Distributed non-invasive system for measuring the arterial input function in PET

Usually, PET images are evaluated statically and qualitatively, seeking for regions with abnormally high or low uptake. Quantification allows for a more thorough understanding of the physiological process, benefiting not only diagnoses but also allowing for longitudinal studies and inter subject normalization. Quantification can be performed using compartmental analysis that requires the knowledge of concentration of the radiotracer present in the arterial blood at each instant - the arterial input function (AIF). Measuring the AIF is usually an invasive and challenging process, making this a real obstacle to performing in patients and in healthy volunteers. In this work, a prototype for measuring the AIF non-invasively is presented. The system is composed by several gamma detectors that are placed near superficial arteries. The main idea is to obtain signals from arteries and their surroundings using different locations and combining the signals in order to accurately estimate the AIF. We assembled proportional radiation detectors comprising essentially a bismuth germanium oxide (BGO) crystal and a photodiode. The first prototypes are being tested for energy resolution, efficiency, linearity and adaptation to the patient body during real PET exams. Energy resolution was estimated to be 14% FWHM (Full Width at Half Maximum), efficiency 1.4% and proportionality between response and activity of radioactivity sources was observed. The detectors were placed on the wrist and close to the ankle in two patients that underwent PET scans. Measures from the wrist are compatible to the AIF but movements of the patients show that shielding the detectors is necessary to prevent contamination from other sources other than the radial artery. Regarding the ankle detector, it is difficult to have it fixed during the exam, which has yet to be tackled. Results so far permit to conclude that the system exhibits suitable characteristics for measuring the AIF for PET quantification..