Feasibility of dual radionuclide brain imaging with I‐123 and Tc‐99 m

A study was conducted to evaluate the feasibility of simultaneous dual radionuclide brainimaging with 123I and 99m Tc using photopeak image subtraction techniques or offset photopeak image acquisition. The contribution of the photons from one radionuclide to a second radionuclide’s photopeak energy window (crosstalk) was evaluated for SPECT and planar imaging of a brain phantom containing 123I and 99m Tc for a range of activity levels and distribution properties approximating those in rCBF images of the adult human brain. Crosstalk was evaluated for 10% symmetrical energy windows centered on the 123I and 99m Tc photopeaks and for 10% energy windows asymmetrically placed to the left and right of the center of the respective photopeaks. Major observations include: (1) in the centered photopeak windows, 99m Tc crosstalk in the 123I window is 8.9% of the 99m Tc seen in the 99m Tc window and ranges from 37.5% to 75.0% of the 123I in the 123I window. 123I crosstalk is 37.8% of the 123I seen in the 123I window and ranges from 4.4% to 8.9% of the 99m Tc seen in the 99m Tc window; (2) the spatial distribution of a radionuclide’s crosstalk photons differs from that observed in the radionuclide’s photopeak window; (3) a 99m Tc photopeak window offset to the left does not decrease 123I crosstalk, and the percentage of 99m Tc scatteredphotons is significantly increased in the window. Offsetting the 123I window to the right decreases 99m Tc crosstalk to 9.0% to 17.9% of the 123I counts, but decreases 123I sensitivity by 39.9%; and (4) offsetting both photopeak windows to the right decreases the 99m Tc scatteredphotons in the 99m Tc window, but increases 123I crosstalk to 17.0% to 33.8% of the 99m Tc counts. The findings show that image quality, spatial resolution, and quantitative accuracy are degraded to unacceptable levels with the combinations of energy windows tested for dual radionuclide imaging of 99m Tc and 123I. This indicates that dual radionuclide imaging must be thoroughly tested and validated before use in clinical studies.