Characterization of system-related geometric distortions in MR images employed in Gamma Knife radiosurgery applications

Abstract

This work provides characterization of system-related geometric distortions present in MRIs used in Gamma Knife (GK) stereotactic radiosurgery (SRS) treatment planning. A custom-made phantom, compatible with the Leksell stereotactic frame model G and encompassing 947 control points (CPs), was utilized. MR images were obtained with and without the frame, thus allowing discrimination of frame-induced distortions. In the absence of the frame and following compensation for field inhomogeneities, measured average CP disposition owing to gradient nonlinearities was 0.53 mm. In presence of the frame, contrarily, detected distortion was greatly increased (up to about 5 mm) in the vicinity of the frame base due to eddy currents induced in the closed loop of its aluminum material. Frame-related distortion was obliterated at approximately 90 mm from the frame base. Although the region with the maximum observed distortion may not lie within the GK treatable volume, the presence of the frame results in distortion of the order of 1.5 mm at a 7 cm distance from the center of the Leksell space. Additionally, severe distortions observed outside the treatable volume could possibly impinge on the delivery accuracy mainly by adversely affecting the registration process (e.g. the position of the lower part of the N-shaped fiducials used to define the stereotactic space may be miss-registered). Images acquired with a modified version of the frame developed by replacing its front side with an acrylic bar, thus interrupting the closed aluminum loop and reducing the induced eddy currents, were shown to benefit from relatively reduced distortion. System-related distortion was also identified in patient MR images. Using corresponding CT angiography images as a reference, an offset of 1.1 mm was detected for two vessels lying in close proximity to the frame base, while excellent spatial agreement was observed for a vessel far apart from the frame base.

Assessment of the accuracy and stability of frameless gamma knife radiosurgery

“…The errors in the fiducial marker‐based MR coordinates were primarily due to image distortion, marker location error, and marker registration algorithms . The MR image distortion, that is, the mean deviation of the fiducial marker‐based MR images compared with the fiducial marker‐based CT images, was 0.8 ± 0.3 mm, which is similar to the mean MR distortion error of 0.95 mm reported by Pappas et al …”

Gamma Knife® icon CBCT offers improved localization workflow for frame‐based treatment

“…Even with modern imaging capabilities, distortion in a phantom with the G‐frame attached is on average 0.53 mm with increasing magnitude with distance from the center of Leksell space (1.5 mm at 7 cm). This study by Pappas et al indicates the frame’s presence itself as a major contributor of this distortion and also that severe distortions can occur outside the treatment area and affect accuracy with the fiducials in this space especially the lower portion near the frame . The further calculated dosimetric problems caused by this distortion are nontrivial and should be considered in addition to the discussion of the localization methods here .…”

Dosimetric impact of rotational errors on the quality of VMAT‐SRS for multiple brain metastases: Comparison between single‐ and two‐isocenter treatment planning techniques

“…Employing a six DOF robotic couch could minimize the required margins and/or number of isocenters, although intrafractional patient motion still remains a concern . Future work will focus on the investigation of the overall spatial uncertainties and corresponding additional margins required, incorporating MR‐related distortions, MR/CT image registration accuracy and mechanical dose delivery uncertainties …”

“…For a cranial case, the MR/CT registration uncertainty was estimated at 1.8 mm in a multi‐institutional study . Although rotational uncertainties were not separately reported, it can be expected that they may considerably contribute to the overall spatial uncertainty, especially for targets lying away from the MR isocenter where MR images inherently exhibit increased geometric warping . Furthermore, geometric uncertainties stemming from the linac rotating parts (i.e., gantry, collimator or couch) or related to the angular alignment accuracy between the (i) on‐couch imaging system (kV or MV CT), (ii) mechanical, and (iii) radiation delivery isocenters should also be taken into consideration .…”

“…32 Although rotational uncertainties were not separately reported, it can be expected that they may considerably contribute to the overall spatial uncertainty, especially for targets lying away from the MR isocenter where MR images inherently exhibit increased geometric warping. [33][34][35] Furthermore, geometric uncertainties stemming from the linac rotating parts (i.e., gantry, collimator or couch) or related to the angular alignment accuracy between the (i) on-couch imaging system (kV or MV CT), (ii) mechanical, and (iii) radiation delivery isocenters should also be taken into consideration. 36,37 Rotational errors are more important in single-isocenter multitarget cases, as lesions may lie several centimeters away from the isocenter and, therefore, induce considerable translations.…”