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Chapter: Basic Radiology : Scope of Diagnostic Imaging

Nuclear Medicine - Radiology

Positron Emission Tomography/CT (PET/CT)



Nuclear medicine studies, in general, are very sensitive, but relatively nonspecific in the detection of pathology. It is very important, therefore, to correlate nuclear medicine examina-tions with pertinent history, physical findings, laboratory data, and other diagnostic imaging studies in order to opti-mize the diagnostic utility of these studies. Nuclear medicine imaging examinations are performed by administering vari-ous radiopharmaceuticals to the patient and subsequently recording in vivo distribution. Radiopharmaceuticals consist of two main components: (1) the main component that is distributed to various organs via a number of different mech-anisms, and (2) the radionuclide that is tagged to the main component, which emits gamma rays, permitting detection of the compound in the body.


Most nuclear medicine studies are performed with gamma cameras, which provide planar (2D) images. Single photon emission computed tomography (SPECT) is a special technique that creates tomographic images using a rotating gamma camera system. Positron emission tomography (PET) is another unique technique that creates tomographic images by detecting gamma rays produced when positrons interact with electrons.

Some  common  nuclear  medicine  procedures  includecardiac studies to evaluate myocardial perfusion and/or ventricular function; (2) skeletal studies to evaluate for early bony metastases (Figure 1-12), skeletal trauma, osteomyelitis, and primary bone neoplasms; (3) renograms and renal scans to evaluate kidney function and morphology; (4) ventilation-perfusion studies to evaluate for suspected pulmonary emboli; and (5) PET studies to diagnose or stage tumors (eg, lung, lymphoma, melanoma, colorectal, breast), evaluate dementia, monitor for brain tumor recurrence, track post-therapy changes, and evaluate myocardial viability.


Less common nuclear medicine studies include (l) thy-roid evaluation of nodules and therapy for hyperthyroidism and thyroid cancer; (2) hepatobiliary studies to evaluate for acute cholecystitis and bile duct patency; (3) brain imaging to evaluate dementia and brain death; (4) white blood cell stud-ies to detect infection and inflammation; (5) gastrointestinal bleeding studies to detect and localize small gastrointestinal bleeds; (6) lymphoscintigraphy to identify sentinel lymph nodes for surgery; and (7) parathyroid studies to identify adenomas and hyperplasia.


Positron Emission Tomography/CT (PET/CT)

Positron emission tomography (PET) with fluorine (18F) fluorodeoxyglucose (FDG) is a functional imaging method that plays an important role in the diagnosis and staging of malignancy, as well as in treatment monitoring. CT is an anatomic imaging modality that provides excellent spatial localization of pathology. The first combined PET/CT scanner was in operation in 2001. Combined PET/CT scanners have separate individual imaging components that reside in the same unit. In general, a CT scan is per-formed first and the PET scan follows. Output from PET/CT imaging includes separate CT and PET images, as well as the coregistered fused images that overlay the anatomic CT and metabolic data. The combined anatomic and functional images can be acquired in a single examina-tion. The use of CT images for attenuation correction of the PET emission data also significantly reduce PET scan time. The combined PET/CT is more sensitive and specific for detecting otherwise occult malignancy, tumor staging, and detecting disease recurrence and/or metastasis. PET/CT has also proven useful for following post-therapy changes, such as squamous-cell carcinoma of the head and neck. Fused PET/CT images consistently outperform sepa-rately collected CT and PET images for the detection of pathology, even when the separate nonfused imagines are viewed simultaneously.

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