European Nuclear Medicine Guide
Historical developments
Positron emission tomography has been successfully used in cardiac, neurological and oncological research applications since the 1970s, and numerous milestone publications have shown the potential of this technique for the characterization of functional tissue properties covering metabolism and perfusion. Since then, PET has been developed into a clinical imaging tool, but, due to high costs and lack of reimbursement in many countries, PET stayed in the research domain for a long time. However, this changed in 2001, when Townsend and Beyer implemented the first hybrid PET/CT system, which combined high-resolution morphological CT images with informationally valuable, albeit at a lower spatial resolution, metabolic specificity of PET[151]. From a logistical point of view, another advantage has to be appreciated: the time-consuming acquisition of a transmission dataset for attenuation correction using external sources, taking up to 15 minutes per bed position, was replaced with a scan lasting a few seconds using the CT. This enabled higher patient throughput in whole-body, oncological imaging and paved the way for the success of PET/CT in today’s medical imaging world. Although this success is almost entirely based on tumour imaging, given the clinical availability of PET/CT systems, neurological and cardiac applications are widely used (Fig. 1).
Figure 1. Three PET image examples from neuro-oncology, prostate imaging and cardiac viability scanning.
Technical principles
PET imaging uses the rapid radioactive decay of positron-emitting isotopes (such as 18F, 11C, 13N, 68Ga, 82Rb, etc.), which are chemically “inserted” in molecules with high biological relevance (e.g. for assessment of metabolism, perfusion, and receptors). These radiopharmaceuticals (otherwise called tracers) are intravenously injected into a patient either prior to the scan or after positioning in a PET scanner. Then, positrons are emitted and will annihilate with electrons within a very short time under the emission of two 511 keV photons (this process is discussed in more detail earlier in this book). PET tomographs utilize the near-simultaneous (coincidence) detection of these photons with rings of detectors (Fig. 2). The high-energy photons are detected with dedicated materials which convert them into lower-energy photons, which are then, in turn, detected and amplified (mostly) with photomultiplier tubes. These electrical signals are subsequently processed to estimate the so-called line of response (LOR) along which the decay must have taken place. This is possible since the two annihilation photons travel in an anti-parallel manner (i.e., they are emitted at an angle of approximately 180o), implying that the decay must have taken place somewhere along the LOR (Fig. 3). Thus, in contrast to SPECT scanners, where collimators are used to associate a detected event with the direction it came from, PET relies on an “electronic collimation” by using a very short-acceptance timing window of typically a few nanoseconds or less for the detection of both events. Fundamentally, three types of events can occur (Fig. 4). The first kind is a true coincidence, if indeed the two annihilation photons arrive unscattered at the two detectors along the LOR. The second, a scattered coincidence, occurs when one or both photons from a single positron decay undergo a scatter event in the body, but arrive within the time window; obviously, this results in a wrong spatial association. Finally, a random coincidence can occur, where rays from two unrelated decay events are registered within the time window. Whereas the first possibility results in a “correct” measurement, the two other cases will yield image degradation. Originally, PET systems operated in 2-dimensional (2D) mode with interplane septa which reduce scattered photons as coincidence measurements. Basically, in this 2D mode, coincidence measurements are performed only in one plane of the PET camera. Removing the septa and accepting coincidences between scanner planes, however, increases sensitivity and consequentially shortens scan time. This so-called 3D mode has shown many advantages especially in oncological imaging protocols, including increased patient throughput[152,153].