X-Ray_Imaging_and_Computed_Tomography
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2012-01-09
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1
X-Ray Imaging
and
Computed Tomography
1.1. GENERAL PRINCIPLES OF IMAGING WITH X-RAYS
X-ray imaging is a transmission-based technique in which X-rays from a source
pass through the patient and are detected either by film or an ionization chamber on
the opposite side
of
the body, as shown in Figure 1.1. Contrast in the image between
different tissues arises from differential attenuation
of
X-rays in the body. For example,
X-ray attenuation is particularly efficient in bone, but less so in soft tissues. In planar
X-ray radiography, the image produced is a simple two-dimensional projection of the
tissues lying between the X-ray source and the film. Planar X-ray radiography is used
for a number
of
different purposes: intravenous pyelography (IVP) to detect diseases
of the genitourinary tract including kidney stones; abdominal radiography to study
the liver, bladder, abdomen, and pelvis; chest radiography for diseases
of
the lung and
broken ribs; and X-ray fluoroscopy (in which images are acquired continuously over a
period of several minutes) for a number
of
different genitourinary and gastrointestinal
diseases.
Planar X-ray radiography
of
overlapping layers of soft tissue or complex bone
structures can often be difficult to interpret, even for a skilled radiologist. In these
cases, X-ray computed tomography (CT) is used. The basic principles
of
CT are
shown in Figure 1.2. The X-ray source is tightly collimated to interrogate a thin
"slice" through the patient. The source and detectors rotate togetheraround the patient,
producing a series
of
one-dimensional projections at a number
of
different angles.
These data are reconstructed to give a two-dimensional image, as shown on the right
of Figure 1.2. CT images have a very high spatial resolution
("v
1 mm) and provide
reasonable contrast between soft tissues. In addition to anatomical imaging, CT is the
imaging method that can produce the highest resolution angiographic images, that is,
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