Variation in scanning line source sensitivity: a significant source of error in simultaneous emission-transmission tomography

Transmission measurement is recommended in order to accurately correct for attenuation in myocardial single-photon emission tomography (SPET) studies. It is important that transmission studies are artefact-free, otherwise the attenuation-corrected SPET studies may also be affected. An assumption in transmission studies is that the measured transmission in air used as a reference scan is valid for any camera orientation. Variation in transmission source sensitivity (both source efficiency and detector sensitivity) with rotation negates this assumption and can produce errors that result in significant reconstructed artefacts. The aim of this study was to investigate the variation in transmission source sensitivity with a view to defining action thresholds for routine quality control tests. Transmission measurements in air were recorded on two commercial scanning line source installations for the 180° arc normally used in myocardial SPET. Significant variation in transmission source sensitivity was observed on one system (exceeding 30%). Comparison was also made with the reference scan recorded at a different time at a fixed angular location. Both systems demonstrated measurable variation between transmission counts and the corresponding reference scan. A simulation study was undertaken using patient data to determine the influence of transmission sensitivity variation on reconstructed myocardial counts. To maintain reconstructed counts to within 15% of that obtained with artefact-free transmission data, the variation in transmission sensitivity with rotation needed to be within 5%. These results have necessitated the addition of quality control procedures and specific maintenance procedures to attempt rectification of the problem. Variation in transmission source sensitivity with rotation is a potential source of error in attenuation-corrected SPET. Steps should be taken to stabilise transmission source mountings so as to minimise this potential source of error.