Monochromatic imaging using collimating polycapillary X-ray optics

Monochromatic parallel beam imaging produces high subject contrast, high resolution, and low patient dose. Monochromatic imaging is typically done with synchrotron sources and free electron lasers. These sources are expensive and not practical for clinical settings. However, conventional laboratory sources normally have insufficient intensity. Polycapillary x-ray optics can be used to efficiently produce an intense parallel beam, which can be diffracted from a crystal to create monochromatic radiation.; Polycapillary collimating optics need to be characterized before applying to monochromatic experiments. Measurements of the optics included transmission, uniformity, and exit angle divergence. The transmission for a prototype with a 250-mm source to optic distance was 37% at 17.5 keV, which matched well to the simulation. Field uniformity across the entire optic output was approximately 7% rms. The output divergence from the optic was about 4 mrad, again in good agreement with simulation results.; Contrast, resolution, and intensity measurements were performed with both high and low angular acceptance crystals. Testing was first done at 8 keV with an intense copper rotating anode source. Preliminary 17.5 keV measurements were then made with a low power molybdenum source. At 8 keV, contrast enhancement was a factor of five relative to the polychromatic case, in good agreement with theoretical values. At 17.5 keV, monochromatic subject contrast was a factor of two times greater than the conventional polychromatic contrast. An additional factor of two increase in contrast is expected from the efficient removal of scatter by using the air gap which is allowable from the parallel beam. The measured angular resolution with a silicon crystal is 0.6 mrad at 8 keV, and 0.2–3 mrad at 17.5 keV. For a 50-mm thick patient, this angle corresponds to 50 lp/mm with an ideal detector. The use of polychromatic collimating optics allowed monochromatic imaging measurements using a conventional rotating anode source and computed radiography plate.