High-resolution x-ray imaging at soft diagnostic energies using a silicon photodiode array

A linear self-scanned silicon photodiode array (PDA) has been used as a high-resolution detector for small object computed tomography using soft diagnostic x rays. The imaging performance of the PDA when used to detect x rays directly has been characterized for x-ray spectra between 24 and 69 kVp. The modulation transfer function (MTF) was measured to 40 cycles/mm (twice the PDA sampling cut-off frequency) by double sampling an edge profile using a half-detector shift. The MTF had no significant energy dependence, a 10% value at 28 cycles/mm, and a 3% low-frequency drop. The detective quantum efficiency zero-frequency value ranged from 1.5% with an 18-keV average-energy beam to 0.4% at 40 keV, and decreased further with increasing spatial frequency. It was found that the effective thickness of the silicon detector was 22 microns. This is typical of the expected diffusion length of minority carriers in doped silicon. An average of 4.6 +/- 1 eV was required to generate one e-h pair in the PDA, consistent with the accepted value of 3.6 eV in silicon. No evidence of radiation damage was observed after an accumulated exposure of over 5000 R. Use of the PDA is demonstrated by generating computed tomographic images using a third-generation (rotate only) reconstruction algorithm. Images of a resolution test phantom, a pen and pencil, a mouse head, and bone biopsy specimen taken from a patient with osteoporosis are shown.