contrast enhancement and injection techniques CT

atomic number iodinated contrast

n=53

does iodine have a higher or lower HU than blood

higher so appears radiopaque

osmolarity describes

number of particles in solution

blood plasma: 290 mOsm/kg of water

hyperosmolar is 

7x the value of blood plasma (290mOsm/kg)

hypo-osmolar is

2x the value of blood plasma (290mOsm/kg)

viscosity describes

the friction of fluid as it flows

the higher concentration of iodine means what in a contrast solution

it is more viscous

ionic vs non-ionic

ionic dissociates into ions when in solution

non-ionic do not but have less adverse reactions

most non-ionic contrast agents have ____ osmolarity

low

if the focus on the study is imaging the urinary system and we want to make sure there is no mass in the area. The patient has hematuria (typically stones or a mass), what can we do to rule out mass

give contrast to highlight masses

stones can sometimes be seen without contrast

what is something we need to keep in mind when imaging the bladder

the urine will dilute the contrast so we want to use a higher osmolality option

if the osmolality of contrast is close to blood what can we assume about adverse reactions

there will be less adverse reactions

how does the body dispose of contrast in approx how long

through filtration in the kidneys 2 hours approx

what is contrast induced nephropathy. who do we need to be concerned about

the contrast causes a temporary drop in kidney function - asymptomatic normally and recovers fast.

sometimes it does not recover quickly and there is a drop in renal function

patients at risk: diabetic, kidney transplant patients, kidney failure patients, 70+ patients, patients with 1 kidney

CIN is dependent on what dose?

the dose of contrast media used

what medications can make CIN worse?

NSAIDS

metformin therapy needs to be stopped - lactic acid buildup

what are the different eGFR numbers associated with in terms of delivering contrast?

eGFR > 45: give contrast

30 < eGFR < 45 with no renal risk factor: give contrast

30 < eGFR < 45 + renal risk factors: written consent, hydration, give contrast

eGFR < 30: NO CONTRAST

idiosyncratic reactions, how fast do they occur, do they produce antibodies

adverse reactions not related to dose

idio means we don’t know the exact reason 

typically occurs within 1 hour of injection 

no antibodies produced

what is the most important factor in screening

idiosyncratic reactions 

if a patient has an eGFR below 30 and has recieved dialysis within the last 24 hours can we give them contrast?

yes we can give contrast

if a patient had a hives reaction last time we used contrast can we give contrast next time? what about an anaphylactic reaction?

hives: give contrast with pre-medication

anaphylactic: no contrast, not worth risking

comparing the brain and lungs which has higher inherent contrast?

the lungs - air and tissue contrast

do contrast agents increase or decrease sensitivity

increase sensitivity

what must be the difference in HU # for us to see a difference in tissues on CT

10 HU difference between tissues

do contrast agents increase or decrease specificity? what does that mean?

increase specificity

we can better see the type of tumour

a patient is coming in for a brainscan with a known meningioma, what does this mean should change with our procedure?

we can image almost immediately after injecting contrast

what are the phases of tissue enhancement in CT?

1. arterial phase (bolus phase)

2. venous phase (non-equilibrium phase)

3. delayed phase (equilibrium phase)

how is the ROI tool used?

we can use it to drop on the region of interest to know the HU number

can tell phases - place one in a artery and one in a vein then compare the HU numbers to tell if we are in an arterial or venous phase

arterial phase means the AVID is what number?

AVID >- 30

what can we expect to see in the venous phase? AVID?

Both artery and vein have unequal contrast but both have

10 HU < AVID < 30 HU

about how long after the arterial phase do we see the delayed phase?

2 min

what can we expect to see in delayed phase? AVID?

contrast is empties from arteries and greatly diluted in veins and soaked into organ parenchyma

AVID < 10 HU

when can delayed phase be useful?

to assess if the metastases uptake contrast to determine if they are tumours. typically the delayed phase will be part of a series 

Iodine dose is determined by:

contrast concentration and contrast volume

contrast volume formula

volume = injection rate x duration of injection

mL = mL/s x s

with an increase in contrast volume (while maintaining everything else) what do we notice about PE (peak enhancement) on a graph?

1. PE increases

2. time to PE increases - takes longer to reach

3. duration of any given enhancement level increases

4. duration of injection increases

when we inject an increased concentration iodine (while maintaining everything else) what do we notice about PE (peak enhancement) on a graph?

1. PE increases

2. duration of any given enhancement level increases

3. time to PE remains unchanged

when we decrease the kVp (while maintaining everything else) what do we notice about PE (peak enhancement) on a graph?

1. PE is higher for any given concentrations of iodine 

what combination of adjustments for PE would give us the highest PE?

low kVp and high concentration would give the highest PE as the ray is being more absorbed since it has lower energy and the iodine is concentrated so it does a better job at attenuating the beam

what parameters determine the phase of enhancement captured?

1. rate of injection (ml/sec or cc/sec)

2. scan delay

what does scan delay describe? what is another name for it?

the amount of time between starting injection and imaging

another name is prep delay

with an increased rate of injection (while maintaining everything else) what do we notice about PE (peak enhancement) on a graph?

1. PE is higher

2. time to PE is shorter

3. duration of PE is shorter

IV gauge and contrast injection rate for arterial phase

18-20G and 4-6cc/sec

IV gauge and contrast injection rate for venous phase

22G and 2-3cc/sec

is an aortic or venous enhancement better for showing a PE

arterial

why are dual syringe power injectors better than hand-injecting?

1. deliver precise flow rates and volume

2. injections are consistent - can be reproduced

3. programmable

4. saline flush 

what is saline test used for? what is the typical flow rate of a saline test?

to make sure the contrast is going in okay

flow rate: should be the same as the flow rate that will be used for contrast 

what is the formula for calculating the volume for injector consoles?

volume = flow rate x duration 

what is the split bolus technique

total contrast dose is split (often in half). first dose given with a delay of 1.5-2 min. this allows the structure that are slower to enhance to opacify. then we inject the second bolus of contrast using the secondary injection to get the arterial vessels opacified.

dual injector protocol with a split bolus technique

1. saline test

   1. hold

2. 50% of contrast volume (lymph nodes + soft tissue enhanced)

   1. pause time - varies

3. other 50% of contrast volume (arterial + venous enhanced)

   1. image so we have tissues and vasculature enhanced to some degree

4. saline flush

when does scan delay start?

as soon as we start injecting the contrast 

importance of saline flush? what does it increase?

pushes the tail of injected contrast bolus into central blood volume 

allows for more contrast to enter the organ being imaged because we are pushing the last bit of contrast in 

increases: efficiency and utilization

how is bolus shaping achieved? what rate should the saline flush be?

beginning the injection with a relatively high flow rate and then using a slower flow rate for the remainder of the injection period.

saline flush should be at the same rate as contrast injection

what artifact does a saline flush help to reduce?

reduces streak artifact - comes from dense contrast medium

what can a streak artifact mimic?

pulmonary embolism

what are the common areas of streak artifacts and what can we do to reduce them?

streak artifacts are from dense contrast medium and typically found in the SVC (pooling) or the arteries that exit when we image while the pt. has their arm up which can pinch vessels. 

reduction: saline flush to reduce density of contrast

if we inject and the pressure curve goes straight up above the pressure threshold what is a common cause?

forgetting to unclamp the IV

what color are saline and contrast typically on graphs?

saline: blue

contrast: green

patient factors affecting contrast enhancement

1. weight and height - BMI

   1. affect on aortic, venous and parenchymal enhancement 

2. cardiac output - affects more

   1. decrease CO = circulation of contrast slows 

   2. the contrast bolus will arrive slowly and clear slowly so there will be a delay in peak and prolonged contrast enhancement profile

as patient BMI increases what can we expect to see on a PE graph?

1. PE decreases

   1. larger BMI = larger blood volume = contrast more diluted

2. time to PE remained unaffected

   1. dependent on flow rate and we are not changing flow rate

what is most important to take into account when calculating the scan delay to use?

patient cardiac output

Timing bolus method

challenges with the test bolus method?

1. increased contrast load

2. timing calculation errors

bolus triggering /automatic triggering

bolus triggering /automatic triggering challenges

1. technologist cannot stay with patient once contrast injection begins due to absence of scan delay

2. the patient may move (most common) causing the ROI to register incorrect reading

3. abnormalities in vasculature may cause ROI placed over the are not receive adequate contrast