MRI-Related Geometric Distortions in Stereotactic Radiotherapy Treatment Planning: Evaluation and Dosimetric Impact

Abstract

In view of their superior soft tissue contrast compared to computed tomography, magnetic resonance images are commonly involved in stereotactic radiosurgery/radiotherapy applications for target delineation purposes. It is known, however, that magnetic resonance images are geometrically distorted, thus deteriorating dose delivery accuracy. The present work focuses on the assessment of geometric distortion inherent in magnetic resonance images used in stereotactic radiosurgery/radiotherapy treatment planning and attempts to quantitively evaluate the consequent impact on dose delivery. The geometric distortions for 3 clinical magnetic resonance protocols (at both 1.5 and 3.0 T) used for stereotactic radiosurgery/radiotherapy treatment planning were evaluated using a recently proposed phantom and methodology. Areas of increased distortion were identified at the edges of the imaged volume which was comparable to a brain scan. Although mean absolute distortion did not exceed 0.5 mm on any spatial axis, maximum detected control point disposition reached 2 mm. In an effort to establish what could be considered as acceptable geometric uncertainty, highly conformal plans were utilized to irradiate targets of different diameters (5-50 mm). The targets were mispositioned by 0.5 up to 3 mm, and dose-volume histograms and plan quality indices clinically used for plan evaluation and acceptance were derived and used to investigate the effect of geometrical uncertainty (distortion) on dose delivery accuracy and plan quality. The latter was found to be strongly dependent on target size. For targets less than 20 mm in diameter, a spatial disposition of the order of 1 mm could significantly affect (>5%) plan acceptance/quality indices. For targets with diameter greater than 2 cm, the corresponding disposition was found greater than 1.5 mm. Overall results of this work suggest that efficacy of stereotactic radiosurgery/radiotherapy applications could be compromised in case of very small targets lying distant from the scanner's isocenter (eg, the periphery of the brain).

Keywords: DVH; MRI; geometric distortion; spatial accuracy; spatial uncertainty; stereotactic radiosurgery; stereotactic radiotherapy.

Figure 1.

A, The phantom utilized in this study filled with copper sulfate solution. B, The phantom being MR scanned using the head coil. MR indicates magnetic resonance.

Figure 2.

Overview of the workflow for distortion detection implemented in this study.

Figure 3.

Total geometric distortion detected at all 947 CP locations for the 3 clinically used imaging protocols. Results are presented against radial distance from the corresponding MR scanner’s isocenter. CP indicates control point; MR, magnetic resonance.

Figure 4.

Total distortion maps (d^R _tot) on a sagittal plane at x = 0 mm for FSPGR BRAVO (left), FSPGR 3DT1w (middle), and T1w MPRAGE (right).

Figure 5.

Distortion vectors on a sagittal plane at x = 0 mm for FSPGR BRAVO (left), FSPGR 3DT1w (middle), and T1w MPRAGE (right). Vectors’ lengths have been magnified by a factor of 3 to increase visibility.

Figure 6.

Calculated DVHs for the original plan (no offset) as well as for the deliberately mispositioned targets toward the x direction (0.5 up to 3 mm offset) for 4 representative target sizes. DVHs indicate dose–volume histograms.

Figure 7.

Geometric uncertainty on x, y, and z axes resulting in difference greater than 5% in D₉₅ value as a function of target diameter.