Thoracic positron emission tomography
- Paul Stark, MD
Paul Stark, MD
- Professor of Radiology
- University of California San Diego
- Section Editor
- Nestor L Muller, MD, PhD
Nestor L Muller, MD, PhD
- Section Editor — Pulmonary Imaging
- Professor of Radiology
- University of British Columbia
- Deputy Editors
- Geraldine Finlay, MD
Geraldine Finlay, MD
- Deputy Editor — Pulmonary, Critical Care, and Sleep Medicine
- Associate Professor
- Tufts University School of Medicine
- Susanna I Lee, MD, PhD
Susanna I Lee, MD, PhD
- Deputy Editor — Radiology
- Associate Professor of Radiology
- Harvard Medical School
- Massachusetts General Hospital
Positron emission tomography (PET) allows in vivo imaging of metabolic, physiologic, and pathologic processes. It has been used as a research tool for many years, and has evolved into a clinically indispensable imaging examination. The increased clinical use is a consequence of regional commercial radiopharmacies being more able to supply hospitals throughout the country with radiopharmaceuticals, while the equipment has become more robust and financially more affordable. Traditional PET scanners with an array of several thousand ring detectors can be replaced by traditional two-headed single photon emission computed tomography (SPECT) gamma cameras that have been retrofitted with coincidence detectors to allow 18-fluoro-2-deoxyglucose (FDG)-PET imaging. Many PET scanners are being replaced by integrated whole-body PET and computed tomography scanners (PET/CT scanners) , while integrated, hybrid PET and magnetic resonance (PET/MR) scanners are being introduced as well [2,3].
Technical issues (ie, radionuclides, technique, sources of error) and the indications for thoracic PET imaging are reviewed here. The use of PET scanning in the evaluation of solitary pulmonary nodules or lung cancer staging is discussed separately. (See "Computed tomographic and positron emission tomographic scanning of pulmonary nodules" and "Imaging of non-small cell lung cancer".)
Radionuclides — Positron-emitting radionuclides are produced in a cyclotron (eg, oxygen-15, nitrogen-13, carbon-11, and fluorine-18) or by a generator (eg, rubidium-82, copper-62). Fluorine-18 is one such radionuclide that is produced by proton bombardment of oxygen-18 in a cyclotron. When it is bound to a D-glucose analog, it forms 18-fluoro-2-deoxyglucose (FDG), which is the most commonly used radionuclide in thoracic positron emission tomography (PET).
FDG is transported into cells where it is phosphorylated by hexokinase to form FDG-6-phosphate, which remains trapped temporarily because it is not further metabolized by the enzyme glucose-6-phosphatase, nor is it easily extruded from the cell. The fluorine-18 emits positrons as it decays, which travel a short distance before they are annihilated by electrons, resulting in the emission of two 0.511 MeV photons oriented at 180 degrees to each other. These events are registered when the photons collide with the sodium iodide or bismuth germanate contained within the scintillation detectors that surround the patient (image 1A-B), thereby triggering an electronic pulse within the scintillation crystal. Three-dimensional localization of the decay is possible because of the coincidence circuitry that detects the annihilation radiation . The end result is measurement of the distribution of FDG throughout the body. The FDG is eventually extruded from the cell, filtered by the glomerulus without being resorbed in the proximal convoluted tubule, and excreted in the urine.
Technique — During a PET scan, the patient lies within a scanner that consists of a moving table and gantry, reminiscent of the equipment used for computed tomography (CT). Before the administration of FDG, scans are obtained using a rotating germanium-68 pinpoint source. This process is called attenuation correction and its purpose is to adjust for the decrement of positron-related radiation caused by soft tissues of varying electron densities (figure 1) . FDG is injected intravenously using a dose of 10 to 20 mCi (370 to 740 MBq) and then multiple (ie, 30 to 45) axial (ie, cross-sectional) images are obtained. A cross-sectional, axial field of view of 16 to 20 cm and a spatial resolution of 5 mm are achieved at full width half maximum. Finally, axial, coronal, and sagittal projections are viewed on a workstation and whole body images can be generated.
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