3D X-ray CT : a multiscale, multi contrast technique

International audienceX-ray computerized tomography (CT), which has been a pioneer technique in medical imaging, has evolved from a single slice to a fully three-dimensional imaging technique. Today, many developments are in progress to achieve time and spatially resolved quantitative images with different contrasts from the organ to the cell level. These developments allowed the spreading out of CT techniques from clinical imaging to pre-clinical and biological imaging. We will describe some of these evolutions and illustrate these methods on the multiscale characterization of bone tissue.Spatially resolved CT is necessary to image thin structures such as bone micro-architecture. Imaging trabecular bone has been a driving application in the development of X-ray micro-Computerized Tomography (CT) ex-vivo. X-ray micro-CT systems made commercially available have received applications in a huge number of fields (biology/medicine, material science, industry, environment…). We will particularly present micro-CT based on synchrotron radiation (SR) and their advantages in terms of image quality over systems using standard X-ray sources, as well as the setups developed at ESRF (European Synchrotron Radiation Facility), Grenoble, France.While the conventional contrast in X Ray CT is based on absorption, other contrasts are becoming available. A very promising field is phase contrast imaging, which is a coherent imaging technique improving the sensitivity of X-rays by exploiting another mode of contrast. This technique can be easily implemented on synchrotron sources by using the free propagation of the beam. This technique has been implemented on different beamlines at the ESRF permitting to achieve phase contrast at different spatial resolutions. While micro-CT permit a detailed analysis of bone micro architecture with spatial resolution of a few micrometers, nano-CT permit to investigate in details the lacuno-canalicular network (LCN) embedding the osteocyte system in bone with spatial resolution up to a few tenth of nanometers. According to the setup, it is possible either to obtain a global assessment of the LCN in entire osteons or a detailed analysis around a few cells (Figure 1)[1] [2] [3].If synchrotron CT is well suited to acquire monochromatic images, yielding to a quantitative map of the linear attenuation coefficient within the object, this possibility is generally not offered on standard CT due to limited X-ray source flux and the use of integration detectors. A new generation of CT, called spectral CT, is emerging, based on the use of photon counting detectors permitting the acquisition of energy resolved projections. This new modality may provide various maps, such as the concentration of different materials in the object or pseudo monochromatic images. With the development of specific reconstruction algorithms is spectral CT, this technique is promising for many applications [4].References[1] Pacureanu A, Langer M, Boller E, Tafforeau P, Peyrin F. Med Phys 2012;39:2229.[2] Langer M, Pacureanu A, Suhonen H, Grimal Q, Cloetens P, Peyrin F. PLoS ONE 2012;7:e35691.[3] Yu B, Weber L, Pacureanu A, Langer M, Olivier C, Cloetens P, Peyrin F, Optics Express 2018, Vol. 26, No. 9 ; pp 11110-11123.[4]. Ducros N., Abascal J.F.P.J., Sixou B., Rit S., Peyrin F., Regularization of Nonlinear Decomposition of Spectral X-ray Projection Images, Med Phys, vol 44, n°9, pp e174-e187, 2017AcknowledgementThis work was supported by the LabEx PRIMES (ANR-11-LABX-0063) within the program "Investissements d'Avenir" (ANR-11-IDEX-0007)