Kv Assist Theory - GE Revolution CT User Manual

The selection of tube voltage in combination with tube current is important because the goal of
technique setup is to pick the parameters that give the desired image quality at the lowest
radiation dose. The behavior of both the image quality and the radiation output of the scanner
are directly impacted by both the tube voltage and tube current selection. To help compensate
for patient size and tissue composition without impacting the clinical workflow, an assisted
approach for kV selection has been integrated into the scanner under the name kV Assist.
kV Assist is a tube voltage selection feature that can suggest the optimal kV based on the
attenuation of the Scout image, and thereby provide a consistent desired image quality despite
the wide range of patients. This feature works in conjunction with both SmartmA and Manual
mA in order to adjust the Noise Index/mA as needed once the kV is selected.
Because the selection of kV and mA may result in differences in the image appearance relative
to historical techniques, WW and WL should be adjusted in order to optimize the image display.
WW/WL adjustment based upon the selected kV can be enabled by the user to optimize
contrast to noise and image quality for image viewing.

2.7.2 kV Assist theory

The selection of the tube voltage has a large impact on the number of X-rays that are used and
their energy. Higher energy X-rays have greater penetrating power, but provide less image
contrast. Lower energy X-rays have lower penetrating power, but provide more image contrast.
Some materials, such as iodine, have a large change in X-ray attenuation with energy (and
therefore have a large change in the resulting image contrast). For example, the image contrast
in an iodine region in an average sized phantom could be approximately 1.6 times higher at 80
kV than 120 kV, and 1.2 times higher at 100 kV than 120 kV. Other materials, such as water,
have little change in X-ray attenuation with energy (and therefore have little change in the
resulting image contrast).
For the materials that do produce greater image contrast at different tube voltages, the resulting
image quality may be improved if the noise is kept constant when the tube voltage is lowered.
This is based on the impact that the CNR has on image quality.
the image noise at a lower tube voltage with improved image contrast, in order to improve the
CNR. Alternatively, you could maintain the image noise and reduce the amount of contrast
agent (if relevant) in order to maintain the CNR. A third option is to increase the image noise in
order to maintain the CNR while reducing the radiation dose to the patient. kV Assist takes this
third approach (while preventing excessive noise increases) in order to maintain image quality
while reducing the radiation dose. For the example above (with contrast at 80 kV ~ 1.6 times
contrast at 120 kV), maintaining the CNR could be achieved with a 60% and 20% increase in
noise at 80 and 100 kV, respectively. This would result in significant dose reduction.
For many cases, maintaining the CNR of a particular material may not be clinically acceptable.
Instead, both CNR and overall image noise should be taken into account.
the consideration of CNR and image noise based on your selection of clinical task.
kV Assist will take into account the patient size information from the Scout, the initial/baseline
kV and the initial/baseline Noise Index/mA as the starting point, along with the user preference
parameters for the selection of kV. The recommendation from kV Assist will be a consistent set
of parameters affecting the scanning of the images.
•
kV
•
mA/NI – to meet CNR for clinical task
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Direction 5480385-1EN, Revision 1
Therefore, you could maintain
1,2
3 kV Assist balances
2 Scan Theory