Radiographic Contrast Media:

• Positive and negative contrast agents.
• The positive contrast media attenuate X-rays more than do the body soft tissues.
• Water soluble iodine agents
• Non-water-soluble barium agents.
• Negative contrast media attenuate X-rays less than do the body soft tissues. No negative contrast media are commercially available i.e air

Barium Contrast Media:

• Barium sulphate preparations used to visualize the gastrointestinal tract consist of a suspension of insoluble barium sulphate particles which are not absorbed from the gut.
• Differences between the different commercially available agents are very minor and relate to the additives in the different barium sulphate preparations.

Iodinated contrast media:

Osmolarity –

    High osmolar, low osmolar, iso-osmolar

Ionic state –

    ionic and non-ionic

No. of benzene rings –

    monomeric and dimeric

Fig 1: Iodinated Contrast Media

Magnetic resonance (MR) imaging contrast agents:

• Contain paramagnetic or superparamagnetic metal ions which affect the MR signal properties of the surrounding tissues.
• They are used to enhance contrast, to characterize lesions and to evaluate perfusion and flow-related abnormalities. They can also provide functional and morphological information.
• The most widely used paramagnetic contrast agents are non-specific extracellular gadolinium chelates.
• Their active constituent is gadolinium, a paramagnetic metal in the lanthanide series, which is characterized by a high magnetic moment and a relatively slow electronic relaxation time.
• Non-specific extracellular gadolinium chelates can be classified by their chemical structure, macrocyclic or linear, and by whether they are ionic or nonionic.
• also include liver specific gadolinium-based agents (gadobenate dimeglumine, Gd-BOPTA and gadoxetate, Gd-EOBDTPA) and manganese-based preparations [manganese chelate (mangafodipir trisodium) and free manganese combined with vitamins and amino acids (to promote the uptake) for oral intake]
• Superparamagnetic contrast agents include
• Superparamagnetic iron oxides (SPIOs).
• Ultra small superparamagnetic iron oxides (USPIOs).
• Two preparations of SPIOs are available: ferumoxides and ferucarbotran.
• These particulate agents are composed of an iron oxide core, 3–5 mm in diameter, covered by low molecular weight dextran for ferumoxides and by carbodextran for ferucarbotran.
• SPIOs are approved for liver imaging and
• USPIOs are under consideration for MR lymphography.
• After injection, SPIO and USPIO particles are metabolised into a soluble, non superparamagnetic form of iron. Iron is incorporated into the body pool of iron (e.g. ferritin, hemosiderin and hemoglobin) within a few days.

Ultrasound contrast media:

• Ultrasound contrast agents produce their effect by increased back-scattering of sound compared to that from blood, other fluids and most tissues.
• Ultrasound contrast agents can be divided into five different classes:

i.Nonencapsulated gas microbubbles (e.g. agitated or sonicated)

ii.Stabilized gas microbubbles (e.g. with sugar particles)

iii.Encapsulated gas microbubbles (e.g. by protein, liposomes or in polymers),

iv.Microparticle suspensions or emulsions [perfluorooctyl bromide (PFOB), phase-shift],