable to achieve phase-matched µ-maps. Lesion update differences between MRACPM-CIRCUS (30 s) versus MRACPM-Default (reference, 100 s) were 0.1%  ±  1.4% (range of  -2.7% to 3.2%) and not significant (P  >  .05); while, the differences between MRACUM versus MRACPM-Default were 0.6%  ±  11.4% with a large variation (range of  -37% to 20%) and significant (P

Background: Diffusion‐weighted imaging (DWI) is common for evaluating pediatric musculoskeletal lesions, but suffers from geometric distortion and intense acoustic noise. Purpose To investigate the performance of a near‐silent and distortion‐free DWI sequence (DW‐SD) relative to standard echo‐planar DWI (DW‐EPI) in pediatric extremity MRI. Study Type: Prospective validation study. Subjects: Thirty‐nine children referred for extremity MRI. Field Strength/Sequence: DW‐EPI and DW‐SD, based on a rotating ultrafast sequence modified with sinusoidal diffusion preparation gradients, at 3T. Assessment DW‐SD image quality (Sanat) was assessed from 0 (nondiagnostic) to 5 (outstanding) and comparative image quality (Scomp) (from –2 = DW‐EPI more delineated to +2 = DW‐SD more delineated, 0 = same). ADC measured by DW‐SD and DW‐EPI were compared in bone marrow, muscle, and lesions. Statistical Tests: Wilcoxon rank‐sum test and confidence interval of proportions (CIOP) were calculated for Scomp, Student's t‐test, coefficient of variation (COV), and Bland–Altman analysis for ADC values, and intraclass correlation coefficient (ICC) for interreader agreement. Results: DW‐SD and DW‐EPI ADC values for bone marrow, muscle, and lesions were not significantly different (P = 0.3, P = 0.2, and P = 0.27, respectively) and had an overall ADC COV of 14.8% (95% confidence interval: 12.3%, 16.9%) and no significant proportional bias on Bland–Altman analysis. Sanat CIOP was rated diagnostic or better (score of 3, 4, or 5) in 72–98% of cases for bone marrow, muscle, and soft tissues. DW‐SD was equivalent to or preferred over DW‐EPI in muscles and soft tissues, with CIOP 86–93% and 93%, respectively. Lesions were equally visualized on DW‐SD and DW‐EPI in 40–51%, with DW‐SD preferred in 44–56% of cases. DW‐SD was rated significantly better than DW‐EPI across all comparative variables that included bone marrow, muscle, soft tissue, cartilage, and lesions (P < 0.05). Readers had moderate to near‐perfect (ICC range = 0.45–0.85). Data Conclusion: DW‐SD of the extremities provided similar ADC values and improved image quality compared with conventional DW‐EPI. Level of Evidence 2 Technical Efficacy Stage 2 J. MAGN. RESON. IMAGING 2021;53:504–513. [ABSTRACT FROM AUTHOR]

Purpose: Because of short signal lifetimes and respiratory motion, 3D lung MRI is still challenging today. Zero‐TE (ZTE) pulse sequences offer promising solutions as they overcome the issue of short T2∗. Nevertheless, as they rely on continuous readout gradients, the trajectories they follow in k‐space are not adapted to retrospective gating and inferred motion correction. Theory and methods: We propose AZTEK (adaptive ZTE k‐space trajectories), a set of 3D radial trajectories featuring three tuning parameters, to adapt the acquisition to any moving organ while keeping seamless transitions between consecutive spokes. Standard ZTE and AZTEK trajectories were compared for static and moving phantom acquisitions as well as for human thoracic imaging performed on 3 volunteers (1 healthy and 2 patients with lung cancer). Results: For the static phantom, we observe comparable image qualities with standard and AZTEK trajectories. For the moving phantom, spatially coherent undersampling artifacts observed on gated images with the standard trajectory are alleviated with AZTEK. The same improvement in image quality is obtained in human, so details are more delineated in the lung with the use of the adaptive trajectory. Conclusion: The AZTEK technique opens the possibility for 3D dynamic ZTE lung imaging with retrospective gating. It enables us to uniformly sample the k‐space for any arbitrary respiratory motion gate, while preserving static image quality, improving dynamic image quality and guaranteeing continuous readout gradient transitions between spokes, which makes it appropriate to ZTE. [ABSTRACT FROM AUTHOR]

Purpose: To develop a near‐silent and distortionless DWI (sd‐DWI) sequence using magnetization‐prepared rotating ultrafast imaging sequence. Methods: A rotating ultrafast imaging sequence was modified with driven‐equilibrium diffusion preparation, including eddy‐current compensation methods. To compensate for the T1 recovery during readout, a phase‐cycling method was used. Both compensation methods were validated in phantoms. The optimized sequence was compared with an EPI diffusion sequence for image distortion, contrast, ADC, and acoustic noise level in phantoms. The sequence was evaluated in 1 brain volunteer, 5 prostate volunteers, and 10 pediatric patients with joint diseases. Results: Combination of several eddy‐current compensation methods reduced the artifact to an acceptable level. Phase cycling reduced T1 recovery contamination during readout. In phantom scans, the optimized sequence generated similar image contrast to the EPI diffusion sequence, and ADC maps between the sequences were comparable; sd‐DWI had significantly lower acoustic noise (P <.05). In vivo brain scan showed reduced image distortion in sd‐DWI compared with the EPI diffusion, although residual motion artifact remains due to brain pulsation. The prostate scans showed that sd‐DWI can provide similar ADC compared with EPI diffusion, with no image distortion. Patient scans showed that the sequence can clearly depict joint lesions. Conclusion: An sd‐DWI sequence was developed and optimized. Compared with conventional EPI diffusion, sd‐DWI provided similar diffusion contrast, accurate ADC measurement, improved image quality, and minimal ambient scanning noise. The sequence showed the ability to obtain in vivo diffusion contrast in relatively motion‐free body regions, such as prostate and joint. [ABSTRACT FROM AUTHOR]

Purpose: To obtain T2* and T2‐weighted images as well as quantitative T2*, T2, and susceptibility maps with a novel, silent 3D imaging method, which combines zero‐echo‐time (ZTE) imaging with gradient‐ and spin‐echo BURST encoding. Methods: After a segment of standard ZTE encoding with multiple 3D radial k‐space spokes, the direction of traversing k‐space is reversed while excitation is switched off. This recalls gradient echoes for each spoke/excitation. This results in multiple images: one FID image from ZTE and multiple BURST echo images at different echo times weighted by a T2* decay. By adding a pair of 180° pulses with an appropriate wait period, it is also possible to obtain spin echoes, leading to T2‐weighted images. Data is reconstructed using standard 3D gridding and Fourier transformation. In vivo feasibility was demonstrated by imaging the brain of multiple healthy volunteers. Results: It is possible to acquire high‐quality T2*‐ and T2‐weighted brain images in a silent manner. From images acquired with gradient‐echo ZTE‐BURST, it is possible to extract quantitative T2* and magnetic susceptibility maps, whereas the spin echo version yields T2 maps. Conclusion: ZTE combined with BURST enables silent acquisition of T2*‐ and T2‐weighted images with good image quality. [ABSTRACT FROM AUTHOR]

Background: Field inhomogeneities in MRI caused by interactions between the radiofrequency field and the patient anatomy can lead to artifacts and contrast variations, consequently degrading the overall image quality and thereby compromising diagnostic value of the images.Purpose: To develop an efficient free-breathing and motion-robust B1+ mapping method that allows for the investigation of spatial homogeneity of the transmitted radiofrequency field in the myocardium at 3.0T. Three joint approaches are used to adapt the dual refocusing echo acquisition mode (DREAM) sequence for cardiac applications: (1) electrocardiograph triggering; (2) a multi-snapshot undersampling scheme, which relies on the Golden Ratio, to accelerate the acquisition; and (3) motion-compensation based on low-resolution images acquired in each snapshot.Study Type: Prospective.Phantom/subjects: Eurospin II T05 system, torso phantom, and five healthy volunteers.Field Strength/sequence: 3.0T/DREAM.Assessment: The proposed method was compared with the Bloch-Siegert shift (BSS) method and validated against the standard DREAM sequence. Cardiac B1+ maps were obtained in free-breathing and breath-hold as a proof of concept of the in vivo performance of the proposed method.Statistical Tests: Mean and standard deviation (SD) values were analyzed for six standard regions of interest within the myocardium. Repeatability was assessed in terms of SD and coefficient of variation.Results: Phantom results indicated low deviation from the BSS method (mean difference = 3%). Equivalent B1+ distributions for free-breathing and breath-hold in vivo experiments demonstrated the motion robustness of this method with good repeatability (SD < 0.05). The amount of B1+ variations was found to be 26% over the myocardium within a short axis slice.Data Conclusion: The feasibility of a cardiac B1+ mapping method with high spatial resolution in a reduced scan time per trigger was demonstrated. The free-breathing characteristic could be beneficial to determine shim components for multi-channel systems, currently limited to two for a single breath-hold.Level Of Evidence: 1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:499-507. [ABSTRACT FROM AUTHOR]

Purpose: To introduce a novel MR pulse sequence, termed Looping Star, for fast, robust, and yet quiet, 3D radial multi‐gradient echo T2* MR imaging. Methods: The Looping Star pulse sequence is based on the 3D radial Rotating Ultra‐Fast Imaging Sequence (RUFIS) extended by a time‐multiplexed gradient‐refocusing mechanism. First, multiple magnetic coherences are excited, which are subsequently gradient‐refocused in form of a looping k‐space trajectory. Accordingly, Looping Star captures an initial FID image followed by gradient echo images at equidistant echo times. Results: Looping Star was demonstrated in phantom and in vivo volunteer experiments for 3D, high resolution T2* weighted imaging, T2* mapping, and quantitative susceptibility mapping (QSM). The method is fast, quiet, and robust against imperfections including Eddy currents, motion, and geometric distortions. When applied to a motor task fMRI experiment a BOLD sensitivity of 5% was achieved at minimal acoustic noise (i.e. 2.7 dB(A) above ambient noise) and with images congruent to other anatomical scans. Conclusions: Looping Star imaging provides new and exciting opportunities for fast, robust and yet quiet T2* MR imaging. Potential applications include T2*‐weighted imaging, T2* mapping, QSM, and fMRI. [ABSTRACT FROM AUTHOR]

Purpose: To evaluate the impact of a novel postprocessing denoising technique on accuracy and precision in myocardial T1 mapping.Materials and Methods: This study introduces a fast and robust denoising method developed for magnetic resonance T1 mapping. The technique imposes edge-preserving regularity and exploits the co-occurence of spatial gradients in the acquired T1 -weighted images. The proposed approach was assessed in simulations, ex vivo data and in vivo imaging on a cohort of 16 healthy volunteers (12 males, average age 39 ± 8 years, 62 ± 9 bpm) both in pre- and postcontrast injection. The method was evaluated in myocardial T1 mapping at 3T with a saturation-recovery technique that is accurate but sensitive to noise. ROIs in the myocardium and left-ventricle blood pool were analyzed by an experienced reader. Mean T1 values and standard deviation were extracted and compared in all studies.Results: Simulations on synthetic phantom showed signal-to-noise ratio and sharpness improvement with the proposed method in comparison with conventional denoising. In vivo results demonstrated that our method preserves accuracy, as no difference in mean T1 values was observed in the myocardium (precontrast: 1433/1426 msec, 95%CI: [-40.7, 55.9], p = 0.75, postcontrast: 766/759 msec, 95%CI: [-60.7, 77.2], p = 0.8). Meanwhile, precision was improved with standard deviations of T1 values being significantly decreased (precontrast: 223/151 msec, 95%CI: [27.3, 116.5], p = 0.003, postcontrast: 176/135 msec, 95%CI: [5.5, 77.1], p = 0.03).Conclusion: The proposed denoising method preserves accuracy and improves precision in myocardial T1 mapping, with the potential to offer better map visualization and analysis.Level Of Evidence: 3 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2017;46:1377-1388. [ABSTRACT FROM AUTHOR]

Purpose: To develop and evaluate a novel MR method that addresses some of the most eminent technical challenges of current BOLD-based fMRI in terms of 1) acoustic noise and 2) geometric distortions and signal dropouts. Methods: A BOLD-sensitive fMRI pulse sequence was designed that first generates T2-weighted magnetization (using a T2 preparation module) and subsequently undergoes three-dimensional (3D) radial encoding using a rotating ultrafast imaging sequence (RUFIS). The method was tested on healthy volunteers at 3Twith motor, visual, and auditory tasks, and compared relative to standard gradient and spin echo planar imaging (EPI) methods. Results: In combination with parallel imaging the method achieves efficient and robust 3D whole brain coverage (3 mm isotropic resolution in 2.65 s scan time). Compared with standard EPI-based fMRI, the method demonstrated 1) T2-weighted imaging clean of geometrical distortions and signal dropout, 2) an acoustic noise reduction of ~40 dB(A), and 3) a consistent BOLD response that is less sensitive (~1.3% BOLD change) but spatially more specific. Conclusion: T2-prepared RUFIS provides quiet and distortionfree whole brain BOLD fMRI with minimal demands on the gradient performance. In particular, auditory fMRI and/or studies involving brain regions near air-tissue interfaces are expected to greatly benefit from the proposed method, especially if performed at ultrahigh field strengths. [ABSTRACT FROM AUTHOR]