Biomedical Engineering Reference
In-Depth Information
During cellular or organ system function studies, gamma or x-rays are emitted and
detected outside the patient, providing physiological (rate of decay or “washout”) as well
as anatomical (imaging) information. More than 90 percent of the diagnostic procedures
in nuclear medicine use emission-imaging techniques. On the other hand, externally
produced radiation, which passes through the patient, is the basis of operation for x-ray
machines and computerized tomography. These devices and the procedures involving
them are housed within departments of radiology. The purpose of this chapter is to present
the fundamental principles of operation for each of these radiation imaging modalities.
15.2 EMISSION IMAGING SYSTEMS
Nuclear medicine is that branch of medicine that employs emission scanning for the
purpose of helping physicians arrive at a proper diagnosis. An outgrowth of the atomic
age and ushered in by the advances made in nuclear physics and technology during World
War II, nuclear medicine emerged as a powerful and effective approach in detecting and
treating specific physiological abnormalities.
The field of nuclear medicine is a classic example of a medical discipline that has
embraced and utilized the concepts developed in the physical sciences. Conceived as a
“joint venture” between the clinician and the physical scientist, it has evolved into an
interdisciplinary field of activity with its own body of knowledge and techniques. In the
process, the domain of nuclear medicine has grown to include studies pertaining to
the following:
The creation and proper utilization of radioactive tracers (or radiopharmaceuticals) that
can be safely administered into the body.
The design and application of nuclear instrumentation devices and systems to detect and
display the activity of these radioactive elements.
The determination of the relationship between the activity of the radioactive tracer and
specific physiological processes.
To better understand this radiation imaging modality, it is necessary to discuss radioactivity,
its detection, and the instruments available to monitor the activity of radioactive materials.
15.2.1 Basic Concepts
In 1895, when Roentgen announced the discovery of a new type of penetrating radiation
that he called “x-rays,” he opened a new realm of scientific inquiry. X-rays are a form of
electromagnetic (EM) energy just like radio waves and light. The main difference between
x-rays and light or radio waves, however, is in their frequency or wavelength. Figure 15.2
shows the EM radiation spectrum. It will be noted that x-rays typically have a wavelength
from 100 nm to 0.01 nm, which is much shorter than radio or light waves.
Spurred on by Roentgen's discovery, the French physicist Henri Becquerel investigated
the possibility that known fluorescent or phosphorescent substances produced a type of
radiation similar to the x-rays discovered by Roentgen. In 1896, Becquerel announced that
certain uranium salts also radiated—that is, emitted penetrating radiations, whether or
