Iodinated Contrast Media (Part 2) Flashcards
Recap
Need for additional radiographic contrast.
Substances which when introduced into the body, demonstrate organs or systems with an enhanced difference in density to adjacent structures.
Classification:
Negative Agents (Radiolucent)
All gasses (air or CO2)
Positive Agents (Radio-opaque)
Barium based (non-soluble)
Iodine based (soluble)
Ionic
Non-ionic
Recap
Negative Contrast Agents
Air/CO2
Advantages of CO2
Double contrast technique
Positive Contrast Agents
Barium Sulphate
Recap
How contrast agents work
Linear attenuation coefficient
Proportional to:
the density of the medium
the number of atoms present in a given volume
Densities/Atomic numbers of Positive/Negative contrast agents
Recap
Barium Sulphate
Properties
Viscosity/Density
Complications
Contraindications
Pharmacology & Pharmacological Agents used with BaSO4
Buscopan
Glucagon
Maxalon
Classification
Negative Agents
Air or CO2
Positive Agents
Barium based
Iodine based
Ionic
Non-ionic
Iodine Based Positive Contrast Agents
Contain iodine atoms
Water soluble
Classification:
Ionic (High Osmolarity)
Non-ionic (Low Osmolarity)
Why Iodine?
Densities:
Iodine: 4940 kg/m3
Barium: 3600 kg/m3
Soft tissue: 1000 kg/m3
Atomic Numbers:
Iodine: 53
Barium: 56
Soft tissue: 7.5
The higher the density and the atomic number, the higher the attenuation coefficient and the more radiation is absorbed.
Ionic Contrast Agents
First type of iodine based contrast agent developed in the 1920s/30s.
Have undergone several iterations to reduce their adverse side effects.
Based on an ionic monomeric/dimeric salt of a tri-iodinated benzoic acid.
Ionic Contrast Agents
Based on an ionic monomeric/dimeric salt of a tri-iodinated benzoic acid
Ionic – dissociates into ions (negatively charged anions and positively charged cations)
Ionic Contrast Agents
Based on an ionic monomeric/dimeric salt of a tri-iodinated benzoic acid
Ionic – dissociates into ions (negatively charged anions and positively charged cations)
Monomeric/Dimeric – one part/two parts
Ionic Contrast Agents
Based on an ionic monomeric/dimeric salt of a tri-iodinated benzoic acid
Ionic – dissociates into ions (negatively charged anions and positively charged cations)
Monomeric/Dimeric – one part/two parts
Tri-iodinated – 3 iodine atoms
Ionic Contrast Agents
Based on an ionic monomeric/dimeric salt of a tri-iodinated benzoic acid
Ionic – dissociates into ions (negatively charged anions and positively charged cations)
Monomeric/Dimeric – one part/two parts
Tri-iodinated – 3 iodine atoms
Benzoic Acid – organic acid based on a benzene ring
Ionic Contrast Agents
Typical ionic, water-soluble, tri-iodinated contrast molecule.
The anion is the benzene ring with the negatively charged acid group attached.
The groups R2 and R3 aid in solubility and excretion.
The positively charged cations are sodium or methylglucamine. Some media contain both cations.
Ionic Contrast Agents
High Osmolarity
Osmolarity (Osmotic Concentration)
The concentration of a solution expressed as the total number of solute particles per litre.
Number of particles in a solution, per unit liquid, as compared to blood.
High osmolarity contrast media (HOCM) are approximately five to eight times the osmolarity of blood/serum.
Ionic Contrast Agents
High osmolarity effect of ionic contrast agents:
as contrast molecules are administered into a blood vessel, interstitial fluid will move across the vessel wall via the principle of osmosis
HOCM have a strong osmotic diuretic effect
Effects of Ionic HOCM
Loss of water from cellular and interstitial spaces causing localised heat and pain
Systemic effects such as:
vasodilatation
alterations to permeability of the vessel endothelium
hyperemia (increased blood flow)
osmotic diuresis
Ionic Contrast Agents
The iodine concentration of a contrast agent determines its radio-opacity.
The higher the iodine ratio, the better the opacification.
Radiographic efficiency and osmolarity both depend on the iodine atom : particle ratio.
Ratio = Number of Iodine atoms / Number of particles
Ionic Contrast Agents
Ionic monomeric agent:
Ratio = 3 iodine atoms = 1.5 / 2 particles
Ionic Contrast Agents
Ionic dimeric agent:
Ratio = 6 iodine atoms = 3/
2 particles
But…
The higher the iodine concentration, the more chance of an adverse reaction.
Fewer particles of contrast medium mean a lower osmotoxic effect.
An improved iodinated contrast agent was required.
In the 1970s/80s, non-ionic contrast media was introduced.
Non-Ionic Contrast Agents
The tri-iodinated benzoic acid anion was converted into an non-ionic molecule by replacing the carboxylic acid radical (COOH) with an amide group.
Non-Ionic Contrast Agents
Non-ionic monomeric agent:
Ratio = 3 iodine atoms = 3/
1 particle
Non-Ionic Contrast Agents
Non-ionic dimeric agent:
Ratio = 6 iodine atoms = 6/
1 particle
Non-Ionic Contrast Agents
Do not ionise in solution
Lower osmolarity compared to ionic agents (Low osmolarity contrast media – LOCM)
Latest ionic contrast agents are iso-osmolar having very similar osmolarity to that of blood/plasma (advantages over LOCM debatable)
- HOCM: >1400 mOsm/kg H2O
- LOCM: 600-800 mOsm/kg H2O
- IOCM: 290-300 mOsm/kg H2O
Non-Ionic Contrast Agents
Advantages:
Reduced osmolarity
Better tolerated
Fewer reactions
Reduced discomfort
Less tissue damage
Less dilution
Wider clinical application
Viscosity
Thickness or resistance to flow
Affected by:
The concentration
The size of the molecules
The temperature of the contrast agent
Ionic/Non-ionic dimers are more viscous than their monomeric counterpart.
Viscosity will affect the rate of injection.
Use of Iodinated Contrast Agents
Water soluble so can be administered intravenously or ingested
Use of Iodinated Contrast Agents
What other imaging examinations make use of iodinated contrast agents?
Name at least three.
Excretion of Iodinated Contrast Agents
Intravenously contrast is excreted via kidneys by glomerular filtration.
Excretion rate will depend on plasma volume and GFR (Glomerular Filtration Rate).
GFR will determine kidney function – cannot be directly measured so this is estimated based on creatinine level, age, race and gender (eGFR).
Excretion of Iodinated Contrast Agents
Creatinine – chemical waste product formed from the breakdown of protein and muscle.
Excreted by the kidneys so high levels of blood creatinine indicates renal impairment.
Creatinine and eGFR are indicators of kidney function.
Prior to IV contrast administration, patients need to have creatinine and eGFR blood tests done.
Risk Factors
Hypersensitivity to iodine
Previous reaction to contrast media
Asthma or significant allergy history
Heart/lung conditions
High blood pressure
Renal impairment
>70 years of age
Allergies to seafood/shellfish is NOT a risk factor
Whilst there is iodine in shellfish, the allergy is due to the certain proteins such as tropomyosins not to iodine itself.
Risk Factors
Diabetes (use of Metformin)
Metformin is used in type 2 diabetes to decrease the amount of glucose produced by the liver and to increase the body’s response to insulin. In patients with renal failure, the renal clearance of metformin is decreased and there is an associated risk of lactic acidosis. Metformin needs to be stopped prior to IV contrast administration.
Impaired thyroid function (hyperthyroidism: overactive thyroid)
A typical dose of 100ml iodinated contrast medium increases free iodide in the body and can overwhelm physiologic thyroid hormone regulation in susceptible individuals.
Adverse Reactions
Despite improvements in iodinated contrast agents, adverse reactions still occur.
Range from mild reactions (majority) to life threatening emergencies (rare).
Reactions can be immediate (up to 1hr post administration of contrast) or delayed (from 1hr to 7 days).
Any reactions need to be documented and recorded under the patients record for future reference.
Other Reactions
Some patients may experience a “metallic” taste after the intravenous injection of a contrast agent.
Taste dysfunction, or a “metallic” taste, can occur because the contrast agent can stimulate taste receptors in taste cells.
A warm flush feeling at the injection site, head/neck and pelvic areas is also common.
Iodine is a vasodilator so large blood vessels will have an even larger volume of blood creating a warm sensation.
Higher rates of injections (more than 3ml/s) generally exacerbate the intensity of these reactions/feelings.
Extravasation
Leakage of intravenously administered contrast media from the normal intravascular compartment into surrounding soft tissues
Complication rather than a reaction.
Patient can experience pain, swelling, tightness, redness depending on the volume extravasated.
Administration of contrast/examination will need to be stopped immediately.
Follow protocol for dealing with extravasation.
Requirements and Good Practice
Follow departmental protocol for administration of all contrast agents and dealing with reactions.
Use the least amount of contrast agent required for the examination (body weight based).
Patient should be well hydrated prior administration of IV contrast.
Patients should not be left alone following administration of contrast media.
Know the location of the resuscitation medication and equipment.
The IV cannula used for the administration of contrast media should be left in situ for ≥ 15 minutes post injection.
To Conclude…
Ideal Qualities of a Contrast Agent
Excellent x-ray absorption characteristics at the x-ray energies used in diagnostic imaging
Chemical and thermal stability
High solubility and low viscosity
Non-antigenic (does not illicit an immune response) and low toxicity
Does not affect physiological homeostasis
Low/Iso-osmolar for minimal osmotic effects
Low renal load and effective excretion
Readily available and inexpensive
