Contents
The emergence of computed tomography, as a method of scanning the human body, became possible only thanks to the discovery by Wilhelm Roentgen, a German physicist, X-rays with a unique ability to penetrate solid objects. Some time after this discovery, the rays were called X-rays, and the scientific and medical world gained an unprecedented way to explore the internal state of the human body without open surgical interventions – X-ray scanning. Radiography, as a method of obtaining images of body parts in one plane, in fact, became the first step towards the emergence of computed tomography – already at the beginning of the 20th century, radiography began to be used in medical institutions. And thanks to the achievements of scientific and technological progress in the 20th century, the results of which were the first computers (electronic computers), in the 70s, computed tomography was first introduced to the medical community around the world.
Development of computed tomography: from Pirogov to Kormak
Despite the fact that CT is considered an achievement of science at the end of the 20th century, the concept of tomography, as well as the method of layer-by-layer removal of information about the human body, first appeared in the 19th century in the works of Nikolai Ivanovich Pirogov, a surgeon and anatomist. He developed a tactic for studying the anatomical structure of internal organs, which he called topographic anatomy.
The essence of the proposed method was not to perform an autopsy immediately according to the standard scheme. First, the body had to be frozen, after which it was possible to perform layer-by-layer cutting in various anatomical projections. Thus, doctors got the opportunity to study the internal states of patients, however, after their death. Helping the deceased in this way, of course, was not possible, but the information collected in this way was an invaluable contribution to science, to develop methods of diagnosis and treatment that could be successfully applied to living patients. The described technique is called anatomical tomography or “ice anatomy” of Pirogov.
A start was made. In 1895, penetrating X-rays were discovered. At the beginning of the 20th century, I. Radon, an Austrian mathematician, deduces a law that justifies the ability of X-rays to be absorbed differently by media of different densities. It is this property of X-ray irradiation that underlies the entire method of computed tomography (CT).
The American and Austrian physicists Cormack and Hounsfield, based on the theory of Radon, independently continue to work in this direction, and in the late 60s they present the world with the first prototypes of computed tomographs. Since 1972, these devices have been used to diagnose patients around the world.
Types of CT scanners
The development process of computed tomographs includes 5 stages, respectively, during this time 5 types of tomographs have been developed.
The first generation tomographs were designed in the likeness of the Hounsfield apparatus. The scientist used a crystal detector with a photomultiplier in his device. A tube connected to a detector acted as a radiation source. The tube alternately made translational and rotational movements with constantly broadcast X-rays. Such devices were used only for examining the brain, since the diameter of the translucent zone did not exceed 24-25 centimeters, in addition, the scanning lasted a long time, and it was problematic to ensure complete immobility of the patient for the entire duration of the scan.
The second generation of CT scanners appeared in 1974, when machines with multiple detectors were first introduced to the world. The difference from the devices of the previous type was that the translational movements of the tube were faster, and after this movement the tube made a turn of 3-10 degrees. Due to this, the obtained images were clearer, and the radiation load on the body decreased. However, the duration of tomography using such a device was still long – up to 60 minutes.
The third stage in the development of tomographic devices for the first time excluded the translational movement of the tube. The diameter of the studied area has increased to 40-50 centimeters, in addition, the computer equipment used has become much more powerful: more modern primary matrices have begun to be used in it.
The fourth generation of tomographs appeared at the turn of the seventies and eighties. They provided for the presence of 1100-1200 fixed detectors located along the ring. Only the X-ray tube was set in motion, resulting in a significant reduction in image acquisition time.
The most modern devices are fifth-generation CT scanners. Their fundamental difference from previous devices is that in them the electron flow is produced by a stationary electron beam gun, which is located behind the tomograph. When passing through a vacuum, the flow is focused and directed by electromagnetic coils to a tungsten target under the table where the patient is located. Large mass targets are placed in four rows and are cooled by a continuous supply of running water. Fixed solid-state detectors are located opposite the targets. Devices of this type were originally used to scan the heart, as they made it possible to obtain a picture without noise and artifacts from the pulsation of the organ, and now they are used everywhere.
The essence of the method of computed tomography
Diagnosis by CT is the process of obtaining an image of a thin layer of tissue by processing data obtained from X-ray detectors by transilluminating this layer in different projections. During scanning, the tube rotates around the object. Differences in the density of various parts of the object of study, which the radiation encounters on its way, cause changes in its intensity, which are fixed by the detector. The received signal is processed by a computer program, which constructs a layered image on its basis.
Modern devices give a minimum layer thickness of 0,5 millimeters.
Classifications of computed tomography according to various criteria
One of the reasons for dividing the procedure into types is the amount of image that it allows you to get in one rotation of the tube:
- single-slice CT provides one image in one projection per rotation;
- multislice CT can scan from 2 to 640 slices per tube rotation cycle.
Depending on the use of the contrast agent in the process, there are:
- CT scan without contrast;
- CT scan with contrast – when a coloring agent is injected intravenously or orally during the procedure.
The use of computed tomography with contrast is due to the need:
- increasing the information content of the obtained images:
- enhancing the differentiation of closely spaced organs in the image;
- separation of pathological and normal structures in the pictures;
- clarification of the nature of the detected pathological changes.
According to the number of detectors and tube revolutions per unit of time, the following types of computed tomography are distinguished:
- serial CT;
- spiral tomography;
- multilayer multislice computed tomography.
Sequential computed tomography
This type of CT assumes that, after each revolution, the X-ray tube stops in order to return to its original position before starting the next cycle. While the tube is stationary, the tomography table with the patient moves forward a certain distance (the so-called “table step”) in order to take an image of the next slice. The thickness of the slice, and, accordingly, the step, is selected depending on the objectives of the survey. When examining the chest and abdomen, the patient uses the tube’s immobility time to exhale or inhale and hold the breath for the next image. Such a scanning process is fragmented, discrete. It is divided into cycles equal to one revolution of the tube around the scanned object.
Serial CT is practically not used today. It was used to examine various organs and parts of the body, but it has a number of drawbacks (significant duration, shift and inconsistency of tomographic sections as a result of patient movements), due to which it was replaced by other types of computed tomography – spiral and multilayer multislice.
How helical tomography works
This type of CT was first proposed in medical practice in 1988. Its essence lies in the continuity of two actions: the rotation of the X-ray tube around the object of study, and the continuous translational movement of the table with the patient along the longitudinal scanning axis through the gantry aperture. The gantry includes a radiation source, signal detectors, and a system that ensures their continuous movement. The gantry aperture diameter is the depth of the area of the object to which the scanning capability extends.
During this type of tomography, the movement of the X-ray tube has a spiral trajectory. In this case, the speed of movement of the table with the patient can take arbitrary values necessary to achieve the goals of the study. This technology made it possible to reduce the duration of the procedure, and, consequently, the radiation load on the subject.
Multispiral multilayer computed tomography
The fundamental difference of this type of computed tomography is the number of detectors – at least 2 rows can be located around the circumference of the gantry, with a total number of up to 1100-1200 pieces.
For the first time, the technology of multislice or multislice scanning was proposed in 1992. Initially, it meant the product of two slices during one cycle of rotation of the X-ray tube, which significantly increased the productivity of the tomograph. Today, devices allow you to get up to 640 sections of an object in one rotation, resulting in not only a high-precision and high-quality picture in the pictures, but also the ability to monitor the state of organs in real time. The time of the procedure has also been significantly reduced – multislice computed tomography, or MSCT, lasts only 5-7 minutes. This type of tomography is preferred for examining bone tissue.
Other types of computed tomography
Another factor that determines the differentiation of types of QDs is the number of sources emitting radiation. Since 2005, the first devices with two X-ray tubes have appeared on the tomography market. Their development was a natural necessity for the derivation of computed tomography of objects that are in very fast, continuous motion, for example, the heart. To achieve the greatest effectiveness and objectivity of the results of the examination of this organ, the period of scanning the slice should be as short as possible. The improvement of existing tomographs with a single X-ray tube stopped at the fact that the technical limit of the speed of its rotation was reached. The use of two radiation sources located at an angle of 90 degrees makes it possible to obtain an image of the heart, regardless of the frequency of its contractions.
An important advantage of devices with two radiation tubes is their complete “autonomy” from each other, that is, the ability of each of them to work independently, with different voltage and current values. Due to this, closely spaced objects of different density can be better differentiated in the image.
According to the areas of scanning, computed tomography is distinguished:
- internal organs;
- bones and joints;
- vascular system;
- brain and spinal cord.
Each type of tomography differs among themselves in the requirements for preparation, the need or lack of need to introduce contrast, as well as the mode of operation of the device.
Computed tomography of internal organs
CT of the internal organs allows you to get clear pictures and a three-dimensional image of the organs of the chest, abdominal cavity, mediastinum, neck, retroperitoneal space, small pelvis, bronchi, soft tissues.
CT scan of the musculoskeletal system
Computed tomography of bones and joints scans the condition and functional disorders in dense bone formations, muscles, articular structures, as well as in subcutaneous fat. If, for example, radiography is also successfully used to examine the condition of bones, then examination of the joints is a process that requires more unique solutions, because the joint is a complex system of interconnected tissue elements. Of course, there are other methods for examining these parts of the body, for example, arthroscopy and arthrography, but they require surgical intervention, sometimes minor, but because of it, various complications can occur after the procedure.
Tomographic examination of blood vessels
Scanning of the human vascular system using computed tomography, most often occurs with contrast. Such an examination makes it possible to see and analyze the structural features of the vessels, the presence of narrowing or expansion, blood clots, dissection, aneurysm, stenosis, arteriovenous malformation.
Scanning the brain and spinal cord using CT technologies
Computed tomography is one of the main ways of visualizing the spinal cord and brain for their study today. The procedure gives good visibility of all brain structures: the corpus callosum, cerebral hemispheres, cerebellum, pons, pituitary gland, medulla oblongata, CSF-conducting areas, sulci of the hemispheres and cerebellum, as well as exit points of the largest cerebral nerves.
As for the spinal cord, for a long time the only way to examine this organ was x-ray myelography, performed with contrast. At its core, it was a process of obtaining x-rays with a preliminary introduction of a coloring substance to the patient.
According to the results of modern computed tomography, it is possible to determine the shape, contour, structure of the spinal cord, while it is well differentiated from the surrounding cerebrospinal fluid. The images show the roots and spinal nerves, as well as the vascular system of the spinal cord.
Perfusion computed tomography
CT perfusion is a computed tomography technique performed to determine the level of blood flow in internal organs, mainly in the brain or liver. Perfusion is defined as the ratio of blood volume to tissue volume in a particular organ. This type of tomography allows you to evaluate the features of inflow, permeability and outflow of blood.
The main advantages and disadvantages of the method
The technology of examining the internal organs and systems of the human body using special computer equipment and the properties of X-ray irradiation, for a number of reasons, is highly appreciated by physicians around the world. CT scan results are images of bones, organs, blood vessels and soft tissues with high image quality. Tomographs of the latest generation make it possible not only to build a three-dimensional model of most of the internal structures of the human body, but also, in practice, to observe them in real time. The information obtained is easy to process, and is easy to study for a radiologist. It is also convenient to save the image in digital form on a special storage device, and, if necessary, print it as many times as necessary.
Unlike MRI, computed tomography is allowed to be prescribed to patients with metal implants, fixed prostheses, wires embedded in the body, and pacemakers.
Patients who underwent the procedure note its painlessness and speed. In rare cases, it may be necessary for the patient to be in the scanner for more than 15-20 minutes.
Compared to conventional x-rays, CT exposes the patient to much less radiation.
However, in addition to undeniable advantages, the examination method using a CT scanner also has some disadvantages, the main of which is the very fact of using X-rays, especially considering that the human body can be examined without their use, for example, by means of MRI. Due to the fact that the procedure exposes the patient to radiation, it is not recommended for children and pregnant women. It is also undesirable to use the CT method more often than 2-3 times a year.
Scanning the state of internal organs, bones, vascular system, tissues is an objective necessity in medicine. All medical activity without a thorough and informative examination, in fact, does not make sense, since it is extremely difficult to establish a diagnosis, determine the tactics of treatment, or check the effectiveness of an already performed therapy without a diagnosis. Thanks to the collective work of scientists – physicists, mathematicians, physicians – computed tomography has appeared in world medical practice. Over the years of its existence and development, it has gone through several stages, during which the devices have changed and improved, equipment has been modernized, new methods and methods of examination have appeared: CT with and without contrast, sequential, spiral, multilayer CT, as well as computed tomography with two sources radiation. Each of these types of computed tomography has its own characteristics and can be used for different purposes – from scanning the brain to examining the condition of the joints.