First performance tests of the 3TMR-BrainPET

Objectives A prototype of the MR-PET system designed by Siemens for human brain studies has been recently installed at our site. Here we report some preliminary performance tests of the PET detector insert called BrainPET.

Methods The BrainPET has an outer diameter of 60 cm fitting in the MR bore. It consists of 32 copper shielded cassettes each with six compact detector modules. The detector’s front-end has 12 x 12 LSO crystals of 2.5 x 2.5 x 20 mm³ which are read out by an array of 3 x 3 APDs. Each cassette is linked by a 10 m long cable to the MRT cabin’s filter plate the exterior side of which is attached to the data acquisition electronics. A combined transmit/receive coil is placed inside the BrainPET. Transaxial resolution was studied with a ¹⁸F-filled 0.5 mm thin line source and axial resolution with a 0.25 mm sized ²²Na point source. The scatter fraction was estimated with a phantom containing three cold rods one of which was filled with air. Countrate performance was tested with a cylinder of 25 cm length and 20 cm diameter with an ¹⁸F-activity concentration ranging from 0.8 to 75 kBq/ml.

Results The tangential resolution (FWHM) was 2.4 ± 0.2 mm (at r = 0, 2.5 and 5 cm) in a central transversal plane and the radial one ranged from 2.1 mm via 3.3 mm to 5.4 mm. Within the 15 central image planes the axial resolution was 2.5 ± 0.2 mm at r = 0 cm and 3.1 ± 0.2 mm at r = 5 cm. The countrate found in the air-filled rod of the OSEM3D reconstructed and attenuation corrected image of the 3rod phantom led to an estimated scatter fraction of 27%. The peak prompt-minus-delayed countrate is 680000 cps at 50 kBq/ml.

Conclusions The first tests yielded very promising results and will be completed by NEMA guided tests.