For medical professionals such as radiologists and physicians to comprehend what’s happening inside our bodies, they may ask patients to undergo contrast media treatment. Contrast media is used in radiology procedures for delineating tissues with poor inherent contrast to help diagnose issues by highlighting internal areas and structures of the body. Also referred to as “contrast materials” or “contrast agents”, contrast media are liquid substances which temporarily color areas of the body to improve the diagnostic effectiveness during imaging procedures.
They were initially utilized to better visualize vascular structures and the gastrointestinal tract. Over time they become one of the key components of medical imaging and their use extended to the evaluation of vascularization and perfusion of tissue in solid organs, specifically using cross-sectional imaging techniques and enhanced visualization and characterization of focal lesions within these organs. The use of contrast agents is indispensable in radiology, and it will remain as such in the foreseeable future.
Contrast media is a valuable imagining solution applied across the following medical imaging exams:
Specific contrast media will use certain chemical combinations depending on the procedure. For example, iodine and barium-based compounds support X-ray and CT exams while gadolinium is for MRI and microbubbles are for ultrasound procedures.
Contrast Media are classified based on different factors such as Modality, Type, Route of Administration, and Indication.
In X-ray and CT exams, there are many ways for categorizing contrast media:
- Atomic weight: Under this classification we have the positive and negative contrast media. Positive contrast media has its name because it possesses a higher atomic weight than surrounding tissue. This means that it will absorb more photons than the tissue that surrounds it and would produce a lower radiographic density than them. E.g. Barium sulphate, Organic Iodine. Negative contrast media: Has a lower atomic weight than surrounding tissue that it would absorb less photons and would produce greater radiographic density than the tissues surrounding it. e.g. air, oxygen, carbon dioxide.
- Solubility: The positive contrast media barium sulfate and organic iodine are further classified based on their solubility in water. Barium sulfate is a water insoluble contrast agent so it is not readily absorbed and excreted by the body. This means that it will stay within the body cavity for a long period of time. Barium sulfate usually produced in a powder or semisolid suspension like a syrup. These contrast media used majority in investigations of the gastrointestinal tract. Depending on the examination the barium sulfate is either taken orally or through the rectum as an enema.
Water soluble contrast media: The organic iodines fall under this group. This type of contrast media usually exists in a liquid form that is soluble in water. When it is introduced into the body it is rapidly absorbed into the body’s water and excreted by the kidneys. Thus, it does not stay within the region of interest for long. This is why speed is important in investigations where this type of contrast media is used. This type of contrast media is used in investigations of the urinary, biliary, cardiovascular, and central nervous systems.
- Osmolarity: Organic iodines are further classified into High Osmolar Contrast Media [HOCM] and Low Osmolar Contrast Media [LOCM]. Osmolality is the concentration of dissolved particles in a solution. A solution with a high osmolality will induce a greater osmotic pressure. This osmotic pressure causes more fluid to flow into the solution. When contrast media with an osmolality far greater than that of body fluid is introduced to the body, the osmotic pressure causes water to move from the low osmolar body fluid to high osmolar contrast media. This leads to dehydration and increased likelihood of intolerance to the contrast media. If contrast media that has an osmolality near the osmolality of the body fluid is used there is a low chance of contrast media reactions occurring. Compared to when contrast media with osmolality far greater than the body fluid is used which has a higher chance of leading to contrast media reactions.
High Osmolar Contrast Media were the first type of organic iodine contrast media to be produced. They are currently the less expensive type on the market. When high osmolar contrast media is introduced into a solution it breaks down into an anion and a cation. The anion is usually iodine while the cation is either sodium or meglumine. This breaking down or dissociation into ions gives the name ionic contrast media. This dissociation into two causes more particles to be present in this type of contrast. As a matter of fact, high osmolarity contrast media have an osmolarity that is five to eight times greater than the body fluid plasma. This type of contrast media has a higher chance of causing a contrast media reaction this is why it has been replaced by low osmolar contrast media in many radiographic examinations. Low Osmolar Contrast Media are advancement over HOCM and are currently more expensive than the high osmolar contrast media.
A summary of the types of contrast Media classification in X-ray and CT Procedures
Some commercially available small molecule iodinated contrast media are listed in below.
Magnetic Resonance Imaging (MRI) Contrast agents:
Magnetic resonance imaging (MRI) contrast agents are categorized according to the following specific features:
- chemical composition including the presence or absence of metal atoms
- route of administration, magnetic properties
- effect on the magnetic resonance image
- biodistribution
- imaging applications
The majority of these agents are either paramagnetic ion complexes or superparamagnetic magnetite particles and contain lanthanide elements such as gadolinium (Gd3+) or transition metal manganese (Mn2+).
Gadolinium based Contrast Agents (GBCAs):
Regarding the chelate type and charge, GBCAs can be divided into linear/macrocyclic and ionic/non-ionic groups. A wide range of GBCAs are commercially available and their stability is dependent on the conditional thermodynamic stability constant (Kcond), thermodynamic stability constant (Ktherm), and kinetic stability.
Multiple studies show that the brain holds onto more gadolinium particles from linear agents than macrocyclic agents. Early studies show that most macrocyclic molecules filter through the kidneys and leave the body within 24 hours after an MRI. The rest exit the body within 72 hours.
Ionic GBCAs are more stable than nonionic ones and the stability of macrocyclic compounds is higher than linear compounds. Hence, ionic macrocyclic agents are the most stable Gd chelates. Macrocyclic molecules bind strongly to Gd in an organized rigid ring; however, linear nonionic GBCAs have open chains and weaker binding to Gd. Compared to linear agents, macrocyclic agents are more stable in vivo. Low-stability GBCAs (linear, nonionic compounds) likely undergo transmetallation, release free Gd that deposits in tissues, attract fibrocytes, and therefore initiate the process of fibrosis.
