Absolute quantitation of iodine-123 epidepride kinetics using single-photon emission tomography: comparison with carbon-11 epidepride and positron emission tomography

Epidepride labelled with iodine-123 is a suitable probe for the in vivo ima ging of striatal and extrastriatal dopamine D-2 receptors using single-phot on emission tomography (SPET), Recently, this molecule has also been labell ed with carbon-11. The goal of this work was to develop a method allowing t he in vivo quantification of radioactivity uptake in baboon brain using SPE T and to validate it using positron emission tomography (PET). SPET studies were performed in Papio anubis baboons using I-123-epidepride. Emission an d transmission measurements were acquired on a dual-headed system with vari able head angulation and low-energy ultra-high resolution (LEUHR) collimati on. The imaging protocol consisted of one transmission measurement (24 min, heads at 90 degrees), obtained with two sliding line sources of gadolinium -153 prior to injection of 0.21-0.46 GBq of I-123-epidepride, and 12 emissi on measurements starting 5 min post injection. For scatter correction (SC) we used a dual-window method adapted to I-123. Collimator blurring correcti on (CBC) was done by deconvolution in Fourier space and attenuation correct ion (AT) was applied on a preliminary (CBC) filtered back-projection recons truction using 12 iterations of a preconditioned, regularized minimal resid ual algorithm. For each reconstruction, a calibration factor was derived fr om a uniform cylinder filled with a I-123 solution of a known radioactivity concentration. Calibration and baboon images were systematically built wit h the same reconstruction parameters. Uncorrected (UNC) and (AT), (SC+AT) a nd (SC+CBC+AT) corrected images were compared. PET acquisitions using 0.11- 0.44 GBq of C-11-epidepride were performed on the same baboons and used as a reference. The radioactive concentrations expressed in percent of the inj ected dose per 100 mi (%ID/100 ml) obtained after (SC+CBC+AT) in SPET are i n good agreement with those obtained with PET and C-11-epidepride. A method for the in vivo absolute quantitation of I-123-epidepride uptake using SPE T has been developed which can be directly applied to other I-123-labelled molecules used in the study of the dopamine system. Further work will consi st in using PET to model the radioligand-receptor interactions and to deriv e a simplified model applicable in SPET.