Investigation of a SPECT/computed tomography imaging artefact and its impact on SPECT quantification.

During optimisation of L-mode parathyroid imaging, an artefact was identified in retrospective patient single photon emission computed tomography (SPECT) which could affect SPECT quantification. This work assessed: (1) H-mode SPECTs for the artefact; (2) differences in the artefact for [ 99m Tc] and [ 123 I], and with and without non-specific background; and (3) the impact on clinically relevant areas. Parathyroid/thyroid phantom studies were performed for L-mode and H-mode using a Perspex torso with saline bags to mimic non-specific background. SPECT/CTs were acquired with clinical parathyroid protocols and using Hermia software, volumes-of-interest were drawn in thyroid and areas of artefact in all reconstructed SPECTs. Standard uptake values (SUVs) for [ 99m Tc] and [ 123 I] were measured. Two clinicians reviewed the images and determined the artefact was more prominent within L-mode SPECTs, correlating to a greater spread of SUVs compared to H-mode. No differences were observed for [ 99m Tc] thyroid SUVs between all acquisitions ( P > 0.5). There were differences in SUVs between L-mode and H-mode for [ 123 I] ( P = 0.04) and with and without non-specific background ( P = 0.03). At the area of artefact, SUVs should be zero, however, there were measurable SUVs for L-mode and [ 123 I]. Based on these findings, H-mode proved more robust to artefacts and was preferred for clinical practice.