X-Ray computed Tomography or CT-scan is a radiological technique. It is, just like radiography, based on the use of X-rays. It therefore rests on the principle of differential absorption of X-rays by tissues, a principle that we made clear under the study of radiography.
The term scanner is often used to refer to this technique. In fact, the word scanner indicates the apparatus used to carry out the examination.
It’s made up of a mobile table on which the patient lies down, and an X-ray emitting tube which turns around the patient. Here is how it works:
Detectors measure the intensity of the rays emitted by the tube.
On the other side, receivers, composed of digital sensors, record the intensity received, after having passed through the patient’s body.
This device is linked to a computer, which will process the digital data. An image in grayscale is obtained by the difference in intensity between incident and emergent rays, which are calculated for different angles.
Given that the rays penetrate perpendicular to the main axis of the body, the images obtained are horizontal sections.
The patient is always lying down. To explore a whole region, the abdomen for example, we produce a succession of sections. While the tube turns and emits radiation the table moves a constant speed over a tiny distance each. time. The images are viewed in real time, on a computer console, by the radiology technician. They are then interpreted by the radiologist
Remember that the radiographic image is a projected view where all of the organs are superimposed in the same plane.
CT-scanning provides views in section. But computer processing of a succession of sections can reconstruct an image in 3 dimensions. The physician thus has a precious tool for locating and evaluating the depth of a lesion, for example.
Furthermore, CT-scan image has better resolution where contrast is concerned.
Some specific low density tissues cannot be observed other than by using a contrast medium. This reconstruction of the cerebral arteries, for instance, could not have been produced except by injection of a contrast medium based on iodine.
One has to recognize that the power of a scanner lies to a very great extent in the computer processing that is associated with it. Thanks to that, the physician can reconstruct planar sections – superimpose the tissues – and thus navigate inside the reconstructed body of the patient.
Virtual endoscopy, as its name indicates, is a non-invasive method for observing the interior of a conduit, like the bronchi in this sequence.
This relatively new technique requires a specific kind of CT scan –called spiral mode – and expensive software, but its use is developing.
Since this technique uses X-rays, we take the same precautions as we do with radiography, that is limiting the number of examinations. The apparatus are becoming more and more high-performance and the amount of radiation the patient receives is not that much greater than in the case of radiography.
There is also a risk of allergic reaction to the iodine contrast media, but that can be limited.
Finally, the use of a scanner is an examination that is costly in both material and time, and it is necessary to keep a close watch to ensure that guidelines are being followed.