<title>Optimizing the x-ray photon energy for digital radiographic imaging systems</title>

Abstract

In this study, we investigated which photon energy results in the lowest patient does when the image contrast to nosie 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 ranging from 6.5 to 53. Mono-energetic photons from 20 keV to 140 keV were investigated with an x-ray detector that had 100 percent 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 as well as higher Z materials. 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 detection ga low Z lesion increased monotonically from approximately 62 keV for small patients to approximately 78 keV for large patients. For high Z lesions, optimum photon energies were approximately 34 keV for small patients, and increased to approximately 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. The optimum photon energy also varied by a factor of two for the range of detection tasks investigated.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.