![]() ![]() The detector’s role in managing noise in medical imaging Now I’ll explain the role of the detector system. These processes are controlled by fundamental laws of nature and, for any given X-ray acquisition, they determine the fundamental limit on image quality. Up to this point, I have been discussing noise associated with the statistical nature of X-ray production and their subsequent absorption by the patient. ![]() You likely know this guiding principle of radiation safety: “as low as reasonably achievable (ALARA)”. Always, there is a balancing act of using sufficient radiation to achieve a confident clinical diagnosis at the lowest dose possible. Radiologists are accepting of more noise for an easy-to-diagnose injury, such as a break in a major bone, versus a subtle pathology where the disease features may be difficult to discern. It tells them that the proper level of radiation, and hence patient exposure, was used. This is the reason that most radiologists like to see “some noise” in an image. While this can result in a visually pleasing image, it may mean that an unnecessarily high exposure level was used, resulting in overexposure to the patient. In contrast, when a large amount of radiation is used, the visibility of the statistical noise is very low, perhaps even imperceptible. It is a representative of higher exposures. The image on the right displays quantum noise, which has a “clumpy” appearance caused by light spread in the scintillator. The image on the left displays electronic noise, which gives a “salt and pepper” appearance. This can lower the radiologist’s diagnostic confidence. In an exam where only a small amount of radiation has been used to create the image (low exposure), the distracting visual appearance of the statistical noise (sometimes known as “salt and pepper” noise) relative to the size of the signal variations generated by the patient’s anatomy, can reduce the visibility of subtle, clinically important features. But overlaying this image “signal” is the inherent statistical “noise” associated with the X-ray production process. The patient’s anatomy has created variations in the X-ray intensity that the imaging system uses to create the image. Once the X-rays have passed through the patient, the image “information” is contained in the spatial distribution of the X-ray fluence. Some are absorbed by the patient while others pass through and are absorbed by the imaging detector – another statistically controlled process with its own inherent noise characteristics. These incident X-rays pass through the patient’s anatomy. This means that for a given exposure level, the number of X-rays impinging on the patient is different at different locations on the patient’s body. From the thermal emission of the electrons from the X-ray tube’s filament (cathode), to the generation of the X-ray photons as these electrons are accelerated and collide with the anode, each step in the generation process is “statistical” in nature. There is uncertainty in the output caused by the laws of nature that control what is going on. The generation and detection of X-rays is – by the very nature of the physical interactions that are taking place – a “random” process. ![]()
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