SNR and DQE analysis of broad spectrum X-ray imaging

A general analysis of the signal to noise ratio SNR, of X-ray imaging with a broad spectrum is presented. The analysis indicates that the energy modulation of the signal together with its degree of matching by the energy response of the image receptor are significant determinants of the SNR for signal detection. This requires a generalisation of the interpretation of detective quantum efficiency, DQE, the transfer function appropriate from SNR, that will be dependent on the image detection or discrimination task. The generalised DQE is similar to the conventional DQE for the task of detecting radiation levels, but may differ substantially from it for the task of discriminating a lesion from its surround, particularly for signals of bone or iodine. The photon counter is shown to be inferior to the ideal detector for these tasks, but to be generally superior to the energy detecting scintillators used in conventional or digital radiography and computed tomography.