Positron Emission Tomography
Positron Emission Tomography is a scanning technique that allows us to measure in detail the functioning of distinct areas of the human brain while the patient is comfortable, conscious and alert. PET represents a type of functional imaging, unlike X-rays or CT scans, which show only structural details within the brain. The differences between these types of imaging don’t end there. In both X-rays and CT scans, a form of radiation is emitted and travels through the body, and a detector receives the unabsorbed rays and transmits them to a computer. The physics behind PET scanning is quite different.
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Basically, a person is injected with a radioactive substance. This substance begins the process of radioactive decay inside of the person and interacts with the tissue to produce gamma radiation. These gamma rays are detected by scintillation crystals and transmitted to a computer, where images are produced. But how does this all take place? The description of PET scans in detail requires the understanding of the radioactive substance injected into the subject. First, a small amount of a biochemical substance is tagged with a positron-emitting radioisotope.
A positron is an “anti-electron. ” Positrons are given off during the decay of the nuclei of the radioisotope. When the positron emitted collides with an electron in the tissue of the subject, both the positron and the electron are annihilated. When this happens, the collision produces two gamma rays having the same energy (511 KeV), but going in opposite directions. These gamma rays, produced by the annihilation of a positron and an electron, leave the patient’s body and are detected by the PET scanner. The detection of positron-annihilation events forms the heart of any PET scanner.
In most systems, the Gamma detector is a BGO (bismuth germinate oxide) crystal, a high-density scintillator. When it is combined with high performance photomultiplier tubes (PMTs), the detection of 511 KeV gamma rays is possible. These BGO crystals are arranged into 64 distinct segments so that the scintillation light from each of the segments can be distributed onto the photocathodes of four photomultiplier tubes to be amplified. These “block detectors” are placed into modules of four arranged as eight columns of 32 rows of crystals each. A ring of these detectors surrounds the patient during the procedure.
An event is recorded by the detectors when two crystals detect gamma rays that occur within a coincidence time window. A line of response (LOR) indicates what two crystals detected the event. A unique line of response is identified by the angle and the radius of a perpendicular back to the center of the field of view. As additional events are detected, the lines of response are recorded. Each LOR is plotted using polar coordinates (angle vs. radius). The composite results in a sinusoidal plot of LORs through a single point and is referred to as a sinogram. The sinogram is comprised of numerous, overlapping single point plots.
The matrix size of the sinogram is related to the size of the transverse field of view. Following acquisition, filtered back projection algorithms are applied to the data to produce the image. The brain function being studied during a PET scan determines which positron-emitting radiopharmaceutical is used. Oxygen-15 can be used to label oxygen gas for the study of oxygen metabolism, carbon monoxide for the study of blood volume, or water for the study of blood flow in the brain. Similarly, fluorine-18 can be attached to a glucose molecule to produce 2-fluoro-2-deoxy-D-glucose (FDG) for use in the observation of the brain’s sugar metabolism.
PET is the only method that can detect and display metabolic changes in tissue, distinguish normal tissue from those that are diseased, such as in cancer, differentiate viable from dead or dying tissue, show regional blood flow, and determine the distribution and fate of drugs in the body. Because of its accuracy, effectiveness, and cost efficiency, PET is becoming indispensable for the diagnosis of disease and management of patients. The applications of PET are far ranging, and vary from things like neurology and cardiology to oncology.
In the field of neurology, PET can assess dementia, cerebro-vascular disease, and movement disorders, as well as intractable seizures. In cardiology, PET determines myocardial viability by assessing regional blood flow and metabolic functions in patients considered for coronary artery bypass and coronary angioplasty procedures. Cardiac transplant candidates can also benefit from PET-generated data. And in oncology, PET is unique and extremely useful in diagnosing malignancies in tissues such as lung, breast, GI tract, ovary, and musculoskeletal system.
PET is also useful in the post-treatment identification of tumor recurrence. As far as functional nuclear imaging goes, PET is the most advanced. Single Photon Emission Computed Tomography (SPECT) is a technique similar to PET, but the radioactive substances used in SPECT (Xenon-133, Technetium-99, Iodine-123) have longer decay times than those used in PET, and emit single instead of double gamma rays. SPECT can provide information about blood flow and the distribution of radioactive substances in the body.
Its images have less sensitivity and are less detailed than PET images, but the SPECT technique is less expensive than PET. Also, SPECT centers are more accessible than PET centers because they do not have to be located near a particle accelerator. Research is under way to develop still more PET radiopharmaceuticals to assist in the exploration of the working human brain. For example, dopa, a chemical active in brain cells, is labeled with positron-emitting fluorine or carbon and applied in research on the communication between certain brain cells which are diseased, as in dystonia, Parkinson’s disease, or schizophrenia.
Recently, new advances have been made in PET technology. A pair of American scientists working in Switzerland came up with a combination PET/CT scanner, which effectively pairs the two techniques. This new combination will be very useful in cancer diagnosis. With the PET/CT, both anatomical and functional imaging can be done and reproduced on the same image. This will be helpful in pinpointing the location of tumors, and also for the early identification of tumors too small to be of concern in CT scanning.