Compared to conventional x-rays, the method is relatively young, but it is almost impossible to imagine day-to-day clinical practice. Their versatile applicability and rapid technical advancements make them indispensable for a wide variety of questions in almost all body regions. Can X-ray measurements taken from different directions of projection be brought together in such a way that, like a puzzle, they provide a complete, overlay-free image of a body layer?
History of computed tomography
Already in 1917, the mathematician Radon came up with a theory whose reversal made it possible for the physicist Cormack in the early sixties to find a mathematical solution to this question. The electrical engineer Hounsfield took advantage of this knowledge and developed a machine with which he scanned from 1967 brains of pigs and oxen. In 1972, the brain of a human being was examined for the first time, and the triumph of computed tomography began. Cormack and Hounsfield were awarded the Nobel Prize for Medicine in 1979 for their pioneering work.
The first prototype computer tomograph needed nine days to collect and two hours to compute 28, 000 measurements. Today's devices manage to process hundreds of thousands of readings in just a few seconds; the examination of, for example, the head takes between two and ten minutes.
How does computed tomography (CT) work?
In conventional X-rays, the rays are transmitted through the body and, depending on how much they are passed through by different tissues, reach the other side. There they are recorded by a kind of photo plate. You get a two-dimensional image, similar to a silhouette on the wall, in which the different structures are superimposed.
What gets lost is the information, in what depth they are. A crux, which can be solved in part by taking pictures in different projection levels - for example, from front to back and from left to right. Computed tomography also works with X-rays, but solves this problem in a different way.
The difference to classic recordings is that the body is imaged in thin layers. Each of these slices, which are only a few millimeters thick, can be assigned to exactly one point in the body - as if it had been cut a thousand times across with a sharp knife.
But the device can do even more: The images can be edited, enlarged, measured, stored and viewed from different angles. And - especially helpful - the spatial images can be put together as needed, which can be viewed from all sides and allow physicians to precisely allocate and expand structures and their environment, for example, to prepare for an operation. To obtain such thin layers, a fine bundle of x-rays is passed through the body and collected on the other side by detectors.
Different types of CT
The trick is that the CT device revolves around the patient once during the examination and takes a lot of measurements. These are transmitted to the computer and from this - according to the differences between sent and arrived intensity of the rays - assembled into a cross-sectional image with different shades of gray.
Then the device is pushed a little further along the patient and the process is repeated layer by layer until the desired area has been scanned. This conventional technique is also referred to as incremental CT. During the recording, the patient must lie still and adjust his breathing movements to the instructions of the staff so that the image does not shake.
The newer devices work even more efficiently by moving the tube continuously around the patient in a spiral shape (spiral CT), often firing several units of x-ray beams, which are recorded by several detector rows (multi-detector CT = multi-slice CT). CT). Thus, large body sections can be scanned very quickly and with high resolution, an advantage especially in mobile structures such as the heart.