Design and implementation of a 3D-MR/CT geometric image distortion phantom/analysis system for stereotactic radiosurgery

Abstract

The design, construction and application of a multimodality, 3D magnetic resonance/computed tomography (MR/CT) image distortion phantom and analysis system for stereotactic radiosurgery (SRS) is presented. The phantom is characterized by (1) a 1 × 1 × 1 (cm) MRI/CT-visible 3D-Cartesian grid; (2) 2002 grid vertices that are 3D-intersections of MR-/CT-visible 'lines' in all three orthogonal planes; (3) a 3D-grid that is MR-signal positive/CT-signal negative; (4) a vertex distribution sufficiently 'dense' to characterize geometrical parameters properly, and (5) a grid/vertex resolution consistent with SRS localization accuracy. When positioned correctly, successive 3D-vertex planes along any orthogonal axis of the phantom appear as 1 × 1 (cm)-2D grids, whereas between vertex planes, images are defined by 1 × 1 (cm)-2D arrays of signal points. Image distortion is evaluated using a centroid algorithm that automatically identifies the center of each 3D-intersection and then calculates the deviations dx, dy, dz and dr for each vertex point; the results are presented as a color-coded 2D or 3D distribution of deviations. The phantom components and 3D-grid are machined to sub-millimeter accuracy, making the device uniquely suited to SRS applications; as such, we present it here in a form adapted for use with a Leksell stereotactic frame. Imaging reproducibility was assessed via repeated phantom imaging across ten back-to-back scans; 80%-90% of the differences in vertex deviations dx, dy, dz and dr between successive 3 T MRI scans were found to be ⩽0.05 mm for both axial and coronal acquisitions, and over >95% of the differences were observed to be ⩽0.05 mm for repeated CT scans, clearly demonstrating excellent reproducibility. Applications of the 3D-phantom/analysis system are presented, using a 32-month time-course assessment of image distortion/gradient stability and statistical control chart for 1.5 T and 3 T GE TwinSpeed MRI systems.

In phantom evaluation of targeting accuracy in MRI-based brain radiosurgery

“…A substantial number of studies on MRI distortion for Gamma Knife [2][3][4][7][8][9][10][11][12]14,15] is aimed at verifying that the extent of residual distortions, after the imaging procedure is optimized, is sufficiently low to reach the high accuracy required for brain stereotactic radiotherapy. These investigations found different results depending on the volumes Fig.…”

“…The source of distortions can usually be found in nonlinearities of the gradient coils and inhomogeneities in the main magnetic field caused by main magnet imperfections and the magnetic effect of varying susceptibility of patient tissues. Once MRI acquisition settings are optimized to minimize the extent of distortions while maintaining a sufficient signal-to-noise ratio for brain radiosurgery, residual distortions are usually measured with geometric phantoms consisting of regular 3D grids of markers [2,3,6,[8][9][10][11][12][13][14][15]. Gamma Knife treatments are planned directly on MR images, and dose calculation is performed assuming the presence of a water-equivalent tissue inside an external contour through a tissue-to-maximum-ratio (TMR10) algorithm [16,17].…”

Evaluation of patient‐specific MR distortion correction schemes for improved target localization accuracy in SRS

“…Several groups have employed plastic or acrylicbased phantoms filled with paramagnetic solution. 12,16,17,35,36 Although such phantoms can be easily constructed or are commercially available, plastic or acrylic does not yield MR signal. Thus, the hypothetical lesions are not depicted as increased signal areas superimposed on a lower signal background, as typically occurs in clinical brain images.…”

“…Selection of a suitable phantom yielding MR signal and contrast and simultaneously simulating brain lesions is challenging. Several groups have employed plastic or acrylic‐based phantoms filled with paramagnetic solution 12,16,17,35,36 . Although such phantoms can be easily constructed or are commercially available, plastic or acrylic does not yield MR signal.…”

“…3 As an instance, a 0.5 mm average distortion magnitude was revealed for a brain scan for SRS applications with maximum detected offset exceeding 1 mm at the edges of the field-of-view 12 and is therefore comparable to typical sequence-dependent distortion levels. 3,11 Since sequence-independent spatial distortion does not relate to the patient being scanned, it can be measured a priori using specially designed phantoms 12,[16][17][18][19] and corrected for in clinical examinations. Contrarily, sequence-dependent distortion cannot be evaluated and corrected beforehand.…”