Medical Applications of Nuclear Physics
Medical Imaging and Diagnostics
Learning Objectives
- Explain the working principle behind an anger camera.
- Describe the SPECT and PET imaging techniques.
A host of medical imaging techniques employ nuclear radiation. What makes nuclear radiation so useful? First,
Medical Application
(Figure) lists certain medical diagnostic uses of radiopharmaceuticals, including isotopes and activities that are typically administered. Many organs can be imaged with a variety of nuclear isotopes replacing a stable element by a radioactive isotope. One common diagnostic employs iodine to image the thyroid, since iodine is concentrated in that organ. The most active thyroid cells, including cancerous cells, concentrate the most iodine and, therefore, emit the most radiation. Conversely, hypothyroidism is indicated by lack of iodine uptake. Note that there is more than one isotope that can be used for several types of scans. Another common nuclear diagnostic is the thallium scan for the cardiovascular system, particularly used to evaluate blockages in the coronary arteries and examine heart activity. The salt TlCl can be used, because it acts like NaCl and follows the blood. Gallium-67 accumulates where there is rapid cell growth, such as in tumors and sites of infection. Hence, it is useful in cancer imaging. Usually, the patient receives the injection one day and has a whole body scan 3 or 4 days later because it can take several days for the gallium to build up.
Procedure, isotope | Typical activity (mCi), where
|
---|---|
Brain scan | |
7.5 | |
7.5 | |
20 | |
20 | |
50 | |
10 | |
Lung scan | |
2 | |
7.5 | |
Cardiovascular blood pool | |
0.2 | |
2 | |
Cardiovascular arterial flow | |
3 | |
7.5 | |
Thyroid scan | |
0.05 | |
0.07 | |
Liver scan | |
0.1 | |
2 | |
Bone scan | |
0.1 | |
10 | |
Kidney scan | |
0.1 | |
1.5 |
Note that (Figure) lists many diagnostic uses for
(Figure) shows one of the simpler methods of imaging the concentration of nuclear activity, employing a device called an Anger camera or gamma camera. A piece of lead with holes bored through it collimates
Imaging techniques much like those in x-ray computed tomography (CT) scans use nuclear activity in patients to form three-dimensional images. (Figure) shows a patient in a circular array of detectors that may be stationary or rotated, with detector output used by a computer to construct a detailed image. This technique is called single-photon-emission computed tomography(SPECT) or sometimes simply SPET. The spatial resolution of this technique is poor, about 1 cm, but the contrast (i.e. the difference in visual properties that makes an object distinguishable from other objects and the background) is good.
Images produced by
Is it a tumor? Magnetic Resonance Imaging (MRI) can tell. Your head is full of tiny radio transmitters (the nuclear spins of the hydrogen nuclei of your water molecules). In an MRI unit, these little radios can be made to broadcast their positions, giving a detailed picture of the inside of your head.
Section Summary
- Radiopharmaceuticals are compounds that are used for medical imaging and therapeutics.
- The process of attaching a radioactive substance is called tagging.
- (Figure) lists certain diagnostic uses of radiopharmaceuticals including the isotope and activity typically used in diagnostics.
- One common imaging device is the Anger camera, which consists of a lead collimator, radiation detectors, and an analysis computer.
- Tomography performed with
-emitting radiopharmaceuticals is called SPECT and has the advantages of x-ray CT scans coupled with organ- and function-specific drugs. - PET is a similar technique that uses
emitters and detects the two annihilation rays, which aid to localize the source.
Conceptual Questions
In terms of radiation dose, what is the major difference between medical diagnostic uses of radiation and medical therapeutic uses?
One of the methods used to limit radiation dose to the patient in medical imaging is to employ isotopes with short half-lives. How would this limit the dose?
Problems & Exercises
A neutron generator uses an
5.701 MeV
Neutrons from a source (perhaps the one discussed in the preceding problem) bombard natural molybdenum, which is 24 percent
The purpose of producing
(a) Two annihilation
(b) How accurately would you need to be able to measure arrival time differences to get a position resolution of 1.00 mm?
(Figure) indicates that 7.50 mCi of
The activities of
(a) Neutron activation of sodium, which is 100%
(b) What mass of
(a) 6.958 MeV
(b)
Glossary
- Anger camera
- a common medical imaging device that uses a scintillator connected to a series of photomultipliers
- gamma camera
- another name for an Anger camera
- positron emission tomography (PET)
- tomography technique that uses
emitters and detects the two annihilation rays, aiding in source localization
- radiopharmaceutical
- compound used for medical imaging
- single-photon-emission computed tomography (SPECT)
- tomography performed with
-emitting radiopharmaceuticals
- tagged
- process of attaching a radioactive substance to a chemical compound