Computed Tomography Contrast Agents

Computed Tomography Contrast Agents

• Adjacent tissues have different densities (attenuation) for clear image depiction.
• High inherent contrast in some areas (e.g., chest).
• Pulmonary vessels and ribs have different densities from adjacent lung.
• Contrast agents fill structures with differing densities.
• Positive agents have higher density than the structure.
• Negative agents have lower density than the structure.

Contrast Administration Methods

• Intravascular and gastrointestinal routes are common CT methods.
• Intrathecal and intraarticular administration are less common.

Contrast Media (CM) Uses

• Iodine-based IV contrast (e.g., Omnipaque 300, lohexol).
• Oral/Rectal barium sulfate contrast.
• Patient must sign consent form before CM administration.

Properties of lodinated Agents

• Water-soluble iodinated agents are easy to administer intravascularly.
• High safety index.
• Iodine atoms (atomic number 53) increase attenuation in the bloodstream.
• Increased attenuation is displayed as a change from darker to lighter on the image.

CM Factors

• Concentration of iodine in the solution.
• Osmolality (number of particles in solution per unit liquid).
  • High-osmolality media has seven times the osmolality of blood.
  • Low-osmolality media has a lower osmolality than blood.
  • Iso-osmolar media has the same osmolality as blood.
• Viscosity (resistance of fluid flow).
  • Viscosity is affected by temperature and concentration.
  • Heating contrast to body temperature decreases viscosity.

Factors Affecting Contrast Enhancement

• Pharmacokinetic factors (concentration, osmolality, viscosity).
  • Low concentration requires higher injection rate and volume.
  • High injection rate can cause contrast extravasation.
  • Low concentration has lower osmolality, fewer adverse effects.
  • Pre-warming contrast decreases viscosity for easier flow.
• Patient factors (age, sex, weight, cardiovascular status, renal function, other diseases).
• Equipment factors (e.g., scanner type).
  • Larger CM volumes increase time for peak enhancement.
  • Injection flow rate affects time to reach and fall off peak enhancement.
  • Large patients require higher injection rates.
  • Reduced cardiac output increases needed scan delay.
  • Slow scanners require larger volume to extend peak plateau.

Methods of CM Delivery

• Drip infusion: contrast is dripped into an IV line.
  • Scanning begins after most of the contrast medium is administered (roughly 2-3 minutes).
  • Dependent on gravity, flow rates are variable.
  • Not ideal for scans of neck, chest, abdomen, or pelvis.
  • Cannot produce peak enhancement for CT angiography.
• Bolus technique: rapid injection of contrast media.
  • Contrast volume of 50-200 mL, injection rate of 1-6 mL/s.
  • Interval between injection and scanning (scan delay) is critical.
  • Can be given by hand (syringes) or mechanical injector.
    • Hand bolus has variable flow rates.
    • Mechanical injectors provide precise flow rates and volumes.

Automatic Injection Triggering

• Two methods: test bolus and bolus triggering.
  • Used to individualize scan delay for patient factors.

Performing a Test Bolus

• Steps to perform a test bolus may vary.
• Obtain scout views, and determine target region/obtain slice.
• Injected contrast of 10-20 mL (same rate as diagnostic scans)
• Begin trial scans after injection (typically every 2 seconds for 10-15 scans).
• Analyze graphs of contrast enhancement and time.
• Determine scan delay by adding 3 seconds to time to peak enhancement + twice the image number taken at peak enhancement.

Using Bolus Triggering Software

• Obtain scout views and single slice at area of interest.
• Plan the diagnostic study, set the trigger threshold.
• Start the contrast injection; threshold determines scan start.
• Instruct patient to hold breath during table motion.

Contrast Extravasation

• Leakage of fluid from vein into surrounding tissue during IV administration.
• Most extravasations are not significant (mild swelling/erythema).
• Severe cases can result in tissue necrosis/ulceration.

Contrast Injection Techniques

• Use indwelling catheter sets with flexible plastic cannulas.
• Avoid metal needles.
• Monitor injection site for swelling (indicates extravasation). Stop injection if swelling occurs.
• Warm contrast to body temperature for easier flow through IV catheters.
• Use low osmolality contrast media (LOCM).

Phases of Tissue Enhancement

• Bolus phase: immediately follows injection; arterial structures are enhanced.
• Nonequilibrium phase: follows bolus; venous structures are enhanced; contrast is still brighter in arteries than parenchyma organs .
• Equilibrium phase: contrast is diluted; characterized by an AVID of <10 HU ; intravascular and interstitial concentrations equilibrate.

Scan Timing for Angiography

• Precise scan timing crucial for CT Angiography.
• Too early: misses contrast bolus.
• Too late: insufficient opacification, particularly in small vessels.

Patient Preparation for CT Contrast Media

• Laboratory tests (serum creatinine, blood urea nitrogen) to assess kidney function.
• Fasting required for some examinations (6-8 hours).

Documenting CM Administration

• Needs to include legal documents.
• Name of agent, dose (volume and concentration), flow rate(s), injection site, and adverse effects.

This quiz covers the principles and administration methods of contrast agents used in computed tomography (CT). It highlights the differences in tissue densities and the properties of iodinated agents. Additionally, it discusses the various routes for contrast media administration and safety considerations.