Unit 5: Image Quality in CT (Part 1) Flashcards
T/F
The aspects that control exposure in X-ray is the same in CT
True
T/F
Increasing mA decreases the number of electrons that will produce x rays
False; Increasing mA increases the number of electrons that will produce x rays
Why is having a smaller filament better for increasing spatial resolution?
Using a smaller filament concentrates the focal spot, which in turn results in increased spatial resolution
What are the disadvantages of a smaller filament?
-Does not tolerate high mA values a well as large filaments so time must be increased to maintain mAs
In CT, what is scan time?
The time it takes for the x ray tube to make one 360 degree rotation
What is the typical scan time in CT?
2s-0.5s
T/F
Shorter scan times completely eliminate artifacts caused by motion entirely.
False; Shorter scan times reduce, but do not eliminate artifacts caused by motion entirely.
(peristalsis, cardiac motion)
T/F
As a rule, if mAs can be maintained, it is preferable to use the shortest scan times available.
True
What happens to heat produced as mAs increases?
As mAs increases, so does the amount of heat being generated within the x-ray tube.
What is the main limiting factor for exposure in all scanners?
Heat
T/F
There is no correlation between mAs levels and interscan delay
False; There is a direct correlation between mAs levels and the interscan delay.
What is Interscan Delay?
Interscan delay is the time between scans (when the tube is not energized).
T/F
The lower the mAs setting, the longer the interscan delay needed to allow for longer tube cool-off time between scanning sequences.
FALSE; The higher the mAs setting, the longer the interscan delay needed to allow for longer tube cool-off time between scanning sequences.
What 2 main factors determine the mAs levels selected?
Patient thickness and density.
T/F
All CT scanners today have AEC systems
True
What is this describing?
The automatic control of the mA intwo directions of the patiention during data acquisition that take into consideration the patient size and the attenuation differences of the various tissues.
Automatic tube current modulation (ACTM)
What is the main goal of ATCM?
To provide consistent image quality despite the size of the patient and the tissue attenuation differences
T/F
ATCM often optimizes patient dose.
True
What is angular TC modulation?
The automatic control of tube current in the x-y axis (in plane)
mA is modulated as a function of what in x-y axis TCM?
mA is modulated as a function of projection angle
What is the purpose of the x-y axis TCM algorithm?
Ensures that a constant (uniform) noise level is maintained during the scanning process
What is longitidinal (z axis) TCm based on?
Longitidinal (z axis) TCm based on differences in attenuation along body parts
What algorithim is responsible for this?
Designed to change the mA automatically as the patient is scanned from head to toe while maintainting constant noise level for different thickness of body parts examined.
Z-axis TCM
mA is modulated as a function of what along the patient in Z axis TCM?
mA is modulated as a function of longitudinal location along the patient
mA changes correspondingly as the patient is moving along the table
What are the 3 ways in which software automatically adjusts mA to fit specific anatomic regions?
- Obtaining attenuation characteristics by analyzing AP and lateral scouts
- obtaining attenuation charecteristics by analyzing (in real time) signal from the detector rows in the first half (180 degrees) of the rotaion
- 1 and 2
What are attenuation values compared to for obtaining references?
- The target mAs level
- The amount of noise in the image
(based on the maufacturer)
What is the oval ratio?
A ratio of the long axis to the short axis of an ellipse/circle
a/b
T/F
From any given z location the system estimates the attenuation level and the oval ratio from the scan.
False; From any given z location the system estimates the attenuation level and the oval ratio from the scout
What does the oval ratio reflect?
How circular or elliptical the patient is at a given location and is based on width information in the scout
T/F
The oval ratio changes based off of changing the isocenter.
True
What is the reference noise measurements in the system determined from?
A set of phantoms representing a wide range of patient sizes and shapes
What is the primary aim of tube current (mA) modulation?
To adjust mA according to the patient’s attenuation and ultimately to reduce the radiation dose to the patient while sustaining diagnostic image quality
What are the choices of kVp in CT system scanners?
80, 100, 120 and 140
Fewer than x ray
T/F
Excessively reducing kVp would result in increased radiation dose to the patient due to a dramatic increase in attenuation of the x ray beams by the patient
True
(especially for larger patients)
