A Computed Tomography (CT) scan is a type of imaging test that uses X-rays, or X-rays. The image is created on the basis of data on the attenuation of radiation after it has passed through the human body. Data is collected by an array of opposing detectors and an X-ray tube. The next generations of devices are built around an increasing number of detectors, and sometimes more than one radiation source. In modern spiral devices, the radiation source and detectors make a circular motion and determine the scanning axis under which the patient moves. If these movements would result in a visible mark on the skin of the subject, the component of the circular motion of the lamp-detector system and the motion of the table would result in a spiral, hence the name of the test. Data collection – scanning takes a relatively short time, on average a dozen or so seconds, which allows for quick imaging of a given area of ​​the body with a single inhalation. In the further stage of image formation, a computer with high computing power processes the obtained data on the weakening of radiation from the scanned area of ​​the body into the actual image created on the monitor.

The collected information on the X-ray absorption coefficient from each point of the examined area of ​​the human body is called volumetric data, because the image is made not of two-dimensional pixels, but of three-dimensional voxels. From such data – volumetric from the entire scanned area – modern workstations obtain a lot of diagnostic information. It is possible to perform cross-sections in any plane – (e.g. adjusting the examination plane to the curvature of the spine), three-dimensional reconstruction (even slight bone displacements, foreign body location are clearly visible), angiographic reconstruction – showing contrasted vessels without the image of overlapping bones, and even the examination the interior of the cavities and lumen of organs, as happens in virtualbronchoscopyor virtualcolonoscopy. In order to better display the lesions, the doctor may freely rotate and record the rotational movement of the examined area while generating the reconstruction, adjust the angle and intensity of the “illumination” of the reconstructed element, which may be, for example, the area of ​​a complex, multi-fragmentfracture, a temporal bone pyramid with elements of the middle ear. The possibility of using multiple (so-called secondary) reconstructions significantly improves the accuracy of lesion assessment and is a valuable source of information, thanks to which the surgeon can better plan the surgical procedure.

Research safety

X-rays show significant interactions with living tissues. There is practically no safe dose of radiation, and either radiation can lead to a number of mutations in cells. Despite the development of technology, the radiation dose is much higher than in the case of conventional radiographic examinations (radiography, X-ray), this difference is mainly due to the negligible dose of X-rays, which is involved in the formation of an image in conventional radiography. The amount of radiation dose taken depends on the type of CT scan. The device settings allow to reduce the radiation dose in children, and in some examinations, such as HRCT (high resolution computed tomography of the lungs) or low-dose CT scan, the effective dose absorbed by the examined person is comparable to classical radiography.

Modern CT machines are systems with intelligent dosing of X-rays. During scanning, the system changes the parameters of the lamp current and reduces them significantly after passing through the areas that require higher values ​​(such as the pelvis, shoulder girdle).

It should be remembered that a person is constantly exposed to ionizing radiation, the exposure increases rapidly at higher altitudes, e.g. in the mountains above 1500 m, flying by plane. This is due to the decreasing thickness of the atmosphere, which protects us from cosmic radiation. On the other hand, humans are constantly exposed to ionizing radiation from everyday objects (0.9 mSv on average). The geographic regions of significant radiation intensity in various regions of the world are known (Table 1), where the incidence ofneoplasmsdoes not actually increase .

Medical personnel performing tests with exposure to X-rays every 5 years take part in a special training in the field of radiological safety of the patient. The training ends with an exam.

Without disregarding the risk, the ALARA principle (as low as reasonably achievable) is adhered to. Before each examination, the patient confirms his consent to perform it, and the woman of childbearing age confirms that she is not pregnant. If it is otherwise, it is necessary to confirm the validity of the examination, which in the case of pregnancy is performed only for vital reasons or when the patient is at risk of permanent loss of health.

Some tests require the intravenous injection of iodinated contrast agent. Currently, only non-ionic contrast agents are used. Side effects may occur after administration. Table 2 lists the type of adverse reaction and the frequency of occurrence.

What are the indications for computed tomography?

The indications for a CT scan are numerous and relate to many different diseases. Due to the greater exposure to X-rays, a CT scan is performed when previous data do not lead to diagnosis (e.g. laboratory tests,ultrasound, X-ray examinations, specialist consultation), or in emergency situations, when prompt diagnosis is necessary for proper treatment ( differentiation of hemorrhagic and ischemic stroke).

X-ray examinations, including computed tomography, are performed only on the recommendation of a doctor, who should also interpret the result. The reason for computed tomography examination may be suspected changes in the brain (tumors, aneurysms),stroke, emergencies and post-traumatic changes in every area of ​​the body, especially when looking for the site of previously diagnosed bleeding into the peritoneal cavity (e.g. ultrasound), diagnostics of ear structures, sinus examination, pain in the musculoskeletal system, diagnosis of neoplastic changes in various parts of the body and assessment of treatment effectiveness, control postoperative. A good example of the use of virtual colonoscopy is a previous unsuccessful attempt to perform an optical colonoscopy, which happens for technical reasons (adhesions, additional bends of the colon, infiltrative wall changes), and in the case of virtual bronchoscopy – the need to assess the local condition after tracheal surgery.

The indications for CT angiography are i.a. suspected vascular dilatation, narrowing or obstruction, e.g.aortic aneurysm,pulmonary embolism, atherosclerotic lesions. The result of such examination may significantly influence the planned treatment.

The most technologically advanced computed tomography examination systems have several hundred rows of detectors, are very fast and accurate, they test with layers less than 1mm thick, allow for precise imaging of wall changes and in the light of the delicate branches of the coronary vessels. The scanning is closely synchronized with the work of the heart (thanks to the inclusion of electrocadiography in the system), which eliminates the artifacts of blurring resulting from the work of the heart.

Contraindications

An important contraindication to computed tomography is:

1. the patient’s pregnancy,
2. overactive thyroidgland
3. impaired kidney function,
4. previously documented hypersensitivity to the contrast agent (contrast agent, contrast agent).

In many CT scans, intravenous contrast is required. In case of well-documented hypersensitivity, the examination with the use of another iodine contrast agent may be performed after prior preparation of the patient. The patient’s qualification for the examination, that is, considering the indications and contraindications for the examination, and the possible preparation of the patient for the CT examination, planning the procedure after the CT examination, are the responsibilities of the doctor who ordered the examination.

In the TK laboratory, only all activities related directly to the examination are performed.

The listed contraindications are relative as there are no absolute contraindications for computed tomography. It is performed even in patients with renal insufficiency, with appropriate dialysis planning.

Preparation for the examination

The patient receives the exact method of preparation upon registration. Depending on the type of examination, the preparation may vary considerably. Some CT examinations are performed without preparation (e.g. high-resolution tomography of the lungs, some head and spine examinations), also in emergency conditions the examination is performed without preparation.

Fasting should be performed prior to examination with intravenous contrast agent administration, approximately 4-6 hours is recommended. About 2 days before the test, remember to hydrate your body properly, as dehydration increases the risk of kidney damage. When examining the abdominal cavity and pelvis, it is often necessary to contrast the intestines (with a contrast solution) or fill them with plenty of water. Watering typically begins 2 hours before the scan and can last up to 30 minutes before the test. Depending on the type of test, you will need 0.5 to 1.5 liters of fluid. For virtual colonoscopy, a thorough colon cleansing is required.

The course of the study

You should submit for the examination with the results of previous imaging examinations (CT, ultrasound, magnetic resonance imaging). If the intravenous administration of contrast is planned, it should be about 30 minutes earlier with the result ofcreatinineconcentration . If there is a plan to fill the intestines with fluid, it will take approximately 2 hours to drink before the test.

After reporting for the examination, the patient obtains important information, e.g. about the possible risk of the examination, especially if the intravenous administration of a contrast agent is planned. If he is using corrective lenses for reading, he should take them with him – some information may be in writing. The patient may be asked to complete a special questionnaire, and also signs the informed consent.

If the examination is with contrast, it is necessary to access the vein through an intravenous cannula (cannula). Then the patient is placed on the movable table. There is constant patient-technician communication, so you can share your feelings with the technician when necessary. It is important that the subject remains motionless throughout the test. Special belts can help.

An electroradiology technician, nurse and doctor are present during the examination. Appropriate action will be taken where necessary.

The length of stay in the computed tomography laboratory varies depending on the area and type of examination. You must reserve between 30 minutes and 3.5 hours.

After the examination

After performing the test without intravenous contrast administration, the patient may immediately move away. In the case of a test with intravenous contrast agent administration, the observation lasts at least 30 minutes and only after this time is the cannula removed.

The inpatient examination result is available after the examination, while the waiting time for the outpatient examination result is usually several days. You should report to the referring physician with the test result.

If you feel unwell after the test (which may result, among other things, from a late reaction to contrast), the referring physician should act accordingly. In some situations, e.g., with a history of kidney disease, control tests of creatinine concentration can be scheduled immediately up to two weeks after the test.