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confidence. A 30 cm diameter water phantom or a 35 cm diameter low density polyethylene
phantom have an attenuation similar to the average adult abdominal patient.
•
It is important to review the image quality that is obtained with the prescribed Noise Index to
optimize the user's maximum and minimum mA range and Noise Index values accordingly.
The maximum mA value set by the user sets the maximum allowed mA, or clipping mA
value.
•
You should also check the mA table on the scan set up screen to see what mA is actually
being used. If you see that it is frequently at the maximum mA range, and you have
determined that you require lower noise in your images than you are currently obtaining,
consider increasing the Noise Index if more noise can be tolerated in your reconstructed
images without compromising the diagnostic value, or increase the maximum mA limit if it is
not at the maximum limit of the X-ray generator.
• If you normally reconstruct images with thin sections for 3D reformatting and thicker slices
for axial viewing it is important to understand that the first prospective reconstructed slice
thickness is used for calculating SmartmA. Generally you would want to set the Noise Index
for the thicker slice images. For example, you might want a Noise Index of 10.0 for 5 mm
thick images for viewing but you may also want 0.625 mm slices for 3D reformatting. If you
prescribe the 0.625 mm slice recon first, followed by the 5 mm recon, SmartmA will calculate
the mA needed to obtain an image noise of 10 for the 0.625 mm slices since it is prescribed
first. In this case, to avoid excessively high mA and high dose, the user needs to readjust
the Noise Index using the following approximation. Please note that the following equation is
to be used as a starting point, with the actual Noise Index for thin slices possibly varying
from the exact solution given by this equation:
Example:
Why is the standard deviation I measure in the image different than the Noise Index I selected
for the scan?
•
There are many factors that can account for this. But, first consider that the prescribed Noise
Index causes the tube current to be adjusted so that the system projects a similar X-ray
intensity through the patient to the detector. Hence it regulates the X-ray noise or quantum
noise in the scan projection data. The noise in the image depends on additional factors
within the image reconstruction chain, including, but not limited to, the selection of
reconstruction algorithms, reconstructed slice thickness selection (if different than your
prospective selection), and the use of image space filters. In addition, it is very difficult to
make standard deviation measurements on patient data since the standard deviation is
affected by small CT number variations of the heterogeneous anatomy and by patient
motion or beam hardening artifacts. Even with uniform phantoms, standard deviation
Revolution CT User Manual
Direction 5480385-1EN, Revision 1
Chapter 11 Scan 235