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Positron Emission Tomography, also called PET imaging or a PET scan, is a diagnostic test that involves acquiring images of the internal workings of the body. This happens when the PET scanner camera detects radiation from the emission of positrons. Positrons are tiny particles emitted from a radioactive substance ("tracer") administered to the patient. The subsequent images of the human body developed with this technique are used to evaluate a variety of diseases, including cancer and Alzheimer's.
PET scans have become very popular because no other imaging technology shows the internal chemistry of the body so well. A PET scan has the unique ability to identify chemical and metabolic changes in diseases such as cancer before anatomic and structural changes have time to develop.
In other words, PET can detect diseases when anatomic imaging studies are still normal, and may be informative in differentiating healthy from diseased tissue. This PET scans very popular in identifying whether disease is present or not, if it has spread, if it is responding to treatment, and if a person is disease free after treatment.
PET is most routinely used to detect cancer, and PET scans are considered particularly effective at detecting lung, head and neck, colorectal, esophageal, lymphoma, melanoma, breast, thyroid, cervical, pancreatic, and brain cancer, as well as others.
How PET Detects Disease
Our Bodies, Our Cells
The body is made up of ten major systems – vascular (blood), digestive (provide nutrients and remove waste), endocrine (chemical communication via hormones), exocrine (skin, hair, nails, sweat, etc.), lymphatic (immunity), muscular (movement), nervous (information), urinary (blood filtering), reproductive (sex), respiratory (air), and skeletal (bones). The systems of the body perform complex tasks and are made up of various organs, like your liver, brain, or lungs. Organs are made up of specialized tissues, which are made up of cells and non-living material between them. Cells are the true building blocks of our life and our body – we contain about 10 trillion of them! As tiny as they are, they contain a variety of machinery (e.g. organelles, membranes, proteins, DNA) that are critical for us to perform all the necessary biological functions that keep us going. One of the most important tasks our cells perform is metabolism, where we convert the food we consume into glucose (a simple sugar) and eventually energy. We can think of glucose as the gas of the cells, since it keeps all the machinery running.
When Good Cells Go Bad
Sometimes, our cells do not behave as normal. We’ve all experienced this when we get the flu and virus has taken over our cells for their own nefarious plans. Other times, our cells go rogue for other reasons and create a cascade effect where they turn other cells, tissues, organs, or systems into a disease state. The more cells that are affected, the more significant the potential for harm, which is why it’s critical to learn the nature and extent of the disease as soon as possible. In other words, early diagnosis is the best way to stop the bad cells from getting out of hand.
Pet to the Rescue
PET (short for Positron Emission Tomography) is a diagnostic tool that can help us better understand disease. It utilizes two parts to do this: 1) A full body camera and 2) a “tracer” that is injected into the body and read by the camera. Tracers can be anything from small molecules like glucose to proteins that bind receptors on cells. The tracer is special because it’s tagged with a very small, non-harmful amount of radiation, so when it goes to wherever it’s meant to in the body – like specific parts of the brain or certain kinds of cancerous cells – the camera can read the area in 3-D. As the patient, you’ll feel nothing, but an amazing amount of information on what your body is doing will be provided to you doctor!
Although PET can be used to look at a variety of diseases, including Parkinson’s and Alzheimer’s, it’s most often used when cancer is suspected. In that case, the tracer looks a lot like glucose, which tricks the cells to taking the tracer inside, which is helpful because cancerous cells use a lot more glucose compared to normal, healthy cells. It isn't dangerous to your cells, however. Once it’s inside, the camera can take a picture that the doctor can read to see if cancer is present, how far along the cancer might be, and what treatments might be better than others for that particular cancer – all without having to open up the body! When used with other diagnostic tools, like MRI, CT, ultrasound, biopsy, and blood tests, your doctor can begin to get a complete idea of your body and your particular biology of disease. This is what we call personalized health. The more information a doctor can learn about a specific patient, the better a patient’s treatment plan can be created and monitored.
So Many Tracers, So Little Time
PET tracers are special because they have to be made right before the patient arrives for the scan. This is because a radioisotope is attached to the molecule. The radioisotope emits just enough radiation for the very sensitive camera to read, but not enough to harm the body, and only stays around in the body for a few hours before disappearing. The tracers are made in chemical synthesizers by specially trained chemists who make it under very stringent FDA guidelines, like any other drug you might take. Once the tracer is made, it’s shipped to the clinic where the patient is injected and put into the PET camera. After, a trained physician will read the photo and interpret the results.
Other Imaging Options
An X-ray is a form of electromagnetic radiation, similar to light but with a shorter wavelength and capable of penetrating solids. It is the most common form of medical imaging and most people have had one by the time they are an adult, whether for a suspected broken bone or at the dentist. This painless, non-invasive diagnostic test uses a controlled beam of radiation that interacts with matter. A radiation detector records the amount of radiation that passes through (or doesn't, depending on whether it's tissue or bone) and provides a picture of the body area under review in different shades of black and white. The results of your X-ray will then be given to a radiologist for interpretation.
Mammography is a specialized medical imaging method that uses a low-dose X-ray to see inside breasts. A mammography unit is equipped with a device that holds and compresses the breast and captures images at various angles. An X-ray machine points directly at the breast and produces a small burst of radiation that passes through the body, capturing an image that can show abnormalities such as tumors or small calcifications.
Computed tomography, commonly known as a CT or CAT scan, is a diagnostic medical test that produces cross-sectional or three-dimensional images of the inside of the body.
The CT scanner is a large machine with a short tunnel and a narrow examination table that slides into and out of the tunnel. The x-ray tube and electronic X-ray detectors are located opposite each other in a ring that rotates around the tube during the scan. Patients may be asked to hold their breath and lay completely still as any motion can lead to a loss of image quality.
CT scans are used to detect spinal problems and trauma related injuries, as well as cancers of the lung, liver, kidney, ovary and pancreas, or acute symptoms such as chest or abdominal pain or difficulty breathing.
Magnetic resonance imaging (MRI) uses a strong magnetic field, radio frequency pulses, and a computer to produce pictures of organs, tissues, bone and other internal body structures. The MRI unit is composed of a cylinder-shaped tube and outer circular magnet. Patients lie on a runway platform that slides through the center of the magnet.
MRI does not utilize ionizing radiation like X-rays or CT scans. Instead, the MR scanner captures the energy movement that’s caused by radiowaves redirecting naturally occurring hydrogen atoms in the body. A computer then generates a series of images to show a “thin slice” of the body.
MRI exams typically involve multiple runs, called sequences, some of which last several minutes “in the tube”. Patients who suffer from claustrophobia or fear of enclosed spaces may experience discomfort or difficulty lying still during the process.
MRIs are used to help diagnose or monitor treatment for a variety of conditions within the chest, abdomen, pelvis, and spine, including musculoskeletal problems, tumors, and vascular abnormalities.
Ultrasound imaging uses sound waves to produce pictures of the inside of the body. This happens when high-frequency sound waves are transmitted from a hand-held wand ("probe") through gel that's placed directly on the body. The probe collects the sounds that bounce back and a computer uses the sound waves to create an image.
Ultrasound images are captured in real-time and can show the structure and movement of the body's internal organs, as well as blood flowing through blood vessels. Ultrasound is noninvasive (the physician doesn't need to open up your body through biopsy or surgery to use it) and does not use the ionizing radiation that’s used in x-rays.
Ultrasound can be used to help diagnose the causes of pain and swelling and infection in the body’s internal organs. It is most commonly used to examine a baby in pregnant women. It’s also used to help guide biopsies, diagnose heart conditions, and assess damage after a heart attack. An ultrasound can also be used to image dense breast that a mammograph has trouble imaging.