IMPROVED TARGET VOLUME DEFINITION IN RADIOSURGERY OF ARTERIOVENOUS-MALFORMATIONS BY STEREOTAXIC CORRELATION OF MRA, MRI, BLOOD BOLUS TAGGING, AND FUNCTIONAL MRI

In this methodological paper the authors report the stereotactic corre lation of different magnetic resonance imaging (MRI) techniques [MR an giography (MRA), MRI, blood bolus tagging (STAR), and functional MRI] in 10 patients with cerebral arteriovenous malformations (AVM) and its radiotherapy planning. The patient's head was fixed in a stereotactic localization system is usable at the MR and the linear accelerator in stallations. By phantom measurements different materials (steel, alumi nium, titanium, plastic, wood, ceramics) used for the stereotactic sys tem were tested for mechanical stability and geometrical MR image dist ortion. All metallic stereotactic rings or less dramatic geometrical d istortion and signal cancellation in the MR images. The best propertie s - nearly no distortion and high mechanical stability - are provided by a ceramic ring. If necessary, the remaining geometrical MR image di stortion can be ''corrected'' (reducing displacements to the size of a pixel) by calculations based on modeling the distortion as a fourth-o rder two-dimensional polynomial. Using this method multimodality match ing can be performed automatically as long as all images are acquired in the same examination and the patient is sufficiently immobilized. P recise definition of the target volume could be performed by the radio therapist 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 t o an improved definition of structures at risk for se application in r adiosurgery. The different imaging techniques of MR provide a sensitiv e, noninvasive, 3D method for defining target volume, critical structu res, and for calculating dose distributions for radiosurgery of cerebr al arteriovenous malformation because dose calculation of radiosurgery at sufficient accuracy can be based on 3D MR data of the geometrical conformation of the patient's head.