Abstract
Purpose:Dynamic MRI has been used to quantify respiratory motion of abdominal organs in radiation treatment planning. Many existing 4D‐MRI methods based on 2D acquisitions suffer from limited slice resolution and additional stitching artifacts when evaluated in 3D1. To address these issues, we developed a 4D‐MRI (3D dynamic) technique with true 3D k‐space encoding and respiratory motion self‐gating.Methods:The 3D k‐space was acquired using a Rotating Cartesian K‐space (ROCK) pattern, where the Cartesian grid was reordered in a quasi‐spiral fashion with each spiral arm rotated using golden angle2. Each quasi‐spiral arm started with the k‐space center‐line, which were used as self‐gating3 signal for respiratory motion estimation. The acquired k‐space data was then binned into 8 respiratory phases and the golden angle ensures a near‐uniform k‐space sampling in each phase. Finally, dynamic 3D images were reconstructed using the ESPIRiT technique4. 4D‐MRI was performed on 6 healthy volunteers, using the following parameters (bSSFP, Fat‐Sat, TE/TR=2ms/4ms, matrix size=500×350×120, resolution=1×1×1.2mm, TA=5min, 8 respiratory phases). Supplemental 2D real‐time images were acquired in 9 different planes. Dynamic locations of the diaphragm dome and left kidney were measured from both 4D and 2D images. The same protocol was also performed on a MRI‐compatible motion phantom where the motion was programmed with different amplitude (10–30mm) and frequency (3–10/min).Results:High resolution 4D‐MRI were obtained successfully in 5 minutes. Quantitative motion measurements from 4D‐MRI agree with the ones from 2D CINE (<5% error). The 4D images are free of the stitching artifacts and their near‐isotropic resolution facilitates 3D visualization and segmentation of abdominal organs such as the liver, kidney and pancreas.Conclusion:Our preliminary studies demonstrated a novel ROCK 4D‐MRI technique with true 3D k‐space encoding and respiratory motion self‐gating. The technique leads to high‐resolution and artifacts‐free 4D images for improved abdominal organ motion studies.K.S acknowledges funding support from NIH R01CA188300
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