Signal-to-noise ratio in a digital radiograph is measured as the mean grey value of a uniform region divided by its standard deviation. On its own it is not comparable between systems, because a detector with coarse pixels averages over a larger area and therefore reports a higher SNR for exactly the same physics. Doubling the sampling area halves the noise standard deviation, so SNR scales with the square root of the area, which is the square root of the basic spatial resolution.

ISO 17636-2 removes that dependence by normalising every measurement to a common reference resolution of 88.6 micrometres – the value corresponding to a 100 micrometre pixel sampled at the standard aperture. The correction is SNRn = SNR_measured x sqrt(88.6 / SRb), with SRb in micrometres. A coarse system with a large SRb is penalised, a fine one is credited, and the resulting number can be compared across detectors and against the code table.

The minimum SNRn required depends on radiation quality, penetrated thickness and test class, and is tabulated in ISO 17636-2. Class B is more demanding than class A, and the requirement falls at higher energies where fewer, more penetrating photons carry the image. Meeting it is one of two independent digital acceptance criteria – the other is basic spatial resolution – and both come from the same image.

Because noise is dominated by photon statistics, SNR rises with the square root of exposure. Falling short by 40 percent means roughly doubling the dose, not adding 40 percent. That is the usual reason a digital technique that passes SRb still fails qualification.

Worked example

Measured SNR	150
Basic spatial resolution SRb	100 um
Minimum SNRn required	100
Normalisation factor	0.9413
Normalised SNR	141.19
Measured SNR needed to meet the requirement	106.24
Margin on SNRn	41.19

The normalisation factor is sqrt(88.6/100) = sqrt(0.886) = 0.9413, so SNRn = 150 x 0.9413 = 141.19. To achieve an SNRn of 100 on this system the measured SNR only needs to be 100/0.9413 = 106.24, so the image passes with 41.19 to spare.

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