Nuclear Diagnostics

In Canada nuclear medicine is an integral part of the health care system. Nuclear medicine is used in the detection, diagnosis and therapeutic aspects of Canadians’ medical treatments. According to the World Nuclear Organization (2008), nuclear medicine began to be developed in the 1950s by doctors who were focused on treating thyroid disease.

Nuclear Medical Treatment

An example of external beam therapy
An example of external beam therapy.

Radiation is used to treat disease, notably cancer, in several ways. Therapy machines using the radioisotope cobalt-60 deliver an external beam of radiation to the cancer (teletherapy). Today, there are some 1,200 cobalt machines operating throughout the world and over 40,000 treatments a day are delivered using this Canadian technology. Cobalt-60 teletherapy was first used at the Victoria Hospital in London, Ontario in October 1951 using a machine designed by Nordion’s predecessor. In other forms of treatment, radiation sources can be inserted directly into or beside tumours to kill cancer cells. Known as brachytherapy, this technique is more suited to certain areas of the body, such as the prostate, cervix and throat. In brachytherapy, radioactive pellets can be used to help shrink cancerous tissue in localized and delicate areas. These concentrated, high doses of radiation are used to help destroy tumours. Exciting applications in isotope technology are making new treatments possible, such as treating liver cancer, non-Hodgkin’s lymphoma and brain cancer. In this form of treatment radioisotopes are attached to antibodies or other substances that seek out cancer cells. Once connected to the cells, they are acted upon by the radiation from the attached isotope thus delivering highly targeted radiation to the tumour from within the body. In leukemia treatment, the cancerous bone marrow of the patient is killed with a lethal dose of radiation before healthy bone marrow is transplanted.

Medical X-rays and CT Scans

x-ray
X-ray.

While not technically nuclear medicine, both medical x-ray and Computed Tomography (CT) imaging use x-rays — a form of electromagnetic radiation. In 1913, Dr. William D. Coolidge developed the x-ray tube.  The x-ray tube consists of a high voltage cathode that emits a stream of electrons which are attracted to the anode, a tungsten plate at the end of the tube. When the electrons strike the tungsten plate, photons are given off which pass through a set of filters that only allow x-rays of a certain energy range to pass through.  The tube is shielded with lead and an outer metal casing.

x-ray schematic
X-ray schematic.

For conventional x-ray imaging, the patient is placed between the x-ray tube and a film plate. As the x-rays pass through the area of the body where the x-ray beam is directed, a shadow image is created on the film. Today, in many hospitals the film has been replaced with digital technology using a CCD or charged coupled device (similar to the sensor that captures light in your digital camera) and a computer to create the image. X-ray imaging is one of the most common medical diagnostic procedures at any hospital and adds very little to your lifetime radiation exposure. A chest x-ray will expose the patient to 0.1 mSv (millisieverts) of ionizing radiation in less than one minute. That is roughly equivalent to the amount of natural background a person would receive in 10 days.

x-ray
X-ray of a patient’s knees.

In addition to being used to image broken bones, sprained ankles or cavities in our teeth, x-rays are used in a wide range of medical procedures, including: angiograms, joint injuries, mammograms, bone density scans, barium enemas, lung disease, artery blockages and CT scans.

ct scan
CT scan.

Computed tomography or CT scan (sometimes referred to as a Computerized Axial Tomography or CAT scan) is a medical diagnostic tool used by certified radiologists and physicians to perform examinations of the body without having to do exploratory surgery.  A CT scanner is an advanced x-ray machine which uses a rotating x-ray source, x-ray detectors and a complex computer system. During a CT scan the patient lies on a narrow table called a gantry which slides through the scanner. A series of x-ray beams are projected through the patient’s body as the patient passes through the scanner. The x-rays are detected by an array of sensors. Information from the sensors is processed by a computer into image slices and then displayed on a video screen. Sometimes an x-ray-absorbing dye is administered orally or intravenously to add contrast to the image. Movement by the patient during the CT scan may cause the image to blur so it is important the patient remain still throughout the procedure.

ct scanner
CT scanner.

A CT scan shows organs of interest at selected levels of the body. They are the visual equivalent of bloodless slices of anatomy, with each scan being a single slice. CT examinations produce detailed organ studies by stacking individual image slices. CT scanners can image the internal portion of organs and separate overlapping structures precisely. A CT scan can clearly show several types of tissue: lung, bone, soft tissue, and blood vessels.

Gamma Imaging

gamma camera
Gamma camera.

Today, nuclear medicine includes a number of diagnostic techniques.  Diagnostic techniques in nuclear medicine often involve the use of radioactive isotopes. These radioactive isotopes begin emitting gamma radiation once they are created and have an extremely short half-life. Diagnostic techniques require the radioisotopes to be injected into or ingested by the patient. The isotopes are attached to specific chemicals that travel to the particular tissue types to be examined. As the isotopes decay in the patient’s body, they are detected using a gamma camera that creates a picture the radiologist will interpret.

The radioisotope most commonly used in gamma scans is technetium-99m.Technetium-99m is the decay product of molybdenum-99 which is produced in nuclear reactors, then processed and shipped to hospitals in special containers.  The “m” identifies this as a metastable radionuclide.  Normal technetium-99 has a half life of 211,000 years.  The nucleus of the metastable version is in an excited state, and decays to the normal state with a half-life of six hours, emitting only one characteristic gamma ray.  After several half-lives, the concentration of this isotope approaches an equilibrium. Technicians at the hospital remove the technetium-99m chemically from the container. This can be repeated over several days as the molybdenum-99 decays with its 66-hour half-life.  Eventually the amount available becomes too small and the container is sent for disposal.

This isotope is used in many different procedures.  For example, cancerous tumours grow faster than normal tissues and consume large amounts of glucose (sugar). Areas of the body consuming glucose containing the technetium-99m tracer will give off gamma rays which are detected by a large array of sensors. The information from the sensors is converted into two-dimensional images by computers.

As technetium-99m has a half-life of just six hours, it may be safely flushed from the body after the procedure by simply by drinking fluids.  Technetium-99m is the most widely used diagnostic medical isotope.

Positron Emission Tomography

pet scanner
PET scanner.

Positron Emission Tomography (PET ) scans, for example, make use of positron-emitting radionuclides that have been injected into the patient. The radioactive isotopes used are beta emitters and give off positrons which are anti-matter particles that are the exact opposite of an electron.  Positrons are attracted to electrons, and when the two meet, they annihilate each other giving off two gamma rays. The gamma rays are then detected by a large array of sensors and the information collected is converted into two-dimensional images by computers. PET scanners can also be used in combination with MRI (magnetic resonance imaging) and CT scanners to create three-dimensional images.

Radioactive isotopes commonly used as tracers for PET scans include: nitrogen-13, oxygen-15, fluorine-18, carbon-11 and bromine-76, all of which have very short half-lives.

Note: Magnetic Resonance Imaging (MRI) is more properly called nuclear magnetic resonance imaging.  This technology does not use ionizing radiation.  It relies on a strong axial magnetic field, radiofrequency probe fields, and computer processing to image soft tissues.

Sources:

Canadian Nuclear Association, Nuclear Facts: What are the health benefits of nuclear medicine? Nuclear_Facts_Health_Benefits.pdf.
Health Canada, Healthy Living, 2006, www.hc-sc.gc.ca/hl-vs/iyh-vsv/med/mri-irm-eng.php, accessed July 26, 2008.
World Nuclear Organization, 2008. Radioisotopes in Medicine.
www.world-nuclear.org/info/inf55.htm accessed on July 28th, 2008.
X ray – The Mayo Clinic — www.mayoclinic.com/health/x-ray/FL00064.
“Radiation Exposure in X-ray Examinations” – RadiologyInfo.org, www.radiologyinfo.org/en/safety/index.cfm?pg=sfty_xray.
Image source: Britannica On Line: http://cache.eb.com/eb/image?id=62660&rendTypeId=4.
Image source: Carleton University: www.carletonsportsmed.com/Pat_alta/xray_pat_alta1and2.jpg.
Computed Tomography — Sick Kids, www.sickkids.ca/diagnosticimaging/what-we-do/computed-tomography-(ct)/body-ct.html.
Prof Jasmina Vujic — “Nuclear Medicine Imaging PPT”, Department of Nuclear Engineering UC Berkeley,
http://www.authorstream.com/Presentation/aSGuest10208-136373-imaging-education-ppt-powerpoint/.
The Mayo Clinic — www.mayoclinic.com/health/pet-scan/CA00052.