Biomedical Engineering Reference
In-Depth Information
A simple gamma camera system, at the present time, is the most important, as well as the
most basic, instrument for diagnostic nuclear medicine. Interfacing with a digital computer
makes the system even more versatile. Digital image-processing systems perform data
collection, storage, and analyses. Data acquisition requires digitization of the image by an
analog-to-digital converter (ADC), which divides a rectangular image area into small elements,
or pixels, usually a 64 by 64, 128 by 128, or 256 by 256 matrix. As a result, one can select a
particular region of interest to obtain certain quantitative information. Digital computation also
improves dynamic studies, since the regional rate of uptake and clearance pattern can easily
be obtained from the serial images for any particular region, making it possible, for example,
to study cardiac wall motion.
15.3.3 Positron Imaging
The positrons emitted through transformation of a radionuclide can travel only a short
distance in a tissue (a few millimeters) and then are annihilated. A pair of 511-keV photons
that travel at 180
to one another is created. A pair of scintillation detectors can sense posi-
tron emission by measuring the two photons in coincidence. Annihilation coincidence
detection provides a well-defined cylindrical path between the two detectors. Multihole
collimators are unnecessary to define the position in a positron camera because electronic
collimation accomplishes the task. Positron cameras have limited use; however, they have
attained great importance in a highly modified form in positron-emitting transaxial tomog-
raphy or positron emission tomography (PET scanning). A large number of small NaI(TI)
detectors are arranged in an annular form so annihilation photons in coincidence at 180
permit
detection of position. Tomographic images are, however, obtained by computer-assisted recon-
struction techniques. Several large centers use positron emission tomography in conjunction
with cyclotron-produced short-lived radiopharmaceuticals (mainly carbon-11, nitrogen-13,
and fluorine-18) to provide structural as well as metabolic information.
15.4 RADIOGRAPHIC IMAGING SYSTEMS
This section discusses externally produced radiation that passes through the patient and
is detected by radiation-sensitive devices behind the patient. These radiographic imaging
systems rely upon the differential attenuation of x-rays to produce an image. Initial systems
required sizable amounts of radiation to produce distinct images of tissues with good contrast
(that is, the ability to differentiate between body parts havingminimally different density, such
as fat and muscle). High-contrast differences such as between soft tissue and air (as in the lung)
or bone and muscle can be differentiated with a much smaller radiation dose. However, with
the development of better film and other types of detectors, the radiation dosage has decreased
for both high- and low-contrast resolutions. Even more significant was the development of
the computerized axial tomography (CAT) scanner in the 1970s. This device produces a
cross-sectional view of a patient instead of the traditional shadowgraph recorded by conven-
tional x-ray systems by using computers to reconstruct the x-ray attenuation data. In the
process, it provides the clinician with high-contrast resolved images of virtually any body part
and can be reconstructed in any body plane. To better understand the significance of this imag-
ing modality, let us review some of the basic concepts underlying routine x-ray imaging.
