Lixian Liu

Photoacoustic and ultrasound characterization of bone composition

This study examines the sensitivity and specificity of backscattered ultrasound (US) and backscattering photoacoustic (PA) signals for bone composition variation assessment. The conventional approach in the evaluation of bone health relies on measurement of bone mineral density (BMD). Although, a crucial and probably the most important parameter, BMD is not the only factor defining the bone health. New trends in osteoporosis research, also pursue the changes in collagen content and cross-links with bone diseases and aging. Therefore, any non-invasive method that can assess any of these parameters can improve the diagnostic tools and also can help with the biomedical studies on the diseases themselves. Our previous studies show that both US and PA are responsive to changes in the BMD, PA is, in addition, sensitive to changes in the collagen content of the bone. Measurements were performed on bone samples before and after mild demineralization and decollagenization at the exact same points. Results show that combining both modalities can enhance the sensitivity and specificity of diagnostic tool.

Ultrabright femtosecond electron sources: perspectives and challenges towards the study of structural dynamics in labile systems

The advances made in femtosecond electron sources over the last thirty years have been remarkable. In particular, the development of ultrabright femtosecond electron sources has made possible the observation of molecular motion in labile organic materials and it is paving the way towards the study of complex protein systems. The principle of radio frequency (RF) rebunching cavities for the compression of ultrabright electron pulses is presented, alongside with a recent demonstration of its capabilities in capturing the relevant photoinduced dynamics in weakly scattering organic systems. Organic and biological systems can easily decompose or lose crystallinity as a consequence of cumulative heating effects or the formation of side reaction photoproducts. Hence, source brightness plays a crucial role in achieving sufficient signal-to-noise ratio before degradation effects become noticeable on the structural properties of the material. The current brightness of electron sources in addition to the high scattering cross section of keV-MeV electrons have made femtosecond electron diffraction a powerful tool for the study of materials composed by low-Z elements.