Summary

Aim: A new software approach uses separately acquiredCT images for attenuation correction after retrospectivefusion with the SPECT data. This study evaluates the effectof this CT-based attenuation correction on indium-111-pentetreotide-SPECT images. Methods: Indium-111-pentetreotide-SPECT imaging using a dual-headgamma camera e.cam (Siemens Medical Solutions, Erlangen,Germany) as well as separate spiral computedtomography (CT) was performed in 13 patients. Afterfusion of SPECT and CT data, the bilinear attenuation coefficientswere calculated for each pixel in the CT imagevolume using their Hounsfield unit values and attenuation-corrected images were reconstructed iteratively(OSEM 2D). Regions of interest (ROIs) were drawn on24 suspicious foci and background, and target to backgroundratios were calculated for corrected (TBAC) anduncorrected (TBNAC) images. The shortest distance fromthe centre of the lesion to the surface of the body (DS)was measured on the corresponding CT slice. Furthermore,ROIs were drawn over the rim and the centre ofthe liver. Ratios of hepatic count rates for corrected(LRAC) and uncorrected (LRNAC) images were alsocompared. Results: In lesions located more centrally,TBAC was up to 52% higher, whereas in peripherallylocated lesions, TBAC was up to 63% lower than TBNAC.The TBAC/TBNAC quotient was linearly correlated withDS. In the liver, attenuation correction resulted in a 35%increase of LRAC compared with LRNAC. Conclusions:Attenuation correction of SPECT images performed byseparately acquired CT data is quick and simple. Itimproves the contrast between target and backgroundfor lesions located more centrally in the body andimproves homogeneity of the visualisation of traceruptake in the liver.