chapter 14

total dose is from the radiation to the selected slice and from scatter radiation or _______ from neighboring slices

"tails"

Two components regarding radiation dose to the patient:

-appropriate patient selection -minimizing the radiation dose without compromising the diagnostic image quality

The level of risk associated with CT exams can be considered acceptable IF :

-the individual is aware of the risk -the individual receives some commensurate benefit -everything reasonable has been done to reduce the risk

Exposure from a chest x ray is about ?

0.1 msv

risks associated with pediatric CT, list 3 recommendations that should be followed by health care professionals

1. the patient -or parent - should be told of the small risk involved 2. the procedure should be restricted to cases in which it is specially indicated 3. every effort should be made to decrease radiation dose

Exposure from a chest, abdomen for pelvis CT is about ?

10 msv

In general, the tails will contribute approximately ____________ to the total dose

25%-40%

average background radiation for americans is about ?

3 msv

When the radiation doses used in adult protocols are used in neonates or young children, the effective dose is up to _____ greater

50%

______________ represents the kinetic energy released in air and collected in air per unit mass when an x ray beam is traveling through the air

Air kerma

accounts for the exposure variation in the z direction

CTDI

Special Considerations for Pediatric CT

Increased sensitivity Higher effective dose Increasing use

-The total dose is the central slice radiation dose, plus scatter overlap (tails); dose calculated from multiple scans is known as: -Will increase if slices overlap and decrease if there are gaps between slices

MSAD

Is patient shielding effective in reducing dose from CT?

No, because of narrow collimation, radiation areas outside that of the selected scan is minimal and usually attributable to the internal scattering of photons that are unaffected by surface shielding

is applied to the absorbed dose -this accounts for the different biologic effects produced from different types of ionizing radiation -It is 1 for the diagnostic rays used in CT

Quality factor

The CGS unit of absorbed dose is called the radiation absorbed dose, or ______. This unit describes the amount of energy absorbed per unit mass

Rad

The unit of x ray exposure in air

Roentgen (R)

unit that represents quality factor being applied to radiation absorbed dose (rad)

Roentgen Equivalent Man (Rem)

The SI equivalent for rem is ?

Sievert (Sv)

List all the possible methods for reducing dose from CT

adjusting mAs ATCM Avoid increasing kvp increased pitch limit use of thin slices limit repeat scans newer reconstruction methods

why are children more radiosensitive than adults?

because of their younger age with more time to develop cancer than adults and because they have more dividing cells and thus suffer more adverse radiation effects

-Allows an estimate of the MSAD to be accomplished with a single scan -Is what manufactures report to the FDA and prospective customers regarding the doses typically delivered for their machines

computed tomography dose index CTDI

The radiosensitivity of a developing fetus is greatest from ? because this is the time of organ and neural crest development

conception to 3 months

However, for multiple slice examinations, _________ slice thickness and using contiguous slices will increase the MSAD because of the increased amount of scattered radiation

deceasing

the uniformity of the dose ________ as the SFOV and the patient thickness increase

decreases

accounts for the length of the scan

dose length product DLP

attempts to account for the effects particular to the patients tissue that has absorbed the radiation dose

effective dose

what are defined as those images obtained beyond the desired area of interest -added approximately 10% to the patients radiation dose

extra images

what affects the radiation dose by removing some of the soft low energy x rays that are easily absorbed by the patient

filtration

The SI unit for absorbed dose AND air kerma is ?

gray (Gy)

what factors contribute to the concern regarding the radiation dose delivered from a CT examination ?

higher use newer scanners that deliver higher doses new information that relates low dose to higher cancer risk lack of knowledge facilities not adjusting scanning parameters

As slice thickness _________, the volume of tissue irradiated ___________ and the dose may ___________ slightly to the adjacent slices

increases

Kerma (kinetic energy released in matter) is defined as the amount of energy transferred from the incident x ray photons to charged particles per unit mass in the __________

medium

The major concerns regarding risks to the fetus with low levels of exposure associated with body CT are ?

neurologic and carcinogenic in nature

A reduction in dose is associated with a subsequent increase in ________

noise

The central dose of a body scan is approximately ________________________ that of the peripheral dose

one third to one half

The spacing of CT slices obtained with a spiral or helical scan process is called ? -is defined as the table distance traveled in one 360 rotation divided by the collimated width of the x ray beam

pitch

what are the factors that affect radiation dose in CT?

radiation beam geometry filtration detector efficiency slice width and spacing pitch patient size and body part thickness collimation

The unit for the equivalent dose is also the _________

rem or the Sv

What does a pitch of 1 mean

that the slices are adjacent and not overlapped

t/f: body scans are less uniform than head scans

true

The radiation dose is more uniform in CT because the beam is heavily filtered as it exits the x ray tube. And the CT exposure comes from all directions, creating a more _________________

uniform exposure

describes variations that occur along the length of the patient

z axis distribution profile