Background Early diagnosis and treatment of prostate cancer (PCa) can be curative; however, prostate‐specific antigen is a suboptimal screening test for clinically significant PCa. While prostate magnetic resonance imaging (MRI) has demonstrated value for the diagnosis of PCa, the acquisition time is too long for a first‐line screening modality. Purpose To accelerate prostate MRI exams, utilizing a variational network (VN) for image reconstruction. Study Type Retrospective. Subjects One hundred and thirteen subjects (train/val/test: 70/13/30) undergoing prostate MRI. Field Strength/Sequence 3.0 T; a T2 turbo spin echo (TSE) T2‐weighted image (T2WI) sequence in axial and coronal planes, and axial echo‐planar diffusion‐weighted imaging (DWI). Assessment Four abdominal radiologists evaluated the image quality of VN reconstructions of retrospectively under‐sampled biparametric MRIs (bp‐MRI), and standard bp‐MRI reconstructions for 20 test subjects (studies). The studies included axial and coronal T2WI, DWI B50 seconds/mm2 and B1000 seconds/mm (4‐fold T2WI, 3‐fold DWI), all of which were evaluated separately for image quality on a Likert scale (1: non‐diagnostic to 5: excellent quality). In another 10 test subjects, three readers graded lesions on bp‐MRI—which additionally included calculated B1500 seconds/mm2, and apparent diffusion coefficient map—according to the Prostate Imaging Reporting and Data System (PI‐RADS v2.1), for both VN and standard reconstructions. Accuracy of PI‐RADS ≥3 for clinically significant cancer was computed. Projected scan time of the retrospectively under‐sampled biparametric exam was also computed. Statistical Tests One‐sided Wilcoxon signed‐rank test was used for comparison of image quality. Sensitivity, specificity, positive predictive value, and negative predictive value were calculated for lesion detection and grading. Generalized estimating equation with cluster effect was used to compare differences between standard and VN bp‐MRI. A P‐value of <0.05 was considered statistically significant. Results Three of four readers rated no significant difference for overall quality between the standard and VN axial T2WI (Reader 1: 4.00 ± 0.56 (Standard), 3.90 ± 0.64 (VN) P = 0.33; Reader 2: 4.35 ± 0.74 (Standard), 3.80 ± 0.89 (VN) P = 0.003; Reader 3: 4.60 ± 0.50 (Standard), 4.55 ± 0.60 (VN) P = 0.39; Reader 4: 3.65 ± 0.99 (Standard), 3.60 ± 1.00 (VN) P = 0.38). All four readers rated no significant difference for overall quality between standard and VN DWI B1000 seconds/mm2 (Reader 1: 2.25 ± 0.62 (Standard), 2.45 ± 0.75 (VN) P = 0.96; Reader 2: 3.60 ± 0.92 (Standard), 3.55 ± 0.82 (VN) P = 0.40; Reader 3: 3.85 ± 0.72 (Standard), 3.55 ± 0.89 (VN) P = 0.07; Reader 4: 4.70 ± 0.76 (Standard); 4.60 ± 0.73 (VN) P = 0.17) and three of four readers rated no significant difference for overall quality between standard and VN DWI B50 seconds/mm2 (Reader 1: 3.20 ± 0.70 (Standard), 3.40 ± 0.75 (VN) P = 0.98; Reader 2: 2.85 ± 0.81 (Standard), 3.00 ± 0.79 (VN) P = 0.93; Reader 3: 4.45 ± 0.72 (Standard), 4.05 ...
Purpose:To design a pulse sequence for efficient 3D T2-weighted imaging and T2 mapping. Methods: A stack-of-stars turbo spin echo pulse sequence with variable refocusing flip angles and a flexible pseudorandom view ordering is proposed for simultaneous T2-weighted imaging and T2 mapping. An analytical framework is introduced for the selection of refocusing flip angles to maximize relative tissue contrast while minimizing T2 estimation errors and maintaining low specific absorption rate. Images at different echo times are generated using a subspace constrained iterative reconstruction algorithm. T2 maps are obtained by modeling the signal evolution using the extended phase graph model. The technique is evaluated using phantoms and demonstrated in vivo for brain, knee, and carotid imaging. Results: Numerical simulations demonstrate an improved point spread function with the proposed pseudorandom view ordering compared to golden angle view ordering. Phantom experiments show that T2 values estimated from the stack-of-stars turbo spin echo pulse sequence with variable refocusing flip angles have good concordance with spin echo reference values. In vivo results show the proposed pulse sequence can generate qualitatively comparable T2-weighted images as conventional Cartesian 3D SPACE in addition to simultaneously generating 3D T2 maps. Conclusion: The proposed stack-of-stars turbo spin echo pulse sequence with pseudorandom view ordering and variable refocusing flip angles allows high resolution isotropic T2 mapping in clinically acceptable scan times. The optimization framework for the selection of refocusing flip angles improves T2 estimation accuracy while generating T2-weighted contrast comparable to conventional Cartesian imaging. K E Y W O R D S 3D T2 mapping, stack-of-stars, turbo spin echo, variable refocusing flip angles | 327 KEERTHIVASAN ET Al.
The use of a union of local subspace constraints coupled with a sparsity promoting penalty leads to improved reconstruction quality of multi-contrast images and parameter maps.
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