Noninvasive imaging technologies have become increasingly important over the past 20 years in the management of human diseases. Diagnostic radiology is the medical specialty that is responsible for imaging, providing critical information in three general areas, namely (i) anatomy/blood flow, (ii) metabolism, and (iii) receptor expression. The first area is the most widely applied in terms of the number of studies. This type of imaging affords an opportunity to detect the abnormality, since many conditions result in the disruption of normal anatomy, function, or blood flow. One example is the detection of a mass in an abnormal location on a chest radiograph, which, with further tests leads to a diagnosis of cancer. Another example is the identification of fractures following traumatic injury, or decreased bone density resulting from osteoporosis. These basic radiology techniques remain an important component of disease management. They are routinely accomplished by radiography and angiography, computed tomography (CT), magnetic resonance imaging (MRI), and ultrasonography (US). Less frequently, gamma-ray imaging (especially PET) studies assess blood flow.
Metabolism is the second general area that can be assessed by noninvasive imaging. This category includes the evaluation of organ function. Examples include noninvasive imaging to assess heart perfusion under stress, gastric emptying, ventilation/perfusion of the lung, renal and liver function, and blood flow to the brain. Metabolite imaging is a further example, since magnetic resonance spectroscopy (MRS) techniques now detect altered metabolites in disease processes. Another aspect of metabolism that can be assessed is energy utilization. The increased metabolic rate of cancerous tissue relative to normal tissue can be imaged using radioactive probes that accumulate in areas of higher metabolic activity. These studies are accomplished by administration of a radioactive drug; the increased uptake of the radioactive drug in the cancerous lesion is imaged with gamma-ray detection instruments. In a similar manner, the glucose or fatty acid metabolism in myocardium can be evaluated following ischemic injury.
The third area that can be assessed by noninvasive imaging is that of receptor expression. While receptors may potentially be assessed with MRI, the most success to date has been demonstrated using radioactive, gamma-emitting drugs followed by imaging with gamma-ray detection instruments as described above. This area represents the latest evolution in imaging; it is often described as "molecular imaging" since disease-specific receptors are detected. One example is the application of somatostatin receptor imaging for detection of neuroendocrine tumors. This recent capability is possible due to in vivo accumulation of a radiolabeled peptide with high affinity for somatostatin receptor expressed on the surface of the tumor cells. Another example is detection and measurement of dopamine receptors, which become altered in Parkinson's disease. Molecular imaging represents a growth area for radiology, and promises to allow early detection and monitoring of disease response during therapeutic intervention.
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