Computed tomography (CT for short) is an imaging examination procedure used primarily for diagnostic purposes. It produces axial slice images of body regions using X-rays and then analyzes them using a computer.
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Computed tomography is a computer-aided procedure for generating cross-sectional images of the body using X-rays. A sectional image is the representation of organs, tissues and other structures of a very specific straight plane or layer.
The slice thickness shown depends on the particular problem and can vary. Nowadays, slice thicknesses of less than one millimeter are possible.
The imaging procedure enables the diagnosis of many diseases and pathological tissue changes, such as tumors. The method was developed at the end of the 1960s by a team led by Godfrey Hounsfield, an English physicist. It has been used clinically since the mid-1970s.
Sectional image of the skull © bunyos | AdobeStock
During image acquisition, an X-ray tube rotates around the patient lying in the so-called gantry. The gantry is the most important element of the tomograph: On one side of this rotating ring-shaped tunnel is the X-ray tube. A detector is mounted on the other side.
The X-ray tube emits a fan-shaped beam of X-rays during the examination. The X-rays are attenuated to varying degrees by the parts of the body through which they pass. The detector on the opposite side catches them and converts them into image information based on their strength.
The attenuation of the X-rays in the layer of the body being examined is shown in shades of gray.
Patient during a CT scan © Gorodenkoff | AdobeStock
The density or attenuation of a tissue is represented using the Hounsfield unit (HE). By definition, water is assigned the value 0 and air the value -1000.
The Hounsfield units, exemplary for different types of tissue in CT diagnostics, are
The human eye can only distinguish between 20 to 40 gray values. Therefore, a sectional image does not show the entire range of possible density values in the corresponding gray levels.
Instead, the
is defined. This window position is displayed as a medium shade of gray. A limited number of gray levels are then distributed across the selected window width. Density values above the window are displayed in white and density values below the window are displayed in black.
For example, when displaying the lungs, the window width is 100 and the window position is -600.
Spiral CT scanners with a continuously rotating tube and an examination table that moves smoothly through the gantry are usually used. These are multi-detector systems with several rotating rows of detectors. These multi-line spiral computer tomographs can process up to 320 lines. They allow whole-body examinations of polytraumatized patients in just a few seconds.
In dual-source computed tomography, two X-ray systems rotate simultaneously at right angles around the object to be examined. The computer tomograph can take twice as many slices per rotation of the X-ray tubes in the same time. This shortens the examination time and the temporal resolution. Moving objects, such as a beating heart, are thus displayed within very short scanning times(volume CT).
The very thin axial scanning technique enables multiplanar reconstruction (MPR). This is the 3D reconstruction of a scanned tissue from the axial slice scans for viewing the body area from different planes.
Contrast agents are used to
are used. These highly iodine-containing substances are administered intravenously and excreted via the kidneys after the examination.
Multiphase examination protocols have proven to be particularly effective in the abdominal area. Diseased tissue, for example of the liver or pancreas, absorbs the contrast medium differently over time than healthy tissue. A distinction is made here between the
CT angiography utilizes the specific phases of contrast medium injection for the targeted examination of arterial and venous vessels.
Visualization of the aorta and other blood vessels using CT angiography © SutthaB | AdobeStock
Computed tomography shows the organs of interest in layers without superimposition. This makes it easier to detect pathological changes. For this reason, CT diagnostics are used where pathological changes in organ structures are involved.
Computed tomography can be used to visualize blood vessels and soft tissue such as
more detailed images than conventional X-ray technology. However, some types of tissue such as cartilage or ligaments can be better visualized using magnetic resonance imaging (MRI).
The most important areas of application for computed tomography include
Computed tomography is also used in many cases to plan and carry out diagnostic or therapeutic interventions. Diagnostic procedures include biopsies or tissue removal. Therapeutic procedures with CT support are
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