PHAROS: a spectrometer-on-a-chip for digital radiology systems with spectral detection

When a beam of X-rays crosses an object, its spectrum is modified depending on both the density and effective atomic number of the material. We describe an integrated (VLSI) spectrometer used in a digital radiology system for performing spectral analysis of the detected X-rays. It integrates 16 independent channels and gives seven bits of energy resolution. This permits the distinction between tissues featuring small differences in effective atomic number, e.g. micro-calcifications and fat cysts, which have the same contrast in a normal radiography. The integrated circuit services a row of solid-state detectors that can travel along the specimen in the radiology system in perpendicular directions, so that bi-dimensional images can be recorded. The circuit performs pulse height analysis and stores the spectral distribution of the detected photons. The circuit is described and its performance analyzed.