X-Ray Imaging and Computed Tomography

Abstract

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 together around 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 (∼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, 1 P1: FCH PB233-01 PB233-Webb-V2.cls October 23, 2002 14:59 2 X-RAY IMAGING AND COMPUTED TOMOGRAPHY X-ray source collimator anti-scatter grid X-ray film FIGURE 1.1. (Left) The basic setup for X-ray imaging. The collimator restricts the beam of X-rays so as to irradiate only the region of interest. The antiscatter grid increases tissue contrast by reducing the number of detected X-rays that have been scattered by tissue. (Right) A typical planar X-ray radiograph of the chest, in which the highly attenuating regions of bone appear white. images that show blood flow i…