Clinical use of FDG PET
Introduction
The purpose of this article is to review the impact of the various clinical applications of positron emission tomography (PET) using the radiopharmaceutical 2-[18F]-fluoro-2-deoxy-d-glucose (FDG). FDG PET has been widely used for research for more than 25 years [1]. More recently, FDG has become the most widely used PET radiopharmaceutical in the clinical imaging of oncologic, cardiac and neurologic applications. Although the same radiopharmaceutical and imaging technology is being used in all these applications, there is a large variation in the actual clinical volume between these various applications, with oncology being the largest, typically more than 90% of the volume. Further understanding of the underlying factors for this variation may give insight to why a particular combination of radiopharmaceutical and imaging technology will or will not gain widespread clinical use. For this analysis, a set of criteria (or questions) is proposed, which may help to predict the success of a new radiopharmaceutical in clinical use. These questions are as follows: (a) Are there competing tests? (b) Is there a cost-effective role in the clinical management? (c) Is the interpretation of the imaging study easy?
To address the above questions, it is important to recognize which of the technological capabilities in PET are important contributing factors that enabled the favorable outcome for each clinical question. The two general features that PET provides as a medical imaging technology can be viewed as (a) the specific biochemical properties of the positron emitting radiopharmaceutical and (b) the imaging properties of the PET scanner. The PET radiopharmaceuticals are small molecules that are more likely to behave as endogenous compounds within the body. This allows the biodistribution of the radiopharmaceutical to mimic the biodistribution of the endogenous compound at the molecular, cellular and tissue levels. The uniform response of the annihilation coincidence detection system, combined with a method for accurate attenuation correction, enables PET to have a proportional relationship between the tissue intensity on the tomographic image and the actual radiopharmaceutical concentration in tissue [2]. This simplifies the accurate and noninvasive measurement of low concentration of endogenous compounds in the living organism. The additional capability of rapid multislice dynamic PET imaging allows serial measurements of radiopharmaceutical biodistribution and regional tissue concentrations over time. The series measurement of tracer concentration over time permits the application of tracer kinetic modeling of biologic processes [3].
Dynamic PET imaging limits the axial field of view to the axial length of the detector system; however, the technology in the current scanners allows multiple static images to be acquired along the length of the patient, which can then be appended to form a whole-body image (Fig. 1) [4], [5]. In current clinical practice, more than 90% of clinical PET imaging involves static imaging after an uptake period and the use of a tracer uptake ratio index for semiquantitative analysis of radiopharmaceutical uptake.
Section snippets
Oncology PET imaging
Current Medicare reimbursable oncologic FDG PET applications include the following: evaluation of solitary pulmonary nodules, staging of lung cancer, staging of malignant melanoma, staging or restaging of colorectal carcinoma, staging of lymphomas, staging of head and neck cancers, staging of gastroesophageal cancer, staging of recurrent breast cancer, evaluation of thyroid cancer metastases and evaluation of response to therapy in breast cancer [6], [7], [8], [9], [10], [11], [12]. Not all
Cardiac PET imaging
The current Medicare reimbursable cardiac FDG PET application is for the evaluation of myocardial viability. The clinical question of cardiac viability may occur when there is decreased left ventricular wall motion. This lack of regional myocardial contraction may be due to an area of previously infarcted myocardium, hibernating myocardium or stunned myocardium. In the case of infarcted myocardium, the tissue is scarred and reversible dysfunction is lacking. In the cases of hibernating
FDG brain imaging
Current reimbursable indications for FDG PET brain imaging include the evaluation of complex partial epilepsy and the evaluation of Alzheimer's dementia (AD) [19]. The role of PET in the evaluation of complex partial epilepsy is to identify which temporal lobe the seizure focus is residing. The localization of the seizure origin is clinically important for the small number of patients who fail treatment with antiepileptic medications and who may be candidates for surgical resection of a portion
Discussion
The widespread acceptance of a PET radiopharmaceutical and PET imaging procedure is highly dependent on multiple factors. Even with the same FDG radiopharmaceutical, there is a large variation in the number of clinical studies within the various CMS-approved applications, with whole-body oncology imaging being close to 90% of all studies performed. The fact that CMS has approved of all of the discussed PET applications confirms that there is sufficient scientific and clinical data to support
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