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Practical SmartmA Example
Patient exposure for a SmartmA scan is reduced when compared to a similar scan employing
fixed mA. This is demonstrated in Illustration 2 which shows the mA as a function of rotation
angle for fixed mA and SmartmA scan of a uniform oval polyethylene phantom. The image on
the left overlays the fixed mA and SmartmA mA profiles as a function of X-ray tube angle, with
the accompanying reduction in overall patient exposure using SmartmA clearly indicated by a
reduction in relative mAs as a function of tube rotation angle. The exact mA reduction range
when using SmartmA, and the consequent reduction in patient dose, depends on the prescribed
Noise Index, and the size and shape of the patient, with a greater reduction expected for a
relatively larger and more asymmetric patient when a relatively low Noise Index is prescribed.
Illustration 2: Sample comparison of SmartmA modulation with a fixed mA scan.
NOTE: Note that values are illustrative, with exact values dependent on the specific
implementation of SmartmA.
While the SmartmA mA table specifies different mA values per rotation, corresponding to the
mA in the anterior-posterior direction and in the lateral direction, i.e. at 0 and 180° and 90° and
270°, respectively, the actual mA is continually varying in a smooth fashion between the
specified mA values as a function of X-ray tube angle to the patient, as shown in Illustration 2 .
2.6.1.3 References
1
L. Kopka and M. Funke, "Automatically adapted CT tube current: Dose reduction and image
quality in phantom and patient studies," Radiology 197 (P), 292 (1995)
2 D. R. Jacobson, W. D. Foley, S. Metz, and A. L. Peterswen, "Variable milliampere CT: Effect
on noise and low contrast detectability," Radiology 210 (P), 326 (1996)
2.6.1.4 SmartmA FAQs
What suggestions do you have for a new SmartmA user?
•
If you are not familiar with the concept of Noise Index, you can use the GE reference
protocols that have AEC enabled. As a general starting point, use the standard deviation
from an acceptable image for approximation of a Noise Index, or consult the literature until
you find the highest Noise Index value that provides acceptable diagnostic quality.
Experiment by scanning some phantoms with different Noise Index values to gain some
Revolution CT User Manual
Direction 5480385-1EN, Revision 1
234 2 Scan Theory