Contrast Media Part 1

• Contrast agent


  Used to distinguish adjacent tissues on a CT image by creating different densities (attenuation) that produce distinct images
• Axial or transverse plane


  Recommended for better spatial resolution of pathology since the patient is in supine position
• Oral or IV contrast media


  Given to create temporary, artificial density differences between objects
• Rectal contrast media


  Increases the density of the lower part of the large intestine
• The goal is to give different tissues which would ordinarily have similar attenuations, different attenuation coefficients making them more readily visible on the image
• Methods of contrast administration

  • Intravascular
  • Gastrointestinal
• Intravascular contrast


  Administered through dorsal and antecubital veins
• Intrathecal contrast


  Administered into the subarachnoid space surrounding the spinal cord
• Intraarticular contrast


  Administered directly into a joint space
• Positive contrast agent


  Material is of a higher density than the structure, appears radiopaque
• Negative contrast agent


  Low-density contrast agents, such as air or carbon dioxide, appear radiolucent
• Neutral contrast agent


  Gastrointestinal agents with density similar to water
• Proper administration of contrast media can easily provide a 40-75 HU increase in the natural difference of attenuation between tissues, thus making them visibly different in the image
• Intravascular Contrast material has high attenuation compared with human soft tissue, and therefore, wherever it distributes it increases the ability of the enhanced structure to attenuate the x-ray beam
• Osmolality


  Structural property of a liquid regarding the number of particles in solution
• Hypertonic


  Cells shrink
• Isotonic


  Cell size remain the same
• Hypotonic


  Cells enlarge
• HOCM (high osmolar contrast media) has osmolality of 1300-2140 mOsm/kg or about 4-7x that of human blood
• LOCM (low osmolar contrast media) has osmolality of 600-850 mOsm/kg or about 2-3x that of human blood
• IOCM (isosmolar contrast media) has osmolality equal to that of blood
• Viscosity


  Physical property; thickness or friction of the fluid as it flows
• Higher the concentration, the more viscous the solution
• Ionicity


  Whether the molecules in the contrast agent will dissociate into charged particles when dissolved in an aqueous solution
• Ionic contrast agents


  Molecules dissociate into ions when in solution
• Nonionic contrast agents


  Molecules do not dissociate
• All types of iodinated contrast media undergo very rapid distribution throughout the entire extracellular space and are excreted by the body nearly exclusively by the kidney via glomerular filtration
• Dose


  Beam attenuation abilities of a given amount of contrast media are directly related to the concentration of iodine
• Different CT scan protocols require different doses of iodine
• Upper limit 200 mL with a concentration of 320 mgI/mL
• Most common formula for Pediatric patients: 2mL/kg
• Radiologist should confer with referring physician and document certain information before administering iodinated contrast media to pregnant patients
• Less than 1% of iodinated contrast media is excreted into breast milk and absorbed by the baby, so the reaction is very low
• Fatal reactions are extremely rare with both HOCM and LOCM (0.9 per 100,000/ <0.001%)
• Radiologist


  Should confer with the referring physician and document in the radiology report or the patient's medical record
• Information the radiologist should document

  • That the information requested and the necessity for contrast material administration cannot be acquired via other means (ultrasonography)
  • The information needed affects the care of the patient and fetus during the pregnancy
  • That the referring physician is of the opinion that is not prudent to wait to obtain this information until after the patient is no longer pregnant
• Lactation

  • Less than 1% of contrast is excreted into the breast milk
  • Less than 1% is absorbed in the Gastrointestinal Tract of the baby
  • Reaction is therefore, very low
• Fatal reactions

  • Extremely rare in both HOCM and LOCM (0.9 per 100,000/ <0.001%)
  • Nonetheless, it is well documented that adverse reactions sometimes occur with their use
• Contrast reaction


  Can be confusing because it is used in a variety of different ways in relation to the effects of iodinated radiologic contrast agents
• Categories of contrast reactions

  • Chemotoxic reactions
  • Idiosyncratic reactions