What’s in the pixel value?

The pixel value (or pixel intensity) may represent different quantities, depending on the imaging modality. Pixel values on photographs are a measure of visible light reflected by objects. Pixel values of CT (computed tomography) images are the Hounsfield Units (HU, or CT numbers) which measure linear attenuation of photons by the objects. Pixel values on PET (positron emission tomography) images measure radiotracer (18F, for example) uptake.