Updated CT IV

for two materials to be differentiated

they have to be atleast 10 HUs away from each other

max dose of contrast medium given in a day

200 ml contrast + 64 g iodine

total volume given: 50-150 ml

studies for chest, abdomen injection rate

1-3 ml/sec

special procedure injection rate

CTAs, cardiac imaging

up to 4-5 ml/sec

which phase of contrast enhancement to use

depends on pt size, cardiac output, contrast agent [], injection rate

multiphase protocol

getting images from different ehancement phases

arterial phase I

immediately follows contrast administration, lasts for 60 s

arterial structure enhancement (can see difference between artery and vein)

arterial phase requires

arterial venous iodine difference of 30 or more Hus

venous phase

follows arterial phase, begins 1 min after injection

contrast still in artery, vein, organ parenchyema

what is the contrast phase that most studies use

venous phase

equilibrium delay phase

last phase, 2 mins after venous

requires arterial venous iodine difference <10 Hus

venous phase requires

difference of 10 to 30 HU of arterial venous iodine difference

equilibrium delay phase II

contrast mostly empty from artery, diluted in vein, entered organ parenchyma, see more contrast in kidney/ureter (bc contrast start to exit bodY0

what is the most common, easiest, and accurate dosimeter

ion chamver

ion chamber process

collect electron charge

electrometer removes charge and measures RT dose

what are the type of dosi phantoms

16 cm (head) or 32 cm (body) (diameter)

16 cm diameter dosi phantom

used to simulate adult head or child abdomen

holes to position ion chamber

32 cm diameter phantom

sim adult abdomen

holes to position ion chamber

CT dose descriptors

Multiple scan average dose

ct dose index

dose length product

effective dose

size specific dose estimate

every scan must have which CT dose descriptors

multiple scan average dose (MSAD)

CT dose index (DLP)

CT Dose index: CTDI FDA

represents mean absorbed dose in mGy

only able to measure dose for 14 slices, up to 7 mm thickness

CTDI 100-what does it account for

accounts for 100 mm length of ion chamber

sum of all dose profiles over 100 mm length

CTDI 100 no account for

no acocunt for slice thicnkess

no account for the non-unfirom dose distribution (surface relative to center)

CTDI weighted (w)

accounts for non uniform distribution (center vs periphery)

no account for pitch

what CT dose index is used for calc today

CTDI volume

what is CTDI vol

standardized measure of radiation output of CT system and no represent actual absorbed dose in patient

no consider the total scan length

what does CTDI vol account for and not account for

account for dose in the z axis by inclusion of pitch

does not consider total scan length

what is dose length product

provides estimate of total amount of radiation absorbed by patient for a series of scans

measurement unit: mGy-cm

what does dose length product account for and not account for

considers total scan length

no consider organ/tissue relative radiosensitivty

iodine is an example of

positive contrast agent

the most accurate method of estimating CT dose involves

ion chamber

which of the follow pathologies can be diagnosed in this CT image

aortic dissecition

pulmonary embolism

lung carcinoma

hemangioma

aortic dissection

focal nodular hyperplasia

hepatic cyst

renal calculus