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2.5 Slice Thickness Reference Noise Index and Noise Index Values
The Noise Index (NI) is the primary SmartmA input parameter that controls the X-ray output and
ultimately provides the tradeoff between the desired image quality and patient dose. Any user-
defined value between 1 and the maximum Noise Index value can be entered for the Noise
Index when building a protocol.
The Noise Index value will approximately equal the standard deviation in the central region of
the image when a uniform phantom (with the patient's attenuation characteristics) is scanned
and reconstructed using the standard reconstruction algorithm.
The Noise Index corresponds to the relative level, or index, of noise in the reconstructed image:
a higher Noise Index means the images will contain relatively more noise and will be obtained
with lower mA, therefore lowering relative patient dose, while a lower Noise Index means the
images will contain relatively less noise and will be obtained with higher mA, leading to relatively
higher patient dose. The Noise Index should always be reviewed and modified by the user to
meet the clinical need, taking into consideration the user preference for image quality and the
tradeoff with patient dose.
For GE reference protocols that use SmartmA, the Noise Index is based on the slice thickness
defined in the protocol. If the slice thickness defined in the protocol is 5 mm and is changed by
the user to 0.625 mm, the corresponding Noise Index should be adjusted by the user to the
desired Noise Index for that slice thickness. A guide for how to adjust for Noise Index at another
slice thickness is given in the equation in Section 2.6.1.4, SmartmA FAQs . In this system, the
Noise Index will not be automatically adjusted to account for a change in slice thickness away
from the slice thickness contained in the GE reference protocol; the user must manually adjust
the prescribed Noise Index appropriately for the specific slice thickness.
Table 3: Noise Index Values
Anatomy 5.0 mm 2.5 mm 1.25 mm 0.625 mm
Head/Orbit 2.7 3.3 4 4.4
Neck 4.2 5.1 6.2 6.9
Abdomen 9.7 12.3 15.6 18
Pelvis/Spine 11.7 14.6 18.4 21
Chest 12.3 15.4 19.3 22
Upper Extremity 7.8 9.6 11.8 13.3
Lower Extremity 4.1 5 6.1 6.7
Revolution CT User Manual
Direction 5480385-1EN, Revision 1
Chapter 11 Scan 231