Microcomputed Tomography for the Study of Hard Tissues and Bone Biomaterials

K E Y W O R D S bone, tooth, biomaterials, 3D analysis, microcomputed tomography A C K N O W L E D G E M E N T S This work was made possible by grants from the EC through the Network in Europe on Male Osteoporosis and from the Contrat de plan Etat-Région Pays de la Loire. Mrs Guénaelle Brossard and Nadine Gaborit are thanked for their skillful technical assistance. A U T H O R D E TA I L S Daniel Chappard, MD, PhD, INSERM, EMI 0335, Universite of Angers, 49045 Angers Cédex, France. Tel: +33 241 73 58 65 Email: [email protected] Microscopy and Analysis, 19 (3): 17-19 (UK), 2005. MI C R O C O M P U T E D TO M O G R A P H Y MI C R O S C O P Y A N D AN A LY S I S • MAY 2005 17 I N T R O D U C T I O N Microcomputed tomography (microCT) is a miniaturized version of computed axial tomography commonly used by radiologists but the technique has a resolution on the order of a few micrometres. MicroCT systems often make use of laptop computers or work stations and provide images that are very close to those provided by synchrotrons (with a resolution on the order of a micrometre.) Up to now, the use of microcomputed tomography has been successfully used in different branches of science for the study of porous or cavity-containing objects: metallic foams, electronics, stones, wood and composite polymers. In biology, the technique is well adapted to the study of hard tissues because of the high linear attenuation coefficient of the calcified bone and dental matrices [1]. The technique is now favoured for the study of trabecular bone loss in osteoporotic patients or in animal models of osteoporosis [2-4]. In bone biology, a great body of literature is concerned with the measurement of the characteristics of the trabecular network. Histomorphometry was developed in the 1970s as a method to quantify bone loss in osteoporosis in 2D histological sections. For decades, osteoporosis was considered a disease associated with a low bone mass. However, it was only in the 1980s that the 3D alterations of trabecular bone were taken into account although bone, being a living biomaterial, adapts to strains by a redistribution of trabeculae by the remodelling process [5]. The importance of microarchitecture in the pathogenesis of bone fragility is now fully recognized and is part of the WHO definition of the disease: “. . . characterized by low bone mass and microarchitectural deterioration of bone tissue, leading to enhanced bone fragility and a consequent increase in fracture risk”. Several attempts have been made recently to measure the architectural characteristics of trabecular bone in 2D histological sections and in 3D models prepared from microCT analyses. We review here the potential interest of microCT applied to calcified tissues in various research fields. M AT E R I A L S A N D M E T H O D S The microtomographic system We have used the Skyscan 1072 X-ray computed microtomograph (Skyscan, Aartselaar, Belgium) in the cone-beam acquisition mode. The system is composed of a sealed microfocus X-ray tube, air cooled with a spot size less than 8 μm and a CCD camera. Images were obtained at 80 V and 100 μA with a 1 mm aluminum filter each time calcified material was present in the specimen. Specimens were studied either in the wet or dry form. Biopsies or fragile bones were placed in an Eppendorf test tube containing the fixative and the vial was fixed to a stub with plasticine; large and dry specimens were fixed directly to the stub with plasticine. For each specimen, a series of 400 projection images was obtained with a rotation of 0.45° between each image. The magnification used depended on the size of the objects: large human bone biopsies and rat femurs were scanned at 21 (pixel size 11.4 μm), mice bones at 58 (pixel 5.26 μm) while human teeth, blocks of biomaterials and large blocks of human bone (vertebra, radius) were analysed at the lowest magnification (14 ; pixel 19.74 μm). Given a series of projection images, a stack of 2D sections was reconstructed for each specimen (the number of sections depending of the desired height) and stored in the .bmp format with indexed grey levels ranging from 0 (black) to 255 (white). 3D software Three dimensional modelling and analysis reconstruction of the specimens were performed with the ANT software (version 2.2.5.0, Skyscan, Aartselaar, Belgium). The program allows reconstruction of objects from the stack of 2D sections after interactive thresholding. The reconstructed 3D models Microcomputed Tomography for the Study of Hard Tissues and Bone Biomaterials Daniel Chappard, Stéphane Blouin, Hélène Libouban, Michel Félix Baslé, Maurice Audran Faculty of Medicine, University of Angers, France