Computed Tomography (CT)

Compared to conventional X-rays, the method of computed tomography (also: computed tomography; CT) is relatively young, but it is hard to imagine everyday clinical practice without it. Their versatile applicability and rapid technical developments make them indispensable for a wide variety of issues in almost all body regions. Can X-ray measurements taken from different projection directions be combined in such a way that – like a jigsaw puzzle – they provide a complete, superimposed image of a body layer? We will inform you about the process and duration of the investigation.

How does computed tomography (CT) work?

With  conventional X -rays  , the rays are sent through the body and reach the other side, depending on how much they are allowed through by different tissues. There they are recorded by a kind of photographic disc. A two-dimensional image is obtained, similar to a silhouette on the wall, in which the various structures overlap.

What is missing is the information about the depth at which they are located. It is true that recordings can be made in different projection planes – for example from front to back and from left to right. But only computer tomography really solves this problem.

Computed tomography also works with X-rays. The difference to classic X-ray images is that in computed tomography, the body is imaged in thin layers. Each of these slices, which are only a few millimeters thick, can be assigned to exactly one part of the body – as if it had been cut in half with a sharp knife a thousand times.

But the device can do even more: The recordings can be edited, enlarged, measured, saved and viewed from different angles. And – particularly helpful – if required, a  three- dimensional image  can be assembled from the sectional images, which can be viewed from all sides and allows doctors to precisely allocate and expand structures and their surroundings, for example in preparation for an  operation  . To obtain such thin layers, a fine beam of X-rays is sent through the body and caught by detectors on the other side.

Different types of CT

The highlight is that the CT device rotates once around the patient during the examination and carries out a large number of measurements. These are transmitted to the computer, which combines them into a cross- sectional image with different shades of gray based on the differences between the transmitted and received intensity of the rays   .

Then the device is pushed along the patient a little further 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 adapt their breathing movements to the staff’s instructions so that the image is not blurred.

The newer devices work even more efficiently, as the tube moves continuously in a spiral form around the patient  (spiral CT)  , often firing several units of X-ray beams that are picked up by several rows of detectors  (multi-detector CT = multi-slice CT). In this way, large body sections can be scanned very quickly and with high resolution, an advantage in particular for mobile structures such as the heart.

Procedure: How does computed tomography work?

The device is in a separate examination room. The patient lies flat on a table and is thus pushed into the central opening. The table slowly moves forward during the examination. The patient should lie as relaxed and quiet as possible.

He has contact with the treating staff via an intercom system and, if necessary, receives instructions, for example to hold his breath for a moment. The duration of the examination depends on the area examined and the type of procedure.

How long does a computed tomography take?

Most  CT scans last  between 10 and 30 minutes. Sometimes preparations are necessary – for example, the patient must not eat anything that causes gas before examinations of the gastrointestinal tract. Contrast medium is usually administered one to two hours beforehand.

CT: What is being investigated?

The range of applications of computed tomography is diverse. Bones can be displayed particularly well, which is why CT is excellently suited to diagnosing bone fractures, signs of wear and tear,  osteoporosis  (CT bone density measurement) or a  herniated disc . But tumors, bleeding, accumulations of water,  cysts ,  abscesses  and  inflammation  are also easy to see.

Computed tomography can be used to examine the following areas of the body:

• Skull  ( C CT  = cranial computed tomography)
• neck and shoulder region
• chest and lungs
• abdomen
• pool
• spine
• peripheral bones and joints

Vessels  (CT angiography)  can be made clearly visible if contrast medium is given by mouth or injection beforehand. This is used, for example, as a  perfusion CT  in the early diagnosis of  strokes  or to display the coronary arteries. In the latter, the extent of  arteriosclerosis can also  be determined  (coronary calcium measurement).

In addition to sonography, computed tomography is also used to take tissue samples under visual control (e.g. from the liver) and fluid accumulations  (CT-guided biopsies)  or to inject targeted anesthetics (e.g. in the case of severe bone pain).

Virtual colonoscopy for early detection of colorectal cancer

Virtual colonoscopy  , a CT scan that simulates a journey through the large intestine with an endoscope, can be used for the early detection of intestinal diseases and for the clarification of abdominal complaints  . However, the extent to which it is equal to or even superior to conventional colonoscopy is still being discussed.

The disadvantage is that no tissue samples can be taken, the advantage is that surrounding structures can also be assessed in the same examination. Many patients find the carbon dioxide gas introduced through the anus to expand the bowel unpleasant; however, even an inserted tube is not exactly comfortable.

CT: What are the advantages and disadvantages?

Die eigentliche CT-Untersuchung ist schmerzlos. Nachteil ist allerdings, dass die Strahlenbelastung höher ist als beim konventionellen Röntgen. Je nach Umfang des gescannten Bereichs, nach Art des Gewebes und nach Dicke der Schichtaufnahmen kann sie ein Mehrfaches der natürlichen jährlichen Strahlendosis betragen. Allerdings überwiegen die Vorteile – verantwortlicher und gezielter Umgang sowie Vermeiden von Doppeluntersuchungen vorausgesetzt.

Geschichte der Computertomografie

Der Mathematiker Radon stellte bereits 1917 eine Theorie auf, deren Umkehrschluss es dem Physiker Cormack Anfang der Sechzigerjahre ermöglichte, eine rechnerische Lösung für diese Fragestellung zu finden. Der Elektrotechniker Hounsfield machte sich diese Erkenntnis zunutze und entwickelte eine Maschine, mit der er ab 1967 Gehirne von Schweinen und Ochsen scannte. 1972 wurde erstmals das Gehirn eines Menschen untersucht, der Siegeszug der Computertomografie begann. Cormack und Hounsfield erhielten 1979 für ihre Pionierarbeit den Nobelpreis für Medizin.

The first prototype of a computer tomograph still needed nine days to collect and two hours to calculate 28,000 measurements. Today’s devices manage to process hundreds of thousands of readings in a few seconds, examining the head, for example, takes between two and ten minutes.