RT Journal Article
SR Electronic
T1 <sup>68</sup>Ga and <sup>18</sup>F quantification, and detectability of hot spots using an ACR Phantom: Contributions of radionuclide physical differences to hot spot detectability
JF Journal of Nuclear Medicine
JO J Nucl Med
FD Society of Nuclear Medicine
SP 1200
OP 1200
VO 60
IS supplement 1
A1 Michael Silosky
A1 Ramesh Karki
A1 Bennett Chin
YR 2019
UL http://jnm.snmjournals.org/content/60/supplement_1/1200.abstract
AB 1200Introduction: 68Ga DOTATATE is currently the standard of care for PET imaging for suspected neuroendocrine tumors. The 68Ga radiometal is easily chelated, and is a promising PET radionuclide for a number of emerging ligands such as PSMA. 68Ga DOTATATE PET has been implemented in clinical protocols similar to those of FDG, however, 68Ga DOTATATE has several different physical properties compared to 18F including: lower administered dose, higher positron mean energy and mean free path length, and a secondary high energy gamma photon emission. PET acquisition parameters and quantification have previously been optimized for 18F, and traditionally accepted for 68Ga. The purpose of this study is to identify and quantify the potential differences in 68Ga compared to 18F with respect to signal to noise ratio (SNR) in an ACR phantom to determine the effects of these differences on hot spot detectability. Methods: A PET ACR phantom was dosed with 68Ga and 18F in accordance with the ACR PET Phantom Instructions for the Evaluation of PET Image Quality. The phantom was scanned in list mode on a Philips Gemini TF PET/CT scanner for a duration of 15 minutes. Images were reconstructed at a variety of image frame durations (1, 1.5, 3, 5, and 15 minutes). Regions of interest were drawn to carefully contour the hot cylinders on the CT images as well as in the background. SUVmax, SUVmean, and the standard deviation were recorded, and SNR and recovery coefficient (RC) were calculated. Multiple 68Ga and 18F ACR phantom scans and reconstructions were performed to investigate SNR and RC differences between 68Ga and 18F, and how lesion detectability is affected by acquisition time, and activity concentrations (target to background ratios; TBR). Results: 68Ga resulted in consistently lower SUV and SNR than 18F for the same activity concentration and scan parameters. RC for 68Ga was approximately 81.6% ± 3.7% of the RC for 18F (P = 0.02). For large detectable lesions, SNR for 68Ga was approximately 55.9% ± 2.6% of the SNR for 18F (P = 0.05) for 5 minute frames. This value was observed to decrease as frame duration increased. At the lowest TBR (2.4), the smallest cylinder (8mm) was visually detectable on some of the 15 minute, 18F images, however, it was not visible on any of the 68Ga images. Increasing TBR for 68Ga by increasing the activity in the hot cylinders substantially improved SNR. An increase in TBR from 2.4 to 10.9 resulted in a minimum increase in SNR of 5.8 times (1 minute frames, 25 mm cylinder) with improvement in SNR increasing as frame duration lengthened. This is reflected in vastly improved visual detectability of small lesions in the 68Ga images at higher target to background ratios. Conclusions: Hot lesion detectability for 68Ga PET images is lower than that of 18F PET images. SNR, however, is strongly influenced by target to background ratio. At higher TBR, analogous to high tumor uptake seen in clinical 68Ga DOTATATE studies, high SNR resulted in visibility of the smallest cylinder even at short imaging times. This emphasizes the value of 68Ga DOTATATE specifically, as well as the development of new radiopharmaceuticals in general. While increases in imaging time and improvement in scanner sensitivity improve SNR and detectability, the effect of radiopharmaceutical biodistributions that result in high lesion activity relative to the background may be much more significant for lesion detectability.