How CT Scanner Works

A CT, or CAT, scanner is a diagnostic device commonly used in hospitals to evaluate head injuries, abdominal problems and stroke victims. It stands for computed tomography, or computed axial tomography, as the image data is processed and resolved by a sophisticated computer system.

The CT scanner uses a low-dose X-ray beam that traverses a select area of the body in an axial (horizontal) or transverse (perpendicular) plane.

The machine itself features a large cavity where the patient is positioned and through which the X-rays pass. Tubes emit X-rays on one side of the scanner, while a special detector located opposite picks up the beam. Similar to a standard X-ray machine, they enter the body and are either soaked up or travel straight through, depending on the body part. For example, bone tends to absorb the rays resulting in an opaque silhouette, while organs and flesh are less capable of blocking them and so yield more translucent results.

Today’s CT machines rotate the X-ray tubes and detectors around the patient to get multiple cross-sections, enabling the computer to create high-detail 3D images that, in some scans, can even be manipulated in real-time.

Image processing

As soon as the raw data has been collected the computer can process images from it. Sometimes a simple cross-section is all that is required, but modern CT scanners are quite capable of layering multiple ‘slices’ through the human body, stacking them up to create a dynamic pass-through of the subject. Depending on the evaluation, the software compiling the data can create an image via a different plane simply by slicing through the body at another angle. This is useful in images of the spine, for example, where axial shots only show individual vertebra but an orthogonal slice through cAan reveal the vertebrae and their discs in their entirety. 3D images can also be rendered if needed. Surface rendering can colour different tissue types but can’t show internal structures, volume rendering is used to show tissue density, and image segmentation is utilised to remove any unwanted components from the shot.

CT evolution

Whereas radiography started way back in 1895 with Wilhelm Rontgen’s X-ray imaging device, computed tomography came much later. It hinged, of course, on the birth of the computer and – significantly – a computer with enough power. CT scanners were invented in 1972 by Godfrey Hounsfield, an engineer for EMI Laboratories, and Allan Cormack of Tufts University, MA. The first machines appeared in hospitals in 1974 and could only scan the head, taking several hours to create a single ‘slice’ of raw data and then days for the computer to actually generate an image. A modern CT scanner, meanwhile, can analyse someone’s chest and create an image in less than a minute.