Computed tomography (synonyms: CT scan, computer axial tomography – from the ancient Greek: tome: the cut; graphein: to write) is an imaging method of radiological diagnostics.With the help of the application of CT has become possible for the first time a creation of axial superposition-free sectional images of the various body regions. In order to achieve this, X-ray radiological images from different directions are processed by computer, so that a three-dimensional sectional image can be created. Furthermore, it is possible to differentiate between structures with a higher radiation absorption and a broadened layer thickness. While it was still the case with an X-ray image that the degree of thickening of a tissue could not be precisely determined, since no three-dimensional examination allowed a highly differentiated assessment of tissues, the application of CT now represents a solution to this problem. However, viewing the object in three dimensions not only ensures accurate assessment of the volume structure, but also eliminates the need for averaging of sectional images. The absorption coefficient (attenuation coefficient) defined in the Hounsfield scale reflects the reproduction of the tissues in the individual gray levels. The degree of absorption can be illustrated by the values of air (absorption value of -1,000), water (absorption value 0) and the various metals (absorption values of well over 1,000). The representation of the tissues is described in medicine by the terms hypodensity (low absorption value) and hyperdensity (high absorption value). This diagnostic procedure was developed in the 1960s by physicist Allan M. Cormack and electrical engineer Godfrey Hounsfield, who were awarded the Nobel Prize in Medicine for their research. However, even before the final developments of computed tomography, there were attempts to create spatial images from radiological sections, thus bypassing the averaging process of X-ray images. As early as the 1920s, the first research results on tomography were presented by the Berlin physician Grossmann.
The procedure
The principle of the computer tomograph is the avoidance of superimposition of blurred planes, so that a higher contrast generation can be achieved. Based on this, it is also possible to examine soft tissues with the computed tomography scanner. This has resulted in the establishment of CT in healthcare facilities, where CT is used as the diagnostic imaging modality of choice for organ imaging. Since the development of the tomograph, there have been various technologies for performing the diagnostic procedure. Since 1989, spiral CT, developed by the German physicist Kalendar, has been the primary method used to perform it. Spiral CT is based on the principle of slip ring technology. Through this, it is possible to scan the patient in a spiral shape, as the X-ray tube is constantly supplied with energy and both energy transmission and data transmission can be completely wireless. The technology of CT is as follows:
- The modern CT scanner consists in each case of a front end, which is the actual scanner, and the back end, which consists of control console and a so-called viewing station (control station).
- As the heart of the tomograph, the front end includes, among other things, the required X-ray tube, the filter and the various apertures, a detector system, a generator and a cooling system. In the X-ray tube, radiation in the wavelength range of 10-8 to 10-18 m is generated by the entry of fast electrons into a metal.
- To perform diagnostics requires the provision of an accelerating voltage, which determines the energy of the X-ray spectrum. In addition, the current of the anode can be used to determine the intensity of the X-ray spectrum.
- The already mentioned accelerated electrons pass through the anode, so that they are both deflected and braked due to the friction on the atoms of the anode. The braking effect forms an electromagnetic wave that enables imaging of the tissue via the generation of photons. Imaging, however, requires an interaction of radiation and matter, resulting in the fact that the simple detection of X-rays is not sufficient for imaging.
- In addition to the X-ray tube, the detector system also plays a crucial role in the function of the CT scanner.
- Moreover, the motor unit including control unit and mechanics is also part of the front end.
To illustrate the development of the computed tomograph over the decades, here are the device generations that are still relevant today for certain issues:
- First-generation devices: this device is a translation-rotation scanner in which there is a mechanical connection between the X-ray tube and the beam detector. A single X-ray beam is used to take a single X-ray image by rotating and translating this unit. The use of first-generation computed tomography scanners began in 1962.
- Second-generation devices: this is also a translation-rotation scanner, but the application of the procedure was carried out with the help of multiple X-rays.
- Devices of the third generation: as an advantage of this further development is the emission of beams as a fan, so that a translational movement of the tube is no longer necessary.
- Devices of the last generation: in this type of device, various electron guns are used in a circle to ensure an overall view of the tissue in a time-saving manner.
As currently the most modern type of device is the dual-source CT traded. In this new development presented by Siemens in 2005, two X-ray emitters offset by a right angle are used simultaneously to reduce the exposure time. A detector system is located opposite each X-ray source. Dual-source CT has outstanding advantages, particularly in imaging the heart:
- Imaging of the heart with a heart rate-independent temporal resolution of a few milliseconds.
- Elimination of the need to administer beta-blockers to enhance imaging.
- Moreover, this advancement ensures a higher degree of plaque differentiation and achieves more accurate in-stent imaging.
- Even in patients with arrhythmias, imaging equivalent to that of patients without pulse abnormalities is ensured.
Dual-source CT can also be used for issues outside of cardiology. Oncology in particular benefits from improved tumor characterization and more accurate differentiation of tissue fluids. CT can be used for many different complaints or diseases.The following CT examinations are very common:
- Abdominal CT (imaging of the abdominal cavity and its organs).
- Angio-CT (imaging of blood vessels).
- Pelvic CT (imaging of the pelvis and its organs).
- CCT (Cranial CT) (imaging of the skull and brain).
- Extremities CT (arms and legs).
- Neck soft tissue CT (imaging of the pharynx, base of the tongue, salivary glands and larynx).
- Thoracic CT (imaging of the chest to assess the lungs, heart and bones).
- Virtual colonoscopy (colonoscopy).
- Spinal CT
In addition to all of these diagnostic capabilities, CT can also be used to perform punctures and biopsies.
Possible sequelae
- Dose-dependent increase in cancer risk; patients who had CTs:
- Had a 2.5-fold increased risk for thyroid cancer and leukemia risk was increased by just over 50%; the risk increase was most pronounced in women up to 45 years of age
- For non-Hodgkin’s lymphoma (NHL), an increase in risk could only be demonstrated up to the age of 45 years; at ages younger than 35 years, CT was associated with a 2.7-fold increase in risk of disease; at ages 36 to 45 years, with a 3.05-fold increase in risk