Computed tomography

• Synonyms
• history
• Basics and technology
• Alternatives

Synonyms

CT, computed tomography, tomography, tube examination, CT scanning

English: cat - scan

definition

Computed tomography is ultimately the further development of the X-ray examination. In computed tomography, X-ray images are recorded from different directions and converted into slice images with the help of the computer while converting these images. The name computed tomography is derived from the Greek for tomós (Cut) and gráphein (Write) off.

history

The procedure of Computed Tomography was founded in 1972 by the American physicist A.M. Cormack and the British engineer G.N. Hounsfield developed. The two researchers received the 1979 Nobel Prize for Medicine for their work.

Basics and technology

In the CT-Examination/ Computed Tomography one with a classic X-ray tube and a narrow X-ray beam (Fan beam) generated. X-rays are absorbed to different degrees by different types of tissue. Strongly absorbent layers are particularly bone tissue. The detectors on the opposite side of the CT´s perceive the transmitted x-ray radiation.
The X-ray tube of the Computed Tomography rotates perpendicular to the patient's body axis and thus bypasses the entire patient and continually emits and detects the transmitted X-rays.
Depending on the X-rays, the detectors produce electrical impulses. The computer now calculates an image in various gray levels from the individual pulses that were collected while driving around the patient.

This process is then repeated layer by layer, creating the individual layer images. In modern computer tomographs, several shifts can be run at the same time.
In general, section thicknesses between 1 mm - 1 cm are chosen.

Compared to the X-ray there is in the Computed Tomography -Examination no overlay effects. All points of the Computed Tomography can be clearly assigned three-dimensionally. Therefore sizes can be clearly determined and structures can be clearly assigned.

With the possibility of digital post-processing, three-dimensional images of bones and ligaments can be created.

In special questions, e.g. B. in tumor diagnostics, the use of contrast media can increase the informative value through stronger contrasting.

indication

The Computed Tomography is ideal for displaying bone tissue.
That is why it is used in many areas of medicine.
Important areas of application are:

• Computed tomography of the head (CCT, cranial computed tomography):
  It is used if bleeding, brain tumors, age-related changes, stroke (Apoplexy / Apolplex) and bony skull injuries applied.
• Whole body CT:
  A whole-body CT is particularly used when looking for tumor metastases or seriously injured people in order to get as much information as possible.
• Skeletal computed tomography:
  It is the most common in the Orthopedics applied examination technique.
  Special indications are:
  
  • disc prolapse (rare indication when an MRI cannot be performed)
  • Osteoporosis (also used to determine bone density as a qCT)
  • Broken bones (Fractures)

Computed Tomography Risks

Since the base of the Computed Tomography - If the examination is X-rays, the examination will result in radiation exposure.
The radiation exposure is depending on the examination between 3 mSv to 10 mSv (1 mSv = 1/1000 Sievert) specified. A classic chest x-ray is about 0.3 m Sv,
For comparison: the natural radiation exposure at sea level in Germany is approx. 2.5 mSv per year. Contrary to popular belief, the radiation exposure is therefore rather low.

Another risk is having a panic attack during the examination because of the depressed conditions. Should be a claustrophobia (Claustrophobia) are known, sedatives may be given prior to the examination.
More and more open ones are coming Computed Tomographyn on the market in which patients only have to be driven through the CT ring.

Contraindication

In the Computed Tomography As mentioned, it is an X-ray examination. For this reason, patients should pregnancy usually not be examined by computer tomography.

Since iodine-containing contrast medium is used for CT examinations with contrast medium, it must be determined in advance of the examination whether the patient has allergic reactions to the contrast medium or to iodine are known. Furthermore, the function of the thyroid gland (Hyperthyroidism) as well as the kidney (restricted excretory function?) to be clarified by laboratory tests.

Computed tomography sequence

To Computed Tomography the patient is placed on an examination table.
Depending on the examination area, the entire patient or just the region to be examined is now moved through the tomograph.
Just like with photography, the quality of the images is important Computed Tomography the better, the calmer the patient lies during the examination.

The radiologist who is doing the examination usually provides further information in an information brochure.
In general, the patient does not have to come to the computed tomography examination on an empty stomach.

Computed tomography head

A computed tomography of the head is often called in everyday clinical practice cCT (where c is for cranial is abbreviated. During the examination, the patient, who is lying on a movable couch, is driven through the device, and numerous cross-sectional images of the head are created within a short time. Depending on the question, the patient Contrast media Injected via the vein to make certain processes more visible or delimitable.

Computed tomography is used for numerous issues and is mainly in the area of neurology widespread. Computed tomography of the head usually provides valuable information when it comes to clarifying acute processes in the brain and skull. One of the most important indications for a timely cCT is the suspicion of intracranial bleeding. This can usually be well delineated in the CT, as it is lighter than the surrounding brain tissue (hyperdense) appears.
Not infrequently, sudden severe to very severe headaches can be an indication of such a cerebral hemorrhage. In this respect, the preparation of a cCT is diagnostically valuable. For mostly younger people who suddenly have a "Annihilation headache“Can describe this reference to a subarachnoid hemorrhage (SAB), often by rupturing a vascular malformation in the brain, a Aneurysm, arises.

If older people complain of headaches, this should make them prick up the ears, especially if they have had a recent fall and are also taking blood thinners. Here, too, bleeding can be the cause, usually in the form of a epidural or subdural hematoma. In patients who present with subacute headaches of moderate intensity and who should also be clarified by means of imaging of the head, there is usually more of a MRI of the head prepared. Another very common indication for performing a head CT is to rule out fractures after falls or accidents. Here, CT is the gold standard, as it has the best resolution in the area of ​​bone structures.
Also a stroke can generally be clarified using a cCT. If it is the rather rare form of the hemorrhagic infarct acts, i.e. a stroke that is triggered by bleeding, this can usually be clearly demarcated with the CT. Is it a stroke that results from reduced blood flow (ischemic infarction), magnetic resonance imaging is usually better suited in the acute stage, and it also has a significantly lower radiation exposure. An ischemic stroke also shows up on CT. As a standard, however, if a stroke is suspected, a cCT is usually performed first in order to gain initial knowledge about the development.

Another possible indication for computed tomography of the head is recurrent dizziness, which can be an indication of circulatory disorders in the brain. Often, however, MRI can also be preferred here, as it can sometimes show the structures that are essential for the development of vertigo in more detail than CT. Depending on the type of cancer, cCT is also often performed in patients with cancer, especially if the patient has symptoms such as dizziness, headaches or neurological deficits Linguistic or Visual disturbances, Paralysis or Sensory disturbances describe. In this case, there is a risk that the tumor has metastasized into the brain or that a brain tumor has developed. This suspicion can first be clarified with a cCT, but MRI usually provides the better resolution for this question.

The MRI is usually given preference over the CT in the clarification of inflammatory processes, for example in the context of a multiple sclerosis, if tumors or metastases are suspected in the brain and to clarify processes in the area of ​​the Cranial nerves, of Cerebellum and des Brain stem. It is therefore clear that it is not that easy to make a very clear indication for cCT and against MRI or vice versa. In a nutshell, however, it can be said that cCT are very important after trauma, suspected cerebral haemorrhage, after a stroke and unconsciousness.

abdomen

The Computed Tomography (=CT) of the abdomen, so the abdomen is either carried out to assess the entire abdomen or only limited areas are assessed with the help of X-rays x-rayed so that you can only assess individual organs. The computed tomogram, as the examination is called, can be used to examine many organs in the abdomen that would otherwise require multiple examinations, or it enables an assessment to be made of whether further special examinations are required for certain organs.

The examination of the entire abdominal cavity as "overview“Is often necessary to at Tumor-affected Patients looking for Daughter tumors (=Metastases) or to carry out an initial assessment of the tumor on the basis of which the therapy will be carried out later. The computed tomogram of the abdomen is often used in this case Gastric cancer, Pancreatic cancer (=Pancreatic cancer) or also Kidneys- or Liver tumors uncover.

In addition, a computed tomogram of the abdomen is also carried out for tumors that are not in the abdomen, since the daughter tumors are then often found in the abdomen, especially the one Lymph nodes and the liver.

By assessing lymph nodes around the intestines and large blood vessels like that Main artery Tumors of the lymph nodes such as the Hodgkin tumor often diagnose with certainty.

Furthermore, the computed tomography has a high priority in the Assessment of the large blood vessels. With the common disease arteriosclerosis almost everyone is affected. A CT scan can reveal the exact extent of the calcification.

One of the emergency indications is the so-called "acute abdomen". This term describes a situation with severe abdominal painthat can be potentially life-threatening and that should be clarified as soon as possible. Computed tomography helps to get a good overview of what is happening in the abdominal cavity very quickly.

With computed tomography it is also sometimes necessary to the patient before the examination Contrast media because structures in the body can be better represented by contrast media. The contrast agent can get into the body in different ways, depending on which organs you want to assess. If you want to assess the intestine, you are given the contrast agent to drink before the examination. For this purpose, you are given a liquid to drink that contains the contrast agent about half an hour before the examination. After half an hour, the time has come Digestive tract hiked to the intestinal components you want to examine. The last portion of the contrast medium is usually given to drink on the examination table directly before the examination.

If other organs of the abdominal cavity are to be examined, the contrast agent is often replaced by the vein administered into the body. For this purpose, an indwelling venous cannula is typically placed on the back of the hand or in the crook of the elbow. This IV cannula consists of a needle over which a small plastic tube is slipped. With a small prick similar to that of a vaccination, the plastic tube with the needle is inserted into the vein. The needle is removed immediately and the plastic tube remains in the jar. You can then use it to administer medication directly without having to take another stab. The contrast agent is then administered via this cannula. If the contrast agent is then injected during the examination, people call it brief feeling of warmth described in the whole body, but it is almost harmless. However, there is a risk of one allergy on the contrast agent. If the person to be examined is already aware of such an allergy, you should urgently point it out or carry an emergency ID card with you!

CT of the lungs

A CT of the lungs provides results on the smallest changes in the lungs and within a few seconds in which the entire lungs can be displayed. Both the blood vessels of the lungs and the lung tissue itself can be better assessed with computed tomography than with almost all other common examinations.

A frequent reason for a CT scan of the lungs is found in examinations for chronic respiratory diseases, especially COPD, which also changes the supporting structure. Here, the result of the examination can have a significant impact on the approach to therapy.

Another field is the examination of changes in the X-ray image that could be a tumor. Computed tomography then allows different causes of changes in the X-ray image to be differentiated, since these all look similar on conventional X-ray images. Since many images of every section of the lung, no matter how small, can be made during a computer tomography, changes in the millimeter range can be assessed and, if it is a tumor, it can be detected at a very early stage.

As with computed tomography of the abdominal cavity, a contrast agent can also be administered with CT of the lungs. This is necessary in order to be able to represent the small and smallest structures well. If an examination with contrast agent is to be carried out, it is still important to take blood in order to check the functionality of the kidneys based on some values, since the contrast agent is excreted through the kidneys and this must be in tact for this, or in patients with impaired kidney function the Dose can be adjusted. Patients with thyroid dysfunction should inform them about this, as the contrast agent contains iodine and this can also lead to disorders in the thyroid gland, especially if its function is already disturbed.

Further information on this topic can be found at: CT of the lungs or CT of the chest / upper body

Computed tomography radiation dose / radiation exposure

As indispensable as computed tomography has become nowadays, its harmfulness due to radiation exposure is also controversial, especially in patients who have to undergo such an examination more frequently. The word radiation dose is a somewhat vague term in radiology. One speaks of the absorbed dose and describes how much of the X-ray radiation is absorbed by the tissue as energy (absorbed) becomes. It is given in gray (Gy), where 1 Gy = J / kg, i.e. energy absorbed by the tissue per kilogram.

Another important parameter is the dose equivalent. In addition to the amount of energy absorbed, it takes into account the type of radiation. This is important insofar as there are different types of radiation that differ significantly in their effects (and harmfulness to the human organism). Therefore, the absorbed dose is multiplied by a radiation weighting or quality factor for the dose equivalent. It is given in Sievert (Sv).
The effective dose is also derived from this, which also takes into account the fact that different organs react differently to radiation. For example, the gonads such as testes and ovaries and the red (blood-forming) bone marrow are very sensitive to radiation, whereas the skin and the surface of the bones are less sensitive. This is taken into account by multiplying the equivalent factor by an organ weighting factor, the unit remains the same, namely Sievert (Sv).

These values ​​can now be used to describe the radiation exposure that a radiological examination such as computed tomography entails. A distinction is made here between which body region is examined using the CT. A computed tomographic examination of the abdomen (abdominal CT) means an effective dose of about 7 mSv for the body. That of the chest cavity (thorax CT) about 10 mSv and that of the skull about 2mSv.
For better comparability, these values ​​are compared with those of a normal X-ray examination. An x-ray of the abdomen (abdomen x-ray) means an effective dose of around 1 mSv, an x-ray of the chest (chest x-ray) in 2 planes around 0.1 mSv and an x-ray of the head around 0.07 mSv. These values ​​can be roughly related to natural radiation exposure. The effective dose of a chest x-ray examination - common in everyday clinical practice - corresponds to a natural radiation exposure that would be achieved in around 15 days of normal everyday life.

Read more on the topic: Chest x-ray (chest x-ray)

A chest CT means a natural radiation exposure of about 3.5 years. It is therefore clear that computed tomography is associated with significantly higher radiation exposure than conventional X-ray examinations. From this it becomes clear why magnetic resonance tomography, which, like CT, enables cross-sectional representation of body structures, is of great importance. It works with magnetic fields, so - in contrast to CT - there is no exposure to radiation at all.

Read more on the topic: Computed tomography and radiation exposure

Computed tomography side effects

Computed tomography itself has no acute side effects. However, it is not uncommon for certain body structures to be examined during the examination so that they can be better assessed Contrast media via the vein (intravenous) applied. This can have several side effects.

On the one hand, an allergic reaction can develop, which can be manifested by malaise, sweating, itching and nausea. Such an allergic reaction can in rare cases lead to circulatory shock Anaphylaxis In this respect, the patient should report immediately if he experiences such symptoms. It is also essential to indicate before a CT examination if such an allergic reaction to contrast media has occurred before.

Another side effect of the contrast agent can trigger an overactive thyroid gland (Hyperthyroidism) be. This is because the contrast agent used for computed tomography contains a lot of iodine. Therefore, a certain thyroid value (TSH) to be controlled. If the value is abnormal, a contrast-enhanced CT must not be performed without prior treatment or prophylaxis.

Significantly reduced kidney function can also be a contraindication for a contrast medium CT, as the contrast medium can critically intensify the functional disorder of the kidney. That is why a kidney value is always taken in addition to the thyroid value before a planned contrast CT (Creatinine) checked. With computed tomography, the potential risk that the radiation exposure entails must always be weighed individually.

Computed tomography costs

After tariff for doctors (GOÄ), each examination has a certain point value from which an amount for the respective examination is calculated, which the doctor can claim with this. These values ​​are given here for the costs of computed tomography, it should be noted that these values ​​are the sole technical investigation mean and no advice. According to the GOÄ, a CT of the head costs 116.57 euros, a computed tomographic examination of the abdomen (abdominal CT) costs 151.55 euros, a CT of the chest (thorax-CT) costs according to GOÄ 134.06 euros.

Computed tomography duration

The total duration of an examination using computed tomography depends on the region of the body to be examined and also on whether or not contrast agent is applied. The examination usually takes between 10 and 30 minutes.

Alternatives

In some cases, computed tomography is the most informative examination method. However, there are compelling reasons one Computed Tomography not to be carried out (see contraindications), the following examination procedures may represent alternatives.

• the x-ray
• the MRI (magnetic resonance imaging)
• the ultrasound examination (sonography)