Quantify total activity by volume‐of‐interest expansion with clinical SPECT/CT systems, a phantom study

PURPOSE: Quantitative measurements of activity in SPECT are important for radioisotope therapy planning and disease diagnosis. The aim of this manuscript is to develop a robust method to quantify the total activity in a volume‐of‐interest (VOI) of different quantitative SPECT reconstructions and validate the estimation accuracy. METHODS: We customized an IEC body phantom using 3D printing technology and made six sphere inserts of 1–6 cm in diameter with at least 3 cm separation. The activity concentration within the spheres was in the range of patient lesion/organ activity. The background activity was then increased from zero to a sphere/background activity concentration of 8:1, 4:1, and 2:1. SPECT data were acquired with Philips Brightview and GE Discovery 670 SPECT/computed tomography (CT) systems under clinical acquisition protocols. Quantitative SPECT images were reconstructed with Hermes SUV‐SPECT (both Philips and GE data) and GE Q.Metrix (GE data only). The quantitative SPECT reconstructions are iterative with scatter, CT attenuation correction, and resolution recovery. We quantified the total activity by expanding the sphere VOI to include a spill‐out region. Background correction was applied by sampling a region outside the spill‐out region. The true fractions (TFs) (total activity/true activity) were measured for all six spheres for all SPECT acquisitions. RESULTS: The TF is close to 100% for 2–6 cm spheres for zero background, 8:1 and 4:1 sphere/background activity ratios. The TF was found to be unreliable for the 1‐cm sphere because of the limit of phantom design. TF accuracy for 2:1 sphere/background ratio was degraded due to significantly large background, inadequate scatter correction and detector count loss. CONCLUSIONS: The results demonstrated that the proposed quantification method is accurate for objects of different sizes in currently clinical quantitative reconstruction and has the potential for improving the accuracy for therapeutic treatment planning or radiation dosimetry calculations.