Three dimensional correlation of CT-MRI and MRI-SPET using SRS plato to optimise stereotactic treatment planning

The combination of CT or MRI and SPET studies allows, a comparison between anatomy and metabolic informations. To merge information from different sources we used a software package SRS PLATO. The algorithm utilizes a fiducial-based coordinate system with 3 'N' shaped markers which allows the determination of the transformation, for each transaxial section, to the stereotactic coordinate system (9 spots in each image). The N shaped markers are filled with fluids (Gadolinium for MRI, Calcium Chloride for CT and Ga-67 for SPET). This frame is relocatable, for the different acquisitions, by means of an head holder in which a head mask is fixed to immobilize the patient. Algorithm validation was performed with a phantom study (an Agar solution containing spheres filled with Tc-99m for SPET and the same fluids for MRI and CT). Each acquisitions were performed with the frame attached to the phantom, as described; its known geometry allows the registration calibration and the errors obtained were 0.8 mm for CT, 1.4 mm for MRI and 1.8 mm for SPET, that are on the order of pixel size. After calibration, the algorithm gives a 3D visualization of the volumes of interest (VOIs) and structures from multimodal images. Image fusion shows good VOIs overlap and differentiation between tumour and necrosis. This fact has immediate clinical utility to optimise the stereotactic treatment planning