Slide on over: Open MRI table movement improves off-center visualization

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 - phantom shifting MRI
T2-weighted MR images of the prostate region.The contour of the external body (yellow contour) was drawn on an isocentric plane image that was located at the isocenter of the right portion of the pelvis (A) and overlaid on the other MR image (B). Shrinking and stretching of the reconstructed image caused by gradient nonlinearity in off-center regions manifests at the peripheral areas of the pelvic region (yellow arrows).
Source: JACR (

Open-configuration MR systems can be limited due to image degradation from geometric and spatial distortion on the edges, but moving the patient table laterally can help achieve image quality visualization for off-center regions, according to an article published online June 2 in the Journal of the American College of Radiology.

Since MRI systems are generally designed with homogenous magnetic fields near the isocenter, main-field homogeneity and gradient-field linearity are guaranteed there relative to parts far from the center, explained Cheolpyo Hong, PhD, of the Korea Research Institute of Standards and Science, Daejeon, Korea, and colleagues. The magnitude of image degradation in off-center regions is increased with open-configuration MRI.

To account for this, Hong and colleagues proposed a simple phantom shifting technique to visualize image quality degradation for off-center regions. Rather than use a special large-sized phantom, wide-area scanning can be implemented by shifting commonly used clinical phantoms on the patient table.

Hong and colleagues estimated geometric accuracy and image intensity uniformity by performing MRI scans on an American College of Radiology MR accreditation phantom, with axial images acquired with the phantom placed at the isocenter and shifted 30, 60, 90 and 120 mm off center both to the left and right by moving the patient table.

“Geometric accuracy was decreased by 10 percent in the off-center areas compared with the isocentric regions, and uniformity was decreased by 46 percent in the off-center regions,” wrote the authors. “Image quality degradation was more severe in the off-center areas. The scanning of objects near the isocenter allowed the effect of geometric distortion to be decreased because of main-field homogeneity, and gradient-field linearity was guaranteed on and near the isocenter relative to points far from the center.”

Hong and colleagues added that this simple phantom-moving scheme also can be implemented in closed-type MR systems by moving the patient table in the anterior-to-posterior direction rather than laterally.