KEYWORDS: Phase imaging, Image resolution, X-ray optics, X-ray imaging, X-rays, Signal attenuation, Absorption, Signal to noise ratio, Digital mammography
The contrast in conventional x-ray imaging is generated by differential attenuation of x rays, which is generally very small in soft tissue. Phase imaging has been shown to improve contrast and signal to noise ratio (SNR) by factors of 100 or more. However, acquiring phase images typically requires a highly spatially coherent source (e.g. a 50 μm or smaller microfocus source or a synchrotron facility), or multiple images acquired with precisely aligned gratings. Here we demonstrate two phase imaging techniques compatible with clinical sources: polycapillary focusing optics to enhance source coherence and mesh-based structured illumination.
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.
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 or other low-Z materials yield poor attenuation contrast. One method to improve the contrast of X-ray images is to utilize phase information since phase depends depend on the real part of the refractive index, which is typically 1000 times larger than the imaginary part. 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.
We will discuss two methods to achieve phase imaging with large-spot sources practical for clinical or security screening. The first method relies on using polycapillary optics to focus the beam and achieve the necessary coherence for traditional propagation-based phase imaging methods based on the transport of intensity equation. The second method relies on a coarse wire mesh which structures the illumination 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.
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