Preparing for a CT scan

Medical history

Before each CT examination, a complete medical history should be obtained regarding contraindications to the use of contrast agents and possible reactions to them. For example, in patients with suspected renal dysfunction, the blood urea and creatinine levels should be determined. It is important to find out whether the patient has previous CT scans for comparison. Information about previous surgical interventions or radiation therapy in the area of the proposed examination is also important. All relevant radiographic data, both previous and current, should be carefully analyzed. The patient's medical history should clearly define the purpose of the diagnostic search so that the differential diagnosis is as reliable as possible.

Kidney function

With rare exceptions (bone examination, fracture evaluation). CT examinations are performed with intravenous administration of iodinated contrast media. Since contrast agents are excreted by the kidneys, they can cause changes in renal hemodynamics and toxic tubular damage. Therefore, to assess renal function, plasma creatinine levels are measured before CT scanning. If renal dysfunction is detected, contrast agents are administered only for very specific indications. In this situation, low-osmolar iodinated contrast agents should be used, since they have very low nephrotoxicity. It is also important to ensure adequate hydration of the patient. Finally, the administration of acetylcysteine in tablets (Mucomyst) has a renoprotective effect. Particular attention should be paid to patients with diabetes mellitus, especially those receiving the oral antidiabetic drug metformin. In these patients, contrast agents can cause lactic acidosis, especially in the presence of concomitant renal dysfunction. Therefore, it is recommended to stop metformin on the day of the study and for the following 48 hours, and to resume it only after the creatinine level has been assessed to confirm the stability of renal function. Until recently, in situations where it was absolutely necessary to administer a contrast agent to dialysis patients, the study was planned so that dialysis immediately followed the CT scan. However, recent observations have shown that emergency dialysis is not necessary. However, it has been noted that residual renal function may suffer from the circulation of contrast in such patients. Otherwise, circulation of the contrast agent for one or two days before the next dialysis session does not cause any complications.

Plasma creatinine testing is a quick and inexpensive test, so play it safe and order it before every CT scan.

Hyperthyroidism

Examination of a patient with hyperthyroidism is expensive and time-consuming. However, the treating physician should exclude hyperthyroidism if he or she suspects it clinically before a CT scan with a contrast agent. In this case, the necessary laboratory tests and scintigraphy are performed. In all other situations, it is sufficient to have a note in the medical history that “there is no clinical evidence for hyperthyroidism” or, even better, documented results of the thyroid function assessment. Then the radiologist can be sure that the patient has been examined. Please note that normal values may differ in different laboratories. Find out what units of measurement and normal values are accepted in your laboratory. In this case, the risk of thyrotoxicosis due to the use of an iodine-containing contrast agent can be eliminated. If hyperthyroidism or thyroid cancer is planned to be treated with radioactive iodine, the intravenous use of a contrast agent can lead to suppression of the iodine-absorbing activity of the thyroid gland for several weeks. In some cases, radioactive iodine therapy should be postponed for some time.

Normal Thyroid Hormones Levels

• Thyroid-stimulating hormone - 0.23-4.0 pg/ml
• Total thyroxine - 45-115 ng/ml
• Free thyroxine - 8.0-20.0 pg/ml
• Total triiodothyronine - 0.8-1.8 ng/ml
• Free triiodothyronine - 3.5-6.0 pg/ml

Adverse reactions when using contrast agents

Since the introduction of non-ionic contrast agents into clinical practice in the late 1970s, adverse reactions have become quite rare. However, previous reactions indicate an increased risk, and the anamnesis should be aimed at carefully clarifying them. Any reaction to contrast agents in the anamnesis is of great importance. If the patient has had itching or urticaria after the previous administration of a contrast agent, it is advisable to premedicate before the examination. In case of a drop in blood pressure or collapse, the contrast agent is either not used at all, or, if necessary, the clinical indications are carefully weighed again and the appropriate premedication is prescribed. The general rule for patients requiring premedication is to abstain from food intake for 6 hours before the examination. This will reduce the risk of aspiration in case of a severe anaphylactic reaction requiring intubation and artificial ventilation.

Premedication (history of adverse reactions to contrast agents)

In case of mild side effects, prednisolone is usually prescribed orally in doses of 50 mg 3 times 13, 8 and 1 hour before the examination. Additionally, 50 mg of an antihistamine (e.g. diphenhydramine) is administered intramuscularly 1 hour before the procedure. In this case, side effects such as increased intraocular pressure and urinary retention may occur. In addition, drowsiness will be observed for 8 hours, so the patient should refrain from driving a vehicle during this period. When planning an outpatient CT examination, the patient should be warned about possible drowsiness and temporary visual impairment, so an escort may be required when returning home.

Oral corticosteroids

The patient takes a liquid contrast agent orally on an empty stomach in small portions for 30-60 minutes before the CT scan. This ensures a continuous, uniform distribution of corticosteroids along the gastrointestinal tract. Therefore, the patient should arrive at least an hour before the abdominal examination. In order to make it easier for the radiologist to choose a contrast agent, the CT application should indicate whether surgery is planned immediately after the examination, whether there is a suspicion of perforation of a hollow organ or the presence of a fistula. In these situations, a water-soluble contrast agent (e.g., gastrografin) should be used instead of a barium sulfate-containing agent. If the patient has undergone a traditional X-ray examination with a barium suspension (e.g., stomach, small or large intestine, passage), then, if possible, the abdominal CT scan should be postponed for 3 days. In this case, the topogram usually clearly shows barium residues along the intestine, which cause significant artifacts during computed tomography, making it uninformative. Therefore, the sequence of diagnostic manipulations in patients with abdominal pathology should be carefully planned.

Informing the patient

Patients are afraid of the harmful effects of X-ray radiation during CT scans. Their anxiety can be reduced by comparing diagnostic X-ray radiation with natural background radiation. Naturally, the patient must have the impression that he is taken seriously and his concerns are understood. Otherwise, the trust in the doctor is at risk.

Many patients find it helpful to know that they can communicate via intercom with the X-ray technician in the control room and that the scan can be paused or stopped at any time if an emergency arises. Claustrophobic patients may feel more comfortable if they close their eyes during the scan. In very rare cases, mild sedation may be necessary.

Breath

Before the examination, the patient is informed of the need to control breathing. With traditional CT, the patient is told to inhale and hold his breath for a few seconds before each new section. With spiral CT, holding the breath for 20-30 seconds is required. If the patient is unable to hold his breath, the movements of the diaphragm will lead to blurring of the image with a clear deterioration in image quality. When examining the neck, swallowing movements deteriorate the image quality even more than breathing.

Removal of metal objects

Naturally, before the head and neck examination, to avoid the appearance of artifacts, jewelry and removable dentures should be removed. For the same reason, before CT scanning of the chest or abdomen, clothing with metal hooks, buttons and zippers should be removed.

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