What is quantum noise (quantum mottle)?

Quantum noise, also called quantum mottle, is the random variation in image intensity caused by the statistical nature of X-ray photon detection. It occurs because the number of photons detected in different regions of the image fluctuates randomly, even when the exposure is uniform.

Quantum noise arises from random fluctuations in the number of detected X-ray photons and becomes more prominent when fewer photons are detected.

Quantum noise becomes more prominent when fewer photons reach the detector, producing a grainy appearance in the image.

Understanding the physics

X-ray production and detection are fundamentally random processes. Even if an X-ray beam is uniform, the exact number of photons detected in each small region of the detector will vary slightly due to statistical fluctuations.

These fluctuations follow Poisson statistics, which describe the behaviour of random counting events. For a mean number of detected photons $N$ , the standard deviation in photon number is:

This means that the magnitude of the noise is proportional to the square root of the number of detected photons.

Because the useful image signal is proportional to $N$ , while noise is proportional to √N, the signal-to-noise ratio is:

This relationship explains why increasing the number of detected photons improves image quality. However, because the improvement follows a square-root relationship, large increases in photon number are required to produce modest improvements in image noise.

When the number of detected photons is low, the relative magnitude of statistical fluctuations becomes larger, and the image appears grainy. This graininess is referred to as quantum mottle.

Where this matters clinically

Quantum noise is a major determinant of image quality in digital radiography. If too few photons reach the detector, the image becomes noisy and fine anatomical detail may be obscured.

Increasing mAs increases the number of detected photons and therefore reduces quantum noise. However, increasing photon number also increases radiation dose to the patient, so imaging protocols aim to use enough photons to achieve adequate image quality while keeping dose as low as reasonably achievable.

Detector design also affects how efficiently photons are converted into signal. Detectors with higher detective quantum efficiency (DQE) can achieve acceptable image noise with fewer photons.