MRI contrast agents Flashcards
what is a contrast agent and why do we need them
alters image contrast
- aids in visualisation and diagnosis
what is întra-articular administration
a type of shot that’s placed directly into a joint to relieve pain.
- can be cortecosteroids or pain relief
what is the main contrast agent used in MRI
gadolinium
what is the general theory behind MRI
- abundance of water in our body is used to produce MRI images
what are the 3 types of MRI contrast agents
- extracellular fluid
- blood pool
- tumour specific
what is extracellular fluid contrast
- distributed in intravascular compartment initially
- rapidly diffuses through extravascular space
what is blood pooling contrast / intravascular contrast
- intravascular contrast agent that lasts in the blood for up to an hour rather than leaving the system in a few mins like others
- differentiated from other contrast agents due to their high molecular weight and higher relativities
what is a tumour specific contrast agent
pharmaceuticals targeted to tumoura
what type of magnetisation do majority contrast agents have?
paramagnetic ion complexes (with applied magnetic field, they are partially / weakly aligned)
most contrast agents in MRI contain a lanthanide element such as Gd3+, how does this affect the T1/2 relaxation time and what happens to the image
- these elements shorten the T1/2 relaxation time
- causing increased signal intensity on T1 weighted images OR reduced signal intensity on T2 weighted images
why are most contrast elements positive agents
- postive agents shorten T1, so enhanced parts appear bright on T1 weighted images
ferromagnetic agents and super paramagnetic agents are negative contrast agents/ how does this show on T2 weighted images
- as they are negative contrast agents, the enhanced parts appear darker on T2 weighted images
reminder :
paramagnetic = weak attraction
ferromagnetic = strongly attracted
diamagnetic = weakly repelled
what is gadolinium (what type of agent is it
- paramagnetic agent
- has large magnetic moment
MRI contrast agents
- increase T1 singnals by rapidly returning protons to their baseline
- allowing quicker regrowth of longitudinal magnetisation
- which produces a brighter t1 signal
(commonly gadolinium used)
why is gadolinium good for being used as contrast
- it is paramagnetic at body temperature
what is an isotonic solution
- isotonic solution is at the same concentration as the solution to which is it being compared to
how does osmolarity affect torleration
the more isotonic a contrast agent is, the better it is tolerated
osmolarity is directly responsible for sensations of heat
what is the blood brain barrier
- protects the central neural tissue
how many more times likely is an ionic contrast to cause a reaction compared to a non-ionic contrast
x4
5.5 times more likely to cause a severe reaction
what is a monomer and dimer
monomer =
- single molecule of contrast agent
dimer =
- 2 molecules of contrast agent joined together
what is the difference in the way linear agents work with gadolinium and how macrocyclic agents work with gadolinium
- linear agents dont fully surround the gadolinium ion
- macrocyclic molecules fully enclose gadolinium with nitrogen
gadolinium has a excellent safety profile but what are some screening criteria relating to reaction
- severe asthma
- prior reaction to Gd
be aware that gadolinium does have risk of nephrogenic systemic fibrosis, what is this
a progressive multiorgan fibrosing condition mainly caused by patients’ exposure to gadolinium-based contrast agents (GBCAs) used for magnetic resonance imaging (MRI)
what is the name of the commonly used contrast agent in MRI containing macrocyclic galdinium
gadovist
how fast will excretion of contrast be in kidneys (glomerular filtration)
50% in 2 hrs
90% in 12 hrs
how fast will the liver excrete the contrast into bile
50% in 56-96 mins
Gd-cm is only used during pregnancy if very necessary but it has no known effect on lactation (going into breast milk)
if lactating woman is given gadolinium, they are advised to excite enough breast milk for 24 hours post injection (only for high risk agents)
low risk agents do not require woman to pre make breast milk
what age group is most at risk of nephrotic systemic fibrosis
over 65’s
Gd-cm designated low, medium or high risk
GFR <30 high risk
GFR <60 low risk
know that linear gadolinium can have a large proportion retained in the brain
- macrocyclic not as much and studies show it can be washed out over time
what is negative contrast agents used to see
visualise mucosal enhancement
what is mannitol (additive)
used to decrease pressure in the eyes, as in glaucoma, and to lower increased intracranial pressure.
USPIO is a new developed contrast, what is its benefit
has longer plasma half life
- biodegradable
what is fMRI
Functional magnetic resonance imaging or functional MRI measures brain activity by detecting changes associated with blood flow
what is the difference between dose-dependent and dose-independent adverse events with contrast
dose-dependant (caused directly by contrast) =
e.g due to osmolatiry
- pain
- heat
dosé-indépendant (as a result of contrast/ not directly caused by agent) =
- allergic or sensitivity reactions
what are the most common contrast reactions
- anaphylactic (allergic) severe (immune triggered)
- anaphylactoid (allergic) minor (no immune trigger)
- hyperosmolar (a condition in which the blood has a high concentration of salt (sodium), glucose, and other substances.)
give examples of minor adverse reaction
mild rash
vomiting
nausea
flushing
give examples of moderate adverse reactions
facial oedema
hypotension
bronchospasm
severe urticarial (rash)
give examples of severe reactions
hypotensive shock
laryngeal oedema
convulsion
respiratory/cardiac arrrest
how do you treat mild reachtions
- monitor
- o2
- ## anaphylactic drugs e.g antihistamine
how do you treat moderate reactions
- MET
- o2
- anaphylactic drugs
how do you treat severe reaction
- MET and crash trolley
how long does it take for a delayed adverse reaction to take pplace
30 mins after administration
how do you deal with reactions to contrast in MRI room
remove patient from room to be treated due to continuous magnet in the rooma
all actions from patient reaction case must be documented and uploaded to where
CRIS
- alarm on CRIS
- fill out yellow card
