conventional sense) are produced in the electron cloud surrounding the nucleus. In medical imaging, X‐rays are artificially or intentionally produced on demand, whereas gamma rays (and other particles) are part of an ongoing nuclear decay, enabling unstable radioactive atoms to reach a stable state. The length of time it takes for one half of the unstable atoms to reach their stable state is called their half‐life. Radionuclide imaging involves the emission of a photon that is imaged using a crystal or solid‐state detector. The detector may be static and produces images of the event in a single plane or the detector may be rotated about the patient to gather three‐dimensional data and reconstruct a tomographic image in the same manner as a CT scanner. This is the basis for single photon emission computed tomography (SPECT). Examples of planar images used in salivary gland diseases include gallium (67Ga) for evaluation of inflammation, infection, and neoplasms (lymphoma). SPECT, which produces tomographic cross‐sectional images, is less commonly used in oncologic imaging, although novel radionuclides and ligands are under investigation. The recent introduction of SPECT/CT, a combined functional and anatomic imaging machine may breathe new life into SPECT imaging.
POSITRON EMISSION TOMOGRAPHY (PET)
Positron emission tomography (PET) is a unique imaging modality that records a series of radioactive decay events. Positron emission is a form of radioactive decay in which a positron (positively charged electron) is emitted from the unstable atom to achieve a more stable state. The positron almost immediately collides with an electron (negatively charged) and undergoes an annihilation event in which both particles are destroyed and converted into pure energy. The annihilation event produces two gamma rays, each with 511 Kev (kilo electron volt) of energy, and traveling in 180‐degree opposition. By using sophisticated solid‐state detectors, and coincidence circuitry, the PET system can record the source of the event, thereby localizing the event in three‐dimensional space. Using a complex algorithm similar to SPECT and CT, a three‐dimensional block of data is produced and can be “sliced” in any plane, but most commonly in axial, coronal, and sagittal planes, as well as a maximum intensity projection (MIP) rendering.
Figure 2.25. Contrast‐enhanced axial CT exam through the parotid gland prior to biopsy demonstrates a relatively large heterogeneous mass with ill‐defined borders (a). (b) Ultrasound‐guided core needle biopsy. Note the biopsy needle passing into the hyperechoic mass within the parotid gland. (c) Fine needle aspiration biopsy: Note the darkly staining cells that appear atypical. A definitive diagnosis cannot be reached utilizing this cryptologic material. (d) Core needle biopsy. Note the darkly staining epithelial cells arranged in ductal groups. This biopsy was consistent with an atypical mixed tumor. (e) Surgical material from a parotidectomy. This lesion represents a carcinoma ex. mixed tumor. Note the similar cells seen in the core needle biopsy.
PET radionuclides are produced in a cyclotron and are relatively short lived. Typical radionuclides include 18F, 11C, 15O, 82Rb, and 13N. A variety of ligands have been labeled and studied for the evaluation of perfusion, metabolism, and cell surface receptors. The most commonly available is 18F‐deoxyglucose (FDG), which is used to study glucose metabolism of cells. Most common uses of FDG include oncology, cardiac viability, and brain metabolism. PET has a higher spatial resolution than SPECT. Both systems are prone to multiple artifacts, especially motion. Acquisition times for both are quite long, limiting the exam to patients who can lie still for prolonged periods of time. Both systems, but PET in particular, are very costly to install and maintain. Radiopharmaceuticals are now widely available to most institutions through a network of nuclear pharmacies.
The oncologic principle behind FDG PET is that neoplastic tissues can have a much higher metabolism than normal tissues and utilize glucose at a higher rate (Warburg 1925). Glucose metabolism in the brain was extensively studied using autoradiography by Sokoloff and colleagues at The National Institutes of Health (NIH) (Sokoloff 1961). The deoxyglucose metabolism is unique in that it mimics glucose and is taken up by cells using the same transporter proteins. Both glucose and deoxyglucose undergo phosphorylation by hexokinase to form glucose‐6‐phosphate. This is where the similarities end. Glucose‐6‐phosphate continues to be metabolized, eventually to form CO2 and H2O. Deoxyglucose‐6‐phosphate cannot be further metabolized and becomes trapped in the cell as it cannot diffuse out through the cell membrane. Therefore, the accumulation of FDG reflects the relative metabolism of tissues (Sokoloff 1986). The characteristic increased rate of glucose metabolism by malignant tumors was initially described by Warburg and is the basis of FDG PET imaging of neoplasms (Warburg 1925).
FDG PET takes advantage of the higher utilization of glucose by neoplastic tissues to produce a map of glucose metabolism. Although the FDG PET system is sensitive, it is not specific. Several processes can elevate glucose metabolism, including neoplastic tissue, inflammatory or infected tissue, and normal tissue in a high metabolic state. An example of the latter includes uptake of FDG in skeletal muscle that was actively contracting during the uptake phase of the study (Figure 2.26). Another peculiar hypermetabolic phenomenon is brown adipose tissue (BAT) FDG uptake (Figure 2.27). BAT is distributed in multiple sites in the body including interscapular, paravertebral, around large blood vessels, deep cervical, axillary, mediastinal, and intercostal fat, but is concentrated in the supraclavicular regions (Cohade et al. 2003b; Tatsumi et al. 2004). BAT functions as a thermogenic organ producing heat in mammals and most commonly demonstrates uptake in the winter (Tatsumi et al. 2004). BAT is innervated by the sympathetic nervous system, has higher concentration of mitochondria, and is stimulated by cold temperatures (Cohade et al. 2003a; Tatsumi et al. 2004). Administration of ketamine anesthesia in rats markedly increased FDG uptake presumably from sympathetic stimulation (Tatsumi et al. 2004). Although typically described on FDG PET/CT exams, it can be demonstrated with 18F‐Fluorodopamine PET/CT, 99mTc‐Tetrofosmin, and 123I‐MIBG SPECT as well as 201TlCl, and 3H‐l‐methionine (Baba et al. 2007; Hadi et al. 2007). Propranalol and reserpine administration appears to decrease the degree of FDG uptake whereas diazepam does not appear to have as significant an effect (Tatsumi et al. 2004). Exposure to nicotine and ephedrine also resulted in increased BAT uptake; therefore, avoiding these substances prior to PET scanning can prevent or reduce BAT uptake (Baba et al. 2007). Preventing BAT uptake of FDG can be accomplished by having the patient stay in a warm ambient temperature for 48 hours before the study and by keeping the patient warm during the uptake phase of FDG PET (Cohade et al. 2003a; Delbeke et al. 2006). Although somewhat controversial, diazepam or lorazepam and propranalol can reduce BAT uptake by blocking sympathetic activity, as well as reducing skeletal muscle uptake from reduced anxiety and improved relaxation (Delbeke et al. 2006). Understanding the distribution of BAT and the physiology that activates BAT, as well as recognizing the uptake of FDG in BAT in clinical studies, is critical in preventing false positive diagnosis of supraclavicular, paravertebral, and cervical masses or lymphadenopathy.
Figure 2.26. CT (a), PET (b), and fused PET/CT (c) images in axial plane and an