PET scanning for evaluation of bone metabolism

737 Open Access-This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the source is credited. Sir–With great interest we read the paper of Ullmark et al. on using positron emission tomography (PET) to assess bone metabolic activity (Ullmark et al. 2009). PET is an imaging tool that can provide functional rather than morphological information, and has thus attracted a lot of attention. The prerequisites for proper analysis and subsequent sufficient interpretation are very important, and most of these originate from the principles of PET. Unfortunately, Ullmark et al. do not address these important issues and, to avoid misinterpretations , we wish to highlight some of these important points. The principle of positron emission tomography PET scanning is based on a short-lived radioactive tracer, which decays by emitting a positron. The tracers can exist as single isotopes or be incorporated in larger molecules, depending on the application. This method suffers from low spatial resolution. Apart from single-positron emission tomography (SPECT), all PET systems utilize coincidence detection of the annihilation pho-tons from positron decay. Since the paired gamma rays from the annihilation of the positron are anti-parallel, the detection of the gamma rays determines a line of response (LOR) along which the annihilation took place. Resolution is thus determined by the size/density of the detectors that are most often placed in a ring around the subject. However, the LOR is not completely linear and the gamma rays are released in a direction of 180° ± 0.5°. This physical phenomenon further limits the resolution. The spatial resolution of most clinical PET scanners is therefore only about 6–8 mm (Townsend 2004). Hence, the resulting images are known to be affected by partial volume effects, which can cause small regions with high tracer uptake to be imaged as having an artificially low concentration and vice versa (Soret et al. 2007). When comparing regions of interest (ROIs), it is therefore important for the accuracy that the size is sufficiently high or that the tissue surrounding the ROIs has a comparable uptake of the tracer. For the reader to be able to interpret PET data, describing the size of the ROI is as important as the obvious adding of standard deviations to the results. The three basic analytical methods with or without blood sampling will not be addressed here. In …