All of the examination designated as special or advanced procedures require the introduction of some type of material into the area of interest to provide contrast. Contrast media are also known as contrast agents are the substances that can be used to highlight areas of the body in radiographic contrast to their surroundings tissue. This is necessary because the differences in density among the various tissues in the body are too small to provide adequate contrast for visualization of anatomic details. To compensate for this, it is necessary to increase or decrease the density of the organ to provide the desire contrast. There are numerous types of radiographic contrast media employed in medical imaging, which have different applications depending upon their chemical and physical properties. Contrast media can be administered by injection, insertion or ingestion when using it for imaging purpose. Contrast media are generally classified as negative and positive contrast agents. Negative contrast agent decreased the attenuation of the x ray beam and produces areas of increased density on the radiograph. Positive contrast agents increase the attenuation of the x ray beam and produces areas of decrease density on the radiograph.
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General types of contrast media are divided into two. There are radiolucent (negative) and radiopaque (positive). Radiopaque (positive) contrast media will absorb x ray photons because these media are opaque to x rays. This is because, the anatomic areas filled by these agents appear light (decreased density) on radiograph. They are also called positive contrast agents. This media are composed of high atomic numbers. Barium and iodine based solutions are used in medical imaging to produce positive contrast. For the radiolucent (negative), the x ray photons are very easy to transmitted or scattered through this media. These media are relatively lucent to x-rays. This is because, the anatomic areas filled by these agents appear dark (increased density) on the radiograph and also called negative contrast agents. This media composed of elements with low atomic number. Both positive and negative contrast agents can be employed together in double contrast examination. Double contrast is used primarily in the alimentary tract but is also used in arthrography of the joints. This positive contrast medium is used to coat the walls of the cavity and the negative contrast medium, in the form of gas is used to distend the area being imaged. Double contrast examination permits optimum visualization by producing a high inherent contrast whilst allowing adequate penetration of the area under examination.
Negative contrast media
Negative contrast media are composed of low-atomic-number elements and are administers as gas (air) or gas producing tablets, crystals, or soda water (carbon dioxide). This is because cell absorb oxygen quickly, this gas is rarely used alone as a contrast agents. Air is introduced by the patient during a radiographic examination. The example is inspiration during chest examination or can also be introduced by the radiographer as part of the examination in a double contrast barium enema. Oxygen introduced into cavities of the body for example in the knee when performing the arthrogram to demonstrate the knee joint. Carbon dioxide introduced into the gastrointestinal tract in conjunction with the barium sulphate solution to demonstrate the mucosal pattern example in double contrast meal. Normally, complications from the admirations of negative contrast agents are minimal. Although the air cause emboli. These small air masses can enter the circulatory system and become lodged in blood vessels, causing pain and loss of oxygen to the area. Patients who receive barium sulphaate with air should be instructed to drink plenty of fluids after the procedure. This way can dilute and eliminate the barium sulphate. Administration of water soluble iodine contrast media along with air in the joint spaces usually does not result in complications.
Example of positive contrast media
Barium and iodine solutions are used to create positive contrast on images. Barium is used because it has an atomic number of 56, thus it is radiopaque. Barium sulphate is and inert powder composed of crystals that are used for examination of the digestive system. The chemical formula for barium sulphate is BaSO4. Barium sulphate, BaSO4 is called as compound because it chemical formula indicates a ratio of one atom of barium to one atom of sulfur to four atoms of oxygen. Barium sulfate must be mixed or shaken into a suspension in water because it is insoluble in water. Barium sulpahet, BaSO4 suspensions must be concentrated enough so that X rays are absorbed. These suspensions must flow easily and yet coat the lining of organ. The concentration of barium in the solution is normally stated as a percentage weight to volume ratio (w/v). The densities of the barium solution is dependent upon the weight volume because a 100% w/v solution contains 1 g of barium sulphate per 100 mL of water. There are many types of barium suspension available and the type used dependent on area of gastrointestinal tract being examined. Positive contrast media also can be classified into subcategories likes iodine based, non-iodine based and others.
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Iodine based contrast media are water soluble. It is the largest group of contrast media in which molecules of iodine are the opaque agents. These compounds contain iodine atom and iodine atomic number is 53 which is bound to carrier molecule. Water soluble type contrast media are usually used for examination like intravenous urography, angiography and for contrast enhancement in computerized tomography. For this contrast type, it can be divided into other 2 subcategories either ionic contrast media or non-ionic contrast media and ionic and non-ionic contrast will be divided into ionic monomers, nonionic monomers, ionic dimers and non-ionic dimers.
Ionic monomer is high osmolar contrast media (HOCM) which the basic molecule of all water soluble, iodine containing contrast media is the benzene ring. Benzene is not water soluble so to make it soluble, carboxyl acids (COOH) is added. The three hydrogen left in the molecule is replaced by the iodine, rendering it radiopaque but still remains quite toxic. To make the compound less toxic and more acceptable to the body, the remaining two hydrogen (R1 and R2) are replaced by a short chain or hydrocarbon.
Ionic dimer is low osmolar contrast media (LOCM). A contrast medium therefore needed with reduced osmotic effects. The higher the effect ratio, the lower the osmolality of the contrast media. To increase the effect ratio, and produce contrast medium with lower osmolality can be achieve by linking together two conventional ionic contrast media molecule. This will resulting dimeric ionic contrast medium was an improvement on the high osmloar contrast media (HOCM). By reducing the osmolality, it will make the contrast more tolerable for the patients.
Non-ionic monomers or low osmolar contrast media (LOCM).these are low osmolar agents and do not dissociate into two particles in a solution, making them more tolerable and safer to use than ionic contrast. For each three iodine molecules in a non-ionic solution, one neutral molecule is produced. The advantages are the negative carboxyl group is eliminated thus decreasing the neurotoxity and the elimination of the positive ion reduces osmolality. Non-ionic LOCM is used for intracheal and vascular procedure.
Non-ionic dimers are dimeric non-dissociating molecules. For each one molecule there are six iodine atoms. They are isotonic because, the contrast solution has similar osmolality with the blood plasma. The osmolality combined with a slower diffusion of the larger molecules across vessel walls from the vascular space, plays significant role in imaging venous phase images following arterial injections.
Before the procedure is carried out, verify the patient identity. Make sure the person you call is the right person. Then greets patients respectfully and introduces you. Read back the form to confirm the examination. Check whether all the pre-procedural requirements (located in patient chart) have been carried out or not. Pre-procedural requirements examples are bowel preparation, fasting fluid restriction, establishing access, hydration and documentation of the baseline vital signs (heart rate, blood pressure, respiratory rate, blood oxygen saturation) before the administration of contrast media. Then, try to check the medical history of the patient by asking them or check their previous medical report. Try to check back their medical report on the renal function status, their bleeding tendencies, what medication they took now, and previous contrast media reaction. History of allergy about the patient is important to avoid any complication when the contrast media is injected into their body. Then the contrast can be injected into the patientâ€™s body. Watch the patient for adverse reactions. Apply pressure to injection site after procedure is completed.
Effect of contrast media
They will be some effect when using contrast media. But, this effect can be prevented. The first effect is general effect. Water soluble iodine contrast media have known physiologic effect. High osmolality and aspect of chemical structure are the major characteristic for water soluble media that are responsible for this effect. Most of ionic agents are higher-osmolality contrast media and therefore have shown greater effects and adverse reaction. Viscosity of the media is influenced by the concentration and size of the media. Heating the media to body temperature will reduces the viscosity and facilitates the ability for rapid injection. The second one is osmotic effects. Because of the ionic media dissociate in water, their injection onto the blood plasma results in a great increase in the number of particles present in blood plasma. This has the effect of displacing water. When the contrast media is displace the plasma water, water from the body cell move to vascular system. This result in hypervolemia and blood vessel dilatation, with pain and discomfort. It is very important to obtain patients medical report to convey any contraindications to the radiologist and other physician.
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Anaphylactoid is allergic reactions to water soluble iodinated contrast media resemble allergic reactions to the foreign substances. Some patients may experience wheezing and edema in the throat, lungs with accompanying bronchospasm, nausea, and vomiting. These reactions are thought to be caused by the release of substances call histamine. Premedication with steroids and athihistamines can reduce or eliminate allergic effect. The fourth one is renal effect. High osmolality contrast media can cause the arteries off the kidney to expend as the result of the osmotic effect. Arterial expansion results in the release of vasoconstrictors that cause constriction of the renal arteries. The end result is diminished blood supply to the kidneys. The last one is other effect. Carotid injection of water soluble iodine contrast media can alter the blood-brain barrier by causing the capillary cells to shrivel because of water loss. Clinical symptoms of the effect are increased blood pressure, slow heartbeat (bradycardia) and fast heartbeat (tachycardia). In patients with sickle cell anemia and those who carry the trait but who are asymptomatic, injection of high osmolality contrast media can cause red blood cell to shrink. It may trapped in small-diameter blood vessel and capillaries, causing pain and blood clots. All of this effect can be classified into their severity; mild ,moderate and severe.
Contrast media can be divided into positive and negative varieties. Negative contrast agents silhouette the anatomy against a radiolucent background, whereas positive contrast agents produce areas of decreased density on the finished radiograph. Commonly used negative contrast agents are air, oxygen and carbon dioxide. The positive contrast agents are primily substituted benzoic acid compounds. They all use the iodine to provide the contrast in the organ, but they vary in the number of particles that provide osmolality of the substances. There are divided into ionic and non-ionic in nature. There is a category of iodinated compounds that are considered to have low osmolality than the purely ionic agents. Non-ionic compound have been proved to cause fewer reactions than other varieties. Reactions contrast media injection cannot be predicted. An emergency drug cart should be present in the special procedures, CT or MRI suite. A common complication during injection of the contrast agent is extravasation; prevention is the best tool for reducing its occurrence. A department protocol should be established for patientâ€™s evaluation and for monitoring and reporting any incidents that might occur.