The relationship between activity administered and time of acquisition per bed position for 68Ga radiopharmaceuticals

Abstract

Background: 68Ga-PSMA is a radiopharmaceutical used to image prostate cancer by Positron Emission Tomography (PET). The administered activity must be balanced with the acquisition time per bed position for a good quality image and radiation protection abiding with the ALARA principle however, there are no clear written standards for this. Objective: Therefore, in order to detect a 13mm diameter lesion, a study was undertaken utilizing the NEMA IQ phantom to establish a correlation between these two factors. The aim was to determine the relationship based on three sphere-to background ratios (SBRs) that closely resemble the Lesion-to-background ratio (LBR) observed in prostate cancer. The selected SBRs were 1.7:1, 2.2:1, and 3.9:1. Research Methodology: The phantom was scanned two consecutive times (20 minutes/bed position) for each SBR to calculate the Percentage Difference (PD), having a threshold of less than 20%. Reconstructions at different acquisition times were performed using list-mode data. The CNR for the smallest sphere (13mm diameter) was also calculated having a threshold to be equal or greater than 3 as per the Rose Criterion. The Coefficient of Variation (COV) indicates the noise level present in the image and the maximum COV was chosen at which both mentioned criteria are met. Results: SBR 1.7:1 did not meet the criteria mentioned, therefore COVmax was not determined and the smallest sphere was not detected, regardless of the acquisition duration. SBRs 2.2:1 and 3.9:1 resulted in a COVmax of 27.9% and 30.57%. This was translated to clinical protocol to acquire the minimum acquisition time for each SBR. For SBR 2.2:1 or higher, a minimum clinical acquisition time of 115 seconds is needed with a clinical activity regimen of 3.83 MBq/kg min and for 3.9:1 and higher, a minimum clinical acquisition time of 91 seconds is needed with clinical activity regimen of 3.03 MB/kg min. Conclusion: Currently, the clinical acquisition time/bed position is 150 seconds, hence with further research by including patient scans and achieving clinical validation, the clinical acquisition time/bed position can be improved to at least 115 seconds.