Kv Assist; Kv Assist Background - GE Revolution CT User Manual

2.7 kV Assist

There is growing interest in optimizing the tube voltage, kV, to minimize the radiation dose of a
CT scan. Lowering the kV has a significantly higher impact on the dose than lowering the mA,
but kV also has a very complex interaction with other scan technique and image quality factors.
The optimization of kV and mA parameters and image quality factors are also clinical task
dependent.
• Lowering kV with a constant mA level will lower the radiation dose and can increase the
image contrast in certain clinical tasks. It will always significantly increase the image noise.
• Lowering the kV with SmartmA can increase the image contrast in certain clinical tasks and
will keep the image noise constant. It will have very little effect on the radiation dose.
Careful optimization of the kV and mA (or NI) settings can maintain or improve image quality
while reducing radiation dose in the context of specific clinical tasks.
When a tube voltage is selected, the desire is to pick the optimal tube voltage for the particular
patient and clinical task, based on some criterion. This optimization could be performed, for
example, in order to lower the amount of radiation dose associated with a CT scan, or to
decrease the amount of contrast agent delivered to the patient or to improve image quality in
terms of image contrast. For kV Assist, the goal is to optimize the tube voltage for radiation dose
reduction. In order to understand how the tube voltage can be optimally selected, it is beneficial
to examine the background of the impact of kV on CT imaging.

2.7.1 kV Assist background

Two significant factors that impact the quality of a CT image are the amount of X-ray quantum
noise and of X-ray contrast (either from contrast agent administered to the patient or from
tissues in the patient) in the scan data used to reconstruct the image. One key CT image quality
factor is the Contrast-to-Noise Ratio (CNR). CNR gives an indication of how well structures can
be seen through the image noise. CNR can be improved by increasing the contrast between
structures, lowering the noise level, or both. If one assumes that only a single tube voltage will
be used, these factors simplify to noise. For example, historically many CT scans have been
performed at 120 kV. In that case, the behavior of how image noise, and therefore CNR, is
impacted by X-ray scan technique factors can be understood from the description of SmartmA.
In this case where a particular tube voltage is used, the use of SmartmA allows the scanner to
adjust the tube current for the patient size and tissue composition in order to obtain a more
consistent image noise.
In a more general look at X-ray scan technique factor setup, the behavior of image noise and
contrast becomes more complicated. In this case, the quality is impacted not only by the
number of X-rays that reach the detector, but also by their energy. This is impacted by what X-
rays the system generates (how many [tube current] and at what energies [tube voltage]), and
how much they are attenuated (patient size and composition). The selection of the tube voltage
is related to the desired image contrast because, for some materials, the amount of X-ray
stopping power varies greatly with the energy of the X-ray. The selection of the tube current is
related to the desired image noise, but it must be noted that it is also greatly impacted by the
tube voltage selection. For example, the number of X-rays produced at 100 mA at 80 kV is
approximately three times less than the number produced at 120 kV. SmartmA, for example,
compensates for this once you select the tube voltage, but does not aid in tube voltage
selection.
Chapter 11 Scan
Revolution CT User Manual
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