Radiology: the eyes that look into our inner body

Computerized Tomography

Plain x-rays show very little contrast between various body parts such as muscle, kidney, liver, pancreas, blood vessels and spleen. Masses wíthin these organs certainly could not be seen unless they happened to calcify. Structures overlap one another on plain x-rays and lesions can be obscured. For example, a lung cancer can be nearly invisible behind a rib. Conventional tomography solved some of the overlap problems by selectively blurring structures, but it took the development of CT to improve contrast to the point where masses within solid organs were readily depicted.

In CT the patient is moved through the aperture of a circular bank of x-ray detectors. At the same time a thin fan-shaped bean of x-rays passes through a thin cross-section of the patient. The detectors measure the amount of transmitted radiation, and the data are fed to a computer that calculates the x-ray absorption of every point within the examined slice of the patient. A series of equations are solved and the x-ray absorption of each point is displayed as a shade of gray, creating the image of the particular slice. The region of interest in the patient is examined slice-by-slice, as you would slice a loaf of bread. The resultant pictures are displayed on a monitor and then photographed for interpretation. In many patients image contrast is further improved by the intravenous administration of radiographic contrast material. Interestingly, the mathematicians had ability to perform CT for many years prior to its development in the early 1970's, but were thwarted until that time by insufficient computing power and speed.

Although many CT applications are being taken over by MRI, the CT machine is still a busy workhorse in any radiologi department, and new applications such as CT angiography and CT virtual colonoscopy continue to emerge.