X-ray scatter background signals in transmission radiography

With monoenergetic x-ray beams incident on polystyrene phantoms, the spectra of the tramsmitted x rays were measured with a scintillation spectrometer. The scattered and unscattered components of the transmitted x-ray fluence at a point on the beam axis were determined as a function of (i) the incident x-ray energy (18, 22, 32, 49, 58, 69, and 660 keV), (ii) the phantom thickness (5.3, 10, and 21 cm), (iii) the scatter solid angle determined by the exposed area of the phantom and the separation distance of the image plane (0.090, 0.31, 0.66, 1.8, 3.5 4.3, 4.8, and 5.1 sr), and (iv) the beam diameter at the image plane (25, 17, and 10 cm). The results indicate that, as the incident x-ray energy decreases from 660 to 30 keV, the contribution of the scattered component to the transmitted fluence increases from approximately 50% to 90% for the 21-cm phantom and from 21% to 50% for the 5.3-cm phantom. For typical cases, the data show the effect of the scatter component on the ratio of the image to the background signals. In addition, the examples show that optimum conditions for maximizing this signal ratio may be obtained by a careful selection of the incident x-ray energy for low-, medium, and high-contrast objects.