Traditional X-Ray Images

William Roentgen made the first x-ray image in 1895, but the technology remained essentially the same until the late 1960s. These images were projected onto flat detectors, such as film or electronic sensors. The x-rays pass through the body and project a shadow of the contents of the body onto the detectors, which record this projection in various shades of gray. The traditional x-ray image of a broken arm shows that bone, which stops a lot of x-rays, appears white. Soft-tissue, which does not stop x-rays as well as bone, appears a darker gray. The air around the patient, which stops hardly any x-rays, appears black.

Traditional x-ray of a severely broken arm (Image courtesy of the Mayo Clinic)

CT Scan

Although bones show up clearly on such x-ray images, soft tissues do not show up as well. Moreover, since three-dimensional body parts are projected onto two-dimensional film, much information is lost. What is needed is a “cross-section” view that displays a thin slab of the body. Computed Tomography (CT) images provide this kind of view, as shown in the drawing. A CT image is composed of pixels, whose brightness corresponds to the absorption of x-rays in a thin rectangular slab of the cross-section, which is called a “voxel.”

Notice that the x-ray tube and detectors rotate around the patient. For each direction of the x-ray beam, the scanner records the x-ray absorption by the patient's body. A computer program then computes the brightness of each pixel from all of these separate recordings. Since the CT scan requires so many x-ray exposures, the amount of radiation used to make a CT scan is typically greater that used to make a traditional x-ray.

Introduced in the early 1970s, CT scanning gained rapid acceptance in clinics and hospitals. A physicist and an engineer, Allan Cormack and Godfrey Hounsfield, shared the Nobel Prize in Physiology or Medicine in 1979 for their contributions to its development.

The geometry of the CT Scan; the x-ray tube and detectors rotate, with the axis of rotation running from the patient's head to toe. (image courtesy of the Mayo Clinic)

Improvements in CT Technology

As shown in the table, the technology of CT scanners has improved dramatically since the first scanner was introduced. Today's scanners can image the entire abdomen and pelvis of most adults, making a total of 80 CT images, in less than 30 seconds. The amount of detail in the image has increased six-fold since 1970.

This pair of CT images of the human brain shows how much image quality has progressed over the last 27 years.

 

Left: Circa 1975, in the early days of the CT scan.
Right: A present-day scan, showing the six-fold increase in detail
(images courtesy Siemens Medical Systems and Imaginis.com)

Here is a cross-sectional image of the abdomen made with present-day technology.

This CT image shows a cross-section of the abdomen. The darker tissue surrounding the abdomen corresponds to a thin layer of fat, and the skin is the very thin white edge of the image. The white, bony structures correspond to the spine (at the bottom center) and short cross-sections of ribs. Blood vessels can be seen at the upper right. (image courtesy of the Mayo Clinic)