Purpose High angular resolution diffusion imaging (HARDI) is a well-established method to help reveal the architecture of nerve bundles, but long scan times and geometric distortions inherent to echo planar imaging (EPI) have limited its integration into clinical protocols. Methods A fast imaging method is proposed here that combines Accelerated Multi-shot Diffusion Imaging (AMDI), Multiplexed Sensitivity Encoding (MUSE) and Crossing Fiber Angular Resolution of Intravoxel Structure (CFARI) to reduce spatial distortions and reduce total scan time. A multi-shot EPI sequence was used to improve geometrical fidelity as compared to a single-shot EPI acquisition, and acceleration in both k-space and diffusion sampling enabled reductions in scan time. The method is regularized and self-navigated for motion correction. Seven volunteers were scanned in this study, including four with volumetric whole brain acquisitions. Results The average similarity of microstructural orientations between under-sampled datasets and their fully sampled counterparts was above 85%, with scan times below 5 minutes for whole brain acquisitions. Up to 2.7-fold scan time acceleration along with four-fold distortion reduction was achieved. Conclusion The proposed imaging strategy can generate HARDI results with relatively good geometrical fidelity and low scan duration, which may help facilitate the transition of HARDI from a successful research tool to a practical clinical one.
Purpose: To reduce image distortion in MR diffusion imaging using an accelerated multi-shot method. Methods: The proposed method exploits the fact that diffusion-encoded data tend to be sparse when represented in the kb-kd space, where kb and kd are the Fourier transform duals of b and d, the b-factor and the diffusion direction, respectively. Aliasing artifacts are displaced toward under-utilized regions of the kb-kd plane, allowing non-aliased signals to be recovered. A main characteristic of the proposed approach is how thoroughly the navigator information gets utilized during reconstruction: The phase of navigator images is used for motion correction, while the magnitude of the navigator signal in kb-kd space is used for regularization purposes. As opposed to most acceleration methods based on compressed sensing, the proposed method reduces the number of ky lines needed for each diffusion-encoded image, but not the total number of images required. Consequently, it tends to be most effective at reducing image distortion rather than reducing total scan time. Results: Results are presented for three volunteers with acceleration factors ranging from 4 to 8, with and without the inclusion of parallel imaging. Conclusion: An accelerated motion-corrected diffusion imaging method was introduced that achieves good image quality at relatively high acceleration factors.
Purpose: To improve the geometric fidelity and spatial-resolution of multi-b diffusion-weighted MRI of the prostate. Materials and Methods: An accelerated segmented diffusion imaging sequence was developed and evaluated in 25 patients undergoing multi-parametric MRI exams of the prostate. A reduced field-of-view was acquired using an endo-rectal coil. The number of sampled diffusion weightings, or b-factors, was increased to allow estimation of tissue perfusion based on the intra-voxel incoherent motion (IVIM) model. Apparent diffusion coefficients (ADCs) measured with the proposed segmented method were compared to those obtained with conventional single-shot echoplanar imaging (EPI). Results: Compared to single-shot EPI, the segmented method resulted in faster acquisition with twofold improvement in spatial resolution and a greater than threefold improvement in geometric fidelity. ADC values measured with the novel sequence demonstrated excellent agreement with those obtained from the conventional scan (R 2 =0.91 for b max =500 s/mm 2 and R 2 =0.89 for b max =1400 s/mm 2). IVIM perfusion fraction was 4.0+/−2.7% for normal peripheral zone, 6.6+/ −3.6% for normal transition zone and 4.4+/−2.9% for suspected tumor lesions. Conclusion: The proposed accelerated segmented prostate diffusion imaging sequence achieved improvements in both spatial resolution and geometric fidelity, along with concurrent quantification of IVIM perfusion.
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