Written by: Ildikó Kalina
4.4. The CT examination
4.4.1. Patient preparation for CT examination
Patient preparation for elective CT examination is the referring physician's responsibility.
Before and after the examination, the patient must be hydrated (with drinking plenty of fluids or with infusion) to avoid the influence of the injected nephrotoxic intravenous contrast agent.
Intravenous contrast material will be administered only by the knowledge of renal function (serum creatinine, GFR), which will be verified in all cases by the radiologist too. Particular attention should be paid to high-risk patients or patients with chronic kidney disease.
Patients with impaired renal function get special (Iomeron, Visipaque) contrast material, for patients with very low GFR we give intravenous contrast material only in case of vital indication.
A contrast-enhanced CT examination takes place after four hours of starvation to avoid the contrast agent side effects (nausea, vomiting and aspiration).
If the patient anti diabetic medication contains metformin, it should be skipped before and after the test 48 hours to avoid the lactic acidosis, which may occur mainly in patients with impaired renal function.
The intravenous injected iodinated contrast material changes the serum levels of iodine by the iodine-containing drug-treated patients, so it can fake the control examination results.
Before the examination, we will tell the patient about the risks of the test, that will he acknowledge and justify by signing the consent form.
4.4.2. Examination technique
Each test includes two basic types of recording: the topogram (scout) and the tomogram (slice, layer).
All CT studies begins with a drift slicing picture, this is the topogram. The study region is included in this picture. In fact, this is a digital X-ray image, which can be taken from postero-anterior or from lateral side, depending on the part of the body which will be examined. We select the beginning and the end of the examination area. After that the tomograms - the transverse slices - will be prepared, of that part of the body.
The measurement parameters can be chosen from the opportunities offered by the computer, according to the clinical question and the examined region, but it is possible to prepare own protocols.
The tomography usually carried out in two series, first natively and after administration of contrast material.
4.4.3. By CT examinations applied contrast materials
A CT scan can be done natively, without introduction of any external material.
However in most cases different contrast agents are used. We can get the contrast media routinely to the examination area one of two ways. For signaling the alimentary tract oral contrast material and for the better general tissue resolution achievement intravenous
iodinated contrast media can be used, most commonly in a cubital vein. In most cases, we use injector for the intravenous application of contrast agent. The injector delivers the contrast material into the vascular system smooth, with constant, administered flow. In special cases we can use contrast media in other ways, such as through the rectum, or a tube into special anatomical region, or through an opening of a fistula.
37 4.5. The clinical use of CT
Most common, routine CT examinations:
Head CT (and / or CTA) scan (brain, bones, blood vessels)
Neck CT (and / or CTA) scan (cervical soft tissue, lymph nodes, blood vessels)
Facial CT scan (sinuses)
Chest-CT (and / or CTA) scan (lung, mediastinum, great vessels, heart)
Abdominal-pelvic CT (and / or CTA) scan (abdominal parenchymal organs, digestive tract, urography, abdominal blood vessels)
Lower limb CT angiography
CT-scan of bone (e.g. spine)
CT scan of the patient takes place commonly in the examination algorithms after some other imaging methods (X-rays, ultrasound) but in some cases it can be the first modality of choice (stroke, polytrauma, pulmonary embolism, suspected aneurysm rupture).
In all cases, the rules applicable to the study are that both written and pictorial documentation has to be prepared. The study design is based on the clinical question.
It is the radiologist competence to determine that, how "routine" is a study and how to implement the examination on the basis of the clinical question(s).
The task of the clinical doctors is to completely inform about the relevant clinical datas.
For the radiologist doctor the examination-request paper and the review of the available clinical data are always essential. We can model our examination based on these facts so, that we can obtain the maximum amount of information with minimum charge for the patient in examinations supporting the clinical diagnosis.
For routine scans we usually use pre-written protocols, which will be tailored and optimized by the patient's individual endowments.
In some cases it may be sufficient to prepare only native series (e.g. consideration of fresh bleeding or urinary tract stones), sometimes we has to do an examination with more than one phase or other special way.
Arterial, parenchymal, venous and late phase images can be produced after administration of intravenous contrast material.
The high-volume, quick data collection allows mapping with various technical parameters even in body regions or even in the whole body, and even several different times after the intravenous administration of contrast material. So this quick data procedure creates the possibility of doing different types of examinations shortly after each other.
Dynamic CT: after the intravenous injection of the contrast material the same body region will be several times scanned, that means we detect the time course of the contrast
enhancement (e.g. by the focal liver lesions).
HRCT (high resolution CT): it visualizes thin layers with high resolution. The measurement time is longer and the radiation exposure is higher (e.g. examination of the lung parenchyma or focused investigation of the inner ear).
Three-dimensional (3D) images can be produced from the large amount of data.
Arterial CT angiography: the measurement is done by bolus detection at injection of the intravenous contrast material. The flow is high 4-5 ml/sec. In the selected altitude (location) the bolus detection may be done under the control of eye or it is possible to set up automatic bolus detection. The images of transverse slices are made by a software solution in 3D.
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Venous CT angiography: with comparison of the arterial angiography more contrast material should be injected with slow flow (1.8-2 ml/sec). Due to the prolonged contrast enhancement automatic 3D representation is not possible; the different planar reconstructions are preferred.
Virtual colonoscopy: instead of colonoscopy it is a CT-scan (with screening purposes, or if the endoscopic examination can not be performed); with this method we can diagnose cancers and pre-cancer conditions (polyps).
Virtual bronchoscopy: a non-invasive assessment of bronchial structures (foreign body, tumor).
CT perfusion: with the help of time-density curves (perfusion maps) it is possible to determine by the acute ischemic stroke the Penumbra, the extent of brain tissue that can be saved.
With the newest machines it is possible to do perfusion examinations in liver, kidneys and lung.
These studies have advanced technical and personnel conditions, the assessment of them is more time-consuming than the average examinations.
The CT can guide invasive diagnostic procedures (FNAB, biopsy) and therapeutic
interventions (drainage, RFA) also when it is not possible with ultrasound. In many cases the CT-guided biopsy can be carried out more accurately and safely, than the US-guided
procedure.
4.6. Advantages and disadvantages of CT examination
The benefits of CT examination
The CT-scan - against the summation effect of X-ray procedures - has an excellent spatial resolution (better than MRI) and gives a good anatomical orientation.
Contrast to the ultrasound the set of the sections is standard, it has a good reproducibility and it has the ability for cross-sectional imaging of the total body.
The contrast resolution, especially with proper contrast-enhancement techniques - except the magnetic resonance imaging - is over other imaging procedures.
Based on the density measurements (Hounsfield-value) the individual lesions can also be ordered into different tissues.
Short examination time
The CTA takes out the diagnostic angiography.
Based on 3D images it is possible to design recunstructive surgery.
The disadvantage of CT examination
The greatest and almost only disadvantage of the CT is, that it uses ionizing radiation that is even multiple fold bigger than by conventional X-Ray examination (in some cases it can be 5-20 fold bigger). Compounded by direct and scattered (1-2 scale smaller) radiation exposure.
In many cases the radiation doses suffered by the patient aren't in the centre of attention in clinical doctors.
39 4.7. Summary
The CT is one of the most effective methods in the diagnostic algorithms based on the great diagnostic accuracy, despite the considerable radiation exposure. The resolution can be further enhanced by application of contrast material. The examination time is short; it is possible to scan even the whole body in seconds that is a unique diagnostic ability.
Translated by Csaba Korom
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