Comparison images between PET and SPECT of ⁹⁰Y: A Monte-Carlo simulation study

Objectives The Single Photon Emission Computed Tomography (SPECT) image for the bremsstrahlung emitted from Yttrium-90 used in targeted radionuclide therapy and the Positron Emission Tomography (PET) image for the internal pair production were generated using our own custom Monte-Carlo simulation codes to examine the detection limit. And we compared the images between PET and SPECT.

Methods We used an NEMA IEC Body Phantom containing six hot spheres. Each sphere included the activity concentrations of 1-6 MBq/mL of Yttrium-90 and the background concentration included 1/10-1/40 for the hot spheres. Acquisition time was set to 30 min. In the case of SPECT, the number of the projections was set to 120/360°. We used the “Prominence Processor®” software to reconstruct the SPECT projection images and to evaluate some reconstructed images. In the case of PET, we used our own custom software to reconstruct the PET images.

Results The contrast recovery coefficient (CRC) and the contrast noise ratio (CNR) for hot spheres decreased with decreasing diameter of the hot sphere and with decreasing hot sphere-background concentration ratio. In the case of 1-6 MBq/mL concentration in the hot spheres, the CNR for a 10 mm hot sphere was lower than 5.0 in all conditions for SPECT. Therefore, a 10 mm hot sphere could not be identified by SPECT. The CNR for a 13 mm or larger hot sphere was higher than 5.0 only for the case of the hot sphere-background ratio of 1/40. In PET, when the hot sphere-background ratio was as high as 1/20 and 1/40, we could identify the 10 mm hot sphere. The profile of PET was steeper and lower background-noise level than that of SPECT.

Conclusions PET image is superior to SPECT image because the CRC and the profile of PET are better than those of SPECT.