Contrast Media (week 12 and 13)

Attenuation vs absorption

Attenuation: the reduction in the total number of x-ray photons in the beam after passing through a given thickness of material. 

Absorption: the atoms actually absorbing the radiation in photoelectric interactions

• What is the purpose of using a contrast media

To increase the radiographic contrast in an area where it was absent or low before

  • To be able to see anatomic detail in an image, the area of interest must differ in radiographic density from its surrounding tissues

• What affects subject attenuation? 

  • Atomic number of the absorber

  • Mass density of the absorber

  • Thickness of the absorber

  • Electron density of the absorber

• Two main types: positive and negative

Positive:

Negative:

  • Radiolucent

  • Low atomic number

  • Allows x-rays to pass through quite easily

More radiation reaches the IR so causes structures to appear black

• Types of negative contrast and when are they used or seen?

Used for:

Trauma/pathology: non-intentional or accidental

  • Perforation

  • Surgical Emphysema

  • Fat Pad signs

Can be:

  • Room air

  • CO2

  • N2O

  • O2

Present due to some kind of injury or disease ie. bowel perforation or the collapse of lung tissue. 

• What is a contraindication to their use?

They must never be injected intra-arterially or intravenously; doing so will have serious if not fatal consequences ????????????????

• How do negative agents perform? 

Decrease the atomic number of the area to be demonstrated since they are composed of low atomic number elements. 

Instilled intentionally or not, they will decrease the attenuation of the x-ray beam and produce areas of increased optical density (black) on the radiograph.

• What is a double contrast study?

Both a negative and positive contrast agent being used

• Types of positive contrast agents

  • Barium based agents: barium has Z of 56

  • Iodine based agents: iodine has Z of 53

            -  water soluble 

            -  non soluble (oil-based)

• When is barium used vs iodine? 

Barium: digestive system and alimentary canal. Is not chemically reactive and not water soluble

Iodine: urinary and cardiovascular systems, absorbed and is water soluble

If there is suspected perforation of the bowel since barium is contraindicated in these situations.

• When can’t barium be used? 

Not water soluble so cannot be administered intravascularly or intrathecally

• Ionic vs non-ionic iodine media

Non-ionic: is almost always used in intravascular studies → less reactions towards it

Ionic: used for biliary and retrograde urinary tract studies and when necessary in GI tract studies. → separates into ions.

• Benefit of non-ionic

Less reactions towards it

• Images showing when contrast agents are used 

Angiography: cardiology:Fluoroscopy: CT:

MRI: Myelography:Arthrography:

• Radiolucent vs radiopaque

Radiolucent: x rays transmit through the body

Radiopaque: x rays are absorbed by the body