Optimizing the x-ray photon energy for digital radiographic imaging systems

In this study, we investigated which photon energy results in the lowest patient dose when the image contrast to noise ratio (CNR) is kept constant. This optimum photon energy was obtained for a range of patient sizes, as well as for the detection of lesions with atomic number (Z) ranging from 6.5 (fat) to 53 (iodine). Mono-energetic photons from 20 kev to 140 kev were investigated with an x-ray detector that had 100% quantum detection efficiency. Patients were modeled as slabs of water with a thickness that ranged from 5 to 30 cm. Image contrast was computed from the x-ray attenuation of small lesions consisting of low Z materials (i.e., fat and water) as well as higher Z materials (i.e., calcium and iodine). Relative values of the CNR as a function of the x-ray photon energy were obtained by assuming that the image noise was proportional to the square root of the incident number of x-ray photons under scatter free conditions. The energy imparted to the patient as a function of photon energy was obtained using published data of the absorbed percentage of the incident energy fluence. For each patient thickness and lesion composition, the CNR was kept constant by appropriate adjustment of the x-ray beam intensity, and the corresponding x-ray photon energy that resulted in the minimum patient dose was determined. The optimum photon energy for detecting a low Z lesion increased monotonically from ∼62 kev for small (i.e., 15 cm) patients to ∼78 kev for large (i.e., 25 cm) patients. For high Z lesions, optimum photon energies were ∼ 34 kev for small patients, and increased to ∼40 kev for large patients. The optimum photon energy was found to vary by about a factor of two over the range of patient thickness investigated (i.e., 5 to 30 cm). The optimum photon energy also varied by a factor of two for the range of detection tasks investigated (i.e., low Z versus high Z).