If we encounter a photon with, for example, 80 keV energy, there is no way to tell whether it is an X-ray or a gamma photon because the names refer to the origin of the photon: X-rays always come from the electron shells, while gamma rays originate from the nucleus.

Since there is a wide energy range (up to ~ 100 keV) where both gamma — and characteristic X-rays can occur, the devices for detecting gamma rays are also capable of measuring X-rays of the same energy. So the gamma camera might also be called “electromagnetic camera”; for instance, when using thallium-201 for imaging, which decays with electron capture, actually we mainly detect its charac­teristic X-rays between 68 and 85 keV.

Note that the number of photons detected by a gamma camera and an X-ray device is different by several orders of magnitude. So a gamma camera would be blinded by a regular X-ray generator, while the radiation of a patient injected with 600 MBq of a Tc-99m-labeled radiopharmaceutical would be under the detection limit of a standard medical X-ray detector.