CT or CT scan - Computed Axial Tomography

CT or CT: Computed Axial Tomography

What does TC mean? The acronym TC it means Computed Tomography, the acronym CT scan (Computed Axial Tomography) even if commonly in use, it is to be considered obsolete, since the machines used today no longer perform Axial scans, but Spirals, multiplanars. Tomography is defined as computerized because it is integrated by software that is able not only to reconstruct the images, but also to modify them or to obtain a three-dimensional reconstruction of the body structures examined.

Physical principles

How does CT work? CT or CT is a diagnostic imaging method a X-ray. These are produced from an X-ray tube, a vacuum-maintained structure with a cathode, which releases electrons at high speed which collide against theanode, thus releasing energy in the form of X-rays, high frequency electromagnetic radiation.

The radiation produced in this way, they are directed in a single strongly concentrated ray towards the bed of the patient, centrally located in the machine. Penetrating into the tissues, X-rays interact with the atoms that compose them, transferring part of their energy to the electrons, thus ionizing the atoms (for this reason they are defined ionizing radiations).

After passing through the tissues, the X-rays are detected by a digital detector which processes the radiological density of the crossed structures, measured in Hounsfield Units (HU), of a variable value between -1000 and 1000 HU, based on the radiation received, allowing them to be displayed . So the structures that absorb more radiation, such as thebone, they will have ahigh density radiological (1000 HU for bone), while the less absorbent structures will have a lower density: theair present in the lungs has the lower radiological density in absolute: -1000 HU. Water, on the other hand, has an intermediate density of 0 HU.

Once the radiological density of each structure has been defined, a computer processes a black and white image, with white corresponding to the maximum density, that of the bone, and black to the minimum density, that of air. All the remaining fabrics take on a gray color of varying intensity depending on the density.

The contrast medium

Some body structures, due to their position or some of their characteristics, are not displayed optimally on CT, therefore to allow better visualization it is necessary to use a half of contrast or contrast medium.  There are various types of contrast medium, they can be iodized (therefore compounds based on iodine) or non-iodized. Nowadays non-iodinated contrast media are mostly used, as they are more soluble and have less toxicity. They are commonly administered intravenously, but can also be administered by other routes (oral or rectal for example) as needed.

Adverse reactions to the contrast medium

The contrast medium it can, in rare cases, cause allergic / anaphylactic reactions, which occur in 0.6% of cases, but are found to be severe only in 0.04% of cases. These reactions are more common in elderly individuals, dehydrated people, people with asthma, with known allergies (not all, only certain types and in any case with an unproven relationship) or with suboptimal renal function or with a known allergy to contrast medium, which it constitutes the only absolute contraindication to the administration of the contrast.

The other conditions listed do not necessarily prevent the administration of the contrast liquid. Even in the case of allergy to the contrast medium, CT alone is not contraindicated. Possible adverse reactions to the contrast medium range from mild reactions, such as headache, hives, nausea and vomiting, to severe reactions, such as shock, pulmonary edema and seizures. With the use of non-iodinated contrast media the risk of reactions of this type is lowered from 0.04% up to 0.0004%.

CT or CT scan: computed tomography

CT, or computed tomography, is a diagnostic method commonly used in modern medicine.

Uses of TC

CT is used in the diagnosis of diseases and in their continuation, it can be useful to define the evolution or not of a pathology after years. It is particularly relevant in urgency, as it can be done quickly, as opposed to other examinations such as MRI which take longer. In this context, it is therefore useful in the diagnosis of stroke, head, pelvic, thoracic or abdominal trauma, pulmonary embolism (CT pulmonary angiography) and kidney stones. It is particularly important in the case of a stroke, as it passes through the Brain CT, allows you to quickly evaluate the brain to understand whether or not there are emboli or hemorrhages. In trauma it is very useful because it clearly defines bone fractures and allows you to see if there are injuries to abdominal organs such as the liver, kidney, spleen or even to the chest and lungs. It is the best test for detecting pulmonary embolism.

Outside of urgency it is used for the diagnosis and delimitation of abdominal and thoracic tumors, allowing to define the size of the tumor and the relationships with other organs, whether these are invaded or not, and the involvement of the lymph nodes. The evaluation of these characteristics allows to establish how aggressive a tumor is. In addition, CT is used for bone diseases especially affecting the spine, hands and feet, lung diseases such as fibrosis, emphysema and bronchial inflammation (through Chest CT scan). CT may be used to define the anatomy of the coronary arteries in heart patients (coronary CT angiography), or for the study of pathologies affecting the spleen, liver, pancreas (CT abdomen), kidney and urinary tract (uro-TC) and for numerous other pathologies.

CT can be used after surgery, to evaluate the outcome, or as a support in performing biopsies, ie tissue samples needed for the diagnosis of diseases such as tumors. In these cases we speak of stereotaxic biopsies. CT can also be used to perform neurosurgical procedures, making them minimally invasive: these are called stereotaxic surgery procedures.

Comparison with other radiological techniques

The other major imaging techniques to compare CT with are the following:

Ultrasound

L'ultrasound, which compared to CT allows, through the use of sound waves, the ultrasound, harmless, the study of numerous organs with a lower resolution but at very low costs and in a short time, so it is often performed to decide whether the patient will then have to perform a CT or an MRI. Ultrasound has the disadvantage of being a method whose validity greatly depends on the experience of the operator who performs it.

X-ray

There radiography. Although the two techniques use the same source of radiation (X-rays), CT allows you to study the organs under examination with more precision, as it allows you to obtain multiple scans that allow you to examine them in a three-dimensional way, obtaining a very more detailed. X-ray instead consists of one or two scans, which provide a uniquely two-dimensional image. Compared to radiography, however, CT has a higher radiation load and a higher cost.

Magnetic resonance

There magnetic resonance. The difference is first of all in the radiation used: the resonance exploits radio waves and magnetic fields, therefore, does not involve exposure to harmful radiation. However, it takes longer to perform, so it cannot be used urgently. Compared to CT, the resonance is better in the study of muscles, tendons, ligaments and intervertebral discs, so it is more useful for mechanical pathologies. It also allows for better visualization of brain structures than CT. Bone structures, on the other hand, are better visualized on CT. The two techniques have similar costs, however the resonance has the advantage of being able to be made with less toxic contrast media, if necessary. Sometimes it may be meaningful to use both of these tests to integrate their characteristics, especially in the study of pathologies inside the skull.

Contraindications and limitations of CT

CT should be done with caution in pregnant patients due to exposure to ionizing radiations, which, however, do not reach the minimum dose causing fetal damage during CT scan. However, it is not recommended, especially in the early stages of pregnancy. If necessary, in the doctor's opinion, you can consider doing ultrasound, resonance or radiography instead (the radiation dose in this case is present but much lower and less dangerous). Children, being growing organisms, also have a greater sensitivity to radiation and therefore it is usually preferred not to perform CT unless strictly necessary to formulate a diagnosis. Even when necessary they should not be scanned repeatedly. Low-dose radiation CT may be done to reduce exposure in children.

As for the contrast medium, the two fundamental contraindications are allergy and nephropathy induced by the contrast medium. In fact, the kidney represents the main route of elimination of this substance, and can be damaged by it. This happens rarely and especially in patients with already compromised kidneys, which is why it is good to check the functioning of the kidney by measuring creatinine between blood tests before administering the contrast medium. In pregnant women the administration of contrast medium it does not seem to accompany fetal damage, and in women during breastfeeding the elimination of these substances through milk is minimal, insufficient to cause harm, although it is generally advisable to stop breastfeeding for 24-48 hours.

Sources:

  1. Principles of CT and CT Technology, Lee W. Goldman
  2. Iodinated contrast media, Dr Dan J Bell◉ and Mr Andrew Murphy◉ et al., Radiopedia
  3. Iodinated contrast media adverse reactions, Mr Andrew Murphy◉ and Dr Bruno Di Muzio◈ et al.
  4. Diagnostic and therapeutic role of CT-guided stereotactic surgery in the management of intracranial tuberculomas, Ersahin M1, Hakan T, Ayan E, Berkman Z, Ekinci O, Ceran N, Aker FV.

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