Radiation Protection Flashcards
what is the main concept to follow for radiation protection in CT?
ALARA
as low as reasonably achievable
what three principles should encompass radiation protection in CT?
strict clinical indication
protocol optimization
shielding
What three scenarios would limiting CT scanning benefit in saving radiation dose?
limiting ct to strict indications
limit scan lengths to clinically indicated areas
minimize multi-phase exams
What determines an optimized protocol?
One that acquires images with acceptable noise levels at low possible dose, while still delivering images of diagnostic quality
What 4 things need to be considered when making changes to scanning parameters?
examination indication, regions of interest, patient’s body habitus and age
What is the process of adjusting parameters such as mA/kVp, slice thickness and pitch for the purpose of reducing patient radiation dose while maintaining adequate image quality?
protocol optimization
What relationship lies between mA setting, scan time and patient dose?
directly proportional
If mAs is doubled, what happens to radiation dose?
doubled
what relationship is between kVp and radiation dose?
direct and exponential
What will happen as result of decreasing only kVp? (mas, scan time and pitch stay the same)
significant decrease in quantity of x-radiation produced by CT tube and subsequent decrease in patient dose
What two situations can a decrease in kvp be applied without a loss in image quality?
pediatric imaging and CT angiography exams
What is the section of the patient being scanned referred to in CT data acquisition?
dose profile
What controls the dimension of the longitudinal (z-axis) of the dose profile and the measurement of it at the isocenter?
collimation
With what type of scan will an increase in collimation result in a decrease in patient radiation dose?
single slice
What happens to slice thickness when collimation is increased?
thinner slice width
How is collimation proportionately related to radiation dose?
indirectly
How does an increase in collimation affect image noise?
increases
although not usually necessary, what is typically adjusted to accommodate for the increased noise from thin sections?
increase in mAs
What type of collimation is used in MDCT and also improves dose efficiency because less overbeaming occurs?
wide collimation
What type of beam width increases overbeaming while reducing dose efficiency?
narrow beam width
reconstructed algorithms and windowing settings have what type of controlling factor with patient dose?
indirect
What are noise levels in CT a function of?
dose
How does a decrease in dose affect image noise?
increases
How can noise be reduced after image is displayed?
appropriate reconstruction algorithm or correct window settings
What is a factor that image noise is directly related to?
patient size
What happens to image noise when patient size decreases?
decreases
What is a protocol optimization for making adjustments for noise and patient size?
size-based dose (mAs) adjustments
Should the clinical indication be considered in attempt to reduce radiation dose?
yes
What is an example of a clinical indication and scan that can be imaged with less dose and tolerate more noise for a diagnosis?
CT Abdomen and Pelvis for renal stones
Why can a scan for kidney stones be acquired with less dose and still obtain diagnostic quality images?
the kidney stones absorb radiation due to high density structures, creating high contrast pathology
during SSCT, pitch greater than what value can allow for the acquisition of a given scan volume in a shorter time and reduce patient dose?
greater than 1
What does an increase in pitch in MDCT result in?
image noise
What technical factor is considered when increasing pitch in order to reduce noise and obtain diagnostic image quality?
increase in mA
What feature has been added to the scanner to manage patient radiation dose on the basis of size, density and overall attenuation of the part being imaged?
AEC - automatic exposure control
What two types of automatic tube current modulation exist?
Angular and Longitudinal
This type of ATCM runs along the x- and y- axis and automatically adjusts mA settings according to slice thickness of the part as the tube rotates from AP to LAT position
angular ATCM
When using the angular option of ATCM, which plane attenuates the least radiation as the tube moves from AP to LAT?
AP position
This type of ATCM runs along the z- axis and adjusts the mA setting based on attenuation differences of the patient as the scan moves longitudinally down the z-axis
longitudinal ATCM
What type of gating is used during MDCT cardiac studies in order to reduce patient exposure?
prospective
What feature in prospective gating allows for pulses of x-radiation rather than continuous exposure to be used?
ECG- triggered tube current modulation
What is reduced during the cardiac phase that is not utilized for image reconstruction?
tube current
The radiation dose savings gained with ECG-triggered tube current modulation depend on the patient’s what?
heart rate
What happens to radiation dose savings when a patient’s heart rate increases?
decreases
What setting is available on some scanners that adjust tube kilovoltage according to varying patient attenuation during CT acquisition?
automated tube voltage selection
what object can be utilized during CT to reduce scatter radiation dose to sensitive tissues that lie outside of the scan acquisition range?
out-of-plane lead or contact shielding
What are two examples of radiosensitive organs that should be shielded?
breast tissue and gonads
Where on the patient should contact shields be placed in order to maximize dose reduction as best as possible?
both above and below the patient
Where does the lead shield need to be located on the patient when using ATCM while scanning?
outside the scan range
what will occur if a lead shield is placed within the anatomy being scanned with ATCM?
increase in radiation exposure to patient
What type of shielding can be utilized in the orbits, thyroid and breast tissue to reduce effective radiation dose?
in-plane bismuth shield
what other material can be added to bismuth shield to reduce dose?
rubber
What is a disadvantage of bismuth shielding that may occur on the image?
streak artifact
What can be placed between the patient and the bismuth shield to reduce scatter artifact?
thin foam spacer
What organization recommends against the use of bismuth shielding in favor of more comprehensive dose reduction techniques?
AAPM- American Association of Physicists in Medicine
What personnel evaluates the room shielding requirements?
qualified radiologic health physicist
What are four considerations taken when evaluating room shielding requirements?
exam workload, scanner position, construction of doors and windows
