Access Type

Open Access Dissertation

Date of Award

January 2016

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Medical Physics

First Advisor

Carri K. Glide-Hurst

Abstract

While MR-only treatment planning has shown promise, there are still several well-known challenges that are currently limiting widespread clinical implementation.

Firstly, MR images are affected by both patient-induced and system-level geometric distortions that can significantly degrade treatment planning accuracy. . In addition, the availability of comprehensive distortion analysis software is currently limited. Also while many groups have been working toward a synthetic CT solution, further study is needed on the implementation of synCTs as the reference datasets for linac-based image-guided radiation therapy (IGRT) to help determine their robustness in an MR-only workflow.

A 36×43×2 cm3 phantom with 255 known landmarks (~1 mm3) was scanned using 1.0T high-field open MR-SIM at isocenter in the transverse, sagittal, and coronal axes, and a 465x350x168mm 3D phantom was scanned by stepping in the superior-inferior direction in 3 overlapping positions to achieve a total 465x350x400mm sampled FOV yielding >13,800 landmarks(3D Gradient-Echo, TE/TR/α = 5.54 ms/30 ms/28°, voxel size =1×1×2mm3). A binary template (reference) was generated from a phantom schematic. An automated program converted MR images to binary via masking, thresholding, and testing for connectivity to identify landmarks. Distortion maps were generated by centroid mapping. Images were corrected via warping with inverse distortion maps, and temporal stability was assessed.

To determine candidate materials for phantom and software development, 1.0 T MR and CT images were acquired of twelve urethane foam samples of various densities and strengths. Samples were precision machined to accommodate 6 mm diameter paintballs used as landmarks. Final material candidates were selected by balancing strength, machinability, weight, and cost. Bore sizes and minimum aperture width resulting from couch position were tabulated from the literature (14 systems, 5 vendors). Bore geometry and couch position were simulated using MATLAB to generate machine-specific models to optimize the phantom build. Previously developed software for distortion characterization was modified for several magnet geometries (1.0 T, 1.5 T, 3.0 T), compared against previously published 1.0 T results, and integrated into the 3DSlicer application platform.

To evaluate the performance of synthetic CTs in an image guided workflow, magnetic resonance simulation and CT simulation images were acquired of an anthropomorphic skull phantom and 12 patient brain cancer cases. SynCTs were generated using fluid attenuation inversion recovery, ultrashort echo time, and Dixon data sets through a voxel-based weighted summation of 5 tissue classifications. The DRRs were generated from the phantom synCT, and geometric fidelity was assessed relative to CT-generated DRRs through bounding box and landmark analysis. An offline retrospective analysis was conducted to register cone beam CTs to synCTs and CTs using automated rigid registration in the treatment planning system. Planar MV and KV images were rigidly registered to synCT and CT DRRs using an in-house script. Planar and volumetric registration reproducibility was assessed and margin differences were characterized by the van Herk formalism.

Over the sampled FOV, non-negligible residual gradient distortions existed as close as 9.5 cm from isocenter, with a maximum distortion of 7.4mm as close as 23 cm from isocenter. Over 6 months, average gradient distortions were -0.07±1.10 mm and 0.10±1.10 mm in the x and y-directions for the transverse plane, 0.03±0.64 and -0.09±0.70 mm in the sagittal plane, and 0.4±1.16 and 0.04±0.40 mm in the coronal plane. After implementing 3D correction maps, distortions were reduced to < 1 pixel width (1mm) for all voxels up to 25 cm from magnet isocenter.

All foam samples provided sufficient MR image contrast with paintball landmarks. Urethane foam (compressive strength ∼1000psi, density ~20lb/ft3) was selected for its accurate machinability and weight characteristics. For smaller bores, a phantom version with the following parameters was used: 15 foam plates, 55×55×37.5 cm3 (L×W×H), 5,082 landmarks, and weight ~30 kg. To accommodate >70 cm wide bores, an extended build used 20 plates spanning 55×55×50 cm3 with 7,497 landmarks and weight ~44 kg. Distortion characterization software was implemented as an external module into 3DSlicer’s plugin framework and results agreed with the literature.

Bounding box and landmark analysis of phantom synCT DRRs were within 1 mm of CT DRRs. Absolute planar registration shift differences ranged from 0.0 to 0.7 mm for phantom DRRs on all treatment platforms and from 0.0 to 0.4 mm for volumetric registrations. For patient planar registrations, the mean shift differences were 0.4±0.5 mm (range, 0.6 to 1.6 mm), 0.0±0.5 mm (range, 0.9 to 1.2 mm), and 0.1±0.3 mm (range, 0.7 to 0.6 mm) for the superior-inferior (S-I), left-right (L-R), and anterior-posterior (A-P) axes, respectively. The mean shift differences in volumetric registrations were 0.6±0.4 mm (range, 0.2 to 1.6 mm), 0.2±0.4 mm (range, 0.3 to 1.2 mm), and 0.2±0.3 mm (range, 0.2 to 1.2 mm) for the S-I, L-R, and A-P axes, respectively. The CT-SIM and synCT derived margins were <0.3mm different.

This work has characterized the inaccuracies related to GNL distortion for a previously uncharacterized MR-SIM system at large FOVs, and established that while distortions are still non-negligible after current vendor corrections are applied, simple post-processing methods can be used to further reduce these distortions to less than 1mm for the entire field of view. Additionally, it was important to not only establish effective corrections, but to establish the previously uncharacterized temporal stability of these corrections. This work also developed methods to improve the accessibility of these distortion characterizations and corrections. We first tested the application of a more readily available 2D phantom as a surrogate for 3D distortion characterization by stepping the table with an integrated batch script file. Later we developed and constructed a large modular distortion phantom using easily obtainable materials, and showed and constructed a large modular distortion phantom using easily obtainable materials, and used it to characterize the distortion on several widely available MR systems. To accompany this phantom, open source software was also developed for easy characterization of system-dependent distortions. Finally, while the dosimetric equivalence of synCT with CT has been well established, it was necessary to characterize any differences that may exist between synCT and CT in an IGRT setting. This work has helped to establish the geometric equivalence of these two modalities, with some caveats that have been discussed at length.

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