Differences
The difference between magnetic resonance imaging (MRI) or also called nuclear spin tomography and computer tomography (CT) lies not only in the respective area of application (different indications), but above all in the physical basis or mode of operation. MRI – unlike CT – is an X-ray independent examination method that uses strong magnetic fields and electromagnetic waves to produce very detailed sectional images of the body or individual body parts or organs in any plane. There is therefore no radiation exposure during an MRI examination.
The electromagnetic waves emitted by the MRI machine cause changes in the position of protons in body tissue, which return to their resting state after the waves are switched off. Signals are emitted, which are recorded by a coil in the device and converted into sectional images by a computer. The patient lies as still as possible in a supine position on an examination couch, which is inserted into the cylindrical MRI machine.
Depending on the proton content of the different body tissues, signals of varying strength are generated, so that information about the tissue type, tissue composition and possible tissue changes can be obtained. In general, MRI is suitable for imaging almost all types of tissue in the body, but the main focus in diagnostics is on imaging soft tissue (e.g. internal organs) and the central nervous system (brain and spinal cord), rather than on bony imaging (skeletal system). A special form is MR angiography, which is used specifically for the exact depiction of the blood vessel system.
An MRI examination takes on average 15-20 minutes, depending on the region of the body to be examined and the additional effort required due to special preparations or the administration of contrast medium, etc. In contrast, CT works with X-rays, which – unlike a conventional X-ray image – do not only scan the patient from one direction, but are “scanned” by the tubular CT device from all directions, so that in the end a high-resolution cross-sectional image of the respective body region is obtained (CT determines “only” cross-sectional images, MRI can produce images in any plane). During a CT examination, the patient is therefore exposed to radiation.
During the examination, the patient lies as still as possible on a supine couch in the CT unit while the unit rotates around the patient in layers. The principle of imaging is identical to that of conventional x-rays: the x-rays pass through the body, are absorbed or reflected to varying degrees depending on the tissue they hit, and are then processed by a computer into a sectional image. The examination usually takes only a few minutes (often only up to 10 minutes), depending on the part of the body being examined and the administration of a contrast medium that may be necessary. The field of application of the CT – like the MRI – is wide, both bony structures and soft tissue can be displayed, whereby the former finds a better display quality in the CT than in the MRI.
