Infographics / CT Physics

Quantum noise, SNR and image quality

Image noise plotted against relative dose (mAs, proportional to detected photon number N). The highlighted curve falls as 1 over the square root of N: doubling mAs reduces noise by root two, quadrupling mAs halves it, and correspondingly SNR rises with the square root of dose. A de-emphasised curve marks the common error of assuming noise falls as 1 over N.

Quantum (Poisson) noise versus dose in CT Image noise plotted against relative dose (mAs, proportional to detected photon number N). The highlighted curve falls as 1 over the square root of N: doubling mAs reduces noise by root two, quadrupling mAs halves it, and correspondingly SNR rises with the square root of dose. A de-emphasised curve marks the common error of assuming noise falls as 1 over N. Quantum (Poisson) noise falls as 1/√N — not 1/N Relative dose (mAs ∝ photon number N) Image noise (relative σ) 1× 2× 4× 8× noise ∝ 1/√N SNR ∝ √N — rises with √dose 1× (reference), σ = 100% 2× mAs → noise ÷ √2 (× 0.71) 4× mAs → noise ÷ 2 (halved) common trainee error: noise ∝ 1/N (falls too fast)
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