Spatial Dependence of Activity Concentration Recovery for a Conjugate Gradient (Siemens XSPECT) Algorithm Using Manufacturer-defined Reconstruction Presets

Siemens absolute quantitative reconstruction, xSPECT, is available with manufacturer-defined reconstruction presets to assist with optimization. This phantom study evaluates the impact of these presets on the spatial dependence of activity concentration recovery (ACR). Single-photon emission computed tomography/computed tomography scans of a 5 : 1 and 10 : 1 (sphere : background) contrast NEMA phantom were performed on a Siemens Intevo 6. Three sphere position configurations, achieved by rotating the sphere mount through 0°, 120° and 240°, were used and three replicate images of each configuration were acquired. xSPECT reconstruction was performed using 'Fast', 'Standard' and 'Best' presets. Maximum voxel and A50 threshold ACR were measured in each sphere. The average ACR per sphere was calculated across replicates. Percentage variation of ACR, about this average, for each sphere within a given configuration across replicates and also alternative configurations was calculated. Within a given sphere configuration, percentage variation for maximum voxel ACR in like-for-like spheres across replicates was within 11% for all three presets across all sphere sizes, and within 3% for 10 : 1 and 9% for 5 : 1 contrast in the three largest spheres. Substantial variation of ACR was observed when comparing like-for-like spheres in different configurations. In the three largest spheres, variation in maximum ACR of up to 35 and 32% was measured for 10 : 1 and 5 : 1 contrast, respectively. Variation in activity concentration may be substantially greater than perceived from using a single phantom configuration. The spatial dependence observed using the manufacturer presets highlights the need for evaluation of user-defined reconstruction parameters.