Physical characteristics of ⁶⁸Ga DOTATATE PET/CT affecting small lesion detectability

⁶⁸Ga DOTATATE PET/CT protocols are similar to ¹⁸F FDG protocols despite differences in physical properties, biodistribution, and tumor uptake. The purpose of this study is to evaluate the impact of scan time (counts), and target activity on signal-to-noise ratio (SNR) in various sized targets, or lesions. To evaluate this, phantom experiments and analysis of clinical ⁶⁸Ga DOTATATE PET/CT studies were performed.

⁶⁸Ga was first compared to ¹⁸F in phantom studies to evaluate recovery coefficients and SNR. ⁶⁸Ga phantom studies were also acquired in list mode, and at varying target activities to evaluate the effects of acquisition time and high target concentrations on SNR in clinically relevant small (8 mm) and larger targets (≥ 12 mm). Clinical studies (n = 50) were analyzed to determine if phantom target concentrations and SNR are present in clinical ⁶⁸Ga DOTATATE studies at similarly very high tumor activity concentrations (n = 159).

In phantoms, recovery coefficient and SUV_max for ⁶⁸Ga were ~87% of ¹⁸F. SNR for ⁶⁸Ga was ~65% of ¹⁸F. For the ⁶⁸Ga small target (8 mm) at standard T/B = 2.4, increasing scan time from 5 to 15 minutes increased SNR from < 1 to 1.6, and did not result in target identification. Increasing T/B from 2.4 to 10.9, however, dramatically increased SNR from < 1 to 22.3. Increased T/B resulted in clear visibility of the 8 mm target, even for 1-minute scans. In patients, high hepatic tumor SUV_max (27.3±29.6), resulted in high SNR (12.5±9.8). For extrahepatic tumors, high SUV_max (41.6±42.8), resulted in high SNR (43.8±49.9).

Very high target or T/B, even in small targets, can offset the physical limitations of ⁶⁸Ga. High target uptake and high T/B are primary factors influencing small lesion detectability.