Z-Axis Modulation
The z-axis–modulation (AutomA, GE
Medical Systems; Real E.C., Toshiba Medical,
Tokyo, Japan) technique functions
somewhat differently than does angular
modulation (21). The AutomA technique
adjusts the tube current automatically to
maintain a user-specified quantum noise
level in the image data. It provides a
noise index to allow users to select the
amount of x-ray noise that will be
present in the reconstructed images. Using
a localizer radiograph, the scanner
computes the tube current needed to obtain
images with a selected noise level.
Hence, z-axis modulation attempts to
make all images have a similar noise irrespective
of patient size and anatomy. The
noise index value is approximately equal
to the image noise (standard deviation)
in the central region of an image of a
uniform phantom.