Biomedical Engineering Reference
In-Depth Information
the patient due to their invasive nature. Since computerized tomography can accurately
locate the areas of the body to be radiated and can monitor the actual progress of the treat-
ment, it is effective not only in diagnosis but in the field of radiotherapy as well.
15.4.2 CT Technology
Since its introduction, computerized tomography has undergone continuous refine-
ment and development. In the process, CT scanning has developed into a technology of
significant importance throughout the world. The manufacture of quality CT systems
requires a strong base in x-ray technology, physics, crystallography, electronics, and data
processing.
The evolution of CT scanners has resulted from improvements in x-ray tube perfor-
mance; scan times; image-reconstruction time; reduction and, in some cases, elimination
of image artifacts; and reduction in the amount of radiation exposure to patients. Achieve-
ments in these areas have focused attention on two key issues. The first is image quality and
its relationship to diagnostic value. The second is scanner efficiency and its relationship to
patient care and the economic impact it entails. In order to attain a better understanding of
the operation and utility of present CT systems, it is important to understand some of the
evolutionary steps.
CT scanners are usually integrated units consisting of three major elements:
1.
The
, which takes the readings in a suitable form and quantity for a
picture to be reconstructed.
2.
The
scanning gantry
, which converts these readings into intelligible picture information,
displays this picture information in a visual format, and provides various manipulative
aids to enhance the image and thereby assist the physician in forming a diagnosis.
3.
A
data-handling unit
, which enables the information to be examined or reexamined at any
time after the actual scan.
storage facility
The Scanning Gantry
The objective of the scanning system is to obtain enough information to reconstruct an
image of the cross section of interest. All the information obtained must be relevant and
accurate, and there must be enough independent readings to reconstruct a picture with
sufficient spatial resolution and density discrimination to permit an accurate diagnosis.
The operation of the scanning system is therefore extremely important.
CT scanners have undergone several major gantry design changes (Figure 15.12). The
earliest generation of gantries used a system known as “traverse and index.” In this system,
the tube and detector were mounted on a frame, and a single beam of x-rays traversed the
slice linearly, providing absorption measurements along one profile. At the end of the traverse,
the frame indexed through 1
and the traverse was repeated. This procedure continued until
180 single traverses were made and 180 profiles were measured. Using the “traverse and
index” approach, the entire scanning procedure took approximately 4-5 minutes. The images
it produced had excellent picture quality and thus high diagnostic utility. However, this
approach had several disadvantages. First, it was relatively slow, resulting in relatively low
patient output. Second, streak artifacts were common, and although they normally were not
