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)
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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
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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
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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
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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
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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
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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
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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
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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
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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
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