Evaluation of quantitation accuracy for xSPECT

Siemens recently introduced xSPECT Quant (xQ) and xSPECT Bone (xB), both enabling quantitative SPECT for 99mTc. In this work, we evaluate the quantitative accuracy of xSPECT. We acquired a Hot-Cold sphere phantom with a clinical representative Symbia Intevo test system in our lab and calibrated using a 3% NIST traceable
⁵⁷
Co source (Calibrated Sensitivity Source (CSS)). Projections are acquired in step-and-shoot mode with 120 views over 360 degrees, and are also subsampled to 60 views (6-degree angular step). We used three reconstruction methods: xQ, xB and xEMAS (xE) at varying updates. xE is based upon OSEM but uses the projection operator employed in xSPECT, and is currently not commercially available. Mean activity concentration (in kBq/ml) is measured for the background and the 6 hot spheres. For the background, a large uniform VOI is chosen. We chose a reduced sphere VOI with 80% of the physical diameter of each sphere for xQ and xE, however we use the interior sphere volume as VOI for xB, as the boundary delinaeation stems from CT and is used in the reconstruction. For the background, all methods give estimations within 1% of error margin compared to truth. For spheres, xE and xQ have almost overlapping curves. Their accuracy gradually improves with the increasing size of spheres, and reaches over 90% accuracy for the largest sphere (16ml). xB gives much higher recovery (>60%) even for the smallest sphere (0.5ml), and rises close to 100% for the largest sphere (16ml). This suggests that xB not only results in considerably higher resolution, but can also yield more accurate quantitation estimation. We did not find significant difference on quantitation between 3-degree and 6-degree sampling.