Scheme of obtaining computer tomograms

A narrow beam of X-rays scans the human body along a circle. Passing through the tissue, the radiation is weakened according to the density and atomic composition of these tissues. On the other side of the patient is installed a circular system of X-ray sensors, each of which (and their number can reach several thousand) converts the energy of radiation into electrical signals. After amplification, these signals are converted into a digital code, which is sent to the computer memory. The detected signals reflect the degree of attenuation of the X-ray beam (and, consequently, the degree of absorption of radiation) in any one direction.

Rotating around the patient, the X-ray emitter "looks" through his body in different angles, in total at an angle of 360 °. By the end of the radiator rotation, all signals from all sensors are fixed in the computer memory. The duration of rotation of the radiator in modern tomographs is very small, only 1-3 seconds, which makes it possible to study moving objects.

When using standard programs, the computer reconstructs the internal structure of the object. As a result, an image of the thin layer of the organ being studied is usually drawn, usually on the order of several millimeters, which is displayed on the display, and the doctor treats it in relation to the task assigned to it: it can scale the image (increase and decrease), isolate the areas of interest (zones of interest) the size of the organ, the number or nature of pathological formations.

In passing, the density of the tissue is determined in individual sections, which is measured in conventional units - Hounsfield units (HU). For the zero mark, the density of water is assumed. The bone density is +1000 HU, the air density is -1000 HU. All other tissues of the human body occupy an intermediate position (usually from 0 to 200-300 HU). Naturally, such a range of densities can not be displayed either on the display or on the film, so the doctor chooses a limited range on the Hounsfield scale - a "window", whose dimensions usually do not exceed several dozen units of Hounsfield. Window parameters (width and location on the entire Hounsfield scale) are always indicated on computer tomograms. After such processing, the image is placed in the computer's long-term memory or discarded on a solid medium - film. We add that with computer tomography, the most insignificant differences in density are revealed, about 0.4-0.5%, whereas the usual X-ray film can display a density factor of only 15-20%.

Usually, when computer tomophagy is not limited to obtaining a single layer. To ensure the recognition of the lesion, several cuts, as a rule, are 5-10, they are performed at a distance of 5-10 mm from each other. For orientation in the arrangement of the separated layers relative to the human body, an overview digital photograph of the area under study is produced on the same apparatus, an x-ray imaging device, on which the levels of tomophages released during further study are displayed.

At present, computer tomographs have been designed in which the vacuum electron guns emitting a beam of fast electrons instead of the X-ray emitter are used as a source of penetrating radiation. The scope of such electron-beam computer tomographs is still limited mainly by cardiology.

In recent years, so-called spiral tomography is rapidly developing, in which the radiator moves in a spiral relative to the body of the patient and thus captures a certain volume of the body in a short time interval, measured in several seconds, which can later be represented by discrete separate layers. Spiral tomography initiated the creation of new, extremely promising methods of visualization - computer angiography, three-dimensional (volumetric) imaging of organs and, finally, the so-called virtual endoscopy, which became the crown of modern medical imaging.

Special preparation of the patient for CT of the head, neck, chest cavity and limbs is not required. In the study of the aorta, inferior vena cava, liver, spleen, kidney, the patient is advised to confine himself to a light breakfast. On the study of the gallbladder, the patient should appear on an empty stomach. Before CT of the pancreas and liver, measures must be taken to reduce flatulence. For a more precise differentiation of the stomach and intestines with CT of the abdominal cavity, they are contrasted by fractional oral administration of approximately 2.5 ml of a 2.5% solution of water-soluble iodide contrast medium before the study.

It should also be noted that if on the eve of the CT scan the patient was subjected to an X-ray examination of the stomach or intestine, then the accumulated barium will create artifacts in the image. In this regard, CT should not be prescribed until the digestive canal is completely emptied of this contrast medium.

An additional CT technique was developed - enhanced CT. It consists in conducting a tomography after intravenous administration of a water-soluble contrast agent to the patient. This method contributes to an increase in X-ray absorption due to the appearance of a contrast solution in the vascular system and parenchyma of the organ. At the same time, on the one hand, the contrast of the image is increased, and on the other, vascularized formations are prominent, for example, vascular tumors, metastases of some tumors. Naturally, against the background of a strengthened shadow image of the parenchyma of the organ, it is better to identify the malovosudistye or completely avascular zones (cysts, tumors).

Some models of computer tomographs are equipped with cardiosynchronizers. They include the emitter at precisely specified times and - in systole and diastole. The transverse sections of the heart obtained from this study make it possible to visually assess the heart condition in systole and diastole, calculate the volume of the heart chambers and the ejection fraction, and analyze the parameters of the general and regional contractile function of the myocardium.

The value of CT is not limited to its use in the diagnosis of diseases. Under the control of CT, punctures and targeted biopsies of various organs and pathological foci are performed. CT plays an important role in monitoring the effectiveness of conservative and surgical treatment of patients. Finally, CT is an accurate method for determining the localization of tumor lesions, which is used to guide the source of radioactive radiation to the focus during radiotherapy of malignant neoplasms.

[1], [2], [3], [4], [5], [6], [7]