Procedures
COMPUTED TOMOGRAPHY (CT)
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Summary
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Synonyms: CT, CT scan

A computed tomography (from the ancient Greek τομή, tome, „a cut” and γράφειν, graphein, „to write“) is the computer-based evaluation of many x-ray images which are taken from different directions. The images are processed to make up a three-dimensional picture. This is also called cross- sectional imaging.

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Technique
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What it is
A CT is an ultra-modern X-ray procedure where the part of the body to be examined is made visible in layers using X-ray images.

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How it works
In CT, X-rays are bundled and continuously rotated around the patient, who is lying down. The recording system is opposite the X-ray source. This equipment is not visible to the patient since it is in a casing (gantry). During the rotation around the body, X-rays fan out through the body while the attenuated X-rays are measured, indicating different tissue structures. The computer recognizes the different tissue structures at each layer and constructs an image in different shades of grey. Calcium for example appears bright while air and fluids appear dark. Since the heart permanently beats and is therefore permanently in motion, extremely short recording times are used for every layer to avoid blurred images. During the examination, contrast agents can additionally be injected into a vein in the crook of the elbow via a synthetic cannula, in order to make particular vessels or organs visible. Sometimes, necessary drugs may also be administered via this cannula.

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Purpose
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Computer tomography can be used to detect a wide variety of disorders, which can then be treated accordingly. The exceptionally good image quality makes the recognition of even the smallest findings possible; a CT is therefore the optimal basis for further medical treatment. A CT can depict the development and the extent of a disorder. Today, the CT has significant value in emergency diagnostics because an examination is very fast, it indicates the even smallest findings very clearly, and it is rapidly available.

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Target Patient Group

1. Tumour search, investigations, operation planning, after-care

2. Investigation of certain infectious diseases

3. Trauma investigations, particularly after accidents

4. Muscular-skeletal disorders, including osteoporosis

5. Investigation of congenital disorders 6. Rapid investigation of emergency patients (e.g. cerebral haemorrhage, stroke, vascular disorders like pulmonary embolism or aortic aneurysm)

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Procedure
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Persons
You will be looked after by an experienced team consisting of a medical radiology technician (MRT) and a radiologist. You will also be supervised from the switch-room via a monitor and, if necessary, via an ECG. Prior to the procedure, the radiologist may inject the contrast agent and any necessary drugs through a cannula in the forearm. After the procedure, when all the images have been evaluated, the radiologist will examine them and draw up a report. The report is either given to you or sent to the doctor who referred you.

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Preparation

1. You can eat a light meal before the procedure. You may drink as much water, soup or juice as you like.

2. You should take your usual daily medication with some liquid in the morning.

3. If you are a diabetic, please ask your doctor how you should take your medication on the day of your procedure. If you think that your blood sugar levels are low, please inform the MRI technician immediately.

4. Before the procedure, please tell the radiologist about all medication you are taking.

5. Please bring up-to-date lab results with you (no more than 7 days old).

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Precautions

Please be sure to inform the doctor if one of the following applies:

1. Pregnancy: If you are pregnant, this examination must NOT be carried out, except in a life-threatening situation that makes an immediate CT examination necessary.

2. Allergy to iodine: If you have ever had an allergic reaction to iodine, there is a high risk associated with administering these substances again. However, if the allergic reaction was only minor, it is possible to minimise the risk of a further allergic reaction by administering an appropriate drug. Please contact the doctor who referred you before the procedure.

3. Diabetes mellitus: If you are a diabetic and need medication daily, please clarify with your family doctor before the procedure which medicines you should stop taking before/after the CT.

4. Thyroid gland disorders: If you suffer from an untreated hyperthyroidism (hyperactivity of the thyroid gland), then the contrast agent can cause a state of shock (thyrotoxic crisis). We therefore ask you to clarify this with your doctor beforehand and to treat this condition with medication. Then there will be no increased risk.

5. Kidney disorder: If you suffer from renal impairment, your laboratory results will determine whether or not you can be given a contrast agent. Otherwise your kidneys must be protected from further damage through special preparation (infusion). Please discuss this beforehand with the doctor who referred you.

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Duration
The entire procedure, including preparation, takes about 15 minutes. There may be delays before the procedure if emergency patients have to take precedence. Please allow at least one hour of waiting time.

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Process
Before the procedure, you will be asked to fill-in a questionnaire. If anything is unclear to you, please discuss this with the MRT or the doctor. Then you will lie down on the examination table of the CT scanner. Depending on the organ to be examined, you will be connected to the ECG and/or a vein cannula will be prepared. The MRT and the doctor then leave the room and the procedure begins. This is a completely painless process and lasts for about 10 seconds. It is important that you hold your breath for this period of time when you are told to do so over the loudspeaker. During the procedure, you will be injected with a contrast agent if necessary, which may cause a short sensation of being hot. This is completely normal. When the examination is over you can take a seat in the waiting room. The doctor/MRT check the images and then remove the vein cannula after 20 minutes at the latest. Then you can go home. The results of the procedure will either be given to you or sent to the doctor who referred you.

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After procedure
You will usually be able to go straight home after the procedure. Please tell an MRT immediately if you feel dizzy or sick. They will advise you to remain still until the symptoms improve, or for at least 30 minutes. In certain cases you may also be given medication to deal with the nausea. If you have come by car, please check with the medical staff after the procedure whether you have been given any medication which might affect your concentration and prevent you from driving home by car. You are asked to drink a lot during the course of the day so that the contrast agent leaves the body as rapidly as possible.

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Consideration
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Risks
The CT is a very low-risk procedure. An intolerance reaction to contrast agents is very rare. You may develop a rash and/or itching in such cases. The symptoms are usually only short-lived and disappear of their own accord. If necessary, medication may also be administered in the form of so-called antihistamines. On extremely rare occasions, severe allergic reactions (anaphylactic shock) may occur, accompanied by breathing and circulation problems and/or loss of consciousness. This can be a life-threatening reaction, depending on its severity, and requires immediate medical attention. CT uses X-ray radiation. To ensure your safety, the X-ray level is reduced to a minimum. However, as X-rays can damage unborn life, this procedure is generally not carried out if you are pregnant.

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Alternatives
Depending on the kind of disorder, there are different alternative or supplementary procedures, which you can read about in the respective sections.

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FAQ

Do I have to be an in-patient in the hospital to have a computed tomography?

No, the procedure can also be performed if you are an out-patient. You can go home again after the procedure.

Which patients should not undergo this procedure?

The risk of a life-threatening incident is too high for patients with a strong contrast agent allergy.

What do I need to bring with me?

You should bring your up-to-date laboratory results (no more than 7 days old), showing results for the kidneys and thyroid gland in particular.

How much radiation will I be exposed to?

The radiation exposure for one computed tomography varies according to the area of the body and approximately corresponds to between about 1-10 mSv, which equates to a natural radiation exposure of 4 months to 3 years (litradiologyinfo.org).

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Citations
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1   Team General Hospital Vienna
 
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