PHASE-CONTRAST IMAGING OF WEAKLY ABSORBING MATERIALS USING HARD X-RAYS

IMAGING with hard X-rays is an important diagnostic tool in medicine, biology and materials science. Contact radiography and tomography usin g hard X-rays provide information on internal structures that cannot b e obtained using other non-destructive methods. The image contrast res ults from variations in the Xray absorption arising from density diffe rences and variations in composition and thickness of the object. But although X-rays penetrate deeply into carbon-based compounds, such as soft biological tissue, polymers and carbon-fibre composites, there is little absorption and therefore poor image contrast. Here we describe a method for enhancing the contrast in hard X-ray images of weakly ab sorbing materials by resolving phase variations across the Xray beam(1 -4). The phase gradients are detected using diffraction from perfect s ilicon crystals. The diffraction properties of the crystal determine t he ultimate spatial resolution in the image; we can readily obtain a r esolution of a fraction of a millimetre. Our method shows dramatic con trast enhancement for weakly absorbing biological and inorganic materi als, compared with conventional radiography using the same X-ray energ y. We present both bright-field and dark-field phase-contrast images, and show evidence of contrast reversal. The method should have the cli nical advantage of good contrast for low absorbed X-ray dose.