Image reconstruction method for dual-isotope positron emission tomography

Abstract

We developed a positron emission tomography (PET) system for multiple-isotope imaging. Our PET system, named multiple-isotope PET (MI-PET), can distinguish between different tracer nuclides using coincidence measurement of prompt γ-rays, which are emitted after positron emission. In MI-PET imaging with a pure positron emitter and prompt-γ emitter, because of the imperfectness of prompt γ-ray detection, an image for a pure positron emitter taken by MI-PET is superposed by a positron-γ emitter. Therefore, in order to make isolated images of the pure positron emitter, we developed image reconstruction methods based on data subtraction specific to MI-PET. We tested two methods, subtraction between reconstructed images and subtraction between sinogram data. In both methods, normalization for position dependence of the prompt γ-ray sensitivity is required in addition to detector sensitivity normalization. For these normalizations, we performed normalization scans using cylindrical phantoms of the positron-γ emitters 44mSc (prompt γ-ray energy: 1157 keV) and 22Na (prompt γ-ray energy: 1274 keV) . A long period measurement using the activity decay of 44mSc (T1/2 = 58.6 hours) elucidated that the acquisition ratio between the prompt γ-rays coincided with PET event and pure PET event changes on the basis of object activities. Therefore, we developed a correction method that involves subtraction parameters dependent on the activities, i.e., the counting rate. We determined that correction for sensitivity normalization in variation of activity can be performed using only the triple-coincidence rate as an index, even if using a different nuclide from that used for normalization. From analysis of dual-tracer phantom images using 18F and 44mSc or 18F and 22Na, data subtraction in the sinogram data with sensitivity correction gives a higher quality of isolated images for the pure positron emitter than those from image subtractions. Furthermore, from dual-isotope (18F-FDG and 44mSc) mouse imaging, we concluded that our developed method can be used for practical imaging of a living organism.