Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_17_3280 - Quantification of geometric distortion in magnetic resonance imaging for radiation therapy treatment planning

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

Quantification of geometric distortion in magnetic resonance imaging for radiation therapy treatment planning
J. M. Slagowski1, Y. Ding1, Z. Wen1, C. D. Fuller1,2, C. Chung1, M. Kadbi3, G. S. Ibbott1, and J. Wang1; 1The University of Texas MD Anderson Cancer Center, Houston, TX, 2University of Texas Graduate School of Biomedical Sciences, Houston, TX, 3Philips HealthTech, Cleveland, OH

Purpose/Objective(s): MRI provides excellent soft tissue contrast that may improve the accuracy of target and normal tissue delineation during treatment planning for radiation therapy (RT). Nevertheless, inhomogeneities in the static magnetic field and gradient nonlinearities can produce geometric distortion that must be considered. The purpose of this work was to quantify geometric distortion for several MR scanners and acquisition protocols.

Materials/Methods: A custom phantom and software were used to measure geometric distortion for 1.5T and 3.0T diagnostic scanners and an MR-Linac. The distortion phantom spanned a 50 cm by 40 cm by 40 cm volume centered about iso-center within the MR bore. The phantom was imaged using 3D T1 and T2 weighted pulse sequences. The pixel bandwidth (BW) ranged from 191-810 Hz/pixel and 376-751 Hz/pixel for T1 and T2 weighted imaging, respectively. The 3D geometric distortion correction option provided with each scanner was applied. After image acquisition, MR visible fiducials within the distortion phantom were segmented and registered with a reference model. Absolute distortion was defined as the Euclidean distance between the MR visible fiducials (n = 828) localized in the acquired images versus the reference model. The mean absolute distortion is reported for the entire imaging field-of-view and within smaller regions-of-interest defined by the diameter of a sphere centered at iso-center. For application to RT treatment planning, iso-lines outlining regions within the bore with 1 mm and 2 mm or less measured distortion were determined and superimposed on distortion maps.

Results: The mean geometric distortion for T1 weighted imaging performed with a 1.5T diagnostic scanner was 1.50 mm at 200 Hz/pixel. Geometric distortion decreased, as expected, with increasing BW to 1.38 mm at 810 Hz/pixel. Distortion was 1.50 mm (BW = 376) and 1.48 mm (BW = 751) for T2 weighted imaging. Geometric distortion increased as a function of distance from iso-center. Example results are provided in the Table below for the 1.5T scanner. Mean geometric distortion was similar for the 1.5T MR-Linac and increased slightly for the 3.0T scanner. Mean distortion over the full imaging field-of-view was 1.6 mm for the MR-Linac and 2.3 mm for the 3.0T scanner for T2 weighted imaging at similar BW.
Mean distortion measured for a 1.5T scanner (mm)
Diameter (mm) T1, BW 200 T1, BW 810 T2, BW 376 T2, BW 751
200 0.52 0.49 0.51 0.48
300 0.74 0.73 0.75 0.74
400 1.15 1.14 1.20 1.15
500 2.51 2.22 2.52 2.50

Conclusion: Geometric distortion was assessed for several MRI scanners and protocols. The methods developed in this work will be used to optimize MR imaging protocols for RT simulation, assess PTV margins, and evaluate geometric distortion correction techniques.

Author Disclosure: J.M. Slagowski: Employee; Baylor College of Medicine. Y. Ding: None. Z. Wen: None. C.D. Fuller: Research Grant; National Institutes of Health, National Science Foundation, Elekta AB. Grant funding; Elekta AB. Honoraria; Nederlandse Organisatie voor Wetenschappelijk Onde. Consultant; Elekta AB, Nederlandse Organisatie voor Wetenschappelijk Onde. Travel Expenses; Elekta AB, Nederlandse Organisatie voor Wetenschappelijk Onde. Reviewer; Radiological Society of North America. Associate Editor; Radiographics. Data Management Task Force Committee Member; MR-LinAc Consortium. Member; National Cancer Institute. Task Group Member; American Association of Physicists in Medicine. C. Chung: None. G.S. Ibbott: Research Grant; Elekta Corp. Travel Expenses; Elekta Corp, Sun Nuclear Corp. Officer; American Board of Radiology. Committee Chair; AAPM. Treasurer; American Board of Radiology Foundation. Convenor, Working Group 1 of Subcommittee 62C; International Electrotechnical Commission. Vice Chair, Commission on Medical Physics; American College of Radiology. J. Wang: Research Grant; GE. Honoraria; Elekta.

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TU_17_3280 - Quantification of geometric distortion in magnetic resonance imaging for radiation therapy treatment planning

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