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A Harmonic Field Approach to Quantifying MRI Spatial Accuracy for MRIgRT

T Tadic

T Tadic*, D Jaffray, T Stanescu, Princess Margaret Cancer Centre, Toronto, ON

TU-G-134-2 Tuesday 4:30PM - 6:00PM Room: 134

Purpose: An outstanding problem in the clinical implementation of MRIgRT is the development of simple and efficient methods for the measurement, characterization, and monitoring of geometrical accuracy in MRI. This study presents the theory, implementation, and validation of a novel harmonic field approach to the quantification of spatial accuracy in MRI.

Methods: A harmonic field description of the 3D distortion vector field in MRI is derived as the solution of a second-order boundary value problem comprised of the Laplace equation and a limited measurement of the distortion at a sparse distribution of points. Harmonic representations of the distortion field were calculated for spherical, cylindrical, and irregular regions of interest (ROIs). These representations were then used to derive volumetric mappings of the distortion field that were compared against reference data obtained on a 3 T full-body scanner. The effects of sampling density were explored and a statistical uncertainty analysis was performed.

Results: The volumetric mappings of the distortion field derived from the harmonic analysis were found to be in excellent agreement with the reference data. For all ROI geometries and distortion vector components, the 1 mm³ voxel size of the images used to acquire the reference data was greater than at least two standard deviations in all discrepancies. The statistical uncertainty analysis demonstrated the robustness of the harmonic approach, as the uncertainties in the volumetric mappings were calculated to be less than or equal to the input reference data.

Conclusion: A novel harmonic approach has been proposed for quantifying MRI spatial accuracy in large imaging volumes for MRIgRT using only limited measurement data. This technique abates the requirement to directly measure the distortion at a dense 3D array of points and thus permits the design of simple, inexpensive phantoms that may feature additional modules for supplementary imaging and dosimetry QA objectives.

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