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Contrast media also known as contrast agents are substances used to highlight areas of the body in radiographic contrast to their surrounding tissues. Contrast media enhance the radioopacity and optical density of the area under investigation so that the tissue or structure absorption differentials are sufficient to produce adequate contrast with adjacent structures. There are numerous types of radiographic contrast media employed in medical imaging, which have different applications depending upon their chemical and physical properties. When used for imaging purposes contrast media can be administered by injection, insertion or ingestion.
In modern imaging department today images of patients are produced using either electromagnetic radiation or ultrasound. X-rays have a frequency and photon energy that are several orders of magnitude higher than those of visible light and they can penetrate the patient's body. Their photon energy is so large that they can break chemical bonds and induce ionization. X-rays can be detected on radiographic film and by different fluorescent materials.
The radio waves used in magnetic resonance imaging have a frequency and photon energy that are many orders of magnitude lower than those of visible light. Like X-rays they can penetrate the human body. They have not sufficient energy to induce any ionization, but they can make molecules vibrate, which means that they produce heat. These radio waves are detected by antennas.
Ultrasound consists of pressure waves of a much higher frequency than those of audible sound. Ultrasound energy is both produced and detected by piezoelectric crystals. Ultrasound propagates through the body and causes vibrations of molecules which again produce heat in the tissues.
All contrast media in diagnostic imaging have one task, to increase the differences between the different "voxels" in the body regarding their ability to absorb and/or reflect energy from electro-magnetic radiation or ultrasound. A "voxel" in this context may mean any structure, such as a piece or slice of normal tissue, or a complete organ, or a pathologic process or any other morphologic detail. Different contrast media influence electro-magnetic radiation or ultrasound by different mechanisms.
To perform their task the contrast media should reach different concentrations in different structures or "voxels". The larger the difference in contrast medium concentration between those structures, the smaller those structures (representing morphological details) can be while remaining detectable in the images.
A good contrast medium must influence electro-magnetic radiation or ultrasound energy inside the body, but should, ideally, not have any other effects on living tissue. Unfortunately, this is impossible and all contrast media have adverse effects.
Requirements of the Ideal Contrast Medium
Currently there is no contrast medium on the market that considered being ideal, but the ideal contrast medium should fulfil certain requirements for safe and effective application. It should be:
Easy to administer
A stable compound
Concentrated in the required area when injected
Rapidly eliminated when necessary
Of appropriate viscosity for administration
Tolerate with patient
Contrast media are divided into two main categories.
The first is negative contrast media which radiolucent or of low atomic number, causing the part in which it is placed to be more readily penetrated by X-rays than the surrounding tissue, as they attenuate the X-ray beam less effectively than body tissue, they appear darker on the X-ray image. Gases are commonly used to produce negative contrast on radiographic images. The second type is positive contrast media, these are radioopaque and are of high atomic number, causing the part in which it is placed to be less readily penetrated by X-rays than the surrounding tissue.
Consequently this contrast agent-filled area appears denser than the body tissue. Barium and iodine based solutions are used in medical imaging to produce positive contrast. Both positive and negative contrast can be employed together in double contrast examinations to produce a radiographic image. Double contrast is used primarily in the alimentary but also used in arthrography of joints. The positive contrast media is used to coat the walls of the cavity and the negative contrast, in the form of a gas, is used to distend the area being imaged. Double contrast examinations permit optimum visualisation by producing a high inherent contrast whilst allowing adequate penetration of the area under examination. Use of a small amount of contrast agent in conjunction with the distended cavity allows coating of the structures in the cavity or in the case of the alimentary tract, the mucosal lining which provides better detail of the area when the thin coating is shown in contrast to the gas-filled area, rather than using large amounts which may be dense enough to mask important information.
Negative Contrast Media
The following gases create negative radiographic contrast on images:
Air: introduced by the patient during a radiographic examination, example inspiration during chest radiography 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 examination in the knee when performing an arthrogram to demonstrate the knee joint
Carbon dioxide: introduced into the gastrointestinal tract in conjunction with a barium sulphate solution to demonstrate the mucosal pattern, example double contrast barium meal. For the barium meal it is formulated as affervescent powder example 'Carbex' granules or ready mixed carbonated barium sulphate (Baritop). Carbon dioxide can also be introduced into the colon when performing double contrast barium enema. It has been recommended that carbon dioxide be used as negative contrast agent in a double contrast barium enema, rather than air, as it causes less immediate abdominal pain as well as less postprocedural pain os discomfort. However, some studies have shown that carbon dioxides produces inferior distension and additional insufflations are required to maintain adequate quality distension. Carbon dioxide can also be used as an alternative contrast to iodinated contrast for diagnostic angiography and vascular interventions in both the arterial and venous circulation. The gas produces the negative contrast due to its low atomic number and low density compared with adjacent tissues
Positive Contrast Media
Barium and iodine solutions are used to create positive contrast on images.
Barium sulphate solutions used in Gastrointestinal Imaging
Barium solutions are the universal contrast media used for radiographic examinations of the gastrointestinal tract. The following characteristics of barium solutions make them suitable for imaging of the gastrointestinal tract:
High atomic number (56) producing good radiographic contrast
Excellent coating properties of the gastrointestinal mucosa
Barium suspensions are composed from pure barium sulphate mixed with additives and dispersing agents, held in suspension in water. Compound to stabilise the suspension are added, these act on the surface tension and increase solution viscosity. A dispersing agent to prevent sedimentation is added, ensuring an even distribution of particles within the suspension. Also added to suspension is a defoaming agent, employed to prevent bubbles that may mimic pathology in the gastrointestinal tract. Flavourings are usually added to oral solutions, making them more palatable for patients. The concentration of barium in the solution is normally stated as a percentage weight to volume ratio (w/v). A 100% w/v solution contains 1 g of barium sulphate per 100 ml of water; the density of the barium solution is therefore dependent upon the weight volume. There are many varieties of barium suspensions available and the type used is dependent upon the area of the gastrointestinal tract being imaged. It also depends greatly upon the individual preferences of the practitioner.
When preparing the barium solutions for administration it is important to check expiry dates and ensure the packaging is intact. Solutions administered rectally should be administered at body temperature to improve patient tolerability and also reduce spasm of the colon. It is important that the administrator knows the patient's medical history and check for any contraindications prior to administration. Barium sulphate solutions are contraindicated for the following pathologies:
Suspected fistula or to check an anastomosis site
Suspected partial or complete stenosis
Haemorrhage in the gastrointestinal tract
Prior to surgery or endoscopy
If the patient has had gastrointestinal wide bore biopsy (usually within 3-5 days)
Iodine-based Contrast Media used in Medical Imaging
The largest group of contrast media used in medical imaging departments are the water-soluble organic preparations in which molecules of iodine are the opaque agent. These compounds contain iodine atoms (iodine has number of 53), bound to carrier molecule. This holds the iodine in a stable compound and carries it to the organ under examination. The carrier molecules are organic, containing carbon, and are of low toxicity and high stability. Iodine is used as it is relatively safe and the K edge = 32 keV (binding edge of iodine K-shell electron) and is therefore close to the mean energy of diagnostic X-rays. The selection of kVp used for imaging examinations utilising iodine-based contrast plays a part in providing optimum attenuation. The absorption edge of iodine (35 kev) predicts that 63-77 kvp is the optimal range. The iodine-based compounds are divided into four groups depending upon their molecules structure; the four groups are:
Ionic monomers - high osmolar contrast media(HOCM)
The basic molecule of all water-soluble, iodine-containing contrast media is the benzene ring, carboxyl acid (COOH) is added. Three of the hydrogen's in this molecule are replaced by iodine, rendering it radiopaque but it still remains quite toxic. The remaining two hydrogen are replaced by a short chain of hydrocarbons, making the compound less toxic and more acceptable to the body. The exact natures of these compounds differ between different contrast media but are usually prepared as sodium or meglumine salts as these help to provide solubility.
Ionic compounds dissociate into charged particles when entering a solution. They dissociate into positively charged cations and negatively charged anions. For every three iodine molecules present in ionic media, one cation and one anion are produced when it enters a solution. Their effect ratio is therefore 3:2. These solution are highly hypertonic with an osmolality approximately five times higher than human plasma.
Ionic monomer (ratio1.5)
Ionic dimmers - los osmolar contrast media (LOCM)
A contrast medium was therefore needed with reduce somatic effects. As already stated, the higher the 'effect' ratio, the lower the osmolarity of the contrast media. An attempt was made to increase the 'effect' ratio and produced a contrast medium with lower osmolarity. This was achieved by linking together two conventional ionic contrast media molecules. The resulting dimeric ionic contrast medium was an improvement on the HOCM. Reduced osmolality (600 mOsm/kg H2O) made the contrast more tolerable for the patients. The ionic molecule still dissociated into two particles, a positive cation and a negative anion. However, there are now twice as many particles in solution with twice the osmolarity. Each molecule now carried six iodines (as opposed to three in the HOCM), hence there is an iodine atom to particle ratio of 6:2;so to achieve the same iodine concentration, only half the number of molecules are needed.
Non- ionic monomers (LOCM)
These are low osmolar agents and do not dissociate into two particles in the solution, making them more tolerable and safer to use than ionic contrast. For every three iodine molecules in a non-ionic solution, one neutral molecule is produced. Non-ionic contrast media are referred to as 3:1 compounds. They substitute the sodium and maglumine side chains with non-ionising radicals (OH)n. Two major advantages arise through the change in chemical structure: the first being that the negative carboxyl group is eliminated, decreasing the neurotoxicity and, the second, the elimination of the positive ion reduces osmolality to 600-700 mOsm/kg H2O.non ionic LOCM are used for intrathecal and vascular procedures.
Non-ionic monomer (ratio 3).
Non-ionic dimmers (isotonic)
These are dimeric non-dissociating molecules; for every one molecule there are six iodine atoms. The ratio is therefore 6:1, double the non-ionic monomers. They are isotonic, example the contrast solution has similar osmolarity to blood plasma (approximately 300 mOsm/kg H2O). Their isoosmolality, combined with a slower diffusion of the larger molecules across vessel walls from the vascular space, plays a significant role in imaging venous phase images following arterial injections (and arterial phase images following venous injections). These compounds represent a gold standard, water-soluble, iodine contrast medium.
Non-ionic dimmer (ratio 6).
Treatment for Adverse Reactions to Contrast Media
Mild reactions simply require careful observation of the patient. Most of the symptoms will pass within a few minutes post administration. Some schools thought have postulated that a great many mild adverse effects are the result of the patient's fear and apprehension. Mild adverse reactions are encountered in as many as 15% of patient after administration of intravenous ionic high-osmolar contrast media and up to 3% of patients after non-ionic, low-osmolar contrast media. Signs and symptoms of a mild reaction include:
A warm feeling that may associated with hot flushing
A metallic taste in the mouth
Pruritis (itching) and diaphoresis (sweating)
Treatment of mild reactions usually only involves observation of the patient and reassurance. Usually no medical treatment is required and the reaction does not interfere significantly with the examination procedure being undertaken.
This is a more severe reaction in which medical treatment is necessary and where the examination procedure is delayed or otherwise affected. Signs and symptoms of a moderate reaction include:
Facial swelling caused by oedema
Treatment of a moderate reaction may vary. Compression and tight clothing should be released and the patient reassured. The patient will need to be seen by a medic and the adverse reaction requires information to be entered in the patient's permanent medical record. All documentation should be completed according to department protocols. Drug therapy may be required, such as administration of antitihismine intravenously, or adrenaline 0.5 ml 1 : 1000 solution subcutaneously, to reduce the symptoms.
Seek medical advice immediately; medical treatment with hospitalisation is necessary. The examination is terminated. The management of severe adverse reactions, including drug treatments, should be handled by the resuscitation team. Signs, symptoms and effects of a severe reaction may include:
It is important that the radiographer recognises the significance of certain signs:
Pulmonary oedema - dyspnoea and cyanosis; the patient develops a cough with white frothy sputum, accompanied with dyspnoea
Anaphylactic shock - dramatic onset; pallor, sweating, nausea, syncope. A weak pulse due to hypotension, bradycardia or tachycardia may be observed. In sever cases, cardiac arrest may occur
Cardiac arrest - dramatic onset; absence of palpable pulse, dilated pupils, pallor, cyanosis
Respiratory arrest - abrupt onset of cyanosis with cessation of breathing
Cerebral oedema - leading to convulsions and possible coma
Administration of oxygen by mask (6-10 L/m) is vital and should be administered as soon as possible, as hypoxia may occur. Severe reactions require immediate recognition and evaluation of the patient's cardiopulmonary status. Cardiopulmonary resuscitation (CPR) equipment should be readily available in any area where contrast media are used. The radiographer should be trained in the techniques of CPR. Treatment of a severe reaction should follow the 'ABCD system':
Drug and definitive therapy
Contrast media should never be injected by anyone unfamiliar with resuscitation procedures. Radiology staff and management should continually review departmental protocols to ensure all staff are aware and are able to accomplish their roles should an event occur.