When doing a certain imaging test, after the plain scan is done, some, depending on the condition, have to do an enhancement test, where a drug is injected into the blood vessels and then scanned. This drug is the contrast agent in imaging examinations, also known as contrast agent. There are many different kinds of contrast agents and different ways of introducing them into the body, and the one that people are most worried about and have side effects is the one that is injected into the blood vessels.
As the name implies, they are used to increase the black and white contrast of the image of the lesion area, which helps to show the lesion more clearly. Another main purpose of some contrast agents such as those used in CT or MRI (i.e., MR) is to reflect certain pathological characteristics of the lesion area, such as the blood supply, internal composition and structure of the lesion, and the integrity of the blood-brain barrier, in order to determine the nature of the lesion.
(Note: Special drugs: are specifically the radiopharmaceuticals used in nuclear medicine imaging, such as iodine-131, technetium-99, fluorine-18, etc.. These drugs belong to the special drugs strictly managed by the relevant state departments. They are safe for patients to use, and their radioactive half-life is very short, so they do not cause any harm to the examinees. (We will not talk more about these drugs here)
Types of contrast agents.
Air, CO2, water, fatty milk, barium sulfate, iodized oil, water-soluble iodine preparations, gadolinium preparations, iron oxide, etc. Depending on the application, there can be: gastrointestinal contrast agents; water-soluble iodine preparations for DSA or CT; gadolinium preparations for nuclear magnetic (MR).
Routes of introduction into the body.
Direct oral administration of contrast agents.
1. Barium sulfate suspension for contrast of the gastrointestinal tract, safe and without side effects. For example, barium meal imaging of the gastrointestinal tract.
2. Iodine preparations, such as compounded pantethine, are commonly used to fill the gastrointestinal tract orally half an hour before CT scanning to reduce gastrointestinal artifacts and increase contrast.
Direct infusion of contrast agent.
1. Water-soluble iodine preparations or iodine oil, water or gas are injected through the catheter for relevant imaging. For example, retrograde pyelography, transendoscopic cholangiopancreatography, sinus tractography, and hysterosalpingography.
2. Transanal perfusion for intestinal imaging, for example, barium gas colonography, CT or MR colonic water angiography, etc.
Intra-arterial injection of contrast agent.
Intra-arterial injection of contrast into the artery via catheter, using DSA method for arteriography or imaging before interventional treatment. For example, abdominal aortography, coronary angiography, etc. It can also be used for CT angiography.
Intravenous contrast injection.
Injection of contrast into the vein is currently used for intravenous pyelography, enhancement scans for CT or MR, CT angiography, and MR angiography.
Safety issues of contrast agents.
1. Iodine preparations.
The local injection area can have short-lived pain caused by drug irritation or extravasation (local reaction: phlebitis, treated with topical magnesium sulfate hot compresses). Here mainly refers to the presence of various adverse or allergic reactions to intravascular injection of contrast agents, commonly seen in iodine preparations. For example, angiography by DSA, CT-enhanced scan, CT angiography. In mild cases, various allergic reactions occur, such as skin scratching, skin paresthesia, sneezing, headache, nausea, vomiting, laryngeal edema, and in severe cases, anaphylaxis or even death. Atypical manifestations, such as abdominal pain, have also occurred. Therefore, an iodine allergy test is required prior to drug administration. Statistically, the chance of an allergic reaction with intravascular injection of iodine preparations has been reported to be 3-10%; symptoms usually appear 10 seconds to 20 minutes after drug administration. The mortality rate related to contrast agents is 1/205,000 examinations (mainly intravascular administration).
In recent years, the use of non-ionic iodine preparations has been advocated, which reduces the chance of allergic reactions, but is more expensive. Patients are especially requested to provide their physicians with a history of drug allergies and whether they are allergic in time to draw attention to them earlier.
Another side effect is the nephrotoxic effect of iodine preparations, which is contraindicated in patients with severe renal insufficiency. Otherwise, it can cause acute renal failure.
2. Contrast agents used in nuclear magnetic (i.e. MR).
The most widely used is ionized non-specific extracellular fluid contrast agent (gadolinium preparation), namely Gd-DTPA, the Chinese name for diethylenetriaminepentaacetic acid gadolinium or gadopentetate glucosamine salt, the trade name of Magenvexian (Magnevist).
The conventional dose of Gd-DTPA for clinical application is 0.1 mmol per kg of body weight, and the maximum permissible FDA dose is 0.3 mmol per kg of body weight.
Gadolinium preparations, unlike CT iodine preparations, are paramagnetic drugs and their respective principles of enhancement are different.MR contrast agents are rarely associated with serious allergic reactions.Gd-DTPA is a very safe contrast agent, with a safety factor (LD50/effective dose) of up to 200 according to the routinely applied dose, compared to 8-10 for iodine contrast agents.The incidence of side effects with Gd-DTPA is very The incidence of side effects of Gd-DTPA is low, with 1.5%-2.5% reported in the literature, mostly manifesting as dizziness, transient headache, nausea and vomiting, and skin rash. The incidence of serious adverse reactions is very low, about 1-2 per million, and most of those with serious reactions have a history of respiratory diseases or allergies. The mechanism of the occurrence of Gd-DTPA side effects is still unclear. The risk factors for Gd-DTPA side effects and their prevention and management are similar to those of water-soluble iodine-containing contrast agents. Several manufacturers have developed and introduced non-ionic extracellular fluid MR contrast agents.
After more than 20 years of application, only one case of allergic death due to MR contrast agent injection was reported in 2006 in China. In addition, the toxic effects on the kidney are also mild. Therefore, allergy testing is not required in clinical applications. Nevertheless, patients with a history of drug allergy or hypersensitivity should inform their doctors in advance.
The FDA issued a notice on its website in May 2007 requiring companies to include a new black box warning for all gadolinium-containing magnetic resonance imaging contrast agents. The warning is that a new condition, nephrogenic systemic fibrosis or nephrogenic fibrosing dermatosis, may develop after use in people with severe renal insufficiency
(Nephrogenic Systemic Fibrosis, NSF/NFD). The mechanism is not well understood, but is still thought to be related to renal damage.
Allergic reactions to contrast agents can occur late, called delayed allergic reactions, and a case of allergic death reported in China in 2006 was an allergic reaction that started 2 hours after drug administration. Therefore, when adverse reactions occur within a few hours after the use of contrast media, especially if they cannot be relieved or aggravated, the patient should immediately go to the hospital for treatment.
3.Contrast agent used for ultrasound examination
Ultrasound contrast agent (Ultrasound contrast: agent), referred to as UCA, is a class of chemical agents that can significantly enhance the intensity of ultrasound backscatter. Its main component is microbubbles, usually 2-10 um in diameter, which can pass through the pulmonary circulation.
Ultrasound contrast agents are extremely safe, with a very low incidence of side effects, much lower than CT or MR, and no cardiac or renal toxicity. However, UCA cannot be used in pregnancy, and is contraindicated in some countries for women who are breastfeeding.
4. Regarding the use of contrast agent family signature.
Regarding family signature, most of the current ones refer to iodine-containing contrast agents for CT enhancement, and some hospitals also require family signature before using gadolinium contrast agents for MRI. This is a requirement for medical procedures, and families do not need to be nervous, but if the patient is allergic or has kidney function problems, it should be raised early. When allergic reactions (mainly to iodine preparations) occur during the use of contrast media, emergency medications are available on site and a special nurse is in charge of handling them. Late reactions to contrast agents also occur, that is, allergic reactions do not occur at that time, but several hours after the examination, or after the patient goes home. Once the above-mentioned allergic manifestations occur, the patient should go to the hospital without delay.