MRI distortion: considerations for MRI based radiotherapy treatment planning

Abstract

Distortion in magnetic resonance images needs to be taken into account for the purposes of radiotherapy treatment planning (RTP). A commercial MRI grid phantom was scanned on four different MRI scanners with multiple sequences to assess variations in the geometric distortion. The distortions present across the field of view were then determined. The effect of varying bandwidth on image distortion and signal to noise was also investigated. Distortion maps were created and these were compared to the location of patient anatomy within the scanner bore to estimate the magnitude and distribution of distortions located within specific clinical regions. Distortion magnitude and patterns varied between MRI sequence protocols and scanners. The magnitude of the distortions increased with increasing distance from the isocentre of the scanner within a 2D imaging plane. Average distortion across the phantom generally remained below 2.0 mm, although towards the edge of the phantom for a turbo spin echo sequence, the distortion increased to a maximum value of 4.1 mm. Application of correction algorithms supplied by each vendor reduced but did not completely remove distortions. Increasing the bandwidth of the acquisition sequence decreased the amount of distortion at the expense of a reduction in signal-to-noise ratio of 13.5 across measured bandwidths. Imaging protocol parameters including bandwidth, slice thickness and phase encoding direction, should be noted for distortion investigations in RTP since each can influence the distortion. The magnitude of distortion varies across different clinical sites.

Quantitative analysis of image quality for acceptance and commissioning of an MRI simulator with a semiautomatic method

“…Some new large geometric phantoms are being developed to simulate and evaluate distortion with large FOV 21, 22. The magnitude of the distortions increases with increasing distance from the isocenter of the scanner 23, 24. Similar to our results, within a distance of 200 mm, the mean distortion in the axial plane can be controlled in an acceptable range for radiotherapy.…”

Prospective observer and software-based assessment of magnetic resonance imaging quality in head and neck cancer: Should standard positioning and immobilization be required for radiation therapy applications?

“…Using MRI as the primary treatment planning tool has the advantage of not having image co-registration induced spatial localization errors yet several other limitations still exist. This include system-related geometric distortions, artifacts caused by motion in the head and neck area, object-induced distortions, and lack of comprehensive electron density information required for dose calculation 22 ; therefore, further efforts are required to investigate solutions to overcome these limitations 23–25 .…”

Medical physics publishing in a changing research environment: the Australasian Physical & Engineering Sciences in Medicine 40th anniversary editorial

“…2 APESM usually rejects commissioning reports on existing or previously published technologies, but this journal is the natural home for investigations of new technologies, or new ways to use existing technologies. Some of APESM's most-cited papers of the past 10 years specifically describe such investigations for film [14][15][16] and imaging modalities [17].…”

“…For example, two of APESM's four most cited papers of the last five years were written by TEAP registrars [17,38].…”