CT Flashcards
name 3 types of contrast media used
- oral
- intravenous
- air or CO2
what is the use of contrast media?
to assist in providing visual representations of information from within the body
aid in providing additional information
why is iodinated contrast media used?
iodine has high atomic number –> ability to absorb X-ray
what are the 4 basic principles of contrast media
- iodinated CM
- osmolarity and viscosity of CM
- ionicity of CM
- properties of an ideal CM
describe osmolality of CM
measurement assessing the number of particles dissolved in 1kg of water
osmolality of CM should be as close as possible to human plasma (300 mOsm/kg H2O)
what are the 3 classifications of CM osmolality
- high osmolar (1000- over 2400 mOsm/kg H2O; 5-8x)
- low-osmolar (900 mOsm/kg H20; 3x)
- iso-osmolar (290 mOsm/kg H2O)
explain osmolar toxicity
when too high osmolar CM is given, fluids in patient’s cell leave the intracellular compartment and enter the extracellular space –> causing cell shrinkage –> crenation of RBC –> impaired cell functioning
describe viscosity of CM
a measure of friction or resistance of liquid to flow
factors affecting viscosity of CM
size and shape of solute particle
temperature
describe ionicity of CM
property of a molecule to break into positively charged cations and negatively charged anions
presence of ionicity: ionic
absence of ionicity: non-ionic
ionic contrast can disturb the normal dynamic ionic equilibrium of the body
ionic toxicity can affect physiological processes such as heart rate, BP and neurotransmission
therefore, non-ionic CM are preferred
how many iodine atoms are in a benzene molecule
3 (tri-iodinated benzenes ring)
how to differentiate between ionic and non-ionic CM molecular structures
ionic molecular structures will contain ions at the R-group, showing positive and negative charges
describe the distribution of CM
- ICM does not cross cellular membrane easily, and is primarily distributed in the bloodstream
- pathological tissue contains membranes that are less obstructive, thus more permeable to ICM (allow CM to enter the cell)
describe neurotoxicity of ionic CM
- may cause seizures in susceptible patients
- high risk of neurotoxicity in patients with disrupted BBB
properties of an ideal CM
- water soluble
- biologically inert
- low viscosity
- non-ionic
- low or iso-osmolar
- increase sensitivity and enhance differentiation of different soft tissues
4 types of risk associated with IV contrast scan
- adverse reactions
- contrast induced nephropathy (CIN) or contrast induced acute kidney injury (CIAKI)
- metformin induced lactic acidosis
- extraversation
probability of acute adverse reaction event occurring with high osmolar CM
5-15%
probability of acute adverse reaction event occurring with low osmolar CM
0.2-0.7%
probability of severe adverse reactions occurring with low osmolar CM
0.04%
how long does it take for the reactions to take place
manifest <1hour
can start to manifest within 5 mins after contrast injection
majority of the life-threatening events <20mins
what are some of the mild reactions signs and symptoms
- vomiting
- warm feeling
- urtcaria
- nausea
- headache
- sweat
- cough
- itching
what are some of the moderate reaction signs and symptoms
- tachycardia
- bradycardia
- hypertension
- hypotension
- dyspnea
what are some severe reaction signs and symptoms
- convulsions
- cardiac arrest
- unresponsiveness
- profound hypotension
risk factors for adverse reactions
- previous history of adverse reaction to CM
- hx of allergic rxn that required medical intervention
- hx of asthma
- hx of significant cardiac diseases
- multiple drug allergy
what is the usual premedication for adverse reaction
prednisolone 30mg 12hours and 2hours before CT scan
what is given for premedication for adverse reaction in the event of emergency
- if scan can wait, prepare IV hydrocortisone 100/200mg every 4 hours until time of scan; with/without IV diphenhydramine 50mg 1hr prior to scan
risk factor for CIN
eGFR + :
- age >60
- hx of kidney disease as an adult, including tumour and transplant
- family hx of kidney failure
- diabetes treated with insulin or other prescribed med
- hypertension
- paraproteinemia syndromes or disease (eg myeloma)
- current use of nephrotoxic medications
metformin use for ICM
- eGFR >45 - continue metformin
- eGFR >30 - stop metformin 48hrs before CM injection and resume after 48hrs in the absence of deteriorating renal function
- eGFR<30 - metformin is contraindicated and should be avoided
how does body weight affect iodine dose
body weight increase = iodine concentration decrease
how does cardiac output affect the arterial peak enhancement
poorer CO = longer time for heart to pump = peak gets pushed back
how does contrast volume affect the iodine concentration
contrast volume increase = iodine concentration increase
contrast volume increase = longer time to inject CM = peak gets pushed back
how does injection flowrate affect iodine concentration
injection FR increase = iodine flux increase = iodine peak is higher
characteristic of saline chaser
- pushes contrast in tubing and peripheral veins into central veins
- allows reduction in contrast vol
- increases peak attenuation
- reduced streak artifacts from veins and right heart
- simpler to implement with dual head injectors
what are some patient preparation for IV contrast scan
- risk of physiologic and allergic-like adverse reactions
- premedication for adverse reactions
- risk of CIN
- fasting
- IV cannula
- metformin induced lactic acidosis
- explained risk of extravasation
what is included in a CT protocol
- region/scan range
- any use of contrast
- phases/sequences
- scan details
- recon details/phases
difference between epidural hematoma and subdural hematoma
epidural hematoma:
- occurs between the skull and dura mater
- convex shape
subdural hematoma:
- occurs between the dura mater and the arachnoid
- concave shape