What is the percentage increase in noise when you halve the mAs?

Halving the milliampere-seconds (mAs) in a computed tomography (CT) scan has a direct effect on image noise. The relationship between mAs and image noise is described by the signal-to-noise ratio (SNR). When mAs is decreased, the quantity of photons reaching the detector is reduced, which leads to increased noise.

Specifically, when mAs is reduced by half, the noise increases by a factor of the square root of the change in mAs. The formula used to calculate the percentage increase in noise due to a halving of mAs is as follows:

1. The original mAs is set to 1 (100%).
2. Halving that gives 0.5 mAs.
3. The noise increase is calculated using the square root: ( \text{Noise increase} = \sqrt{\frac{1}{0.5}} = \sqrt{2} \approx 1.414 ).
4. To find the percentage increase, you would take ( 1.414 - 1 = 0.414 ) or 41.4%.

Thus, when you halve the mAs, the noise increases by approximately 41%, making option C the