Search

Purpose: For reliable DCE MRI parameter estimation, k‐space undersampling is essential to meet resolution, coverage, and signal‐to‐noise requirements. Pseudo‐spiral (PS) sampling achieves this by sampling k‐space on a Cartesian grid following a spiral trajectory. The goal was to optimize PS k‐space sampling patterns for abdomin al DCE MRI. Methods: The optimal PS k‐space sampling pattern was determined using an anthropomorphic digital phantom. Contrast agent inflow was simulated in the liver, spleen, pancreas, and pancreatic ductal adenocarcinoma (PDAC). A total of 704 variable sampling and reconstruction approaches were created using three algorithms using different parametrizations to control sampling density, halfscan and compressed sensing regularization. The sampling patterns were evaluated based on image quality scores and the accuracy and precision of the DCE pharmacokinetic parameters. The best and worst strategies were assessed in vivo in five healthy volunteers without contrast agent administration. The best strategy was tested in a DCE scan of a PDAC patient. Results: The best PS reconstruction was found to be PS‐diffuse based, with quadratic distribution of readouts on a spiral, without random shuffling, halfscan factor of 0.8, and total variation regularization of 0.05 in the spatial and temporal domains. The best scoring strategy showed sharper images with less prominent artifacts in healthy volunteers compared to the worst strategy. Our suggested DCE sampling strategy also showed high quality DCE images in the PDAC patient. Conclusion: Using an anthropomorphic digital phantom, we identified an optimal PS sampling strategy for abdominal DCE MRI, and demonstrated feasibility in a PDAC patient. [ABSTRACT FROM AUTHOR]

Purpose: Aortic motion has direct impact on the mechanical stresses acting on the aorta. In aortic disease, increased stiffness of the aorta may lead to decreased aortic motion over time, which could be a predictor for aortic dissection or rupture. This study investigates the reproducibility of obtaining 3D displacement and diameter maps quantified using accelerated 3D cine MRI at 3 T. Methods: A noncontrast‐enhanced, free‐breathing 3D cine sequence based on balanced SSFP and pseudo‐spiral undersampling with high spatial isotropic resolution was developed (spatial/temporal resolution [1.6 mm]3/67 ms). The thoracic aorta of 14 healthy volunteers was prospectively scanned three times at 3 T: twice on the same day and a third time 2 weeks later. Aortic displacement was calculated using iterative closest point nonrigid registration of manual segmentations of the 3D aorta at end‐systole and mid‐diastole. Interexamination and interobserver regional analysis of mean displacement for five regions of interest was performed using Bland–Altman analysis. Additionally, a complementary voxel‐by‐voxel analysis was done, allowing a more local inspection of the method. Results: No significant differences were found in mean and maximum displacement for any of the regions of interest for the interexamination and interobserver analysis. The maximum displacement measured in the lower half of the ascending aorta was 11.0 ± 3.4 mm (range: 3.0–17.5 mm) for the first scan. The smallest detectable change in mean displacement in the lower half of the ascending aorta was 3 mm. Conclusion: Detailed 3D cine balanced SSFP at 3 T allows for reproducible quantification of systolic–diastolic mean aortic displacement within acceptable limits. [ABSTRACT FROM AUTHOR]

Purpose: To obtain homogeneous signal throughout the human liver at 7 T. Flip angle (FA) shimming in 7T whole‐liver imaging was performed through parallel‐transmit kT‐point pulses based on subject‐specific multichannel absolute B1+$$ {\mathrm{B}}_1^{+} $$ maps from Fourier phase‐encoded dual refocusing echo acquisition mode (PE‐DREAM). Methods: The optimal number of Fourier phase‐encoding steps for PE‐DREAM B1+$$ {\mathrm{B}}_1^{+} $$ mapping was determined for a 7T eight‐channel parallel‐transmission system. FA shimming experiments were performed in the liver of 7 healthy subjects with varying body mass index. In these subjects, first B0$$ {\mathrm{B}}_0 $$ shimming and Fourier PE‐DREAM B1+$$ {\mathrm{B}}_1^{+} $$ mapping were performed. Subsequently, three small‐flip‐angle 3D gradient‐echo scans were acquired, comparing a circularly polarized (CP) mode, a phase shim, and a kT‐point pulse. Resulting homogeneity was assessed and compared with estimated FA maps and distributions. Results: Fourier PE‐DREAM with 13 phase‐encoding steps resulted in a good tradeoff between B1+$$ {\mathrm{B}}_1^{+} $$ accuracy and scan time. Lower coefficient of variation values (average [min‐max] across subjects) of the estimated FA in the volume of interest were observed using kT‐points (7.4 [6.6%–8.0%]), compared with phase shimming (18.8 [12.9%–23.4%], p < 0.001) and CP (43.2 [39.4%–47.1%], p < 0.001). kT‐points delivered whole‐liver images with the nominal FA and the highest degree of homogeneity. CP and phase shimming resulted in either inaccurate or imprecise FA distributions. Here, locations having suboptimal FA in the estimated FA maps corresponded to liver areas suffering from inconsistent signal intensity and T1‐weighting in the gradient‐echo scans. Conclusion: Homogeneous whole‐liver 3D gradient‐echo acquisitions at 7 T can be obtained with eight‐channel kT‐point pulses calculated based on subject‐specific multichannel absolute Fourier PE‐DREAM B1+$$ {\mathrm{B}}_1^{+} $$ maps. [ABSTRACT FROM AUTHOR]

Purpose: Cerebral T2 oximetry is a non‐invasive imaging method to measure blood T2 and cerebral venous oxygenation. Measured T2 values are converted to oximetry estimates using carefully validated and potentially disease‐specific calibrations. In sickle cell disease, red blood cells have abnormal cell shape and membrane properties that alter T2 oximetry calibration relationships in clinically meaningful ways. Previous in vitro works by two independent groups established potentially competing calibration models. Methods: This study analyzed pooled datasets from these two studies to establish a unified and more robust sickle‐specific calibration to serve as a reference standard in the field. Results: Even though the combined calibration did not demonstrate statistical superiority compared to previous models, the calibration was unbiased compared to blood‐gas co‐oximetry and yielded limits of agreement of (−10.1%, 11.6%) in non‐transfused subjects with sickle cell disease. In transfused patients, this study proposed a simple correction method based on individual hemoglobin S percentage that demonstrated reduced bias in saturation measurement compared to previous uncorrected sickle calibrations. Conclusion: The combined calibration is based on a larger range of hematocrit, providing greater confidence in the hematocrit‐dependent model parameters, and yielded unbiased estimates to blood‐gas co‐oximetry measurements from both sites. Additionally, this work also demonstrated the need to correct for transfusion in T2 oximetry measurements for hyper‐transfused sickle cell disease patients and proposes a correction method based on patient‐specific hemoglobin S concentration. [ABSTRACT FROM AUTHOR]

Purpose: Gadolinium‐based dynamic susceptibility contrast (DSC) is commonly used to characterize blood flow in patients with stroke and brain tumors. Unfortunately, gadolinium contrast administration has been associated with adverse reactions and long‐term accumulation in tissues. In this work, we propose an alternative deoxygenation‐based DSC (dDSC) method that uses a transient hypoxia gas paradigm to deliver a bolus of paramagnetic deoxygenated hemoglobin to the cerebral vasculature for perfusion imaging. Methods: Through traditional DSC tracer kinetic modeling, the MR signal change induced by this hypoxic bolus can be used to generate regional perfusion maps of cerebral blood flow, cerebral blood volume, and mean transit time. This gas paradigm and blood‐oxygen‐level‐dependent (BOLD)‐MRI were performed concurrently on a cohort of 66 healthy and chronically anemic subjects (age 23.5 ± 9.7, female 64%). Results: Our results showed reasonable global and regional agreement between dDSC and other flow techniques, such as phase contrast and arterial spin labeling. Conclusion: In this proof‐of‐concept study, we demonstrated the feasibility of using transient hypoxia to generate a contrast bolus that mimics the effect of gadolinium and yields reasonable perfusion estimates. Looking forward, optimization of the hypoxia boluses and measurement of the arterial‐input function is necessary to improve the accuracy of dDSC. Additionally, a cross‐validation study of dDSC and DSC in brain tumor and ischemic stroke subjects is warranted to evaluate the clinical diagnostic utility of this approach. [ABSTRACT FROM AUTHOR]

Purpose: To compare cerebral blood flow (CBF) and cerebrovascular reserve (CVR) quantification from Turbo‐QUASAR (quantitative signal targeting with alternating radiofrequency labeling of arterial regions) arterial spin labeling (ASL) and single post‐labeling delay pseudo‐continuous ASL (PCASL). Methods: A model‐based method was developed to quantify CBF and arterial transit time (ATT) from Turbo‐QUASAR, including a correction for magnetization transfer effects caused by the repeated labeling pulses. Simulations were performed to assess the accuracy of the model‐based method. Data from an in vivo experiment conducted on a healthy cohort were retrospectively analyzed to compare the CBF and CVR (induced by acetazolamide) measurement from Turbo‐QUASAR and PCASL on the basis of global and regional differences. The quality of the two ASL data sets was examined using the coefficient of variation (CoV). Results: The model‐based method for Turbo‐QUASAR was accurate for CBF estimation (relative error was 8% for signal‐to‐noise ratio = 5) in simulations if the bolus duration was known. In the in vivo experiment, the mean global CVR estimated by Turbo‐QUASAR and PCASL was between 63% and 64% and not significantly different. Although global CBF values of the two ASL techniques were not significantly different, regional CBF differences were found in deep gray matter in both pre‐ and postacetazolamide conditions. The CoV of Turbo‐QUASAR data was significantly higher than PCASL. Conclusion: Both ASL techniques were effective for quantifying CBF and CVR, despite the regional differences observed. Although CBF estimated from Turbo‐QUASAR demonstrated a higher variability than PCASL, Turbo‐QUASAR offers the advantage of being able to measure and control for variation in ATT. [ABSTRACT FROM AUTHOR]

Purpose: 19F‐MRI is gaining widespread interest for cell tracking and quantification of immune and inflammatory cells in vivo. Different fluorinated compounds can be discriminated based on their characteristic MR spectra, allowing in vivo imaging of multiple 19F compounds simultaneously, so‐called multicolor 19F‐MRI. We introduce a method for multicolor 19F‐MRI using an iterative sparse deconvolution method to separate different 19F compounds and remove chemical shift artifacts arising from multiple resonances. Methods: The method employs cycling of the readout gradient direction to alternate the spatial orientation of the off‐resonance chemical shift artifacts, which are subsequently removed by iterative sparse deconvolution. Noise robustness and separation was investigated by numerical simulations. Mixtures of fluorinated oils (PFCE and PFOB) were measured on a 7T MR scanner to identify the relation between 19F signal intensity and compound concentration. The method was validated in a mouse model after intramuscular injection of fluorine probes, as well as after intravascular injection. Results: Numerical simulations show efficient separation of 19F compounds, even at low signal‐to‐noise ratio. Reliable chemical shift artifact removal and separation of PFCE and PFOB signals was achieved in phantoms and in vivo. Signal intensities correlated excellently to the relative 19F compound concentrations (r−2 = 0.966/0.990 for PFOB/PFCE). Conclusions: The method requires minimal sequence adaptation and is therefore easily implemented on different MRI systems. Simulations, phantom experiments, and in‐vivo measurements in mice showed effective separation and removal of chemical shift artifacts below noise level. We foresee applicability for simultaneous in‐vivo imaging of 19F‐containing fluorine probes or for detection of 19F‐labeled cell populations. [ABSTRACT FROM AUTHOR]

Purpose: To introduce a new method for motion‐insensitive 3D multishot diffusion imaging using 3D spiral‐encoded navigators from stimulated echoes (3D‐DISPENSE). Methods: The 3D‐DISPENSE sequence contains a 3D stack‐of‐spiral navigator generated between the diffusion preparation and the turbo spin‐echo image acquisition from the twin pathway of a stimulated echo. Unlike normal navigator methods, 3D‐DISPENSE separates the navigator acquisition from the imaging readout without compromising the image SNR. Phase information from the navigators was included in an iterative image reconstruction algorithm to correct for intershot phase incoherence caused by motion. Results: In a phantom experiment, 3D‐DISPENSE correctly estimated deliberately introduced phase errors. In a moving phantom, motion‐induced artifacts in the DWI were greatly mitigated by 3D‐DISPENSE. The ADC after 3D‐DISPENSE correction was identical to the reference. In a brain diffusion tensor experiment, phase‐incoherence artifacts from breathing, cardiac, and subject motion were removed almost perfectly in all view angles, resulting in distortion‐free DWI and color‐coded fractional anisotropy maps with 1.5‐mm isotropic resolution and nearly full brain coverage. Conclusion: Three‐dimensional DISPENSE corrects motion‐induced phase‐incoherence artifacts in 3D multishot diffusion imaging and produces high‐quality 3D DWI and DTI. [ABSTRACT FROM AUTHOR]

Purpose To introduce a method for obtaining fat-suppressed images in real-time MRI of moving joints at 3 Tesla (T) using a bSSFP sequence with phase detection to enhance visualization of soft tissue structures during motion. Methods The wrist and knee of nine volunteers were imaged with a real-time bSSFP sequence while performing dynamic tasks. For appropriate choice of sequence timing parameters, water and fat pixels showed an out-of-phase behavior, which was exploited to reconstruct water and fat images. Additionally, a 2-point Dixon sequence was used for dynamic imaging of the joints, and resulting water and fat images were compared with our proposed method. Results The joints could be visualized with good water-fat separation and signal-to-noise ratio (SNR), while maintaining a relatively high temporal resolution (5 fps in knee imaging and 10 fps in wrist imaging). The proposed method produced images of moving joints with higher SNR and higher image quality when compared with the Dixon method. Conclusions Water-fat separation is feasible in real-time MRI of moving knee and wrist at 3 T. PS-bSSFP offers movies with higher SNR and higher diagnostic quality when compared with Dixon scans. Magn Reson Med 78:58-68, 2017. © 2016 International Society for Magnetic Resonance in Medicine [ABSTRACT FROM AUTHOR]

Purpose A novel three-dimensional (3D) T1 and T2 mapping protocol for the carotid artery is presented. Methods A 3D black-blood imaging sequence was adapted allowing carotid T1 and T2 mapping using multiple flip angles and echo time (TE) preparation times. B1 mapping was performed to correct for spatially varying deviations from the nominal flip angle. The protocol was optimized using simulations and phantom experiments. In vivo scans were performed on six healthy volunteers in two sessions, and in a patient with advanced atherosclerosis. Compensation for patient motion was achieved by 3D registration of the inter/intrasession scans. Subsequently, T1 and T2 maps were obtained by maximum likelihood estimation. Results Simulations and phantom experiments showed that the bias in T1 and T2 estimation was < 10% within the range of physiological values. In vivo T1 and T2 values for carotid vessel wall were 844 ± 96 and 39 ± 5 ms, with good repeatability across scans. Patient data revealed altered T1 and T2 values in regions of atherosclerotic plaque. Conclusion The 3D T1 and T2 mapping of the carotid artery is feasible using variable flip angle and variable TE preparation acquisitions. We foresee application of this technique for plaque characterization and monitoring plaque progression in atherosclerotic patients. Magn Reson Med 75:1008-1017, 2016. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Purpose To compute cohort-averaged wall shear stress (WSS) maps in the thoracic aorta of patients with aortic dilatation or valvular stenosis and to detect abnormal regional WSS. Methods Systolic WSS vectors, estimated from four-dimensional flow MRI data, were calculated along the thoracic aorta lumen in 10 controls, 10 patients with dilated aortas, and 10 patients with aortic valve stenosis. Three-dimensional segmentations of each aorta were coregistered by group and used to create a cohort-specific aortic geometry. The WSS vectors of each subject were interpolated onto the corresponding cohort-specific geometry to create cohort-averaged WSS maps. A Wilcoxon rank sum test was used to generate aortic P-value maps ( P<0.05) representing regional relative WSS differences between groups. Results Cohort-averaged systolic WSS maps and P-value maps were successfully created for all cohorts and comparisons. The dilation cohort showed significantly lower WSS on 7% of the ascending aorta surface, whereas the stenosis cohort showed significantly higher WSS on 34% of the ascending aorta surface. Conclusions The findings of this study demonstrated the feasibility of generating cohort-averaged WSS maps for the visualization and identification of regionally altered WSS in the presence of disease, compared with healthy controls. Magn Reson Med 73:1216-1227, 2015. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]