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The presented analysis of multisite, multiplatform clinical oncology trial data sought to enhance quantitative utility of the apparent diffusion coefficient (ADC) metric, derived from diffusion-weighted magnetic resonance imaging, by reducing technical interplatform variability owing to systematic gradient nonlinearity (GNL). This study tested the feasibility and effectiveness of a retrospective GNL correction (GNC) implementation for quantitative quality control phantom data, as well as in a representative subset of 60 subjects from the ACRIN 6698 breast cancer therapy response trial who were scanned on 6 different gradient systems. The GNL ADC correction based on a previously developed formalism was applied to trace-DWI using system-specific gradient-channel fields derived from vendor-provided spherical harmonic tables. For quantitative DWI phantom images acquired in typical breast imaging positions, the GNC improved interplatform accuracy from a median of 6% down to 0.5% and reproducibility of 11% down to 2.5%. Across studied trial subjects, GNC increased low ADC (

Partial Fourier encoding is popular in single‐shot (ss) diffusion‐weighted (DW) echo planar imaging (EPI) because it enables a shorter echo time (TE) and, hence, improves the signal‐to‐noise‐ratio. Motion during diffusion encoding causes k‐space shifting and dispersion, which compromises the quality of the homodyne reconstruction. This work provides a comprehensive understanding of the artifacts in homodyne reconstruction of partial Fourier ss‐DW‐EPI data in the presence of motion‐induced phase and proposes the motion‐induced phase‐corrected homodyne (mpc‐hdyne) reconstruction method to ameliorate these artifacts. Simulations with different types of motion‐induced phase were performed to provide an understanding of the potential artifacts that occur in the homodyne reconstruction of partial Fourier ss‐DW‐EPI data. To correct for the artifacts, the mpc‐hdyne reconstruction is proposed. The algorithm recenters k‐space, updates the partial Fourier factor according to detected global k‐space shifts, and removes low‐resolution nonlinear phase before the conventional homodyne reconstruction. The mpc‐hdyne reconstruction is tested on both simulation and in vivo data. Motion‐induced phase can cause signal overestimation, worm artifacts, and signal loss in partial Fourier ss‐DW‐EPI data with the conventional homodyne reconstruction. Simulation and in vivo data showed that the proposed mpc‐hdyne reconstruction ameliorated artifacts, yielding higher quality DW images compared with conventional homodyne reconstruction. Based on the understanding of the artifacts in homodyne reconstruction of partial Fourier ss‐DW‐EPI data, the mpc‐hdyne reconstruction was proposed and showed superior performance compared with the conventional homodyne reconstruction on both simulation and in vivo data. [ABSTRACT FROM AUTHOR]

Longer examination time, need for anesthesia in smaller children and the inability of most children to hold their breath are major limitations of MRI in pediatric body imaging. Fortunately, with technical advances, many new and upcoming MRI sequences are overcoming these limitations. Advances in data acquisition and k-space sampling methods have enabled sequences with improved temporal and spatial resolution, and minimal artifacts. Sequences to minimize movement artifacts mainly utilize radial k-space filling, and examples include the stack-of-stars method for T1-weighted imaging and the periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER)/BLADE method for T2-weighted imaging. Similarly, the sequences with improved temporal resolution and the ability to obtain multiple phases in a single breath-hold in dynamic imaging mainly use some form of partial k-space filling method. New sequences use a variable combination of data sampling methods like compressed sensing, golden-angle radial k-space filling, parallel imaging and partial k-space filling to achieve free-breathing, faster sequences that could be useful for pediatric abdominal and thoracic imaging. Simultaneous multi-slice method has improved diffusion-weighted imaging (DWI) with reduction in scan time and artifacts. In this review, we provide an overview of data sampling methods like parallel imaging, compressed sensing, radial k-space sampling, partial k-space sampling and simultaneous multi-slice. This is followed by newer available and upcoming sequences for T1-, T2- and DWI based on these other advances. We also discuss the Dixon method and newer approaches to reducing metal artifacts. [ABSTRACT FROM AUTHOR]

Purpose: To assess the effect of respiratory motion and cardiac driven pulsation in renal DWI and to examine asymmetrical velocity‐compensated diffusion encoding waveforms for robust ADC mapping in the kidneys. Methods: The standard monopolar Stejskal‐Tanner pulsed gradient spin echo (pgse) and the asymmetric bipolar velocity‐compensated (asym‐vc) diffusion encoding waveforms were used for coronal renal DWI at 3T. The robustness of the ADC quantification in the kidneys was tested with the aforementioned waveforms in respiratory‐triggered and breath‐held cardiac‐triggered scans at different trigger delays in 10 healthy subjects. Results: The pgse waveform showed higher ADC values in the right kidney at short trigger delays in comparison to longer trigger delays in the respiratory triggered scans when the diffusion gradient was applied in the feet‐head (FH) direction. The coefficient of variation over all respiratory trigger delays, averaged over all subjects was 0.15 for the pgse waveform in the right kidney when diffusion was measured in the FH direction; the corresponding coefficient of variation for the asym‐vc waveform was 0.06. The effect of cardiac driven pulsation was found to be small in comparison to the effect of respiratory motion. Conclusion: Short trigger delays in respiratory‐triggered scans can cause higher ADC values in comparison to longer trigger delays in renal DWI, especially in the right kidney when diffusion is measured in the FH direction. The asym‐vc waveform can reduce ADC variation due to respiratory motion in respiratory‐triggered scans at the cost of reduced SNR compared to the pgse waveform. [ABSTRACT FROM AUTHOR]

Purpose: To evaluate the performance of a multi‐echo spin‐echo sequence with k‐t undersampling scheme (k‐t T2) in prostate cancer. Methods: Phantom experiments were performed at five systems to estimate the bias, short‐term repeatability, and reproducibility across all systems expressed with the within‐subject coefficient of variation (wCV). Monthly measurements were performed on two systems for long‐term repeatability estimation. To evaluate clinical repeatability, two T2 maps (voxel size 0.8 × 0.8 × 3 mm3; 5 min) were acquired at separate visits on one system for 13 prostate cancer patients. Repeatability was assessed per patient in relation to spatial resolution. T2 values were compared for tumor, peripheral zone, and transition zone. Results: Phantom measurements showed a small bias (median = −0.9 ms) and good short‐term repeatability (median wCV = 0.5%). Long‐term repeatability was 0.9 and 1.1% and reproducibility between systems was 1.7%. The median bias observed in patients was −1.1 ms. At voxel level, the median wCV was 15%, dropping to 4% for structures of 0.5 cm3. The median tumor T2 values (79 ms) were significantly lower (P < 0.001) than in the peripheral zone (149 ms), but overlapped with the transition zone (91 ms). Conclusions: Reproducible T2 mapping of the prostate is feasible with good spatial resolution in a clinically reasonable scan time, allowing reliable measurement of T2 in structures as small as 0.5 cm3. Magn Reson Med 79:1586–1594, 2018. © 2017 International Society for Magnetic Resonance in Medicine. [ABSTRACT FROM AUTHOR]

Purpose To explore the effects of high permittivity dielectric pads on the transmit and receive characteristics of a 3 Tesla body coil centered at the thighs, and their implications on image uniformity in receive array applications. Theory and Methods Transmit and receive profiles of the body coil with and without dielectric pads were simulated and measured in healthy volunteers. Parallel imaging was performed using sensitivity encoding (SENSE) with and without pads. An intensity correction filter was constructed from the measured receive profile of the body coil. Results Measured and simulated data show that the dielectric pads improve the transmit homogeneity of the body coil in the thighs, but decrease its receive homogeneity, which propagates into reconstruction algorithms in which the body coil is used as a reference. However, by correcting for the body coil reception profile this effect can be mitigated. Conclusion Combining high permittivity dielectric pads with an appropriate body coil receive sensitivity filter improves the image uniformity substantially compared with the situation without pads. Magn Reson Med 76:1951-1956, 2016. © 2015 International Society for Magnetic Resonance in Medicine [ABSTRACT FROM AUTHOR]

Magnetic resonance images are prone to artifacts caused by metallic objects. Apart from being a source of image degradation, such artifacts can also provide information about the magnetic properties of the foreign object. In this work, we aim to explore the potential of magnetic resonance imaging to detect and characterize changes in magnetic properties of nitinol undergoing temperature‐ or strain‐induced phase changes. A spin echo and a gradient echo method were used to measure the magnetization changes related to the phase transformations. Results of both methods were in agreement and in accordance with the independent measurements using a vibrating sample magnetometer. Magnetic resonance imaging turned out to be a suitable method to visualize and quantify magnetization and phase changes in situ. It is not restricted to a single imaging strategy and does not require any modification of the test object. The results indicate the potential of magnetic resonance imaging to provide direct feedback of the thermomechanical state of the alloy. © 2006 Wiley Periodicals, Inc. J Biomed Mater, 2006 [ABSTRACT FROM AUTHOR]

Phase contrast flow measurements will be increasingly biased at eccentric positions, where nonlinearity of gradients and inhomogeneity of the main field become important. In theory, they scale the result of phase contrast flow values in two ways: incorrect velocity encoding of moving spins and geometric distortion of the vessel cross-sectional area. A flow phantom, consisting of a 3D grid of interconnected tubes, was used to determine the spatial dependence of the associated scaling factors, which demonstrate that scaling errors in flow can be as large as 20% within the examined volume of 336 × 336 × 336 mm3. The same phantom was also used to determine and minimize concomitant gradient effects. Correction of the off-center flow values with the local scaling factors and the concomitant gradient phase improves the measurement accuracy substantially, both in the flow phantom and in a volunteer study. Magn Reson Med 53:126-133, 2005. © 2004 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]