PET/CT “Cometh the hour, cometh the machine?”

Abstract

Clinical positron emission tomography (PET) and PET/computed tomgraphy (CT) are rapidly expanding and there are many plans for new installations in the UK. This is because PET has been shown to change the management of 30% of cancer patients by adding unique functional information to that obtained from anatomical imaging such as CT and magnetic resonance imaging (MRI). The Health Technology Board of Scotland has recently accepted that PET is more sensitive and specific than other techniques in the investigation of a number of common malignancies, and there is also increasing evidence of the cost-effectiveness of this technique. Most malignant tumours exhibit increased glycolysis due to over-expression of glucose transport membrane proteins and upregulation of intracellular hexokinase activity. The most commonly used radiopharmaceutical in PET is [18F]2-fluoro-2-deoxy-D-glucose (FDG), which behaves as a glucose analogue and shows high uptake in many different types of tumour. Due to its ability to image and quantify increased glucose metabolism in tumour cells PET is more accurate than other non-invasive imaging techniques in differentiating benign and malignant lesions such as solitary pulmonary nodules. PET also allows more accurate staging of malignant processes due to its ability to detect increased glucose metabolism in non-enlarged lymph nodes (and conversely exclude malignant infiltration of nodes enlarged for other reasons), and to detect metastatic disease not evident on conventional imaging. In addition it is a whole-body technique and therefore detects disease in regions not routinely included in anatomical studies. It has also proved highly valuable in the follow-up of oncology patients as it can accurately differentiate recurrence from fibrosis and can demonstrate diminished glucose metabolism in response to chemotherapy before any reduction in lesion size. Oncology accounts for 85-90% of all PET examinations, but there are also important applications in cardiology and psychiatry. PET does, however, have some disadvantages. It demonstrates normal anatomy with only poor (if any) contrast. Functional abnormalities may therefore not be accurately located in specific anatomical structures. Precise anatomical localization of abnormalities is, however, essential for the diagnosis and staging of oncology patients and for accurate planning of treatment. In addition PET examinations are time-consuming (approximately 1 h for a whole-body study) due to the need to perform a transmission scan for attenuation correction. To address some of these problems the concept of hybrid or fusion imaging has arisen—the combination of functional and anatomical studies in the one examination. Post-processing fusion of anatomical and functional images obtained on separate machines has been used successfully in the brain, but is problematic in other regions of the body due to differences in patient positioning and scanner table top contours and due to movement of internal organs between scans. In addition the process is very time-consuming and labour intensive and is not suitable for routine clinical practice. The combination of a CT scanner and a PET camera in one machine overcomes many of these problems: both anatomical and functional images can be obtained consecutively in one examination, which therefore minimizes spatial and temporal differences between the images and considerably shortens the total scanning time for each patient. Such systems are now commercially available from several manufacturers. In PET/CT the CT data are used for attenuation correction after appropriate energy scaling. Attenuation correction with CT (CTAC) can be performed in under 2 min for a whole-body study, which is much quicker than using a transmission