A refractive x-ray lens for scanned-slit digital mammography

We present a refractive x-ray lens to be used in a scanned-slit detector system for mammography. The objective is to increase the x-ray flux on the detector and thus reduce scan-times. The lens consists of 600 diamond-cut parabolic cylinders with a radius of curvature of 100 mu m cut from a 60 mm long slab of PMMA. The refractive effect from each void is very small, but the cummulative effect gives a focal length of approximately 200 mm. A 50 mu m source was projected onto a 30 mu m slit 970 mm away with a gain in flux of 4.3 for 27 keV. Since the index of refraction depends on the energy, the gain reaches a peak value for a certain energy, and by changing the geometry and the parameters of the lens, this peak can be tuned to coincide with the optimal energy for the imaging task. In mammography, the favourable energy region is quite narrow, and the spectrum shaping will improve the signal-to-noise ratio. Calculations show about 20% dose reduction with preserved image quality. Furthermore, less or even no filtering of the beam is needed, which corresponds to an additional gain of flux of about a factor of 2.