Many people when they learn that they have to undergo a Positron Emission Tomography (PET) scan they are often very concerned. This is mainly due to the use of radiation in this medical procedure that is given to the patient in order to make diagnostic imaging possible. The presence of radiation within their bodies often makes patients squeamish and uncomfortable, which is unnecessary as PET imaging is actually a highly safe medical procedure. However, if a person is considering undergoing a PET scan, it is usually because their physician has recommended this procedure. Considering how informative this treatment is and the severity of the diseases that PET detects, it is usually in a patient’s best interest to undergo this treatment.
Before discussing what the PET scan procedure is, it is important to review what a PET scan actually is and how it differentiates itself from other medical imaging procedures. Positron Emission Tomography is a relatively new medical procedure that is a subset of nuclear medicine. This procedure uses the injection or ingestion of a radiopharmaceutical (radioactive tracers) into the body that produces gamma rays, which through specialized medical technology produces an image that the physician uses to form the basis of their diagnosis.
A subset of nuclear medicine, PET technology only began to be used in a clinical setting in the early 1990s. Since this short time, the use of positron emission tomography has exploded as it is primarily used to measure abnormal molecular cell activity. PET imaging can detect such diseases as cancer, Alzheimer’s Disease, Parkinson’s disease, epilepsy, and coronary artery disease. A safe procedure, it is estimated that approximately 350,000 PET scans were performed in 2002. As it becomes more established within clinical medicine, it is believed that PET scan use will explode within the next five years and it is believed that over 2,000,000 PET scans will be performed per year in the near future.
For those that often wonder, “How is a PET scan performed,” the Positron Emission Tomography procedure works like a camera that produces detailed images of biological functions from inside the human body. The PET scan is a unique, non-invasive diagnostic imaging tool that is a metabolic imaging tool. Other commonly used medical imaging tools such as computed tomagraphy (CT), magnetic resonance imaging (MRI), and conventional x-rays are anatomic imaging tools.
What this means is that a PET scan produces images detailing the biochemical functioning of an organ or tissue whereas anatomic imaging tools such as CT and MRI are only able to show body structure. Although CT and MRI scans are highly effective medical imaging tools that are commonly used by physicians in making diagnoses, these anatomical imaging tools are limited as it is detects disease that have caused change in anatomical structure. The PET scan, as a metabolic imaging tool, is able to visualize biochemical changes caused by disease. For example, Alzheimer’s Disease is a disease that does not produce gross structure abnormality, so is difficult to detect in CT and MRI. However, Alzheimer’s Disease produces biochemical change, which a PET scan is able to detect.
Additionally, PET imaging can also be used in conjunction with anatomical imaging tools as a way to examine what biochemical changes have occurred in an organ or tissue that has been shown to has changed anatomically. Consequently, PET imaging is an important imaging tool in cancer detection as it is able to distinguish between benign (alive tissue) disorders and malignant (dead tissue) disorders, whereas anatomic imaging tools are limited in their capacity to confirm the presence of mass. PET scans are therefore important medical tools that provide the necessary information about chemical functions of vital organs like the heart and brain to physicians when they are making their recommendations of further medical actions.
A PET scan usually takes place in a major medical center that contains a small cyclotron. A cyclotron is a highly advanced nuclear medical machine that is used to produce radioisotopes (radioactive chemical elements) that are used to synthesize radiopharmaceuticals, the radioactive tracer that is either injected or ingested by the patient that produces signals that are then interpreted to create the functional images of the body. The cyclotron produces radioisotopes that are easily incorporated into a radiopharmaceutical through chemical synthesis. Additionally, for individuals fearing the radiation component of the PET scan, the radioisotopes that the cyclotron produces have a short period of instability and after a short amount of time are transformed into a stable element.
The radioisotopes that are produced by the cyclotron are then tagged to a natural body compound through a process called radiolabeling. This process creates a radiopharmaceutical, which are then either injected or ingested by the patient. The function of the radiopharmaceutical is that its radioactive component produces signals that are recorded by the PET scanner. These signals record the movement of the radiopharmaceutical as it travels through the body and collects in the targeted body organ or tissue. These recorded signals are then reassembled by a computer, which produces images detailing the chemical functioning of the targeted organ or tissue. From these images, a physician will be able to make an informed diagnosis, as these images are able to show whether the target organ is normal and healthy or if there is the presence of disease and system failure.
Although the injection or ingestion of a radiopharmaceutical may seem dangerous, the patient is exposed to radiation levels that are equivalent to two chest x-rays when undergoing a PET scan. The radioisotopes that are produced by the cyclotron are then attached to a natural body compound to create a radiopharmaceutical. As it is attached to natural body compounds that are produced by the naturally, the radiopharmaceutical is chemically and biologically identical to the original body compound, with the only difference being that it is now traceable with a PET scanner. Consequently, radiopharmaceuticals will have no adverse effect when it is in your body. Additionally, these radiopharmaceuticals contain radioisotopes with a short period of instability that quickly lose its radioactive element.
Radiopharmaceuticals are usually comprised of radioisotopes with natural body compounds like glucose, oxygen, and carbon. The most common radiopharmaceutical uses glucose as its body compound. Fluorodeoxyglucose or FDG is a radiopharmaceutical that shares a similar structure to glucose but contains the radioisotope Fluorine-18. Fluorine-18 contains a half-life (the time it takes for the radioactive nuclei in the radioisotope to reduce to half of its previous value) of 110 min. As it is similar in structure to glucose, it is often used to trace parts of the brain. It is predominantly used in PET imaging for cancer as glucose is absorbed at a faster rate by cancerous cells compared to healthy cells.
Although FDG is the principle radiopharmaceutical used in PET imaging, other radiopharmaceuticals that can be used are:
Additionally, there are hundreds of other radiopharmaceuticals that are designed to study specific functions in the brain. Examples of these radiopharmaceuticals are C-11 or F-18-N Methylspiperone, which is used to map out dopamine and seratonine, as well as C-11 Carfentanil, which is used to study the function of opiate receptors.
Regardless of the radiopharmaceutical that is used, the PET scan procedure is a safe one with the only pain coming from the injection of the radiopharmaceutical into the patient. The patient arrives to the medical center that the PET scan procedure is taking place and lies on the scanning table. In instances where the patient’s head is being scanned, the patient is given special conditions to place against their head to hold their head in place throughout the screening process. The scanning table is attached to the PET scanner, a doughnut-shaped apparatus. Often depicted in movies and television, the PET scan process involves the table that the patient is lying on slowly moving through the opening in the scanner ring. Sometimes, one or two scans are taken prior to the administering of the radiopharmaceutical in the patient. The radiopharmaceutical is usually administered through injection, although it can also be given through an existing intravenous line or inhaled as a gas.
Following the administration of the radiopharmaceutical, the patient is usually taken to a partially darkened room. This is because the radiopharmaceutical usually takes between 30 to 60 minutes to be properly absorbed by the targeted tissue. In order for the radiopharmaceutical to effectively travel to its intended destination, the patient is usually asked to rest quietly and to avoid significant movement or talking, as these actions may alter the localization of the radiopharmaceutical.
Regardless of how it is administered, the radioisotopes within the radiopharmaceuticals lead to the ejection of positive particles known as positrons. Positrons travel approximately one to two millimeters until they collide with an electron. Due to the collision of these opposite forces, mass is converted to energy, which causes two gamma rays to be emitted and travel in opposite directions. Photomultiplier-scintillator detectors that are contained within the PET scanner detect these gamma rays, which triggers the specialized camera also contained in the PET scanner to record the millions of gamma rays that have now been emitted in the body. Another device on the PET scanner, a computer analyzes, integrates, and reconstructs this information through the use of complex algorithms (a complex mathematical formula) produce a 3D image of the area where the radioactive substance has accumulated.
The entire PET scan process lasts between 30 minutes to two hours and is completely painless. No anesthesia is used during this procedure, so following treatment you will be able to resume your normal activities. Results of the PET scan usually take one to three days to be interpreted, reported, and delivered back to the patient. A radiologist that specializes in Positron Emission Tomography will interpret the images produced by the PET scanner. A report will be written that will be forwarded to the physician that had referred the patient to them. From there, the patient learns of the results of their PET scan.
Positron Emission Tomography is predominantly used in clinical situations to evaluate patients that have or suspect that they have such medical conditions as:
For individuals that suspect they may have one of these diseases, early detection through PET scanning increases the possibility of treating the disease in the simplest way. In the case of cancer, PET scans work as a metabolic imaging technique as it uses FDG, a radiopharmaceutical similar in structure to glucose, during its scan. By producing a visible representation of how body tissues use glucose for energy, PET scans are able to determine the presence of cancerous tumors, the spread of cancer, and the effectiveness of cancer treatments. As cancerous tissues use more glucose than healthy tissue, a visual representation of this phenomenon will be present in the readout of the PET scan as the cancerous tissues will appear brighter. This is because a difference in colors as well as different degrees of brightness on a PET image are interpreted by the specialized radiologist to determine the patient’s health.