Dose Measurement Flashcards
What term is used to describe the reconstructed CT section?
slice sensitivity profile
What term describes the section of tissue exposed to ionizing radiation and has a greater width than SSP?
dose profile
What term is used to describe the ability of x-rays to ionize a volume of air?
exposure
What is “exposure” measured in?
roentgens (R)
What term describes the amount of x-ray energy absorbed in a unit of mass?
absorbed dose
What is “absorbed dose” measured in?
grays (Gy)
What is another term used to describe absorbed dose?
kerma
What describes the amount of radiation absorbed in a quantity of air?
air kerma
What accounts for the type of tissue radiation is deposited in?
effective dose (EfD)
What factor determines the weighting factors that are assigned to different tissues?
individual radiosensitivity
What is effective dose used to calculate from exposure to ionizing radiation?
risk
What is effective dose measured in?
sieverts (Sv)
What is also the estimation of the whole-body dose equivalent that results from the partial body exposure the patient receives during a CT procedure?
effective dose
What is an approximate measurement of the dose received in a sinlge CT section or slice?
CT dose index (CTDI)
What is calculated from the central slice of a series when 7 slices surround each side?
CTDI
What measurement is taken with a 100-mm-long pencil ionization chamber and makes no reference to a specific number of slices?
CTDI-100
What causes dose to vary within the image across the acquired FOV?
absorption
What measurement is calculated by the sum of 2/3 of periphery and 1/3 of central exposure of the field?
CTDI-w
What dosimeter measures and calculates radiation exposure for CTDI-w?
TLD’s
What is measured using the axial or “step-and-shoot” method of scanning?
CTDI-w
What is used to measure radiation dose from the helical scan method?
CTDI-vol
What is the formula for calculating CTDI-vol?
CTDI-vol = CDTI-w/pitch
As pitch increases, what happens to the dose per section?
decreases
What axis planes does the CTDI-w approximate dose and what is it measured in?
x- and y- axis; mGy
What axis planes does the CTDI-vol approximate dose and what is it measured in?
z-axis; mGy
What is CTDI-vol similar to?
MSAD- multiple scan average dose
What is a calculation of the average cumulative dose to each slice within the center of a scan consisting of multiple slices?
MSAD
What does MSAD take in account for which makes the average dose higher in a single slice?
scatter radiation
Are the MSAD doses from the beginning and end slices more or less than the central slice?
less
What is the formula for calculate MSAD?
MSAD = T/I x CTDI
What does MSAD account for when calculating dose from an axial scan?
image spacing or “bed index”
Do overlapping scans in axial scanning increase or decrease dose?
increase
Does having gaps between slices in axial scanning increase or decrease dose?
decrease
When slice thickness is greater than image spacing, does MSAD increase or decrease?
increase
When slice thickness is less than image spacing, does MSAD increase or decrease?
decrease
What is MSAD controlled by during a helical scan?
pitch
What is the formula for calculating MSAD of a helical scan?
MSAD= CTDI/pitch
What is the MSAD dose when slice thickness is equal to image spacing?
equal
What term describes the amount of table travel per tube rotation divided by the collimation?
pitch
What part of a scan series is MSAD most accurate?
center
What two measurements are used to calculate the average radiation dose within a scan volume?
CTDI-vol and MSAD
dose amongst what scan plane is not considered in the average radiation dose calculated by CTDI-vol and MSAD?
z-axis
What is the formula for DLP?
DLP= CTDI-vol x scan length (collimation) x No. of slices in scan volume
What is the formula for DLP when using CTDI-vol and scan length to calculate the total dose and what is it measured in?
DLP= CTDI-vol x scan length; mGy-cm
What setting controls the CTDI-vol?
prepatient collimator
What object is used to determine the measurement of effective dose since calculating a direct measurement is not possible?
phantom
What factor is not accounted for by CTDI-vol and DLP and typically leads to under- or over-estimation of dose?
patient size
What two other ways may CTDI-vol and DLP be expressed as?
computed tomography air kerma index (Ca) and air kerma length product (Pkl)
What report is included by DICOM to give information regarding tube’s radiation output during CT acquisition like: tube current and voltage, scan time, acquisition length, pitch, and collimation width?
RDSR- Radiation Dose Structured Report
Does the RDSR report automatically get archived into the patient’s electronic medical record?
yes
What is a new automated alert system that indicates when the anticipated dose for a given acquisition exceeds the recommended maximum?
dose check
What are the two levels of check associated with dose check systems?
dose notification and dose alert
Which dose check system informs the technologist when the prescribed technical settings for an individual CT acquisition may result in a CTDI-vol or DLP higher than a preset recommended value?
dose notification
Which dose check system alerts the technologist when the prescribed acquisitions for an entire CT exam results in a CTDI-vol and DLP higher than preset value?
dose alert
What four requirements have CT systems been designed to obtain to reduce patient exposure?
automated CT dose check
separate protocols for adults and pediatrics
DICOM structured reporting
AEC
