GE Revolution CT User Manual page 236

measurements will produce some variability in measured results because of the inherent
nature of quantum statistics.
• Another situation that can cause significant differences between the selected Noise Index
and the image standard deviation is when very large patients provide insufficient detector
signal. In these cases, electronic noise sources can become the dominant image noise
source instead of X-ray noise. In these cases at various threshold levels, special projection
data dependent filters begin to be applied to help preserve image quality. These filters on
the projection data affect the noise properties of the reconstructed image. The highest kV is
recommended when excessively large patients need to be scanned.
• Another factor is how well the patient is centered in the SFOV. Image noise can increase
significantly if the patient is mis-centered. This occurs because the bowtie filter projects
maximum X-rays intensity at isocenter since this is the region of maximum attenuation if the
patient is centered. If the patient is mis-centered, there are fewer X-rays projected to the
thickest part of the patient, and hence image noise will increase. The optimum strategy is to
find the highest Noise Index sufficient for the clinical task and let SmartmA select the mA
without using significant constraints.
Will I get a dose reduction when I use AEC?
SmartmA will use a patient exposure that depends on the Noise Index you select and the size of
the patient you are scanning. If, you do not obtain a dose reduction over a population of
patients, you may have selected a lower Noise Index than you really need, resulting in, on
average, higher mA values than fixed mA protocols. One strategy to avoid using more exposure
is to set the maximum mA parameter to the same level as your fixed mA protocols, capping the
maximum patient exposure to the same level as your fixed mA protocol. Hence, SmartmA will
never be allowed to use more exposure then previously employed. However, image noise will
increase in regions where the mA is limited by the maximum mA selection and the IQ will
degrade with increasing patient size. The optimum strategy is to find the highest Noise Index
sufficient for the clinical task and let SmartmA select the mA without using significant mA limits.
Why do my images seem noiser with SmartmA?
•
SmartmA estimates the attenuation (size and shape) of the patient from a single scout
image and produces an X-ray intensity to maintain the prescribed Noise Index. The
estimation of patient attenuation may not match the actual attenuation of the heterogeneous
anatomy being imaged. Thus, if images appear noisier when SmartmA is employed, the
user may need to use a lower Noise Index. This may be the case if the user finds that the
average mA employed for the population of patients in question using SmartmA is generally
lower than using previous fixed mA protocols. This situation indicates that the user is now
employing lower dose compared to previous protocols and hence higher noise levels would
be expected.
• Certain patient images may also be noisier than the experience of the user suggests. For
example, experience leads the user to expect significantly lower noise in thin patients than
obese patients, while SmartmA attempts to make the image noise approximately similar for
all patient sizes. Most importantly, the user needs to find the highest Noise Index to enable a
confident diagnosis of the clinical indication since this results in the lowest patient dose.
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Revolution CT User Manual
Direction 5480385-1EN, Revision 1
2 Scan Theory