Part 1: L2, Angiographic Contrast Agents Flashcards
1
Q
Why are imaging agents necessary? How do they typically interact with the body?
A
- Trying to image areas of interest that do not have good contrast i.e. differential tissue density or elemental composition
- ‘Drug’ enhances image to elucidate disease or abnormal pathology -> interacts with spectroscopic technique being used
- Usually, these agents are not target specific, but affect the entire body (thus the high dosage)
2
Q
Contrast agent requirements:
A
- Must enhance the contrast of the imaging tecnhique
- Must be essentially non-toxic, with minimal side effects
- Must be readily removed from body following injection (renal/hepatic/lymphatic systems)
- Must not soluble in aqueous solution if it is to be injected intravenously - also, ionic in nature and of appropriate viscosity
- Must not leach into cells (reduced lipophilicity) to avoid prolonged retention and reducing the contrast enhancement of various tissues
3
Q
Early barium contrast agents:
A
- Used to image gastrointestinal system
- Insoluble so used as a milled suspension in water
- Often there is a thickening agent that coats the intestine (such as carboxymethyl cellulose)
- pH adjusted to 5.3 to ensure the stomach pH does not affect it too much
- Barium can also be used to coat the colon in a barium enema -> technique replaced by endoscopy
4
Q
Modern contrast agents:
A
- Iondinated e.g. iodixanol, iohexol, iopamidol
- Extremely high volumes (~300 mg iodine / mL)
- Iodinated can be….Water insoluble, oily contrast media or water-soluble
5
Q
Oily contrast media:
A
- Historically, lipiodol
- A stable compound of 40% iodine in poppy seed oil, introduced in 1920s
- These oils are still to some extent used for lymphography, and also for hysterosalpingography
- Myelography -> imaging spinal nerves by injecting spinal canal
6
Q
Water soluble iodinated contrast agents:
A
- Largest group
- Early: monoiodinated pyridine derivatives
- Soon followed by di-iodinated pyridones with higher solubility
- By 1950s, tri-iodinated agents were in use -> monomeric agents for oral and cholegraphic media, uro/angiographic media
- The cations of the salt are usually either sodium, meglumine or both (sodium can upset the balance existing in the body -> mixture has lower cardiotoxicity than either alone
- Dissociate in aqueous solution -> ratio 1.5 agents (3 iodine atoms per 2 parts) -> osmolality issue
7
Q
Osmolality description:
A
- biological membranes are semi permeable and allow water and small molecules to pass freely; osmosis
- The force exerted is called osmotic pressure; determined solely by concentration of dissolved particles
8
Q
Definition of osmolality vs osmolarity:
A
- Osmolality: Concentration of particles per unit mass of solvent
- Osmolarity: Concentration of particles per unit volume of solvent
9
Q
Biological implications of osmolality? Give the possible clinically significant effects:
A
- Closer the osmolality of the radiological contrast media is to that of body fluids, the better to general tolerance
- Osmolality of blood and cerebrospinal fluid is 290 mOsm/kg (most agents in use are well in excess of this by 7-8x)
- Responsible for sensations of heat/discomfort/pain
- Clinically significant effects: damage to BBB, renal damage, electrolyte balance in small children, thrombosis and thrombophlebitis
10
Q
Monomeric non-ionic contrast:
A
- Low osmolality contrast media has substantially reduced side effects
- Early example: metrizamide (Analogue of sugar)
- Used for uro/angiography