The quantification of PET-CT radiotracers to determine minimal scan time using quadratic formulation

¹⁸F is the most extensively used radioisotope in current clinical practices of PET imaging. This selection is based on the several criteria of pure PET radioisotopes with an optimum half-life, and low positron energy that contributes to a smaller positron range. In addition to ¹⁸F, other radioisotopes such as ⁶⁸Ga and ¹²⁴I are currently gained much attention with the increase in interest in new PET tracers entering the clinical trials. This study aims to determine the minimal scan time per bed position (T_min) for the ¹²⁴I and ⁶⁸Ga based on the quantitative differences in PET imaging of ⁶⁸Ga and ¹²⁴I relative to ¹⁸F.

The European Association of Nuclear Medicine (EANM) procedure guidelines version 2.0 for FDG-PET tumor imaging has adhered for this purpose. A NEMA2012/IEC2008 phantom was filled with tumor to background ratio of 10:1 with the activity concentration of 30 kBq/ml ± 10 and 3 kBq/ml ± 10% for each radioisotope. The phantom was scanned using different acquisition times per bed position (1, 5, 7, 10 and 15 min) to determine the T_min. The definition of T_min was performed using an image coefficient of variations (COV) of 15%.

T_min obtained for ¹⁸F, ⁶⁸Ga and ¹²⁴I were 3.08, 3.24 and 32.93 min, respectively. Quantitative analyses among ¹⁸F, ⁶⁸Ga and ¹²⁴I images were performed. Signal-to-noise ratio (SNR), contrast recovery coefficients (CRC), and visibility (V_H) are the image quality parameters analysed in this study. Generally, ⁶⁸Ga and ¹⁸F gave better image quality as compared to ¹²⁴I for all the parameters studied.

We have defined T_min for ¹⁸F, ⁶⁸Ga and ¹²⁴I SPECT CT imaging based on NEMA2012/IEC2008 phantom imaging. Despite the long scanning time suggested by T_min, improvement in the image quality is acquired especially for ¹²⁴I. In clinical practice, the long acquisition time, nevertheless, may cause patient discomfort and motion artifact.