- Only organs with a high radionuclide concentration can be separated from their environment
- The resolution is limited, such that improving resolution reduces the detector efficiency
- The radioisotopes widely used have low-energy gamma radiation, which is subject to much absorption in the body causing the detector efficiency to vary significantly with depth, and to be affected by the presence of bone overlying the organ of interest.
Because of all the possible illnesses and injuries suffered as a result of nuclear technology, there was a need to increase our knowledge on medical technology. Whether it'd be a nuclear reactor meltdown or a nuclear war, scientists around the world are progressing in the development of technology to assess and treat problems faced by victims. Here we mention medical imaging technologies. For more information on the injuries and illnesses, consult the radiation effects on humans page.
Computer Aided Tomography
Computer
Aided Tomography (CAT) is causing a revolution in the medical fields such
as radiology, neurology, and nuclear medicine. By combining "ordinary"
X-ray technology with sophisticated computer signal processing, it is possible
to generate an image of the tissues of the body which is unobscured by
other organs. An ordinary X-ray system takes pictures by passing X-rays
through the body and recording the interference patterns onto a photograph.
The different tissues in the body absorb the X-ray beams to varying degrees,
and the film responds to the intensity of the X-rays received. The resulting
photograph displays the accumulated absorption patterns to the tissue.
CAT is similar to an "ordinary" X-ray system, but it uses multiple X-rays oriented at different angles around the body. A computer is then used to extrapolate a three-dimensional image from the various two-dimensional images.
In
medical nuclear imaging, CAT scans are used to view organs containing
a type of radiation known as gamma-emitting radionuclides. Original imaging
methods called scintillation cameras are analogous to conventional X-ray
pictures. However, the usefulness of scintillation cameras is limtied because:
Recent developments use tomographic reconstruction to provide a high-contrast image of organs and structures containing positron-emitting radioisotopes.
Nuclear Magnetic Resonance Imaging
Why was it developed?
Nuclear magnetic resonance, or NMR, can reveal the distribution of atoms in a sample of material. It can do the same in the body, generating images of internal structure without the use of X-rays. The medical need to see inside the human body from the outside has been met for many decades by recording the differential absorption of X-rays such as in an ordinary X-ray system. However, a major deficiency of the standard method of radiography is its inability to discriminate among overlapping structures.
This deficiency has been remedied in recent years by the development of X-ray computerized tomography, or CAT scanning (see above). Although CAT scanning has proved to be an extremely useful diagnostic tool, the information its images provide is basically physical--what the organ looks like. They tell little about the functional or physiological state of the internal organs. Moreover, a type of structure known as pathological lesions can go undetected in a CAT scan unless the lesions are large enough to change the size or shape of the organ. Beyond that X-rays, even in small doses, carry a finite risk of doing physiological harm.
What is NMR imaging?
A new technique for obtaining cross-sectional pictures through the human body without exposing the patient to ionizing radiation is nuclear magnetic resonance imaging. NMR imaging not only yields physical information comparable in many ways to the information supplied by a CAT scan but also promises to discriminate more sensitively between healthy and diseased tissue. This is founded on the well-established ability of NMR spectroscopy to elucidate the intricate structures of organic molecules and to provide insight into dynamic chemical processes. For several years biochemists have exploited NMR techniques to monitor metabolic reactions in animals and human beings. It is the recent development of methods for presenting NMR information in pictorial form that is now providing clinicians with a powerful diagnostic tool.
A more technical explanation of how it works
The experimental foundations of NMR spectroscopy were laid by scientists at Stanford University and Harvard University more than four decades ago; work for which they were awarded a Nobel prize in 1952. It had been known since the 1920's that many atomic nuclei have an angular momentum arising from their inherent property of rotation, or spin. Since nuclei are electrically charged, the spin causes a current which in turn generates a small magnetic field. Each nucleus of nonzero spin therefore has a magnetic moment, or dipole, associated with it. Only nuclei with an odd number of nucleons (protons or neutrons) exhibit a net spin and therefore lend themselves to NMR spectroscopy.
In general the magnetic dipoles of the nuclei with spin will be points in random directions. When they are placed in a magnetic field, however, they will orient themselves with the field's lines of force. For nuclei of the spin designated 1/2, such as protons (hydrogen nuclei), the only allowed orientations of the dipoles are parallel to the field or antiparallel to it (in the opposite direction). The two orientations have slightly different energies. In the case of protons the difference between the number of protons with spin "up" (parallel) and spin "down" (antiparallel) is very small: only about one part in 108, with a slight excess in spin up.
Nuclear magnetic resonance is inherently a three-dimensional phenomenon. The spatial resolution of a three-dimensional set of data is usually equal in all three dimensions. With three-dimensional data in hand, surfaces can be detected mathematically, enabling the clinician to determine the volume of organs or of pathologiacal lesions. In medical practice many factors must be considered when a particular imaging method is being chosen, particularly the time scale of involuntary movements of the tissue being studied. The head, for example, is a good subject for true three dimentional imaging because it can be held still for the duration of the scan. The heart, on the other hand, which beats incessantly, requires either a high-speed imaging method or one that can synchronize the data collected over a series of cardiac cycles.
The Future of NMR imaging
Perhaps the greatest potential of all lies in the imaging of nuclei other than hydrogen, particularly the phosphorus nucleus. Phosphorus is a major constituent of the high-energy molecules adenosine triphosphate (ATP) and phosphocreatine, which mediate the transfer of energy in the living cell. From knowledge of such concentrations it is possible to infer the metabolic status of internal organs, and it many eventually be possible to add this capability to an imaging instrument. The future will undoubtedly see both an improvement in the quality of NMR images and a growing diversity of applications for nuclear magnetic resonance in clinical practice.