Which type of noise cannot be reduced?

• A. Quantum Noise
• B. System Noise
• C. Ambient Noise
• D. Scatter

Answer: (C)

Ambient noise is the background fluctuations that come from outside the imaging signal, and it isn’t tied to the x‑ray photons or the detector’s internal electronics. Because it originates from the environment, you can’t reliably reduce it by changing exposure settings or processing in the same way you can with the other noise sources. By comparison, quantum (photon) noise decreases when you increase the number of detected photons, system (electronic) noise can be reduced with better detector design and cooler, cleaner electronics, and scatter can be lowered with grids, collimation, and other anti-scatter measures. So ambient noise stands out as the type that cannot be reduced through typical imaging adjustments.