Feasibility of 177Lu quantifcation with a 360° CZT gamma camera
Résumé
Aim/Introduction: CZT gamma cameras are known for having better sensitivity and energy resolution than conventional gamma cameras. The 360° CZT camera, consisting of a system of 12 robotic CZT detector, is however unable to detect gammas with energies above 200keV. In this work, we investigate if this system, which allows a faster acquisition time than conventional camera, can be used for 177Lu-based treatments monitoring. Materials and Methods: Two phantoms were used: (1) an uniform phantom flled with 832MBq of 177Lu for calibration and (2) a NEMA IEC Body with 799MBq for the background and 585,35kBq/mL in the six spheres. Acquisitions were performed on the energy peak centered at 112.9keV± 10% with two scatter windows ([79-101.6] and [124.2- 146.8]keV) followed by a CT acquisition. Acquisition time was 15 min. Reconstructions were perform with CT-based attenuation correction, triple-energy window scatter correction, 8 subsets, 10 iterations, PSF compensation, convolution pre-flter (parameter: 0.08, frequency: 2), median post-flter (parameter: 3) and High Peak Correction (HPC, with values 0.4 and 0.2 to correct the penetration of high energy photons into collimator). We assessed the penalizedlikelihood (PL) option by varying its parameter from 0 (no PL) to 2 in steps of 0.2. Reconstructions have also been done without the preflter. Activity and contrast recovery coefcients (ACR and CRC) were computed for all reconstructions and were compared with the ACR obtained from a conventional camera with the same acquisition time. Results: Increased PL values allow to visually decrease the image noise. However, the lower the PL, the better are the ARC and CRC. ARC (CRC) of the largest sphere (37mm) were 0.70 (86%) with 2, 0.79 (105%) with 0.2 and 0.84 (112%) without PL for the biggest sphere. These fndings remain whether the pre-flter is removed or not. The decrease in the HPC parameter leads to a decrease in ARCs and CRCs (0.84vs0.72 and 112%vs81%). For ARC, best results were obtained without the PL option, with a HPC parameter at 0.4 and without pre-flter. They were slightly better than those obtained with the conventional camera (0.45 for the 113keV, and even with the highest 208keV gamma peak: 0.69). Conclusion: 177Lu quantifcation using the 113 keV peak is feasible with a 360° CZT camera. The acquisition time of large 3D whole-body image, which is very important for 177Lu-PSMA monitoring, is reduced by a factor 1.5 compared to conventional gamma cameras.