Comparison of imaging characteristics of I-124 PET for determination of optimal energy window on the Siemens Inveon PET

Objectives I-124 has a half-life of 4.2 days, which makes it suitable for imaging over several days the radiolabeled antibody tracer biological uptake and washout phases. However, it has a low positron branching ratio (23%), because prompt gamma coincidence due to high-energy γ- photons (602 to 1,691 keV), which degrade image quality, are emitted in cascade with positrons. In this study, we assessed the image quality of reconstructed PET to determine the optimal energy window in I-124 PET with various parameters on Siemens Inveon PET. Image qualities such as non-uniformity, recovery coefficient and spill over ratio were assessed according to NEMA NU4-2008.

Methods Recently, we developed the method for determination of prompt gamma coincidence fraction of I-124 (JINST P03006). Briefly, to measure the prompt gamma coincidence fraction (PGF) of I-124, we measured the sensitivities of I-124 and F-18 from energy windows. PGF was predicted using branching ratio corrected sensitivities of I-124 and F-18. In this study, we investigated the optimal PET energy window for I-124 PET based on image characteristics of reconstructed PET. NECR (noise equivalent count rate) was measured using Monte Carlo simulation to compare the results of image quality evaluation. Image characteristics such as non-uniformities, recovery coefficients (RCs) and the spill over ratios (SORs) of I-124 were measured as described in NEMA NU4-2008 standards. All image data were implemented corrections including prompt gamma coincidence correction by considering the PGF. In addition, we proposed a new figure of merit (FOM) to consider multiple parameters in I-124 PET.

Results The difference between best and worst uniformity in the various energy windows was less than 1%.There is no significant difference of RCs among the energy windows. The lowest SORs of I-124 were obtained at 350~750 keV in non-radioactive water compartment. The best FOM was obtained with an energy window of 350~750 keV, although 350~600 and 400~590 keV was reported as NECR based optimal energy window.

Conclusions The present study described the imaging characteristics of I-124 PET for Siemens Inveon PET scanner. Optimal energy window should be determined based on image characteristics. Our proposed FOM was useful to determine the optimal energy window for I-124 PET. For I-124 PET, an energy window of 350~750 keV was optimal energy window based on our proposed FOM.