Smartma/Odm; Smartma; Smartma Background - GE Revolution CT User Manual

2.6 SmartmA/ODM

2.6.1 SmartmA

2.6.1.1 SmartmA background

A significant factor in the quality of a CT image is the amount of X-ray quantum noise contained
in the scan data used to reconstruct the image. Technologists understand how the choice of X-
ray scan technique factors affect noise contained in the reconstructed image. In theory, noise
decreases approximately inversely with kV and with the inverse square root of the mAs and
slice thickness. For example, increasing the mA from 50 to 200 (a factor of 4) theoretically
decreases quantum noise by a factor of 2 (the square root of 4). The final noise in the image is
further influenced by the specific mathematical algorithm employed in the image reconstruction
process, which may alter the theoretical relationships described above. In helical imaging,
quantum noise increases with increasing helical pitch; however, the exact relationship is
likewise dependent on the details of the helical image reconstruction process.
The most significant factor that influences the quantum noise in scan data is the X-ray
attenuation, related to the size and tissue composition of the scanned anatomy.
Illustration 1: Example small patient (120 kVp, 1.25 mm, 0.5 s axial) with factor of 5 noise
As an example, for a given fixed exposure scan technique, the quantum noise can vary by at
least a factor of 5 from the smallest to the largest patient.
five times noise increase simulated for a small patient. With a fixed mA scan protocol, the
technologist must select the mA using a qualitative estimate of the patient attenuation. This may
be accomplished using the patient's weight and/or waist diameter measurements, body mass
index, or with qualitative visual classification. Because these methods provide rough X-ray
attenuation estimates and do not account for attenuation changes within the patient region
being scanned, the technologist must use a high enough technique margin to avoid
compromising the diagnostic quality of the images. Theoretically, radiation dose is inversely
related to the square of the noise, so many patients are likely receiving more dose than
necessary for the required diagnostic quality using such manual methods.
GE's suite of Automatic Exposure Control (AEC) features enable substantial patient dose
reduction, while maintaining diagnostically acceptable image quality. Specifically, SmartmA is
an automatic tube current modulation feature that makes the necessary mA adjustments much
more accurately than those manually estimated for the patient, and can thereby achieve a more
232
increase (simulated): 1 = SD 8 @ 640 mA, 2 = SD 40 @ 25 mA
Revolution CT User Manual
Direction 5480385-1EN, Revision 1
Illustration 1 shows an example of a
2 Scan Theory