Iodinated contrast agents are universally used in different radiology studies because they are water soluble, easy to administer intravascularly, and have a high safety index. Over the years many different agents other than iodine have been tested for intravenous (IV) use, but none have become commercially available for widespread clinical use because of unacceptable toxic side effects.
Properties of Iodinated Agents
Intravascular contrast material had high attenuation compared with human soft tissue, and therefore, wherever it distributes it increase the ability of the enhanced structure to attenuate the xray beam. Because of their relatively high atomic number of 53, the iodine atoms in the contrast material are responsible for this increase in attenuation. Adding an iodinated contrast agent to the bloodstream will temporarily increase the beam attenuation ability of the blood; structure with an adequate blood supply show an increase in attenuation, which is displayed as a change from darker to lighter on the image.
Two (2) tissues must differ by at least 10HU to be visible different on a CT scan images. Because different tissues often enhance differently, and became intravascular contrast material is handled differently in normal versus abnormal tissue, contrast agents can serve to widen the inherent difference in attenuation. This difference of the allows tissues, tumors and disease processes to be more easily discernible. The proper administration of contrast media can easily provide a 40 – 70 Hounsfield Units (HU) increase in the natural difference of attenuation between tissues, thus making them visibly different in the image.
Intravascular (IV) contrast material can serve to widen the difference in attenuation between adjacent structures because different tissues often enhance differently and because intravascular contrast material is handled differently in normal versus abnormal tissue.
Contrast Agent Vs Other Pharmaceuticals
Contrast agent administration varies significantly from other pharmaceuticals that are administered intravascularly. Unlike other medications, iodinated contrast media are not used for their therapeutic qualities, but rather for their distribution and elimination from the human body. The difference is even more dramatic when dose and delivery are considered. Therapeutic agents are given in very small quantities at regularly spaced intervals, whereas relatively large quantities of contrast media are typically given in a bolus lasting only a minute to two, with the intention of having no untoward physical effects. To illustrate the difference, consider the use of morphine sulfate, given specifically to alleviate pain, at a typical dose of 2 to 10mg diluted in 5 to 15mL of sterile water, and given at regular intervals of 4 hours. Compare this with iodinated contrast agents that are given in a single dose, typically between 100 and 150mL, and often delivered in less than a minute. Because of these dramatic differences, guidelines for the use of other pharmaceuticals are of little value when applied to the use of IV contrast agents.
Osmolality
Most intravascular drugs are nearly isotonic, that is, they have nearly the same number of particles in solution per unit of liquid as blood. Contrast agents may have up to seven times the number of particles in solution per unit of liquid as blood. The structural in solution is known as osmolality. Osmolality is measured in milliosmole per kilogram (mOsm/kg) of water. The osmolality of blood plasma is approximately 290mosm/ kg.
Osmolality is a property of intravascular contrast media that refers to the number of particles in solution, per unit liquid as compared with blood.
- High osmolality contrast media may have as much as seven times the osmolality of blood.
- Low osmolality contrast media has roughly twice the osmolality of blood.
- Contrast media that is isosmolar has the same osmolality as blood.
Most brands of iodinated contrast medium have a greater osmolality than blood plasma. Therefore, most contrast agents are said to be hyperosmolar or hypertonic solutions. Older iodinated agents, now less commonly used for intravascular injections, are considered high-osmolality contrast media HOCM. The osmolality of these agents range from approximately 1300 of 2140 mOsm / kg or about 4 to7 times that of human blood.
Viscocity of Contrast Used
Contrast material is much more viscous than most other intravascular (IV) agents. Viscosity is a physical property that may be describe as the thickness or friction of the fluid as it flows. It is an important property that will influence the injectability of intravascular agents through small bore needles and IV catheters. Molecular structure and concentration affect viscosity; therefore, different brands of iodinated contrast media will possess varying viscosities. The viscosity of the contrast material can be significantly decrease by heating the liquid to body temperature for injection. Warming a contrast agent reduces viscosity in a way similar to warming maple syrup. Finally, the concentration of iodine in the agent affects it viscosity; the higher the concentration of iodine, the more viscous the contrast media is.
Ionicity
Intravascular (IV) contrast agent can be classified as to whether the molecules they contain will separate in charged particles like ions, when dissolved in an aqueous solution. Ionic contrast agents are composed of molecules that will dissociate into ions when in solution. The molecules contained in nonionic contrast media do not dissociate. Although most nonionic contrast agents also have low osmolality, the two terms are not synonymous.
Hexabrix (Malinckrodt, Inc., St. Louis MO) Contrast Agent
Hexabrix is an example of contrast agent that is low osmolality but ionic. On the table below here are the other brand of contrast agent along with their osmolality, viscosity and whether they are ionic or nonionic.
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| Contrast Media Characteristics |
Viscosity is a physical property of intravenous (IV) contrast media. It can be described as the thickness of friction of the fluid as it flows. The brand, temperature and concentration of the contrast affect it viscosity.
In solution, ionic contrast agent contains molecules that will form ions. The molecules contained in nonionic contrast medial to do disassociate. Although most ionic contrast media also have low osmolality, the term are not synonymous.
Clearance of Contrast Media
Once the contrast media is injected, all type of iodinated contrast media undergo very rapid distribution throughout the entire extracellular space. They are not metabolized and are excreted by the body nearly exclusively by the kidneys via glomerular filtration. The half life like time it take for half of the dose to be eliminated from the body, in patients with normal renal function is approximately 2 hours.
Dose of Iodinated Contrast Media
To accurately assess the dose of iodinated contrast agent to be delivered, both the iodine concentration and the volume must be considered. The beam attenuation abilities of a given amount of contrast media are directly related to the concentration of iodine. Many concentrations are commercially available. Low Osmolality Contrast Media (LOCM) are measured in milligrams of iodine per milliliter mgI/mL of solution, whereas most High Osmolality Contrast Media (HOCM) are labelled in terms of their percent weight per volume, but the concentration of iodine in mg/mL is easily found in tables or on the via label. When comparing doses between different contrast concentrations and volumes, it is often useful to look at the total grams of iodine delivered.
CT Scan Protocols on Iodinated Contrast Agent
Different CT scan protocols require different doses of iodine. These will be listed in the solution that deals will imaging protocols. In some instances, the injection rate and the delay from injection to scanning will affect the contrast dose selected for the examination. In addition, the iodine concentration of the contrast will affect the selection of injection flow rate. This is because contrast enhancement in CT depends on the iodine concentration depends on the injection rate of iodine on mg/s. Therefore, a concentration of 400mg/mL injected at 3mL/s will provide the same enhancement as concentration of 300mg/mL injected at 4mL/s.
Undoubtedly, an adequate dose of iodinated contrast is an essential component on producing examinations of consistently high quality. The dose must also be safe for the patient. Although they rarely occur, an overdose of iodinated contrast media is possible. Actually, death have occurred as a result of volumes of 250 to 300mL of undiluted, High Osmolality Contrast Media (HOMC) ionic contrast media.
Over Dosage on Contrast Agents
The adverse effects of overdosage effect mainly the pulmonary and cardiovascular systems. Because of the risk of overdose, most facilities set guidelines as to the upper limit on the total volume of contrast media given all at once for routine examinations, regardless of the patient’s size. Individual factors, such as a patient’s level of hydration, can influence what is a safe dose. Therefore, this upper limit is often quite cautious, typically 200mL of an agent with a concentration of 320mgI/mL it is a total of 64 grams of iodine. It should be noted, however, that specific circumstances may necessitate exceeding this 200mL guideline. A radiologist is responsible for determining in what instances, and to what extent, a facility’s guidelines can be safely exceeded.
Regardless of the type of iodinated contrast agent, the lowest dose necessary to obtain adequate visualization should be used. A lower dose may necessary to obtain adequate visualization should be used. A lower dose may reduce the possibility of an adverse reaction, particularly those affecting renal function. Seldom does a CT procedure require either the maximum volume of the highest concentration of an iodinated contrast.
Factors should be individualized accounting of combination of volume and concentration to be used:
- Age
- Body weight
- Size of the vessel into which it will be injected
Other Factors Includes:
- Anticipated Pathology
- Degree and Extend of opacification required
- Structure or Area to examined
- Disease processes affecting Patient
- Specific equipment available
Contrast Media Dosage on Pediatric and Adult Patient
In most clinical practices, the dose used to perform CT examination on pediatric patients is calculated by weight. The most common formula used is 2mL/kg. It is interesting to note that formula are typically abandoned when it comes to scanning adults, and the same dose is given whether the patient is a 100 pound woman or a 350 pound man. Table below demonstrate the dose variation, in mL/kg, when a uniform dose of contrast agent is given to all adult patients. In this example, a 100 lbs woman would receive more than 3 times the dose per kg as the 350lbs man.
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| Examples of Dose Variation When a Uniform Contrast Media Dose Is Used on All Adult Patients Patient Weight in kg (lb) mL |
It is unclear how the practice of administering a uniform dose of an IV contrast media, regardless of the size of the adult patient, because so well established. Did the practice arise from the convenience of using the same dose for each patient? Ultimately, the important question is whether this system is best for the patient. Does a protocol of uniform dosing result in under dosing the heavier patients and overdosing the lighter one? Studies have been conducted that compared the examination quality from uniform dosing protocols to those that result when a weight based system is used. The result of these studies suggest that the quality of examinations are the same, or better, when a weight based dosing protocol is used. An example of a weight based calculation for routine body scanning is 1.5mL/kg not to exceed 200mL. Furthermore, when compared with using a standard dose of 150mL, a significant cost saving can be realized using a weigh based dose this are provided prefilled syringes are not used.
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