A Compton camera for nuclear medicine applications using 113mIn1

Theoretical studies show our prototype Compton camera, C-SPRINT, matches the 99mTc performance of clinically available mechanically collimated systems if an advantage in sensitivity of ∼45 can be achieved. Imaging at higher energies substantially reduces the required sensitivity advantage. At ∼400 keV, our Compton camera system needs only five times the raw count rate of a mechanically collimated system imaging at 99mTc energy to reach the performance “break even” point. We analyze our C-SPRINT system performance for the isotope 113mIn (391.7 keV), and compare it to a collimated system imaging 99mTc. 113mIn has been used in nuclear medicine applications in the past, and can potentially be used to label many of the same radiopharmaceuticals as 99mTc. In order to fully compare the two systems, their relative sensitivities are combined with the relative amount of useful gamma rays that escape the object being imaged (the patient) for the same patient radiation dose. Results for uniformly distributed sources show that for equal lifetime radiation dose, the ratio of useful 99mTc to 113mIn gamma rays is 1.59. For a point source of activity centered inside the ellipsoid, the useful ratio decreases to 1.33. These fractions scale up the required raw sensitivity advantage to yield a required sensitivity advantage of 5 – 8. Monte Carlo simulations have shown that a raw sensitivity advantage of 25 can be achieved by improving C-SPRINT geometry and using a larger volume of silicon detectors. We conclude that gains of 3–5 in noise equivalent sensitivity are achievable when imaging 113mIn with our Compton camera relative to a collimated system imaging 99mTc.