Preparing for computed tomography

Disease history

Before each CT scan it is necessary to collect a complete history of contraindications to the use of contrast agents and possible reactions to them. For example, in patients with suspected renal dysfunction, the amount of urea and creatinine in the blood should be determined. It is important to find out if the patient has previous computer tomograms for comparison. Information on previous surgical interventions or radiation therapy in the area of the proposed study is also important. It is necessary to carefully analyze all the radiologic data relevant to the study, both previous and current. In the patient's medical history, the purpose of the diagnostic search must be clearly defined, so that differential diagnosis is the most reliable.

Kidney function

With rare exceptions (bone examination, fracture evaluation). CT examinations are performed with intravenous administration of iodine-containing contrast medium. Because contrast preparations are excreted by the kidneys, they can cause changes in renal hemodynamics and toxic damage to the tubules. Therefore, to assess the function of the kidney before computed tomography, measure the level of plasma creatinine. If there is a violation of kidney function, contrast preparations are administered only for very narrow indications. In this situation, low-osmolarity iodine-containing contrast agents should be used because they have very low nephrotoxicity. It is also important to ensure adequate hydration of the patient. Finally, the appointment of acetylcysteine in tablets (Mucomyst) has a renoprotective effect. Special attention should be given to patients with diabetes mellitus, especially those receiving oral protivodiabetic drug metformin. In these patients, contrast agents can cause lactic acidosis, especially with concomitant renal dysfunction. Therefore, it is recommended to stop taking metformin on the day of the study and the next 48 hours, and resume reception only after evaluating the creatinine content to confirm the stability of kidney function. Until recently, in situations where it was absolutely necessary to introduce a contrast agent to dialysis patients, the study was planned in such a way that dialysis followed immediately after the CT scan. But recent observations have shown that there is no need for emergency dialysis. However, it was noted that the residual function of the kidneys may suffer from the circulation of contrast in the blood in such patients. In all other cases, the circulation of the contrast agent does not cause any complications for one or two days before the next dialysis session.

A study of creatinine in plasma is a quick and inexpensive study. Therefore, reinsert and prescribe it before each CT scan.

Hyperthyroidism

Examination of a patient with hyperthyroidism is expensive and time-consuming. Nevertheless, the attending physician must exclude hyperthyroidism. If before a CT scan using a contrast medium, it is suspected clinically. In this case, perform the necessary laboratory tests and scintigraphy. In all other situations, it is sufficient to have a note in the medical history that "there is no clinical data for hyperthyroidism," or, even better, a documentary evaluation of the thyroid function. Then the radiologist can be sure that the patient is examined. Note that in different laboratories the norm indicators may differ. Find out what units of measure and standard values are accepted in your laboratory. In this case, the risk of thyrotoxicosis can be eliminated by using an iodine-containing contrast agent. If you plan to treat thyroid hyperthyroidism or thyroid cancer with radioactive iodine, IV use of a contrast medium may lead to a suppression for a few weeks of iodine-absorbing thyroid activity. Polo therapy with radioactive iodine should be postponed for some time.

Normal levels of thyroid hormones

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

Adverse reactions with contrast agents

After the introduction of non-ionic contrast agents into clinical practice from the late 70's, side reactions were observed rarely. However, previous reactions indicate an increased risk, and the history should be directed to a thorough examination of them. Any reaction to contrasting substances in a history is of great importance. If the patient has an itching or urticaria after the previous administration of a contrast agent, premedication is desirable before the test. In the event of a drop in blood pressure or collapse, the contrast agent is either not used at all or, if necessary, the clinical indications are again carefully weighed, and appropriate premedication is given. The general rule for patients who need premedication is refusal to eat 6 hours before the test. This will reduce the risk of aspiration in the event of a severe anaphylactic reaction requiring intubation and artificial ventilation.

Premedication (history of adverse reactions to contrast agents)

In the case of mild adverse reactions, 3 oral prednisolone administrations of 50 mg are usually prescribed for 13, 8 and 1 hour before the study. Additionally, 1 hour before the procedure, 50 mg of an antihistamine drug (eg, diphenhydramine) is administered in / m. In this case, side effects may occur in the form of increased intraocular pressure and retention of urination. In addition, there will be drowsiness during 8 hours, therefore for this period the patient should refrain from driving the vehicle. When planning an outpatient CT scan, the patient should be warned about possible drowsiness and temporary loss of vision, so when you return home, escort may be required.

Oral administration of corticosteroids

The patient takes in the liquid contrast preparation on an empty stomach in small portions for 30 to 60 minutes before the CT scan. This achieves a continuous uniform distribution of corticosteroids along the GIT. Therefore, the patient should arrive at least an hour before the study of the abdominal cavity. In order to make it easier for the radiologist to navigate in the choice of contrast medium. The application for CT should indicate whether surgery is planned immediately after the study, whether there is a suspicion of perforation of the hollow organ or the presence of a fistula. In these situations, instead of a drug containing barium sulfate, a water-soluble contrast agent (for example, gastrographin) should be used. If a traditional X-ray study with a barium suspension (eg, stomach, small intestine or large intestine, passage) was performed, then, if possible, CT of the abdominal cavity should be postponed for 3 days. On the topogram in this case, it is usually easy to see the remains of barium along the bowels, which cause the appearance of significant artifacts in computed tomography, which makes it uninformative. Therefore, the sequence of diagnostic manipulations in patients with pathology of the abdominal cavity should be carefully planned.

Informing the patient

Patients are afraid of the harmful effects of X-rays in computed tomography. Their anxiety can be reduced by comparing diagnostic X-rays with a natural radiation background. Naturally, the patient should have the impression that they are taken seriously and understand his anxiety. Otherwise, trust in the doctor is under threat.

Many patients are helped by the knowledge that they can communicate through an intercom device with an X-ray lab assistant in the control room and that the study can be suspended or terminated at any time if an unforeseen situation arises. Patients with claustrophobia feel more comfortable if they close their eyes while scanning. In very rare cases, you may need to use light sedatives.

Breath

Before starting the study, the patient is informed of the need to control breathing. With traditional computed tomography, the patient is explained that before each new cut, it is necessary to inhale and hold his breath for a few seconds. With spiral computed tomography, breath holding is required for 20 to 30 seconds. If the patient can not hold his breath, the movements of the diaphragm will lead to an unclear image with a clear deterioration in image quality. When examining the neck, swallowing movements worsen image quality even more than breathing.

Removal of metal objects

Naturally, before the study of the head and neck, to avoid the appearance of artifacts must be removed jewelry and removable dentures. For the same reason, computer tomography of the thoracic or abdominal cavity should be removed clothing with metal hooks, buttons and zippers.

[1], [2], [3], [4], [5]