LAUSR

Positron emission tomography (PET) is a unique molecular imaging modality, which allows for non-invasive in vivo assessments of regional tissue function in man. It represents the most selective and sensitive (picoto nano-molar range) method for measuring molecular pathways and interactions in vivo. A multitude of physiological, biochemical, and pharmacological parameters can be measured, including blood flow (perfusion), blood volume (vascularity), oxygen utilization, glucose metabolism, preand post-synaptic receptor density and affinity, neurotransmitter release, enzyme activity, drug delivery and uptake, gene expression, etc. Within cardiology the method has mainly been used for assessing myocardial blood flow (MBF) and glucose metabolism (MMRglu). In fact, the so-called MBF/MMRglu match/mismatch approach probably was the first diagnostic application of PET and for many years has been the gold standard for assessing myocardial viability. Apart from its use in diagnostic imaging, PET has the capacity to provide new information on human disease, for example by using novel radiotracers. In recent years, PET plays an increasing role in the objective assessment of both progression of disease and therapeutic efficacy. At present, the latter application has found its way in the development of new drugs, but at the same time there is an enormous potential to use PET as a guiding tool in precision medicine. For many diagnostic applications visual inspection of reconstructed images is sufficient. One notable exception is, however, the presence of three-vessel disease, where the relative distribution may seem normal and only a quantitative assessment will indicate that there is a global reduction in, for example, MBF. In addition, recently it has been shown that quantitative MBF measurements have a high accuracy in selecting appropriate patients for catheterisation, much higher than the accuracy of qualitative SPECT assessments. In addition, quantitative measurements are essential not only for many research applications, but also for longitudinal studies (response assessments), especially in the presence of global effects. Quantification is not a single-step procedure. On the contrary, many issues need to be addressed when performing quantitative PET studies. These issues are related to data acquisition, data processing, and data analysis (Table 1), although a strict separation is not always possible. For example, in myocardial studies, motion correction usually requires gated acquisitions, i.e., this data processing step can only be carried out by adapting data acquisition accordingly. In a thorough study published in this issue, Turco and co-workers combine three issues (image reconstruction, partial volume correction and motion correction) in a single step by defining the reconstruction technique that maximizes quantitative accuracy. To this end, they incorporate partial volume correction in the reconstruction algorithm itself and use gating to correct for respiratory and cardiac motions. It should be noted that both partial volume and motion corrections are not new. The awareness that the limited resolution of PET scanners may result in partial volume effects (i.e., underestimation of regional tracer concentrations for structures that are smaller than twice the resolution of the scanner) is as old as PET itself, as are methods developed to correct for these effects. Interestingly, over the years, partial volume effects have become smaller with improvements in spatial resolution (from * 2 cm in 1980 to * 4 mm today). Nevertheless, Reprint requests: Adriaan A. Lammertsma, PhD, Department of Radiology & Nuclear Medicine, VU University Medical Center, Amsterdam UMC, P.O. Box 7057, 1007 MB, Amsterdam, The Netherlands; [email protected] J Nucl Cardiol 2019;26:2045–7. 1071-3581/$34.00 Copyright 2019 American Society of Nuclear Cardiology.