Radiographic Contrast in Imaging

Contrast in Imaging

• We use contrast in imaging to distinguish between bones and tissue and to highlight different pathologies by changing differential absorption using contrast media.

Subject and Image Contrast

• Subject contrast is caused by the difference in density of surrounding tissue and cannot be changed, whereas image contrast is what is displayed on the image and can be modified using windowing, etc.

Factors Affecting Subject Contrast

• Density and thickness of anatomy affect subject contrast.
• kVp controls contrast, with low kVp increasing contrast and resulting in better subject contrast.
• Beam strength also affects contrast, with a lower beam strength resulting in increased contrast.

Negatively Impacting Contrast

• Compton scatter negatively impacts contrast and can be reduced using grids or air gap technique or collimation.
• Contrast media can be administered to patients to see densities more clearly, especially in areas of low subject contrast.

Pharmacodynamics and Pharmacokinetics

• Pharmacodynamics is the study of the biochemical and physiological effects of a drug on the body.
• Pharmacokinetics is the study of the absorption, distribution, metabolism, and excretion of a drug by the body.

Types of Contrast Media

• Positive contrast media include radiopaque materials like barium and iodine, which appear white on images.
• Negative contrast media include radiolucent materials like air, which appear black on images.
• Double contrast agent techniques use both positive and negative contrast media to increase clarity.

Administration of Contrast Media

• Contrast media can be introduced intravenously, orally, or directly into the site using a canula.
• Oral administration includes barium meals and small bowel follow-through, with careful timing and mixing with flavouring to make it easier to drink.
• Injected contrast media include iodinated compounds and gadolinium, which are administered intravenously or intra-arterially.

Ideal Properties of Contrast Media

• Ideal contrast media should be non-toxic, inert, stable, physiologically inert, and produce adequate radiographic contrast, with low osmolarity and suitable viscosity and persistence.

Reactions to Contrast Media

• Reactions to contrast media can be mild, moderate, severe, or delayed, with symptoms ranging from chills and sneezing to cardiac arrest and respiratory arrest.
• Management of reactions involves discontinuing injection, remaining calm, and following department protocol.

Precautions and Sustainability

• Precautions include screening patients through questionnaires, informed consent, and checking contrast media contents and route of administration.
• Sustainability efforts include proper disposal of unused contrast media and reducing waste.
• Data shows that contrast media can be found in sewage water, surface water, and drinking water, highlighting the need for sustainable practices.

Contrast in Imaging

• Contrast is used to distinguish between bones and tissue and different pathologies in imaging.
• Contrast media changes the differential absorption of X-rays to enhance image quality.

Types of Contrast

• Positive Contrast Media: • Radiopaque, white on X-rays, high Z (atomic number) • Examples: Barium (Z=56), Iodine (Z=53) • Increases beam attenuation, producing areas of decreased density on the image • Used for imaging GI tract, blood vessels, urinary tract, and biliary ducts
• Negative Contrast Media: • Radiolucent, black on images, low Z • Examples: Air, oxygen, gas-producing tablets • Appears black on image, sometimes used with positive contrast for increased clarity
• Double Contrast Agent Techniques: • Easier to pick up pathology, higher contrast • Used in combination with positive and negative contrast media

Pharmacodynamics and Pharmacokinetics

• Pharmacodynamics: • Study of the biochemical and physiological effects of a drug on the body • What the drug does to the body
• Pharmacokinetics: • Study of the absorption, distribution, metabolism, and excretion of a drug • What the body does with the contrast agent

Introduction of Contrast Media

• Methods of Introduction: • Intravenous injection • Orally (barium meal, Ba swallow, small bowel follow-through) • Introduced directly into the site (canula, etc.)
• Pros and Cons of Oral Contrast Media: • Barium sulphate: high attenuation, not water soluble, may cause allergic reactions • Gastrografin: safe for patients with bowel perforation, bitter flavor, can be mixed with flavoring

Contrast Media in Clinical Settings

• Gadolinium: • Used in MRI • Administered intravenously • Interacts with nearby water molecules to affect signal intensity • Brighter signal on MRI, eliminated from the body through kidneys
• Gas-filled Microbubbles: • Used in US • Injected gas, high degree of echogenicity • Structures with high echogenicity appear brighter • Dissolve in 10-15 minutes

Ideal Properties of Contrast Media

• Non-toxic
• Inert
• Stable compound
• Physiologically inert
• Produces adequate radiographic contrast
• Low osmolarity
• Suitable viscosity
• Suitable persistence
• Miscibility (homogenous)

Reactions to Contrast Media

• Classification:
  1. Mild: localized reactions (chills, sweats, nausea, vomiting, headaches)
  2. Moderate: widespread reactions (hives, severe vomiting, respiratory systems, edema, vasovagal attacks)
  3. Severe: life-threatening reactions (cardiac arrest, hypovolaemic shock, respiratory arrest, convulsion)
  4. Delayed: reactions occurring after 1 week or 1 hour
• Management: • Discontinue injection • Remain calm • Follow department protocol • Alert for assistance • Have easy access to emergency trolley

Precautions and Sustainability

• Precautions: • Screening of patients through questionnaire • Informed consent • Check contents of contrast media, volume, exp, date, and route of administration • Leave canula in the arm through injection • Observe start • Maintain visual of patient throughout scan • Ensure emergency trolley available • React if reaction occurs
• Sustainability: • Approx 300 million CT scans per year, 40% contrast-enhanced • Data shows contrast media is found in sewage water, surface water, and drinking water in many regions • Unused contrast media should have proper disposal • Need to find sustainable solutions for contrast media use