X - ray imaging for biological tissue

Collagen has an important role in many biological tissues such as bone, tendon, teeth, skin and cartilage. Despite significant research efforts over the past five decades, the nanostructure of collagen is still not well known. Coherent X-ray diffraction imaging is a good candidate for addressing this issue, but it is limited as a result of radiation damage and the inability to invert coherent diffraction patterns from continuous objects under finite illumination from an aperture. Felisa Berenguer de la Cuesta and co-workers from University College London, UK, have got around this problem by incorporating X-ray ptychography into a coherent X-ray diffraction scheme. Ptychography is a technique that can solve the diffractionpattern phase problem by interfering adjacent Bragg reflections coherently and thus determining their relative phase. It combines diffraction patterns from overlapping regions and often uses a raster scan of the coherent probe beam. In the scheme proposed by Berenguer de la Cuesta et al., an X-ray beam of wavelength 0.14 nm was collimated down to 10 μm × 10 μm and incident on a rat tail tendon sample measuring 5 mm × 5 mm × 0.7 mm. The tendons contained micrometre-scale fibrils of collagen that were approximately 1 μm in diameter, tightly packed and wellaligned in the tissue. Owing to the array of collagen molecules within each fibril, speckle patterns at a resolution of 60–70 nm were obtained and found to be changing gradually as the beam was scanned across the tendon. Exposure time was fixed at 50 s to avoid thermal denaturation. By using a fast Fourier-transformation calculation, the team proved that these speckle patterns were associated with absorption and phase shifts in the sample. When inverted, the patterns show the disposition and orientation of the tissue. high-resolution neuroimaging Neuron 63, 429–437 (2009)