Improved target volume definition in radiosurgery of arteriovenous malformations by stereotactic correlation of MRA, MRI, blood bolus tagging, and functional MRI.

In this methodological paper the authors report the stereotactic correlation of different magnetic resonance imaging (MRI) techniques [MR angiography (MRA), MRI, blood bolus tagging (STAR), and functional MRI] in 10 patients with cerebral arteriovenous malformations (AVM) and its application in precision radiotherapy planning. The patient's head was fixed in a stereotactic localization system that is usable at the MR and the linear accelerator installations. By phantom measurements different materials (steel, aluminium, titanium, plastic, wood, ceramics) used for the stereotactic system were tested for mechanical stability and geometrical MR image distortion. All metallic stereotactic rings (closed rings made of massive metal) led to a more or less dramatic geometrical distortion and signal cancellation in the MR images. The best properties-nearly no distortion and high mechanical stability-are provided by a ceramic ring. If necessary, the remaining geometrical MR image distortion can be "corrected" (reducing displacements to the size of a pixel) by calculations based on modeling the distortion as a fourth-order two-dimensional polynomial. Using this method multimodality matching can be performed automatically as long as all images are acquired in the same examination and the patient is sufficiently immobilized. Precise definition of the target volume could be performed by the radiotherapist either directly in MR images or in calculated projection MR angiograms obtained by a maximum-intensity projection algorithm. As a result, information about the hemodynamics of the AVM was provided by a three-dimensional (3D) phase-contrast flow measurement and a dynamic MRA with the STAR technique leading to an improved definition of the size of the nidus, the origin of the feeding arteries, and the pattern of the venous drainage. In addition, functional MRI was performed in patients with lesions close to the primary motor cortex area leading to an improved definition of structures at risk for high-dose application in radiosurgery. The different imaging techniques of MR provide a sensitive, noninvasive, 3D method for defining target volume, critical structures, and for calculating dose distributions for radiosurgery of cerebral arteriovenous malformations, because dose calculation of radiosurgery at sufficient accuracy can be based on 3D MR data of the geometrical conformation of the patient's head.