Tissue volume and activity mapping using total intensity projection of PET/CT images

Autoradiography using phosphor imaging screens is often used to characterize tissue distribution of positron emission tomography (PET) radiotracers. PET tracers emit positrons with limited penetration range, and valid quantitative autoradiography can therefore only be achieved in thin tissue slices. However, in some settings, quantitative tracer profiling in thick tissues is required. Our aim was to develop a reliable method for this purpose. In this paper, we present a method based on total intensity projections (TIPs) of PET and computed tomography (CT) images. We show theoretically and experimentally that tissue total activity and tissue volume maps can be derived from the TIPs of PET and CT images, respectively. We also show that these maps are free of signal displacement artifacts in the direction of projection. To demonstrate the utility of the approach, we obtain and compare TIP-based maps and autoradiography of ex-vivo atherosclerotic minipig aortas following in-vivo injection of ¹⁸F-fluorodeoxyglucose. We show that autoradiography of the thick aortas yields distorted results due to positron range effects, whereas TIP-mapping is free from such bias. The TIP-based maps may, thus, provide a low-resolution alternative to autoradiography, when tracer accumulation profiling in thick tissues is required.