Nanoscale imaging of the bone cell network with synchrotron X-ray tomography: optimization of acquisition setup

The fundamental role of the osteocyte cell network in regulating the bone remodeling has become evident in the last years. This has raised the necessity to explore this complex three-dimensional interconnected structure, but the existing investigation methods cannot provide an adequate assessment. The authors propose to use parallel beam synchrotron radiation computed tomography at the nanoscale to image in three dimensions the osteocyte lacunocanalicular network. To this aim, the authors study the feasibility of this technique and present an optimized imaging protocol suited for the bone cell network. Moreover, they demonstrate the multifaceted information provided by this method.

The high brilliance of synchrotron radiation combined with state of art detectors permits reaching nanoscale spatial resolution. With a nominal pixel size of 280 nm, the parallel beam computed tomography setup at the ID19 experimental station of the ESRF is capable of imaging the bone lacunocanalicular network, considering that the reported diameter of canaliculi is in the range 300-600 nm. However, the actual resolution is limited by the detector and by the radiation dose causing sample damage during the scan. The authors sought to overcome these limitations by optimizing the imaging setup and the acquisition parameters in order to minimize the necessary radiation dose to create the images and to improve the spatial resolution of the detector.

The authors achieved imaging of the osteocyte cell network in human bone. Due to the optimization of the imaging setup and acquisition parameters, they obtained simultaneously a radiation dose reduction and an increase of the signal to noise ratio in the images. This permitted the authors to generate the first three-dimensional images of the lacunocanalicular network in an area covering several osteons, the fundamental functional units in the bone cortex. The method enables assessment of both architectural parameters of the microporosity and of mineralization degree in the bone matrix. The authors found that the cell network is dense and connected inside osteonal tissue. Conversely, the cell lacunae are sparse, unorganized, and disconnected in interstitial tissue.

The authors show that synchrotron radiation computed tomography is a feasible technique to assess the lacunocanalicular network in three dimensions. This is possible due to an optimal imaging setup in which the detector plays an important role. The authors could establish two valid setups, based on two different insertion devices. These results give access to new information on the bone cell network architecture, covering a number of cells two orders of magnitude greater than existing techniques. This enables biomedical studies on series of samples, paving the way to better understanding of bone fragility and to new treatments for bone diseases.