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

BACKGROUND AND PURPOSE

Ga DOTATATE PET/CT protocols are similar to F 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 Ga DOTATATE PET/CT studies were performed.

MATERIALS AND METHODS

Ga was first compared to F in phantom studies to evaluate recovery coefficients and SNR. Ga 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 Ga DOTATATE studies at similarly very high tumor activity concentrations (n = 159).

RESULTS

In phantoms, recovery coefficient and SUV for Ga were ~87% of F. SNR for Ga was ~65% of F. For the Ga 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 (27.3±29.6), resulted in high SNR (12.5±9.8). For extrahepatic tumors, high SUV (41.6±42.8), resulted in high SNR (43.8±49.9).

CONCLUSION

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