Radioactivity and radioactive isotopes are often feared by the general public. However, the have revolutionizing medical application. This module will specifically address nuclear imaging.
Nuclear medicine utilizes radioactive materials in trace amounts to diagnose and treat a broad array of diseases. These procedures are often noninvasive, meaning that a person need not be cut open to see or deliver therapy inside the body. Different radioactive substances are chosen for different procedures. For example, to image the brain, scientists will choose an isotope that would occupy the brain at the time necessary to show the metabolism or function of the brain. The half-life of the substance is also an important consideration in this process. The goal is to look at or supply therapy to certain parts of the body without disturbing any functions, so only trace amounts of certain radioactive isotopes are used depending on what procedure is being administered.
Looking at nuclear imaging specifically, radiotracers are swallowed, injected, or inhaled. Next, they concentrate at the organ of focus. Gamma cameras, which as suggested by the name detect the emission of gamma rays, capture the movement and metabolism of these traces. This information can ultimately be used to extrapolate something about the structure and/or function of the focus region/organ.
PET scans produce high-resolution 3-dimensional images. One of the commonly used tracer for PET scans is 18 Fluoro 2 Deoxy glucose. The radioactively labeled glucose is absorbed by the cells in the same manner as regular glucose. Once inside the cell, it undergoes phosphorylaimagingtion and gets trapped inside the cell and cannot undergo further metabolism unlike glucose. This phosphorylated molecule is unstable and it breaks down releasing a proton, which releases two photons. The PET unit camera producing 3D images detects these photons.
Some of the important applications of PET scans are in diagnosing, staging, and monitoring treatment of certain kinds of cancer. PET neuro-imaging can help with early diagnosis of Alzheimer and to distinguish Alzheimer's from other form of dementia.
Figure 1: PET image. Image used with permission from Wikipedia
Another type of scan using nuclear isotopes is thallium scan. The thallium scan is used to study the blood flow to the heart muscle. The radioactive isotope of thallium used spreads throughout the heart muscle. The regions not receiving much of the tracer are dead or damaged. The procedure also used gamma cameras to track the movement of the tracers through the heart muscle.
Gallium scan involves injecting radioactive nuclide in the veins of the patient. The tracer travels via the blood stream and gathers in areas where there is accumulation of white blood cells, which show up as “hot spots” on the images. The white blood cell “hot spots” correlate to inflammation in body tissue. As it takes time of the tracer to accumulate, images are taken at two-three days after the injection.
SPECT is the short for single photon emission computed tomography. This involves the patient receiving an injection of Technicium-99, Iodine-123, or Indium-111. Once again the gamma cameras capture gamma rays emitted by tracers. Practically, it can be used to image tumors, infections (leukocytes), etc. This procedure is also concerned with studying the blood flow in the tissue of interest.
Is Nuclear Imaging Safe?
If this question was asked forty years ago, one might have received a completely different answer. However, thanks to advancements in technology in the field of nuclear medicine, most nuclear imaging is very safe. The radiation caused by nuclear imaging is equivalent to that of a standard x-ray. Furthermore, the radiation is less than the radiation experienced by an average adult due to natural sources over the course of a few months. Natural sources of radiation include cosmic rays, decaying rocks, soil, and internal radiation from within the body.
- Baert, A., and Sartor K. Diagnostic Nuclear Medicine. New York: Springer, 2006.
- Petrucci, Ralph H. General Chemistry. 9th ed. Upper Saddle River: Prentice Hall, 2007. Print
- Sadava, Heller, Orians, Purves, Hillis. Life The Science of Biology. 8th ed. Sunderland, MA: W.H. Freeman, 2008.
- What is a PET scan useful for?
- What is a gamma camera?
- What are some natural sources of radiation we are exposed to in our everyday live?
- What are some benefits of nuclear imaging?
- What are some disadvantages of nuclear imaging?
- Ridhi Sachdev (UC Davis)