This is a branch of medicine that detects the functioning of a person's specific organs or to treat disease. In most cases, the information is used by physicians to make a quick, accurate diagnosis of the patient's illness. The thyroid, bones, heart, liver and many other organs can be easily imaged, and disorders in their function revealed. Five Nobel Laureates have been intimately involved with the use of radioactive tracers in medicine.
Nuclear medicine was developed in the 1950s by physicians with an endocrine emphasis, initially using iodine-131 to diagnose and treat thyroid disease. In recent years, specialists have also come from radiology, as dual CT/PET (Computerized Tomography and Positron Emissions Tomography) procedures have become established.
What are Radioisotopes?
Many of the chemical elements have a number of isotopes. The isotopes of an element have the same number of protons in their atoms (atomic number) but different masses due to different numbers of neutrons. In an atom in the neutral state, the number of external electrons also equals the atomic number. These electrons determine the chemistry of the atom. The atomic mass is the sum of the protons and neutrons. There are 82 stable elements and about 275 stable isotopes of these elements.
At present there are up to 200 radioisotopes used on a regular basis, and most must be produced artificially.
Radioisotopes can be manufactured in several ways. The most common is by neutron activation in a nuclear reactor. This involves the capture of a neutron by the nucleus of an atom resulting in an excess of neutrons (neutron rich). Some radioisotopes are manufactured in a cyclotron in which protons are introduced to the nucleus resulting in a deficiency of neutrons (proton rich).
Cyclotron Produced Radioisotopes
Carbon-11, Nitrogen-13, Oxygen-15, and Fluorine-18: These are positron emitters used in PET for studying brain physiology and pathology, in particular for localizing epileptic focus, and in dementia, psychiatry and neuropharmacology studies. They also have a significant role in cardiology. F-18 in FDG (fluorodeoxyglucose) has become very important in the detection of cancers and the monitoring of progress in cancer treatment.
How PET Radioisotopes are Converted into Radiopharmaceuticals
Every organ in our bodies acts differently from a chemical point of view. Doctors and chemists have identified a number of chemicals which are absorbed by specific organs. The thyroid, for example, takes up iodine; the brain consumes quantities of glucose, and so on. With this knowledge, radiopharmacists are able to attach various radioisotopes to biologically active substances. Once a radioactive form of one of these substances enters the body, it is incorporated into the normal biological processes and excreted in the usual ways.
PET Radiopharmaceuticals play a major role in PET Imaging, also known as Molecular Imaging. PET imaging is now commonly used as key imaging tool in treating cancer, heart disease and brain disorders.
Key Characteristics of PET Imaging
· Earlier diagnosis
· Accurate staging and localization
· Precise treatment and monitoring
PET is a major diagnostic imaging modality used predominantly in determining the presence and severity of cancers, neurological conditions and cardiovascular disease. It is currently the most effective way to check for cancer recurrences and it offers significant advantages over other forms of imaging such as CT or MRI (magnetic resonance imaging) scans in detecting disease in many patients. In 2010, an estimated two million plus clinical PET patient studies were performed at approximately 2,000 sites around the country. PET images demonstrate the chemistry of organs and other tissues such as tumors. A radiopharmaceutical, such as FDG, which includes both sugar (glucose) and a radionuclide that gives off signals, is injected into the patient, and its emissions are measured by a PET scanner.
PET and Cancer
PET is considered particularly effective in identifying whether cancer is present or not, if it has spread, if it is responding to treatment and if a person is cancer free after treatment. Cancers for which PET is considered particularly effective include lung: head and neck, colorectal, esophageal, lymphoma, melanoma, breast, thyroid, cervical, pancreatic and brain, as well as other less-frequently occurring cancers.
PET and Neurological Disease
PET's ability to measure metabolism also has significant implications in diagnosing Alzheimer’s disease, Parkinson’s disease, epilepsy and other neurological conditions, because it can vividly illustrate areas where brain activity differs from the normal brain images.
PET and Cardiovascular Disease
By measuring both blood flow (perfusion) and metabolic rate within the heart, physicians using PET scans can pinpoint areas of decreased blood flow, such as those with blockages and differentiate healthy muscle from damaged muscle, which has inadequate blood flow (myocardial viability). This information is particularly important in patients who have had previous myocardial infarction (heart attack) and who are being considered for a procedure such as angioplasty or coronary artery bypass surgery.
PET Imaging Summary
1. Specially designed drugs used both for diagnoses and therapy
2. Radioactive materials produced on a cyclotron
3. Drugs are mixed with radioactive material
4. Drugs mixed with radioactive material are called radiopharmaceuticals
5. Radiopharmaceuticals are injected in a patient
6. Radiopharmaceuticals attach to target cells in various organs
7. Radiopharmaceuticals emit special signals (gamma rays)
8. Emitted signals can be traced by external cameras (e.g., PET Scanners)
9. Emitted signals are recorded by a PET scan, which radiologists interpret for detection of a disease or progress of a particular treatment
The Future of Molecular Imaging
PET procedure volume has been growing at about 10% per year. In recent year PET imaging providers have become more focused in selecting procedures that are useful and meet the necessary treatment guidelines. An encouraging trend is that researchers are actively developing new PET agents that will extend its range beyond tissue metabolism. The objective is to characterize diseased tissue more accurately at an earlier stage, so that effective drugs can be used for treatment. This should benefit producers of PET radiopharmaceuticals as well as PET imaging systems, encouraging greater utilization of the PET technology.
The market for molecular imaging agents in the United States currently exceeds $1.3 billion annually and is expected to grow to over $3 billion by 2020.