γTools: A modular multifunction phantom for quality assurance in GammaKnife treatments

New developments in MRI: System characterization, technical advances and radiotherapy applications

A Special Issue of Physica Medica - European Journal of Medical Physics, focused on some important points of contact between the world of magnetic resonance and that of medical physics, was published during 2021. This Editorial describes and comments on the content of this Focus Issue, which contains articles from leading groups invited by the Guest Editors.

Clinical impact of magnetic resonance imaging distortions on gamma knife radiosurgery

INTRODUCTION

Magnetic resonance imaging (MRI) is the preferred imaging modality for Leksell Gamma Knife® (LGK) stereotactic radiosurgery (SRS) treatment planning (TP) due to superior soft tissue definition compared to computed tomography (CT). However, inherent distortions in MRI can affect treatment accuracy. The aim of this study was to develop a model to visualise the effect of MRI distortion on LGK SRS target coverage.

METHODS

A model was developed using MR images of a QUASARTM GRID3D QA phantom. One hundred and twenty-five points were compared against known phantom geometry. Using linear interpolation, the model was applied retrospectively to 10 brain metastases patient data sets treated with LGK. The model estimated the corrected shot position accounting for distortion. A total of 44 metastases were investigated regarding the effects of MRI distortion on target coverage.

RESULTS

The model indicated significantly reduced mean error by 0.30 mm and variance by 0.09 mm (P = 0.008). After model application, 23 (53%) metastases showed reduced coverage. Six of the 23 metastases were deemed to be potentially clinically significant changes. Results indicated MRI distortion had a greater effect on smaller targets (mean 0.06cc) located further away from the image isocentre (mean 64.88 mm).

CONCLUSION

This study developed a model to visualise the effect of MRI distortion on LGK SRS target coverage. Results suggest that MRI distortion can affect target coverage and the developed model may be one method to assess its impact. These results indicate that MRI distortion may have a greater effect on smaller targets located at the image periphery.

In phantom evaluation of targeting accuracy in MRI-based brain radiosurgery

PURPOSE

To determine the targeting accuracy of brain radiosurgery when planning procedures employing different MRI and MRI + CT combinations are adopted.

MATERIALS AND METHOD

A new phantom, the BrainTool, has been designed and realized to test image co-registration and targeting accuracy in a realistic anatomical situation. The phantom was created with a 3D printer and materials that mimic realistic brain MRI and CT contrast using a model extracted from a synthetic MRI study of a human brain. Eight markers distributed within the BrainTool provide for assessment of the accuracy of image registrations while two cavities that host an ionization chamber are used to perform targeting accuracy measurements with an iterative cross-scan method. Two procedures employing 1.5 T MRI-only or a combination of MRI (taken with 1.5 T or 3 T scanners) and CT to carry out Gamma Knife treatments were investigated. As distortions can impact targeting accuracy, MR images were preliminary evaluated to assess image deformation extent using GammaTool phantom.

RESULTS

MR images taken with both scanners showed average and maximum distortion of 0.3 mm and 1 mm respectively. The marker distances in co-registered images resulted below 0.5 mm for both MRI scans. The targeting mismatches obtained were 0.8 mm, 1.0 mm and 1.2 mm for MRI-only and MRI + CT (1,5T and 3 T), respectively.

CONCLUSIONS

Procedures using a combination of MR and CT images provide targeting accuracies comparable to those of MRI-only procedures. The BrainTool proved to be a suitable tool for carrying out co-registration and targeting accuracy of Gamma Knife brain radiosurgery treatments.