X-Ray Imaging and Computed Tomography
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to or during the imaging procedure. The use of contrast agents is fairly common in x-ray
imaging. Contrast agents are often radiopaque or radiolucent media introduced to
enhance contrast. Several examples of contrast agents are as follows: Barium sulfate
for use in the gastrointestinal tract. Not all of the methods to introduce artificial
contrast are without hazard. A barium meal is used to explore the digestive system. Iodine compounds are used in blood vessels, which is a nontoxic radiopaque
substance. The iodine is injected to visualize blood vessels in angiography, which
attempts to visualize the constriction of coronary vessels in particular. Carbon
dioxide is used in CT of the colon or in angiography if the patient is allergic to other
contrasting agents. Carbon dioxide is radiolucent and will appear as dark spots
in images.
Knowing the technologies to create and detect x-ray, next, we focus on the physical
factors affecting the quality and resolution of x-ray imaging.
14.5 IMAGE QUALITY
While resolution of the resulting x-ray images can be improved with the image
processing methods introduced in the previous chapters, all existing technologies
attempt to improve the quality of the image in the acquisition stage to have a much
higher quality image.
The four issues that affect the resolution of the details in x-ray imaging are the
size of x-ray beam and source, the motion artifacts, and the quantum noise. These
factors influence the resolution and quality of all x-ray imaging technologies regardless of the specific technology used for detection and image processing.
The x-ray focus is the momentarily active section of the rotating anode that is emitting x-ray radiation. In the mathematical analysis (in particular, in CT), the x-ray focus
should be a point source, providing a single source–image relationship. In other words,
almost all mathematical formulations of x-ray imaging assume that the source creates
infinitely narrow x-ray beams. In reality, the x-ray source, having finite dimensions,
produces a cylindrical beam that irradiates all points from one edge of the irradiated
object to the opposite edge in a single illumination, thus smearing the shadow image
out over a finite width or breadth. This smearing effect of wide x-ray sources is illustrated in Figure 14.9 that describes a more realistic diagram of x-ray image formation
using practical nonpoint x-ray sources. As can be seen in Figure 14.9, geometric distortions and shadowing artifacts are primarily caused by the fact that the source is not a
point source.
The finite width source can be considered as an array of an infinite number of
point sources strung together. The beam overlap between these point sources can
be minimized by reducing the distance from source to object as much as possible
without significant loss in intensity. In addition, placing the detector as close to
the object as possible reduces diffraction and divergence. An additional measure
is to place the source at an angle to the line connecting the source to the object,
thus reducing the source dimensions by multiplication with the sine of the angle of
the normal of the source surface with the connecting line. By rotating the anode
disk, the exposure of the target area of the anode can be reduced without risk of
overheating.
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to or during the imaging procedure. The use of contrast agents is fairly common in x-ray
imaging. Contrast agents are often radiopaque or radiolucent media introduced to
enhance contrast. Several examples of contrast agents are as follows: Barium sulfate
for use in the gastrointestinal tract. Not all of the methods to introduce artificial
contrast are without hazard. A barium meal is used to explore the digestive system. Iodine compounds are used in blood vessels, which is a nontoxic radiopaque
substance. The iodine is injected to visualize blood vessels in angiography, which
attempts to visualize the constriction of coronary vessels in particular. Carbon
dioxide is used in CT of the colon or in angiography if the patient is allergic to other
contrasting agents. Carbon dioxide is radiolucent and will appear as dark spots
in images.
Knowing the technologies to create and detect x-ray, next, we focus on the physical
factors affecting the quality and resolution of x-ray imaging.
14.5 IMAGE QUALITY
While resolution of the resulting x-ray images can be improved with the image
processing methods introduced in the previous chapters, all existing technologies
attempt to improve the quality of the image in the acquisition stage to have a much
higher quality image.
The four issues that affect the resolution of the details in x-ray imaging are the
size of x-ray beam and source, the motion artifacts, and the quantum noise. These
factors influence the resolution and quality of all x-ray imaging technologies regardless of the specific technology used for detection and image processing.
The x-ray focus is the momentarily active section of the rotating anode that is emitting x-ray radiation. In the mathematical analysis (in particular, in CT), the x-ray focus
should be a point source, providing a single source–image relationship. In other words,
almost all mathematical formulations of x-ray imaging assume that the source creates
infinitely narrow x-ray beams. In reality, the x-ray source, having finite dimensions,
produces a cylindrical beam that irradiates all points from one edge of the irradiated
object to the opposite edge in a single illumination, thus smearing the shadow image
out over a finite width or breadth. This smearing effect of wide x-ray sources is illustrated in Figure 14.9 that describes a more realistic diagram of x-ray image formation
using practical nonpoint x-ray sources. As can be seen in Figure 14.9, geometric distortions and shadowing artifacts are primarily caused by the fact that the source is not a
point source.
The finite width source can be considered as an array of an infinite number of
point sources strung together. The beam overlap between these point sources can
be minimized by reducing the distance from source to object as much as possible
without significant loss in intensity. In addition, placing the detector as close to
the object as possible reduces diffraction and divergence. An additional measure
is to place the source at an angle to the line connecting the source to the object,
thus reducing the source dimensions by multiplication with the sine of the angle of
the normal of the source surface with the connecting line. By rotating the anode
disk, the exposure of the target area of the anode can be reduced without risk of
overheating.