Radiosurgical treatment planning of brain metastases based on a fast, three-dimensional MR imaging technique

Abstract

A fast, three-dimensional (3D) sequence for magnetic resonance (MR) imaging of the brain and its application in radiosurgical treatment planning of brain metastases is reported. The measuring sequence (MPRAGE) requires magnetization-prepared 180° inversion pulses followed by rapid low angle excitation pulses and gradient-echoes for image generation. The resulting T 1-weighted MPRAGE images were compared with two-dimensional (2D) T 1-weighted spin-echo (SE) images after administration of 0.1 mmol/kg b.w. Gd-DTPA in 10 patients with known brain metastases. Original or multiplanar reformatted images obtained from a 128 partition data set of the 3D MPRAGE sequence offered comparable diagnostic quality to that of 2D SE imaging. Gd-DTPA enhancement and lesion targeting was similar in most of the patients in SE as well as MPRAGE imaging. During imaging and therapy the patient's head was fixed in a stereotactic localization system which is usable at the MR and the linear accelerator installations. The dose calculation of the radiosurgery planning was based on 3D MR imaging data assuming a homogenous attenuation value inside the head which was sufficient for an accurate dose calculation since tissue inhomogeneities do not significantly influence the shape of the relative dose distribution especially for radiosurgery of the brain. Under this circumstance the dose calculation can be based only on the 3D geometric conformation of the patient's head. A simple algorithm for treatment planning can be used if the MR data are free of geometric distortion. Target point definition, external head contour, and critical structures could be precisely defined in 3D MPRAGE images offering important advantages (i.e., improved spatial resolution, reformatted image reconstruction, rapid 3D imaging and short examination time under mask fixation resulting in better patient comfort, better slice profile, reduced vascular pulsation artefacts, superior and variable choice of soft tissue contrast due to centric phase-encoding order, lower specific absorption rate) over conventional 2D SE imaging. In addition, target volume, calculated dose distribution, and critical structures can be displayed as 3D shaded structures for better assessment for matching of target volume and dose distribution. These findings, along with other advantages of 3D imaging, indicate that the MPRAGE sequence may provide an alternative to T 1-weighted 2D SE imaging in the radiosurgical treatment planning of brain metastases.