Conventionally, the contrast of X-ray images is due to the attenuation of intensity of x-ray beams after penetrating materials, which is proportional to the imaginary part of the complex refractive index. Subtle density variations within soft tissue yields poor contrast. One method to improve the contrast of x-ray images is to utilize phase information, which could provide a signature 1000 times larger than attenuation. However, phase imaging relies critically on the spatial coherence of the x-ray beam which traditionally requires synchrotron sources, small-spot, low power laboratory sources, or precisely aligned gratings and multiple exposures. An additional source of tissue-typing information, which is simply discarded in a conventional mammogram, is coherent scatter. Coherent scatter imaging relies on diffraction within the tissue and hence produces a signature that depends on the molecular structure, but as conventionally collected requires raster-scanning of the beam and multiple exposures. None of these methods is compatible with conventional screening mammography.
We will discuss two methods to achieve phase imaging with large-spot sources practical for clinical use. The first uses polycapillary optics to focus x-rays from a large-spot source and achieve the necessary coherence for propagation-based phase imaging. The second uses structured illumination implemented with a coarse wire mesh to enhance phase signatures and relax the coherence requirement. We will present recent results from both methods, including computational algorithms for phase contrast, phase retrieval and resolution enhancement.
We will also present a slot-scanning coherent scatter system which utilizes a slot to shape the beam and shielding placed at specific angles to capture specific coherent scatter signatures in a geometry that is compatible with slot-scan mammograpy.