Biomedical Engineering Reference
In-Depth Information
1.
Photography
. The blackening of film when it is exposed to a specific type of radiation such
as x-rays (which are the equivalent of gamma rays and will be discussed in the section
on externally produced radiation).
. The passage of radiation through a volume of gas established in the probe of
a gas detector produces ion pairs. The function of this type of detector depends on the
collection of these ion pairs in such a way that they may be counted. This technique has
been most effective in measuring alpha radiation and least effective in measuring gamma
radiation.
2.
Ionization
3.
Luminescence
. The emission of light not due to incandescence. Since the flash of light
produced by the bombardment of a certain type of material with penetrating-type
radiation can be detected and processed, this technique is extremely useful. As a
matter of fact, the fluorescent effect produced by ionizing radiation is the basis of the
scintillation detector discussed following. This type of indirect detection scheme is
excellent for observing the presence of all three types of radiation.
15.3.1 Scintillation Detectors
Since the majority of modern detector systems in nuclear medicine utilize probes based
on the scintillation principle, it will be described in greater detail. Scintillation is defined
as a flash of light emitted when a substance is struck by radioactive material. Detectors
may be used for all types of radiation, depending on the particular type of scintillator used
and its configuration. Regardless of the application, however, the general technique is the
same for all scintillation probes. Certain materials—for example, zinc sulfide and sodium
iodide—have the property of emitting a flash of light or scintillation when struck by ioniz-
ing radiation. The amount of light emitted is, over a wide range, proportional to the energy
expended by the particle in this material. When the scintillator material is placed next to the
sensitive surface of an electronic device called a
, the light from the scintil-
lator is then converted into a series of small electrical pulses whose height is directly
proportional to the energy of the incident gamma ray. These electrical pulses can then be
amplified and processed in such a way as to provide the operator with information regard-
ing the amount and nature of the radioactivity striking the scintillation detector. Thus, scin-
tillators may be used for diagnostic purposes to determine the amount and/or distribution
of radionuclides in one or more organs of a patient.
Figure 15.3 illustrates the basic scintillation detection system. It consists of (1) a detector,
which usually includes the scintillation crystal, photomultiplier tubes, and preamplifier;
(2) signal processing equipment such as the linear amplifier and the single-channel pulse
analyzer; and (3) data display units such as the scaler, scanner, and oscilloscope. Once
the radioactive event is detected by the crystal and an appropriate pulse is generated by
the photomultiplier circuitry, the resulting voltage pulses are still very small. To avoid
any serious loss of information caused by distortion from unwanted signals (such as noise)
and to provide a strong enough signal to be processed and displayed, the amplifier is used
to increase the amplitude of the pulses by a constant factor. This process is called linear
amplification.
In such a system, it should be apparent that because of the wide variation in energies of
gamma rays striking the scintillation crystal, the linear amplifier receives pulses having a
photomultiplier
