GE Revolution CT User Manual page 237

•
If the user desires somewhat lower noise in small patients, the user may want to create
Small, Nominal, and Large patient protocols. The user can then use a slightly lower Noise
Index for the smaller patients and a slightly higher Noise Index for larger patients.
• A noise limiting strategy that can be employed is to increase the minimum mA setting in the
SmartmA protocol. If the user determines that image quality is generally not acceptable
below some minimum mA value, then the user may set this value as the low mA range limit.
This will prevent the SmartmA feature from using mA values that are lower than desired.
Note that this strategy can sometimes defeat the overall purpose of SmartmA, causing
image noise to fall below the prescribed Noise Index, and furthermore increase dose to the
patient.
•
Another possibility for higher noise than is expected is if the user looks at multiple
reconstructed images that have thinner slices than the prospective scan prescribed slice
thickness. SmartmA uses prospective slice thickness as a factor when the mA table is
generated. The user must ensure that the Noise Index is set for the first prospective image
based on image thickness used for axial image viewing (see the first FAQ). This caveat
applies equally for fixed mA and SmartmA scanning.
•
Higher noise images can occur when patients are not well centered in the scan field of view.
The bowtie filter attenuation increases with distance from isocenter, hence the thickest part
of the patient should be approximately centered in the scan field of view, in line with the
minimum filter attenuation, otherwise image noise will increase since the patient thickness
adds to the bowtie filter thickness. This is especially important for highly asymmetric
anatomy such as shoulder anatomy. Again, this effect applies equally for fixed mA and
SmartmA scanning.
•
Recognize also that there are some obese patients that exceed the capabilities of the tube
and generator to satisfy the selected Noise Index. This is also no different than fixed mA
scanning. For such obese patients, one strategy is to select a higher kV setting when
possible.
Why is the mA annotated on the image sometimes slightly different than the mA I see in the mA
table?
The mA displayed on the image is determined by measuring the generator mA during the scan
and averaging the measured result over the total number of views used to reconstruct the
image. The number of views used to produce the image may be more than one gantry rotation
for a helical scan. Hence the annotated value is a combination of the mA table values that
depends on how many views from each rotation were used for the image. In addition, the
generator is automatically adjusting the filament current to account for changing conditions
during the scan to keep the mA within the desired tolerance of the commanded mA table. For
example, this is why you may see an mA value of 41 in the image where the mA table indicated
40.
I understand that noise in the image changes with reconstruction parameter selections, but why
is the noise sometimes different when I retro reconstruct the same scan data at a different
display field of view?
When you select a parameter within a reconstruction algorithm, the system may sometimes
readjust the actual filter kernel. This readjustment will change the image standard deviation.
This will happen if the display field of view (DFOV) selection exceeds a certain dimension for
Chapter 11 Scan
Revolution CT User Manual
Direction 5480385-1EN, Revision 1
237