Use of contrast agents

Oral administration of contrast agents

In CT scanning of the abdominal cavity and pelvic organs, it is very important to clearly differentiate intestinal loops from adjacent muscles and other organs. This problem can be solved by contrasting the intestinal lumen after oral administration of a contrast agent. For example, without a contrast agent, it is difficult to distinguish the duodenum from the head of the pancreas.

The rest of the gastrointestinal tract is also very similar to the surrounding structures. After taking oral contrast, the duodenum and pancreas become clearly visible. To obtain the best quality image, the contrast agent is taken orally on an empty stomach.

Selecting the Right Contrast Agent

Better mucosal coating is achieved with barium sulfate, but it is not water-soluble. Therefore, this oral contrast agent cannot be used if surgical intervention is planned that involves opening the intestinal lumen, such as partial resection with anastomosis, or if there is a risk of intestinal damage. Also, barium suspension cannot be used if there is a suspicion of a fistula or perforation of intestinal loops. In these situations, it is necessary to use a water-soluble contrast agent, such as Gastrografin, since it is easily absorbed when it enters the abdominal cavity.

For better evaluation of the stomach walls, plain water is often used as a hypodense contrast agent, with buscopan administered intravenously to relax the smooth muscles. If the bladder is removed and a reservoir is created from the ileum, the abdominal cavity is first examined with intravenous administration of a contrast agent, which is excreted with urine into the reservoir and does not enter other parts of the intestine. If it is necessary to study other parts of the gastrointestinal tract, additional scanning is performed after taking the contrast agent orally.

Time factor

To fill the proximal sections of the gastrointestinal tract, 20-30 minutes are sufficient. The patient drinks the contrast agent on an empty stomach in small portions in several doses. If it is necessary to fill the colon and especially the rectum with barium sulfate, at least 45-60 minutes may be required. A water-soluble contrast agent (for example, gastrografin) moves through the intestines somewhat faster. When examining the pelvic organs (bladder, cervix, ovaries), rectal administration of 100-200 ml of contrast agent guarantees their clear demarcation from the rectum.

Dosage

To contrast the entire gastrointestinal tract, 250-300 ml of barium sulfate suspension must be thoroughly mixed with water, bringing the volume to 1000 ml. If it is necessary to use a water-soluble preparation, 10-20 ml of gastrografin (in 1000 ml of water) is sufficient for a full examination of the gastrointestinal tract. If it is necessary to contrast only the upper sections of the gastrointestinal tract, 500 ml of any oral contrast agent will be sufficient.

Intravenous administration of contrast agents

Increased blood vessel density not only allows for better differentiation of blood vessels from surrounding structures, but also helps to assess perfusion (accumulation of contrast agent) of pathologically altered tissues. This is important in cases of blood-brain barrier disruption, assessment of abscess boundaries, or non-homogeneous accumulation of contrast agent in tumor-like formations. This phenomenon is called contrast enhancement. In this case, signal amplification occurs due to accumulation of contrast agent in tissues and the associated increase in their density.

Depending on the clinical task, before intravenous administration of the contrast agent, the area of interest is usually scanned without contrast enhancement - native scanning. When comparing normal and enhanced images, the assessment of vascular grafts, inflammatory changes in bones and the abscess capsule is simplified. The same technique is used in traditional CT examination of focal liver lesions. If spiral CT of the liver is used, the venous phase of contrast agent perfusion can be used as an analogue of the image without enhancement for comparison with the early arterial phase. This makes it possible to detect even small focal lesions.

Intravenous administration of contrast agent

Contrast agents are administered intravenously in such a way that the bolus (high concentration) in the vessels is maintained as long as possible before it is diluted in the pulmonary circulation. Therefore, to achieve a sufficient degree of vascular enhancement, contrast agents should be administered quickly (2-6 ml/s). Intravenous cannulas with an outer diameter of at least 1.0 mm (20G) are used, but 1.2-1.4 mm (18G, 17G) is better. It is very important to ensure that the cannula is correctly positioned in the lumen of the vessel. Before administering the contrast agent, a test injection of sterile saline solution is administered intravenously at the same rate. The absence of subcutaneous swelling at the puncture site confirms the correct positioning of the cannula. This also confirms the possibility of passing the required amount of contrast agent through the punctured vein.

Dosage

The dose of the contrast agent is calculated based on the patient's body weight and the diagnostic task. For example, the concentration of the contrast agent in the examination of the neck or aortic aneurysm (to exclude its dissection) should be higher than in the CT examination of the head. In most cases, good quality contrast is achieved by administering 1.2 ml of the agent per 1 kg of the patient's body weight with an iopromide concentration of 0.623 g/ml. This allows achieving a combination of optimal vascular contrast and good tolerability of the contrast agent.

The phenomenon of influx

The image of the superior vena cava lumen may show enhanced and non-enhanced areas due to the fact that contrasted and non-contrast blood enters the vein at the same time. This phenomenon occurs due to the short time interval between the start of contrast administration and the start of scanning. The contrast agent is injected from one side and enters the superior vena cava through the axillary, subclavian, and brachiocephalic veins, where a filling defect is detected within the lumen. If one is unaware of the inflow phenomenon, one may mistakenly diagnose venous thrombosis. This artifact is more likely to occur when contrast concentrations are too high, especially with spiral CT. The inflow phenomenon will be analyzed in more detail on the following pages.

Effects of the initial phase of contrast

In the inferior vena cava at the level of the renal veins, the phenomenon of tidal flow can be seen. This phenomenon occurs due to the simultaneous visualization in the lumen of the vena cava of non-contrast blood flowing from the pelvic organs and lower extremities, and blood from the renal veins containing a fairly high concentration of contrast agent. In the initial phase of contrasting, the inferior vena cava below (caudal) the renal veins is hypodense compared to the descending aorta.

Just above the level of the renal veins, the lumen of the inferior vena cava in the central part remains without enhancement, and enhancement is determined parietal on both sides due to the contrast of blood flowing from the kidneys. If the kidney is removed or the renal veins flow into the inferior vena cava at different levels, contrast enhancement is determined only on one side. Such density differences should not be mistaken for thrombosis of the inferior vena cava.

The phenomenon of the tide

If we follow the lumen of the inferior vena cava towards the right atrium, then after other veins with contrasted blood flow into it, an additional phenomenon of tide appears. In the lumen of the hollow thing, areas of non-uniform density are determined, which arose as a result of turbulent flow movement and mixing of blood with and without contrast agent. This phenomenon does not last long, and after a short time the densities of the lumen of the inferior vena cava and the aorta equalize.

Specific features of spiral CT

If spiral scanning is started immediately after intravenous administration of contrast agent and the concentration of the agent in the axillary, subclavian and brachiocephalic veins is very high, then significant artifacts will inevitably appear on the image in the area of the upper aperture of the chest on the corresponding side. Therefore, with spiral CT of the chest, the examination begins from below and continues upwards (from the caudal to the cranial part). Scanning begins from the diaphragm with surrounding structures and, when it reaches the cranial part, the contrast agent is already sufficiently diluted in the pulmonary circulation. This examination technique allows avoiding artifacts.

Adverse reactions to the administration of contrast agents

Side effects from the administration of contrast agents are quite rare. Most of them appear within 30 minutes after the injection, and in 70% of cases - in the first 5 minutes. The need to observe the patient for more than 30 minutes arises only if he has risk factors. Usually, information about the possible occurrence of side effects is in the patient's medical history, and they receive appropriate premedication before the examination.

If, despite all precautions, after intravenous administration of a contrast agent the patient develops erythema, urticaria, itching, nausea, vomiting or, in severe cases, a drop in blood pressure, shock, loss of consciousness, then treatment measures should be started immediately according to the tables below. It should be remembered that the effect of antihistamines after intravenous administration does not occur immediately, but after a certain latent period. Severe reactions (pulmonary edema, convulsions, anaphylactic shock) when using modern X-ray contrast agents are very rare and, if they occur, require emergency intensive care.

All possible reactions to contrast agents observed in the patient should be recorded in his medical history. In this way, the radiologist, planning future studies, will be warned in advance about the patient's increased sensitivity to contrast agents.

Treatment of adverse reactions to the administration of radiocontrast agents

Hives

1. Stop administration of contrast agent immediately.
2. In most cases, no treatment is necessary.
3. Take orally or administer intramuscularly or intravenously an antihistamine: diphenhydramine (diphenhydramine) in an amount of 25-50 mg.

In case of severe urticaria and a tendency for the lesion to spread, an adrenomimetic is administered subcutaneously: adrenaline (1: 1,000) in an amount of 0.1 - 0.3 ml (= 0.1 - 0.3 mg) in the absence of contraindications from the heart.

Quincke's edema and laryngeal edema

1. Administer subcutaneously or intramuscularly an adrenomimetic: adrenaline (1: 1,000) in the amount of 0.1 - 0.3 ml (= 0.1 - 0.3 mg) or, if arterial pressure drops, adrenaline (1: 10,000) intravenously slowly 1 ml (= 0.1 mg). If necessary, the injection can be repeated, but the total dose of the administered drug should not exceed 1 mg.
2. Oxygen inhalation through a mask (6-8 l per minute). If after this therapy the symptoms of edema do not go away or continue to increase, a resuscitation team should be called immediately.

Bronchospasm

1. Oxygen inhalation through a mask (6-8 l per minute). Set up patient monitoring: ECG, blood oxygen saturation (pulse oximeter), arterial pressure level.
2. 2-3 inhalations of beta-adrenergic aerosol: metaproterenol (alupent), terbutaline (brethaire, bricanil) or albuterol (proventil, ventolin, salbutamol). If necessary, inhalations can be repeated. If inhalations are ineffective, adrenaline should be used.
3. Administer subcutaneously or intramuscularly an adrenomimetic: adrenaline (1:1,000) in the amount of 0.1 - 0.3 ml (= 0.1 - 0.3) mg or, if arterial pressure drops, adrenaline (1:10,000) intravenously slowly 1 ml (= 0.1 mg). If necessary, the injection can be repeated, but the total dose of the administered drug should not exceed 1 mg.

Alternative therapy:

Aminophylline (euphyllin) is administered intravenously by drip at 6 mg/kg of body weight in a 5% glucose solution over 10-20 minutes (loading dose), then 0.4 - 1 mg/kg/h (if necessary). Blood pressure should be monitored, since it may decrease significantly.

If bronchospasm cannot be relieved or blood oxygen saturation is below 88%, a resuscitation team should be called immediately.

Drop in blood pressure with tachycardia

1. Elevate the patient's legs to 60° or more, or place the patient in the Trendelenburg position.
2. Monitor: ECG, blood oxygen saturation (pulse oximeter), patient's blood pressure level.
3. Oxygen inhalation through a mask (6 - 8 l per minute).
4. Provide prompt intravenous fluids (saline or Ringer's solution)

If therapy is ineffective:

Adrenaline (1:10,000) is slowly administered intravenously in a volume of 1 ml (= 0.1 mg), unless there are contraindications from the heart). If necessary, the injection can be repeated, but the total dose of the administered drug should not exceed 1 mg. If the pressure cannot be raised, a resuscitation team should be called.

Treatment of adverse reactions to the administration of radiocontrast agents

Drop in blood pressure with bradycardia (vagal response)

1. Monitor: ECG, blood oxygen saturation (pulse oximeter), patient's blood pressure level.
2. Elevate the patient's legs to 60° or more while lying down, or place the patient in the Trendelenburg position.
3. Oxygen inhalation through a mask (6 - 8 l per minute).
4. Provide prompt intravenous fluids (saline or Ringer's solution).
5. Slowly administer 0.6 mg atropine intravenously. If the patient's condition does not improve, return to steps 2-4.
6. Atropine may be administered repeatedly, but the total dose should not exceed 0.04 mg/kg of adult body weight (2 - 3 mg).
7. The patient leaves the office only after blood pressure and heart rate have normalized.

Increased blood pressure

1. Oxygen inhalation through a mask (6 - 10 l per minute)
2. Monitor: ECG, blood oxygen saturation (pulse oximeter), patient's blood pressure level.
3. Nitroglycerin: 0.4 mg tablet under the tongue (can be repeated 3 times) or as an ointment (squeeze out a 1-inch (~2.54 cm) strip from the tube and rub into the skin).
4. Transfer the patient to the intensive care unit.
5. If the patient has pheochromocytoma, 5 mg of phentolamine should be administered intravenously.

Epileptic seizure or convulsions

1. Oxygen inhalation through a mask (6 - 10 l per minute)
2. It is necessary to administer 5 mg of diazepam (Valium) (the dose can be increased) or midazolam (versed) 0.5 - 1 mg intravenously.
3. If a longer lasting effect is required, a specialist should be consulted (usually intravenous drip administration of phenytoin (dilantin) is used - 15 - 18 mg/kg at a rate of 50 mg/min).
4. Monitor the patient, particularly with regard to oxygen saturation levels, due to possible respiratory depression due to benzodiazepine use.
5. If there is a need to intubate a patient, a resuscitation team should be called.

Pulmonary edema

1. Raise the body and apply venous tourniquets.
2. Oxygen inhalation through a mask (6 - 10 l per minute)
3. Slowly administer a diuretic intravenously: furosemide (lasix) 20-40 mg.
4. Morphine (1-3 mg) can be administered intravenously.
5. Transfer the patient to the intensive care unit.
6. Use corticosteroids if necessary.

Thyrotoxic crisis

Fortunately, this complication is very rare when using modern non-ionic iodine-containing drugs. Patients with a history of hyperthyroidism should block the thyroid function with a thyreostatic drug, such as perchlorate, before intravenous administration of KB. Mercazolil is also used to reduce thyroxine synthesis. In both cases, the effect of taking the drugs occurs in about a week. It is necessary to ensure the effectiveness of antithyroid therapy, for which the thyroid hormone level should be repeated.

If the patient's hyperthyroidism has an obscure clinical picture and is not recognized in time, the introduction of iodine-containing contrast agents can aggravate the disease and provoke a vivid clinical picture of thyrotoxicosis. In this case, the patient develops diarrhea, muscle weakness, increased body temperature, increased sweating, signs of dehydration, unmotivated fear and anxiety, and necessarily tachycardia. The main problem in this situation is the long latent period before the vivid manifestation of thyrotoxic crisis.

Delayed iodine-induced hyperthyroidism develops in some patients with latent hyperthyroidism or suffering from other thyroid pathology (especially those living in iodine-deficient areas) 4-6 weeks after intravenous administration of a contrast agent, regardless of the ionicity and osmolarity of the contrast agent. No special treatment is required, and the symptoms disappear on their own after a certain period of time.

Patients with thyroid cancer who require intravascular or oral administration of iodinated contrast media (ionic or non-ionic) should be approached with particular care. This is because one week after administration of iodinated contrast media, thyroid uptake of I-131 decreases by an average of 50% and recovers after several weeks. Therefore, if treatment with radioactive iodine is planned, administration of iodinated contrast media (intravenously or orally) for diagnostic purposes may be contraindicated. In this case, additional consultation with the attending physician who prescribed the examination using the contrast media is required.