Scheme for obtaining computed tomograms

A narrow beam of X-rays scans the human body in a circle. Passing through tissue, the radiation is weakened according to the density and atomic composition of these tissues. On the other side of the patient, a circular system of X-ray sensors is installed, each of which (there can be several thousand of them) converts the radiation energy into electrical signals. After amplification, these signals are converted into a digital code, which is sent to the computer memory. The recorded signals reflect the degree of weakening 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" at his body from different angles, at a total angle of 360°. By the end of the emitter's rotation, all signals from all sensors are recorded in the computer's memory. The duration of the emitter's rotation in modern tomographs is very short, only 1-3 seconds, which allows studying moving objects.

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

Along the way, the tissue density in individual areas is determined, which is measured in conventional units - Hounsfield units (HU). The density of water is taken as zero. The density of bone is +1000 HU, the density of air 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 cannot be displayed either on a display or on a photographic film, so the doctor selects a limited range on the Hounsfield scale - a "window", the dimensions of which usually do not exceed several dozen Hounsfield units. The parameters of the window (width and location on the entire Hounsfield scale) are always indicated on computer tomograms. After such processing, the image is placed in the long-term memory of the computer or dumped onto a solid medium - photographic film. Let us add that computed tomography reveals the most insignificant density differences, about 0.4-0.5%, whereas conventional X-ray imaging can display a density gradient of only 15-20%.

Usually, computer tomography is not limited to obtaining one layer. For confident recognition of the lesion, several slices are needed, usually 5-10, they are performed at a distance of 5-10 mm from each other. For orientation in the location of the layers being isolated relative to the human body, a survey digital image of the area being studied is produced on the same device - a radiotopograph, on which the tomography levels isolated during further examination are displayed.

Currently, computer tomographs have been designed in which vacuum electron guns emitting a beam of fast electrons are used as a source of penetrating radiation instead of an X-ray emitter. The scope of application of such electron-beam computer tomographs is currently limited mainly to cardiology.

In recent years, the so-called spiral tomography has been rapidly developing, in which the emitter moves in a spiral relative to the patient's body and thus captures, in a short period of time, measured in several seconds, a certain volume of the body, which can subsequently be represented by separate discrete 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 has become the pinnacle of modern medical visualization.

No special preparation of the patient for CT of the head, neck, chest and extremities is required. When examining the aorta, inferior vena cava, liver, spleen and kidneys, the patient is recommended to limit himself to a light breakfast. For examination of the gallbladder, the patient should come on an empty stomach. Before CT of the pancreas and liver, it is necessary to take measures to reduce flatulence. For more precise differentiation of the stomach and intestines during CT of the abdominal cavity, they are contrasted by fractional oral administration of about 500 ml of a 2.5% solution of water-soluble iodine contrast agent by the patient before the examination.

It should also be taken into account that if the patient had an X-ray examination of the stomach or intestines the day before the CT scan, the barium accumulated in them will create artifacts on the image. In this regard, CT should not be prescribed until the digestive tract is completely emptied of this contrast agent.

An additional method of performing CT has been developed - enhanced CT. It involves performing tomography after intravenous administration of a water-soluble contrast agent to the patient. This technique increases the absorption of X-ray radiation due to the appearance of a contrast solution in the vascular system and parenchyma of the organ. In this case, on the one hand, the contrast of the image increases, and on the other, highly vascularized formations are highlighted, such as vascular tumors, metastases of some tumors. Naturally, against the background of an enhanced shadow image of the organ parenchyma, low-vascular or completely avascular zones (cysts, tumors) are better identified in it.

Some models of computer tomographs are equipped with cardiac synchronizers. They turn on the emitter at precisely specified moments of time and - in systole and diastole. The transverse sections of the heart obtained as a result of such a study allow visually assessing the condition of the heart in systole and diastole, calculating the volume of the heart chambers and the ejection fraction, and analyzing the indicators of the general and regional contractile function of the myocardium.

The importance of CT is not limited to its use in diagnosing diseases. Under CT control, 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 target the source of radioactive radiation to the lesion during radiation therapy of malignant neoplasms.

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