Positron Emission Tomography (PET) Unit
The positron emission tomography (PET) unit is a device used to produce images of the body that reflect biochemical changes taking place in the body. Among the body imaging technologies used in medicine, the PET unit is characterized by its use of positron-emitting tracer substances. Because of its use of short-lived positron emitting tracers, the PET unit can provide images of biochemical processes. This feature of PET technology distinguishes it from computer tomography (CT) and magnetic resonance imaging (MRI) technologies, which can provide only images of the structure of the body.
The purpose of the PET unit is to provide images reflecting biochemical changes occurring within the body. The PET unit can also, when used in conjunction with mathematical models of organ systems, quantify biochemical activity (e.g. blood flow, metabolic activity in tissue).
Because the PET unit can provide information on biochemical function, it is particularly helpful in assessing tissue viability and biological processes related to tissue health. The PET unit is used for these purposes most often in the fields of neurology (study of the brain ), cardiology (study of the heart ), and oncology (cancer ).
In neurology, the PET unit is used to diagnose and differentiate among different types of epilepsy, dementia, and cerebrovascular disease. Because the regions of the brain that are affected by these abnormalities have blood flow and glucose utilization patterns that are different from healthy parts of the brain, the PET unit can—by using tracers that follow cerebral blood flow, glucose pathways, and oxygen metabolism—identify areas of the brain that are affected. During epileptic seizures, blood flow and glucose use increase in the area of the epileptogenic focus (site originating the seizure). PET scans are used to identify these foci in patients with drug-resistant epilepsy so that surgical intervention can target these seizure-prone areas. In dementia, the PET unit is used to distinguish Alzheimer's disease from other types of dementia, because each type of dementia has a characteristic glucose utilization pattern in the brain. The PET unit is also used to evaluate and monitor treatments for stroke patients by measuring cerebral blood flow, glucose metabolism, and oxygen levels.
In cardiology, the PET unit is used to assess the metabolism and function of myocardial tissue. Blood flow and fatty acid metabolism are measured by the PET unit, and areas that are affected by coronary artery disease are easily identified. The state of the myocardial tissue, as reflected in the PET scan, also helps the cardiologist determine the best intervention, e.g. an angiogram rather than a heart bypass.
In oncology, the PET unit has had a long history of being used for the diagnosis and localization of brain tumors. Because tumors have greater blood flow directed to them than normal brain cells, the PET scan can identify where the tumor is localized by pinpointing the area with abnormally high blood flow. More broadly, the PET unit can be used in many parts of the body to grade the severity of tumors and identify metastatic processes. Moreover, because PET identifies variations in metabolic activity, the scans are particularly useful in assessing the effectiveness of radiological treatment of cancer; unlike other types of imaging, PET scans can distinguish between (non-viable) scar tissue caused by the radiological treatment of tumors, and viable tumor cells that might have been missed by the treatment.
The components of the PET unit are best understood in the context of the procedures required for positron emission tomography.
The first step in the PET process is the creation of the radioisotope (radioactive version of a chemical element) that is to be used in the tracer compound. The creation of the radioisotope takes place in a device called the cyclotron. The cyclotron is a particle accelerator that speeds up a particle so quickly that it strips electrons from the particle. In most PET units, the particle used is hydrogen, and the resulting stripped particle is a proton (represented as H+). A beam of protons created in this way is then used to bombard a stable isotope (non-radioactive version of a chemical element). The bombardment of the stable version of a chemical element with H+ produces a radioactive version of the element.
The most common radionuclides created by a PET cyclotron are C-11 (carbon), N-13 (nitrogen), O-15 (oxygen), and F-18 (flourine). These elements are popular because many of the compounds in the human body are based on these elements or on analogs of these elements, so that a biochemical compound the body naturally uses can be created from these radio-nuclides. Note that, because these positron-emitting radionuclides decay in a short amount of time (depending on the element, 2-110 minutes), the radio-nuclide must be produced by a cyclotron within a short distance from the location of the other PET procedures.
Once the radionuclide is generated by the cyclotron, it enters the biosynthesizer unit, where it is used to create radioactive biochemical compounds. Examples of compounds synthesized are 15-C (to measure blood volume), 13-N-glutamine (to measure myocardial metabolism), 15-O2 (to measure oxygen metabolism), and 18-F-deoxyglucose (to measure glucose metabolism).
The patient is then injected with or inhales the radioactive labeled tracer and is positioned in the gantry of the PET scanner. The scanner consists of a ring of detectors designed to find the location of and quantify the photon emissions from positron-electron reactions. Note that although the decaying radioactive compounds emit positrons, the positrons do not leave the body. Instead, the positrons emitted by the compounds go a short distance within the body before colliding with electrons. In the annihilation reaction that results from this positron-electron collision, high-energy photons are released, and it is these photons that pass through the body and are detected by the PET scanner. The two released photons, in an annihilation reaction, go in exactly opposite directions (180 degrees from each other), so that the PET scanner is able to reconstruct the three-dimensional spatial distribution of the compound by reconstructing the paths of photons and pinpointing the reaction site.
Since the photons released are not detectable visually, the detector ring in the scanner uses scintillation compounds (compounds that detect the photon flashes from the reaction) that convert the detected photon energy into visible forms. The scanner then uses sophisticated mathematical programs to construct coherent PET images from the visible data. When quantitative information is needed, a tracer kinetic model (mathematical model of tracer behavior) is used in conjunction with the PET data to quantify metabolic and functional processes.
The PET scanner is controlled by a computer monitor that allows for entry of text and commands. The images are previewed on an image monitor, which can be separate from or on the same screen as the control monitor. Because clinicians are often reluctant to diagnose using solely the image monitor, many PET systems allow for the conversion of these images to sheet film that can then be viewed on a standard light-box. Many systems also have an archival system that saves image data for future retrieval.
Variations of equipment
Because PET facilities differ in their imaging needs and financial resources, there may be variations in the features of cyclotrons and scanners among facilities. For radiotracer production, two different systems are often seen—the remote semiautomated system and the remote automated system. The remote semiautomated system allows the operator to determine the order and timing of the reactions and visually monitor the radionuclide synthesis. The semiautomated system is less expensive than the automated system, and is popular in research settings that do not have a standard set of radiotracers to be routinely produced. In clinical settings, where there is a regular flow of patients, the more costly automated system is used because automated synthesis requires less personnel time and production time, and there is less variation in the radionuclides required.
Cyclotrons are available as "Proton Only" devices or "Dual Particle" devices. Although "Proton Only" machines are cheaper, they restrict the chemical synthesis options through which particular radionuclides can be produced. Large quantities of O-15 used for brain research, for example, would be infeasible with "Proton Only" machines because of the prohibitive costs of the source materials required to synthesize O-15 with this type of machine. In general, institutions that have both clinical and research groups using the PET unit (and thus a broad range of radionuclide needs) use "Dual Particle" devices.
Some cyclotrons have what is called dual irradiation capacity, which allows them to produce two different radioisotopes at the same time. These cyclotrons can also, if both ion beams are used to produce the same type of radioisotope, produce large amounts of a single isotope. Dual irradiation cyclotrons are more expensive than single irradiation cyclotrons and are generally found in institutions with both clinical and research demands.
Although many PET scanners are single-ring scanners, multiple-ring scanners are emerging at advanced research and clinical institutions. These multiple-ring scanners allow for the simultaneous imaging of contiguous cross-sections. These types of scanners allow for faster scanning and more dynamic visualization of body processes.
Because of the high cost of the PET devices, the PET unit is used primarily in research institutions and advanced clinical (tertiary care) settings. In 1992, Michel Ter-Pergossian, a prominent researcher in PET technology, noted that there were 50-80 PET centers internationally.
Partly because of radiation safety regulations strictly governing the use and disposal of radioactive materials, radiotracer production and cyclotron operation is mostly automated. The nuclear medicine technologist, typically through a menu-driven computer control unit, designates the specific radiotracer to be synthesized and selects the chemical processes desired. Because the cyclotron is shielded, either in a protected room with concrete walls or behind a shield accompanying the cyclotron unit, the technologist is exposed to very little radiation.
After the radiotracer is produced, quality control (QC) testing is conducted daily. The technologist transfers the materials (in a lead container) from the biosynthesizer unit to the radiochemistry area for QC testing. The compounds are tested for radiochemical purity, radionuclidic purity, correct pH, and sterility. As the pharmacist performs the spectrometry and chromatography tests, he or she stands behind lead shielding. A monitor in the QC testing area indicates the level of radiation exposure in the area to allow the pharmacist to gauge his/her exposure. Staff periodically check their gloves for radioactive contamination, and contaminated items (such as gloves, shoe covers, and syringes) are immediately placed in protected radioactive waste containers.
The scanner operator brings the patient to the scanning room and aligns the patient in a relatively immobile position on the gantry. In the radiochemistry room, he or she or another technologist measures the appropriate patient dose for the radiotracer, and the radiotracer is placed into a Lucite-shielded syringe. The syringe is placed in a lead container, and the technologist carries the lead container to the scanning room, where he or she administers the radiotracer to the patient.
In the control room—an attached room with a window through which the patient can be viewed throughout the entire scan—the technologist controls the scanning and image processing at the control computer. He or she selects the appropriate scanning procedure for the area of interest and enters parameters related to image processing. When all parameters have been verified and entered, the technologist enters the commands to execute the scan. Depending on the nature of the question that the scan seeks to answer, the scan may take 10-90 minutes. The technologist assesses the quality of the images, and should be able to identify artifacts from problems arising in the PET detector or image processor. If the PET images are acceptable, they are stored for the physician or researcher to review at a later date.
Health care team roles
A typical team directly involved with using the PET unit consists of a radiochemist, a pharmacist, two nuclear medicine technologists (a scanner operator and a cyclotron operator), and a medical physicist. The radiochemist oversees the radiochemistry facility and supervises all radiotracer production. He or she is the primary cyclotron operator. The pharmacist performs quality control on the radiotracers. He or she can also operate the cyclotron and administer radiotracers to patients. Both nuclear medicine technologists assist in preparing patients for the PET scan. In addition, the scanner operator and cyclotron operator perform quality control on their respective devices. The medical physicist is the radiation safety officer of the facility. He or she ensures that the facility meets the legal safety requirements for dealing with radioactive materials, and makes sure that personnel are properly trained and monitored.
Nurses and nuclear medicine radiologists are also involved with the PET process. Nurses prepare the patients for the PET scan, monitor patients through the process, and may take blood samples as needed. Nuclear medicine radiologists are the physicians reviewing and interpreting the PET images in the course of patient work-up.
Nuclear medicine technologists, researchers, and physicians who will be using the PET computer stations typically take a week-long course, offered by the makers of the PET unit, to learn how to operate the control computers. Technologists also require a more advanced course in the physics and instrumentation of PET, in radiation safety, and in quality control during image processing. Clinicians who interpret the scans require nuclear medicine training through fellowships or continuing education.
Annihilation reaction— Reaction between electron and positron in which both are destroyed and each particle's mass is converted into photon energy.
Electron— Negatively charged particle of an atom.
Gantry— Frame in which patient is placed, over which the PET scanner moves.
Photon— High-energy light waves.
Positron— Negatively charged electrons, often symbolized as H+.
Proton— Positively charged particle of an atom.
Radionuclide— A radioactive element.
Radiotracer— A tracer compound with a radio-active element.
Tracer— Substance that can be followed through the course of a biochemical process.
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