Quantitative ¹³¹I SPECT with triple energy window Compton scatter correction

In this work accuracy of quantitative ¹³¹I SPECT with triple energy window (TEW) scatter correction is evaluated by phantom measurements. The application is tumor imaging of B-cell lymphoma patients treated with ¹³¹I radioimmunotherapy. The TEW method is a pixel by pixel correction where the scatter fraction in the photopeak window is estimated by linear interpolation between two adjacent narrow sub-windows. For the phantom measurements performed in this work the TEW estimate of scattered counts was close to one half of the total photopeak window counts. Quantification procedure includes marker based X-ray CT-SPECT image fusion to determine object boundaries and to generate attenuation maps. TEW scatter correction significantly reduces the effect of background activity on reconstructed counts within an object, but it still exists due to the finite spatial resolution of the system. Therefore, a background dependent calibration factor had to be used to achieve good quantitative accuracy. Quantitative accuracy with TEW correction was 5% and 14% for a tumor and lung respectively of a physical phantom with non-uniform activity and non-uniform scattering medium. With no scatter subtraction but using a background dependent calibration the quantitative accuracy was 7% and 18% for the tumor and lung respectively