Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_11_3217 - An Analysis of Geometric Distortion in a 3T MRI Scanner Used for Intracranial Radiosurgery

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

An Analysis of Geometric Distortion in a 3T MRI Scanner Used for Intracranial Radiosurgery
E. D. Subashi, Z. Wang, Y. Cui, Z. Chang, J. P. Kirkpatrick, and F. F. Yin; Duke University Medical Center, Durham, NC

Purpose/Objective(s): The accuracy and precision of MRI-guided radiosurgery treatment planning may be impaired by geometric distortion. This study characterizes the residual and patient-specific geometric distortions in a 3T MRI scanner used for radiosurgery treatment planning.

Materials/Methods: Machine-specific distortions were measured in a 3T MRI scanner using an anthropomorphic head phantom. The phantom consists of a 3D grid of 3.0 mm rods, at a spacing of 15.0 mm, encased in a rigid shell of clear polymer. The coordinates of the intersection points in the grid were identified in a high resolution CT (voxel size = 0.75 x 0.75 x 1.00 mm3) and used as the ground truth for distortion calculations. The total number of intersection points was 335. The phantom was routinely scanned over the course of several weeks in a 3T MRI scanner using various 2D and 3D imaging sequences at a sampling rate equal to or higher than the rate used for CT. Geometric distortions due to gradient non-linearity were corrected using an approach based on spherical harmonics deconvolution (2D and 3D). Distortion magnitude was estimated as the distance between the coordinates of the intersection points in MRI images and those in the CT image. Patient-specific geometric distortions were estimated from measurements of B0-inhomogeneities based on a dual-echo GRE pulse sequence (voxel size ~ 3.0 x 3.0 x 3.0 mm3).

Results: More than 300 intersection points were typically detected in both modalities. Distortion measured with a T1-weighted 3D spoiled-GRE sequence was found to increase approximately linearly with distance from imaging isocenter, irrespective of correction method. The magnitude and linear dependence of the estimated distortion decreased significantly (paired t-test, regression analysis, p<0.05) after each correction method (2D or 3D). In un-corrected, 2D-corrected, and 3D-corrected images the distortion magnitude was estimated to be (mean±s.d.) 0.56±0.29 mm, 0.50±0.26 mm, and 0.46±0.25 mm respectively. The mean distortion magnitude was consistent over several weeks. Similar results were observed in datasets from 2D multi-slice acquisitions. Patient-specific distortions in regions of high susceptibility differences were found to be up to an order of magnitude higher than the mean machine-specific residual distortions.

Conclusion: Residual and patient-specific geometric distortions may be above tolerance even in MRI scanners that operate within vendor specifications. Distortions above 1.0 mm are frequent at large distances from the imaging isocenter and in regions of high susceptibility differences. A protocol for measurement and quality assurance of high-field MRI geometric distortion is needed in scanners used for radiosurgery treatment planning.

Author Disclosure: E.D. Subashi: None. Z. Wang: None. Y. Cui: None. Z. Chang: Consultant; Varian Medical Systems. Member of editorial board; RSNA. J.P. Kirkpatrick: Research Grant; Varian Medical Systems. Partnership; ClearSight RT Products, LLP. F. Yin: Research Grant; Varian Medical Systems. Honoraria; Brainlab inc. Speaker's Bureau; Varian Medical Systems. Travel Expenses; Varian Medical Systems. Borad of Directors; SANTRO. Board of Directors Member at Large, committees; AAPM. Board member; The International Stereotactic Radiosurger Society.

Ergys Subashi, PhD

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