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use of contrast media in ct

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Victor Micallef

on 21 October 2016

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Transcript of use of contrast media in ct

contrast media in ct
why?
relatively a very good contrast resolution
about 1%
CM is used to change attenuation of one structure to differentiate it from an adjacent structure of similar attenuation
Classified into
positive
neutral
negative
have an attenuation which is higher than the average attenuation of body tissue
(higher attenuation than
water)
neutral contrast has an attenuation similar to that the average attenuation of body soft tissue
normally water or a water based liquid
negative contrast has a lower attenuation that body soft tissues
gas
modes of administering CM
other cavities

injected intravenously
oral administration
rectal administration
inhaled
usually an iodine compound
highly vascular organs or tissues
or
used to investigate the blood vessels themselves
this makes the blood appear bright on the CT image
when ivcm flows through the blood vessels, the high attenuation of iodine increased the average attenuation of the blood
types of ivcm
intraluminal cm
for decades the standard agent used was a
water soluble
dissolves in water to form a homogenous solution
containing iodine
oil based agents
relatively high atomic number
ease of attachment to organic compounds
53
excreted via urinary system
hysterosalpinography
sialography
iodinated
urographic
contrast media
ionic vs non ionic
the way the compound is bound can be ionic or non ionic

organic
may result in additional complications
they do not dissociate into component molecules
2 or more ions held together by attraction
covalently bind the iodine
have fewer side effects
A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms.
non-ionic CM


Difference between Osmolarity and Osmolality






much more expensive
lower osmolality results in less frequent allergic reactions
blood - 282 - 295 mOsm
non-ionic CM - 520 mOsm
ionic CM - 1500 mOsm (milliosmoles)
have much lower osmolality
Since the amount of solvent will remain constant regardless of changes in temperature and pressure, osmolality is easier to evaluate and is more commonly used, and often preferred.
Osmolality is a measure of the moles of solute per kilogram of solvent
Since the volume of solution changes with the amount of solute added as well as with changes in temperature and pressure, osmolarity is difficult to determine.
Osmolarity is a measure of the osmoles of solute per liter of solution.
Measurements of osmolar concentration are often expressed in osmolarity and osmolality.
advantages

less side effects
lower or no levels of
larger volumes can be used more safely
sensation of heat
vomiting
nausea
less severe effects of extravasation
extravasation is a known risk especially when injecting cm via a power injector
studies have shown that the complications from extravasation of ionic CM are much more severe
these may include tissue necrosis
other risks of IVCM include allergic reactions and nephrotoxicity

to manage these risks we follow established guidelines such as those issued by the RCR or ESUR
royal
college
of radiologists
european society of urogenital radiologists
non-renal adverse reactions
can be classified into
acute
late
very late
occurs in the 1st hour after injection
mild
moderate
severe
convulsion
nausea
mild vomiting
itching
severe vomiting
marked urticaria
bronchospasm
facial/laringeal edema
vasavagal attack
hypotensive shock
respiratory arrest
risk factors
patient related
contrast related
patient who had a previous moderate or severe acute reaction to an iodinated agent
asthmatic patient
previous allergy which required medical treatment
high osmolality ionic agents
occurs 1 hour to 1 week after injection
variety of late symptoms have been described in literature
nausea
vomiting
headaches
msk pain
fever
skin reactions
usually mild to moderate and self limiting
more than a week after administration of contrast
thyrotoxicosis
patient with normal thyroid function are NOT at risk
at risk
untreated Graves' disease
multinodular goiter
thyroid autonomy
renal adverse reactions
Contrast nephrotoxicity
defined as a condition in which an impairment in renal function occurs within 3 days following the administration of CM in the absence of alternative etiology
patient related
CM related
impaired renal function
high osmolality agents
large doses of CM
reducing risks
identify patients at risk
risk of CM may not always be higher than the benefits
withholding CM may deprive the patient of the benefits of valuable diagnostic information
the ultimate responsibility for CM admin rests with the person who prescribes it.

the delivery of injection is often delegated to others under local rules and protocols
a formal record of the decision to inject IVCM should always be made
if radiologist accepts a scan which under normal protocol includes the administration of IVCM, he is there prescribing the IVCM.
His vetting of the request on RIS is a record of him prescribing IVCM
of last 3 months
(or of last 7 days for patients with acute illness or known to have renal disease)
prior to IVCM administration all patients should have their renal function checked
this is done by checking the eGFR (blood test)
estimated glomerular filtration rate
in the presence of renal impairment, all CM are nephrotoxic
if this is combined with diabetes, the risk is significantly higher
congestive heart failure, old age and concurrent administration of nephrotoxic drugs are other risk factors for nephroxicity.
action
risk benefit assessment
reduce dose volume as much as possible without adversely affecting the diagnostability of the scan
the need for the investigation or intervention should be reexamined
if the use of IVCM is deemed necessary, steps should be taken to reduce the risk
use low or iso-osmolar non-ionic contrast
ensure the patient is well hydrated before and after the administration of IVCM
the role of iso-osmolar CM in terms of contrast induced nephropathy of these agents is not yet clear.
there is also insufficient evidence to advocate any prophylactic treatment which might reduce the incidence of CIN
metformin
this is an anti diabetic drug which is exclusively excreted via the urinary system
in the event of a poor renal function, this drug may accumulate in the blood and cause lactic acidosis.
renal impairment is a contraindication for metformin
there is no evidence that patients with normal renal function are at risk of lactic acidosis when taking metformin and are administered with IVCM
if eGFR is normal, there is no need to stop metformin before or after the admin of IVCM
if eGFR is lower than 45, any decision to stop metformin should involve the referring clinic.
IVCM and pregnancy
exceptional circumstances
thyroid function of baby is measured in 1st week after birth
this is due to a small theoretical risk of thyroid suppression on the fetus
lactation
no special precaution
a small percentage of the injected dose does enter the breast milk but virtually none is absorbed across the normal gut of the baby.
thyroid
IVCM is contraindicated in patients with hyperthyroidism
no radio-iodine treatment can be given for 2 months after IVCM
isotope thyroid imaging should be avoided for 2 months after IVCM
neutral CM
when water is used as oral neutral CM, it has the advantage of being better tolerated by most patients
Also, it does not interfere with post processing of images
Especially important when looking for small lesions in the small bowels
Better contrast resolution especially when used together with IVCM
Small changes in attenuation which might be an indication of a lesion or inflammatory process is more conspicuous.
disadvantages of water as an intraluminal contrast
because of its low viscosity it passes very fast through small bowels
the use of spasmolytic agent for the smooth muscle of the bowels will counteract this disadvantage
water is also absorbed very quickly by the intestinal mucosa
a non-reabsorbable salt or an aqueous suspension of long chain molecules can be used to slow this absorption
one example of such molecule is cellulose
Mannitol can also be used to slow absorption of water.
the suspension of this crystalline solid will create osmotic stress to this effect
for small bowel, contrast can be introduced via a trans-gastric catheter to provide a more uniform opacification of the small intestines. This is called enteroclysis.
rectal administration of intraluminal CM is used for fast opacification of the colon
Better imaging of the colon can be achieved with gas insuflation
air or carbon dioxide is used
the colon is distended making virtual colonscopy possible.
Both barium and iodinated agents are available for this purpose and the optimal concentration for use with CT is very low.

the use of positive oral CM is on the decline after many studies have shown that the advantages are few
positive CM
Power injector
accurate control on volume and rate of injection
fast scan time
injection protocol more critical
significantly less room for error
optimum contrast enhancement can be achieved with a smaller volume of injected contrast
risks
air embolus
extravasation
also present in MRI
but since injected volumes are much smaller and the injection rates slower the risks are smaller
Small and moderate-sized air emboli are estimated to occur in 12% to 23% of patients undergoing CE CT examination.
most are asymptomatic as air is easily absorbed. therefore they go undetected
the accidental injection of air into the blood stream
venous air emboli more common in medical imaging procedures
arterial air embolism have much more serious consequences than venous
mechanism of introduction of air into blood steam
connection between vascular system
pressure gradient
can be introduced during insertion of cannula or when connecting injector to cannula
can be also injected from improperly flushed syringes and tubing
flow of injected air bubbles
brachial veins drain into right heart atrium

enter pulmonary circulation and travel to the lungs
small emboli are absorbed in the blood or alveoli of the lungs
larger emboli may obstruct the outflow from right ventricle or block pulmonary arterioles
the severity of the symptoms depend on
volume of injected air
injection rate
patient's state of health
25% to 35% of the general population retain a patent foramen ovale
this and other atrial or ventricular septal defects may allow venous air emboli to cross into the arterial circulation
this is significant because an arterial air embolus as small as 1ml may cause serious consequences
cerebral or coronary arteries
symptoms of air emboli
most asymptomatic
reflexive gasp
cardiovascular
reduced cardiac output
drop in blood pressure
collapse
diagnosis
trans-oesophageal ultrasound
doppler
CE CT
minimizing the harm
identify air entry point and close it to prevent further air entry
place patient in Durant position
left lateral decubitus and trendelinburg position
this might relieve the air lock by floating the air bubble out of the pulmonary vasculature.
intervention
multi-oriface central line is used to aspirate the air
patient is observed in an ITU setting
ECGs
cardiac biomarkers
coagulation studies
repeated imaging
arterial blood gases
hyperbaric oxygen therapy
may reduce the size of the embolus and improve cardiac and cerebral symptoms
minimising the risks
education
competency
written procedures
good practice
equipment
only staff who are adequately trained
familiar with operating procedures
familiar with the risks of air emboli
periodic assessment of radiographers performance
readily available operating procures and injector operators instructions
inspect for air before the injection
inject only when the injector head is pointing downwards
abort procedure if air is noticed in the system
air detection devices to minimize risk
not foolproof
transparent syringes
oval markers
fluid dot system
tilt sensor
prefilled syringes
ultrasound technology
incidence varies
0.001 - 0.4 %
Reducing the risks
release tourniquet
injection only via plastic cannula
extra care taken when venous access was difficult
recognizing extravasation
early detection is important to minimize the damage
extravasation should be suspected when one of the following is present
moderate or severe pain, burning or stinging or any acute change at injection site
redness , swelling or leakage of CM or blood at injection site
resistance to free flow of CM
(fill with injector head up)
when possible, injection site is observed during actual injection
Cannula of 20G min is used to avoid jet effect
Best vein available is used
antecubital vein is preferred
palmar aspect of wrist and other small veins are not used
Xenon is inhaled during the Ct to assess changes in cerebral blood flow in the period shortly after a traumatic brain injury, or to detect or indicate the location of a stroke.
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