contrast media part 1 Flashcards
What is (Radiographic) Contrast?
The difference in density between adjacent structures.
Of critical importance in radiographic images as it is the way various structures are identified.
What are Contrast Agents?
Any substance which when introduced into the body demonstrates organs or systems of the body in radiographic contrast to the surrounding tissue.
These substances therefore enhance the radiographic contrast of the area being imaged.
Classification of Contrast Agents
Negative Agents (Radiolucent)
All gasses (air or CO2)
Positive Agents (Radio-opaque)
Barium based
Iodine based
These are generally used in projectional radiography, fluoroscopy and CT scanning.
Other Types of Contrast Agents
MRI
Gadolinium based (most commonly used)
Manganese based
(all need to have some magnetic properties)
US
Microbubbles based
(an inert gas encapsulated by a shell which affects the vibrations/intensity of the US signal )
Negative Contrast Agents
Air
Occurs naturally in the body (in the airways, lungs, gastrointestinal tract).
Readily available!
CO2
Available in tablet, granules and powder form.
Both can be combined with a positive contrast medium to create a ‘double contrast’ effect.
Refers to the use of positive and negative contrast agents to increase the sensitivity of the examination.
Why CO2?
Ease of swallowing
Rapid disintegration
No residue
No bubble formation
No interference with Barium coating
Adequate gastric distension (200-500ml)
Quicker reabsorption into the body
Generally preferred over ‘room air’
Positive Contrast Agents
Barium Based
Barium Sulphate (non-soluble)
Iodine Based (soluble)
Ionic
Non-ionic (main form currently used)
(Can be administered by ingestion, per rectum or intravenously)
How do Contrast Agents Work?
The transmission of the an x-ray beam depends on the attenuation caused by the intervening structures within the body.
The attenuation caused by the contrast agent and soft tissue are responsible for the visualisation of the structures on the radiograph.
How do Contrast Agents Work?
(Linear) Attenuation Coefficient
The attenuation coefficient (µ) describes the fraction of a beam of x-rays that is absorbed or scattered per unit thickness of the absorber.
Attenuation Coefficient
It = Ioe-µx
Io = incident beam
It = transmitted beam
µ = attenuation coefficient
X = thickness of body part imaged
How do Contrast Agents Work?
Positive Contrast Agents
The intensity of radiation transmitted through the contrast agent is less than that transmitted through the soft tissue.
This is because the linear attenuation coefficient of the soft tissue is less than that for the contrast agent.
How do Contrast Agents Work?
Attenuation Coefficient
Attenuation coefficients are proportional to:
the density of the medium
the number of atoms present in a given volume
Positive Contrast Agents
Densities:
Iodine: 4940 kg/m3
Barium: 3600 kg/m3
Soft tissue: 1000 kg/m3
Atomic Numbers:
Iodine: 53
Barium: 56
Soft tissue: 7.5
The higher the density and the atomic number, the higher the attenuation coefficient and the more radiation is absorbed.
How do Contrast Agents Work?
Negative Contrast Agents
The intensity of radiation transmitted through the contrast agent is more than that transmitted through the soft tissue.
The linear attenuation coefficient of the soft tissue is more than that for the contrast agent.
Negative Contrast Agents
Atomic Numbers:
Air: 7.6 (effective)
Soft tissue: 7.5
Densities:
Air: 1.3 kg/m3
CO2: 2 kg/m3
Soft tissue: 1000 kg/m3
Soft tissue has a very high density compared to air and CO2, meaning a higher attenuation coefficient and more radiation absorbed.
