As PET is a physiological/functional imaging modality, which provides information about the physiology in tissue, it is therefore complementary to the traditional tomographic imaging techniques, such as CT and MRI, that can provide anatomical (or structural) information of the tissue only. These latter techniques are method of choice when normal anatomy is expected to be disrupted by disease. However, there are many situations where functional changes precede anatomic changes or anatomic changes may be absent. Examples include cancers in their early stage, and various neurodegenerative diseases such as Alzheimer's, Huntington's, and Parkinson's diseases, epilepsy and psychiatric disorders, [97-99], in addition to a wide variety of neuroreceptor studies .
Historically, clinical applications of PET were centered around neurology and cardiology. The clinical role of PET has evolved considerably during the past 10 years, and it is well recognized that PET has a preeminent clinical role in oncology. Currently, oncological PET studies contribute to over 80% of clinical studies performed worldwide . It is well recognized that PET is useful for monitoring patient response to cancer treatment and assessing whether lesions seen with CT and MRI are cancerous, and is capable of grading degree of malignancy of tumors, detecting early developing disease, staging the extent of disease, detecting primary site of tumor, measuring myocardial perfusion, differentiating residual tumor or recurrence from radiation-induced necrosis or chemonecrosis, and monitoring cancer treatment efficacy [102-107]. FDG is the primary radiopharmaceutical used in oncological PET studies to assess glucose metabolism. Improvements in instrumentation in the late 1980s overcame the limitation of the restricted imaging aperture and enabled three-dimensional whole-body to be imaged. Whole-body PET imaging has been proven highly accurate in the detection of a number of different malignancies, particularly in cancers of the colon, breast, pancreas, head and neck, lungs, liver, lymphoma, melanoma, thyroid, and skeletal system, depending on the use of specific radiotracers. Figures 2.11 and 2.12 show examples of neuro-oncologic and whole-body coronal FDG-PET images.
As mentioned in Section 2.3, PET offers some unique features that cannot be found in other imaging modalities. The radiolabeled compounds used in PET are usually carbon (nC), nitrogen (13N), oxygen (15O), and fluorine (18F), which can be used to label a wide variety of natural substances, metabolites, and drugs, without perturbing their natural biochemical and physiological properties. In particular, these labeled compounds are the major elemental constituents of the body, making them very suitable to trace the biological processes in the body. As the measurements are obtained noninvasively using external detectors,
experiments can be performed repeatedly without sacrificing the small laboratory animals, such as mice and rats. This is not possible with in vitro tests which involve sacrifice of the animal at a specified time after radiotracer injection and preclude the kinetics of the radiotracer to be studied in the same animal. The greater flexibility in producing natural labeled probes for imaging on a macroscopic level in PET has raised the possibility of in vivo imaging on a cellular or genetic level. Recent advances in this field appear promising, particularly in the imaging of gene expression. Progress is being made and PET is expected to assume a pivotal role in the development of new genetic markers .
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