Attenuation correction of SPECT images based on separately performed CT

Summary

Aim: A new software approach uses separately acquired CT images for attenuation correction after retrospective fusion with the SPECT data. This study evaluates the effect of this CT-based attenuation correction on indium- 111-pentetreotide-SPECT images. Methods: Indium- 111-pentetreotide-SPECT imaging using a dual-head gamma camera e.cam (Siemens Medical Solutions, Erlangen, Germany) as well as separate spiral computed tomography (CT) was performed in 13 patients. After fusion of SPECT and CT data, the bilinear attenuation coefficients were calculated for each pixel in the CT image volume using their Hounsfield unit values and attenuation- corrected images were reconstructed iteratively (OSEM 2D). Regions of interest (ROIs) were drawn on 24 suspicious foci and background, and target to background ratios were calculated for corrected (TBAC) and uncorrected (TBNAC) images. The shortest distance from the 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 of the liver. Ratios of hepatic count rates for corrected (LRAC) and uncorrected (LRNAC) images were also compared. Results: In lesions located more centrally, TBAC was up to 52% higher, whereas in peripherally located lesions, TBAC was up to 63% lower than TBNAC. The TBAC/TBNAC quotient was linearly correlated with DS. In the liver, attenuation correction resulted in a 35% increase of LRAC compared with LRNAC. Conclusions: Attenuation correction of SPECT images performed by separately acquired CT data is quick and simple. It improves the contrast between target and background for lesions located more centrally in the body and improves homogeneity of the visualisation of tracer uptake in the liver.

Zusammenfassung

Ziel: Mit Hilfe einer neuen Software können SPECT-Bilder nach Fusion mit separat aufgenommenen CT-Bildern schwächungskorrigiert werden. Wir untersuchten den Effekt dieser CT-basierten Schwächungskorrektur auf Indium-111-Pentetreotid-SPECT-Bilder. Methoden: 13 Patienten mit neuroendokrinen Tumoren erhielten ein Indium-111-Pentetreotid-SPECT unter Verwendung einer Doppelkopf-SPECT-Kamera e.cam (Siemens Medical Solutions, Erlangen, Germany) sowie zeitlich davon getrennt ein Spiral-CT. Nach Fusion der SPECT- und CTBilder wurden pixelweise bilinear Schwächungskoeffizienten anhand der Hounsfield-Werte aus den CT-Daten errechnet und schwächungskorrigierte Bilder mittels iterativer Rekonstruktion erzeugt (OSEM 2D). Über fokalen Mehranreicherungen (n = 24) und über dem Hintergrund wurden ROIs (regions of interest) gezeichnet und Target/Hintergrund-Quotienten für die schwächungskorrigierten (TBAC) und nicht schwächungskorrigierten (TBNAC) Datensätze ermittelt. Auf den korrespondierenden CT-Schichten wurde der minimale Abstand des Zentrums der Läsion von der Körperoberfläche (DS) gemessen. Außerdem wurde je eine ROI über der Leberperipherie und der Lebermitte positioniert und die Countratenverhältnisse mit (LRAC) und ohne (LRNAC) Schwächungskorrektur verglichen. Ergebnisse: TBAC war bei mehr zentralen Herden bis zu 52% höher, bei peripheren Läsionen dagegen bis zu 63% niedriger als TBNAC. Der Target/Hintergrund-Quotient zeigt eine lineare Korrelation mit DS. In der Leber führte die Schwächungskorrektur zu einem Anstieg von LRAC um 35% verglichen mit LRNAC. Schlussfolgerung: Die Schwächungskorrektur von SPECT-Bildern mit separaten CT-Daten ist schnell und einfach. Der Kontrast zwischen zentralen Läsionen und dem Hintergrund wird verbessert, die Traceraufnahme in der Leber kommt homogener zur Darstellung.

Keywords

SPECT - attenuation correction - image fusion - spiral computed tomography - indium-111-pentetreotide

Schlüsselwörter

SPECT - Schwächungskorrektur - Bildfusion - Spiral-CT - Indium-111-Pentetreotide

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