Radiation Dosimetry in CT-Unit 6 Flashcards
Read over the following CT parameters that affect radiation dose:
Make a chart
- Tube Current (mA)
- Exposure Time
- Tube Current Modulation
- Tube voltage
- Pitch
- Effective tube-current time
- Slice Thickness
- Reconstruction Algorithm
- Filtered Back Projection
What happens to dose when mA increases
Dose increases
What is another name for exposure time in CT?
tube rotation time
What does tube rotation/exposure time mean?
Time for one complete rotation of the CT gantry
As the speed of rotation increases, what happens to dose?
Dose decreases
What is the relationship between x ray tube rotation time and dose?
They are directly proportional to each other
What is the relationship between radiation dose and rotation speed?
They are inversely proportional
What is the primary use of tube current modulation (TCM)?
To maintain image quality (SNR) by providing proper tube current settings for variable patient sizes and exam indications.
What are the three ways in which tube current is modulated in a CT scanner?
- Tube current is modulated in the x, y and z axis based on oval ratio, attenuation values from the CT localizer
- Modulated from attenuation values obtained in the first 180 degrees of tube rotation during image acquisition
- (or both).
As kVp increases, what happens to dose?
Dose increases
What is the relationship between kVp and dose?
kVp is directly (non-linearly) related to dose
For example, a 14% decrease in tube voltage from 140 to 120 kV will reduce patient exposure and decrease radiation dose by up to 30%–35%.
What is the disadvantage of decreasing kVp to try to reduce dose? How can this disadvantage be compensated?
-Disadvantage is Increased image noise
-To compensate for the increased noise, one can increase the mAs setting (if TCM is not utilized).
T/F
The low-tube-voltage, high-tube-current CT technique decreases dose while maintaining image quality.
True
In what types of patients is the low-tube-voltage, high-tube-current CT technique efficient for?
Small and average-sized patients
T/F
For large patients, lowering the kVp results in increased noise that may not be overcome by increasing the tube current.
True
In large patients, why is a higher tube voltage the most dose efficient strategy?
There is better penetration through the organs of interest.
Why is lowering the kVp for smaller/average size patients beneficial for attenuation produced by iodine based contrast material?
Attenuation produced by iodine-based contrast material increases up to 100% if tube voltage is decreased from 140 to 80 kVp because of the higher probability of photoelectric interactions at the lower tube voltage.
(dont do this if its a large patient)
VERY BENEFICIAL FOR angiographic studies!
T/F
Tube volatage cannot be modulated in real time.
True
(mA can be in real time)
What is automated tube voltage selection software?
This software automates/aids tube voltage selection based of the patien’ts attenuation profile from the scout and user’s chosen examination.
Does Tube voltage modification provide diagnostically acceptable image quality?
Yes
Initial reports show that when compared with fixed 120-kV protocols, automated tube voltage software frequently suggests a tube voltage of 100 or 80 kV in more than 50% of contrast-enhanced abdominal CT examinations.
Importantly, the reduced tube voltage values were found to provide diagnostically acceptable image quality.
What is the relationship between Pitch and dose?
They are inversely proportional to each other
If pitch is increased, what factor compensates for the change to maintain image quality?
Tube current automatically increases
(only in systems that use an effective mAs setting)
What is the relationship between patient dose and slice thickness?
Inversely proportional
As slice thickness decreases, what happens to dose?
It needs to increase