radiation protection ct (ch. 10/11)

ionization in air

exposure

exposure measured in

Roentgen

1 R is equal to

2.58 × 10^-4

measures energy of ionzations

air kerma

air kerma measured using

Gray (Gy)

air kerma and exposure can be used to measure

radiation from CT xray tube

scatter

increase in MA does what to exposure and air kerma?

increases

increase in kVP does what to exposure and air kerma?

increases

absorbed dose measured in

Gray (Gy)

accounts for different types of radiation

equivalent dose

equivalent dose measured in

Sv

accounts for radiosensitivity of tissues (represents risk of radiation induced cancer)

effective dose

effective dose measured in

Sv

Wr for x-rays, beta, and gamma

1

Wr for protons, slow neutrons

5

Wr for fast neutrons

10

Wr for alpha

20

Wr in CT is always

1

removal of an electron

ionization

interactions that ionize

photoelectric and compton

what can happen to a cell during ionization?

cell may/may not repair or repairs incompletely

decrease in beam intensity because of interactions with matter

attenuation

interactions that cause attenuation

coherent

photoelectric

compton

increasing patient size, tissue density & atomic number does what to attenuation?

increases

increasing kVp does what to attenuation?

decreases

dose determines the effect (increased dose increases severity of the effect)

deterministic effect

another name for deterministic effects

non-stochastic

deterministic effects measured by

threshold dose (below that amount, no effect)

deterministic effects usually only related to

very high doses of radiation (not those used in CT)

examples of deterministic effects

radiation induced miscarriage

erythema (skin redness)

sperm depletion

epilation (hair loss)

probability of effect with ANY dose (increased dose increases probability of effect)

stochastic effect

deterministic effects is (early/late) and stochastic effects is (early/late)

early

late

average dose to any area of the patient in the scan field (average of entire area)

CTDI

when does CTDI compare technique?

between scans (high or low means setting was incorrect)

dose length product formula

CTDI x scan length

dose to tissues and total volume of tissues exposed

dose length product

DLP is good for

estimating biologic effects (total amount received by patient)

adult head CTDI

75 mGy

adult abdomen-pelvis CTDI

25 mGy

adult chest CTDI

20 mGy

pediatric head CTDI

40 mGy

pediatric abdomen CTDI

20 mGy

what must be documented for every procedure and every scan series within the procedure?

CTDI and DLP

most facilities record CTDI and DLP in

DICOM

dose that produces quality images with the lowest dose

optimal dose

factors that change optimal dose

mA

kVp

pitch

beam width

patient positioning

number of x-rays exiting the tube

mA

increasing mA does what to patient dose

increases (directly proportional)

if mA is too low, image will have

noise

having same mA for entire scan, increased patient dose

fixed mA

adjusts for variations in part thickness

varying mA

controls energy and penetrability of beam but also affects number of x-rays in beam

kVp

as kvp increases, patient dose

increases

distance table moves compared to beam width

pitch

as pitch increases, patient dose

decreases (inversely proportional)

pitch can only be used with

helical scans

detector width x number of active detector rows

beam width (collimation)

as beam width increases, patient dose

decreases

patient should be where on the gantry

center

what kind of positioning increases patient dose?

too high, too low, or off center

when scan length extends beyond the planned image boundaries

overranging

overranging occurs automatically with

helical scans

when scanning radiosensitive areas use

axial

lower pitch or detector collimation

scanner changes the mA during the scan based on tissue being scanned

automatic dose modulation

scanner uses __ to find densest parts of patient

localizer images

superior-inferior modulation

z-axis

if a scanner adjusts AP and lateral

x and y axis

other names for automatic dose modulation

AEC and ATCM

displayed on control panel when CTDI for the scan is set to exceed predetermined notification values

dose notification

dose notification is designed to

prevent overexposure (tech knows they’re using a high dose)

displayed on control panel when cumulative CTDI for a series of scans is set to exceed 1000 mGy

dose alert

if change is not made for dose alert, tech must enter

name and reason for high dose

lead shielding can be used if

not in scan field area

only attenuate low energy photons and reduce entrance dose

bismuth shields

bismuth shields can be placed in scan field when?

after localizer images are acquired

reduce image noise and minimize higher radiation dose inherent in filtered back projection algorithm

iterative reconstrion and deep learning image reconstruction

IR uses measured projections to create simulated projections which are compared to initial measured projections to determine

noise

overall IR images reduce

noise and artifact

2 ways to practice radiation safety

-wear lead aprons

-minimum shielding in walls and control room window

campain to increase awareness about radiation protection of adults

image wisely

campaign to improve safe and effective imaging care of children worldwide

image gently