Feature Of The Week 10/10/10: Comparison of OCT, uCT, and Histology at 3D Imaging of Trabecular Bone Samples
Feature Of The Week 10/10/10: Recently researchers from the Institute of Photonics and Terahertz Technology at Ruhr-University, the Experimental Orthopaedics and Biomechanics Department at Philipps University Marburg, the Division of Biomechanics and Engineering Design at Katholieke Universiteit Leuven, and the Department of Dermatology and Allergology at St. Josef Hospital in Bochum, Germany published a very interesting paper on the application of OCT to trabecular bone samples and compared those results with conventional imaging techniques. The results show areas where there were clear advantages of OCT.
Imaging of the fine structure of bone, especially trabecular bone, is important to understand the processes of bone formation and resorption. This knowledge might help to investigate osteoporosis for example or bone invading tumours. Presently, bone structure is investigated using mainly two standard methods, namely bone histomorphometry/histochemistry (“histology”) and micro computed tomography (µCT).
Small animal (rats and mice) and bone culturing systems have been developed for the purpose of investigation of a wide variety of processes in metabolic bone. They keep bone samples in a metabolic state for several weeks, allowing the monitoring of these processes. Especially the interaction between continuous bone growth and resorption in the marrow region can be observed in culture chambers, which is inherently a critical requirement for investigation of osteoporotic or cancerous processes.
Furthermore, dedicated culture chambers, such as the ZETOS offer the possibility of applying particular forces onto the bone which e.g. could simulate natural loading forces onto the bone. These forces can be varied regarding frequency, amplitude and direction and they stimulate a reacting bone growth upon existing bone. The trabecular bone consists of a solid framework of small bone girders as struts and/or plates, called trabeculae. The single trabeculae are covered by bone lining cells and surrounded by bone marrow cells. However, the average thickness of trabeculae, the average density of single trabeculae (mineralization density) and the bone framework (macro architecture) density are important parameters, which influence the total bone strength and therefore have to be observed for investigations regarding e.g. osteoporosis. For analyzing bone growth processes in long-time studies, an appropriate tomographic method has to be used, which should provide three-dimensional, high resolution and in-vitro imaging of bone samples.
A very well accepted imaging technique for monitoring bone structure in-vitro/ex-vivo is µCT. It provides imaging capabilities with isotropic resolutions ranging from a few millimeters (clinical CT), down to few micrometers (µCT), and even further down to one hundred nanometers with synchrotron radiation sources (nanoCT). Desktop µCT is a precise and validated technique, and has been used extensively for different research projects involving bone cultures, small animal bones and trabecular bone specimens from larger animals and humans. However, it uses ionizing X-rays for imaging.
In contrast, optical coherence tomography (OCT) is a non-ionizing imaging method. Furthermore, it is an approved method to generate high-resolution, contactless, tomographic images of biological samples. Thus, OCT is exactly matched to the specifications for observing bone growth processes within a bone culturing chamber. However, up to now, bone samples have been investigated with OCT rather sparsely and the conditions for the applicability of OCT for monitoring bone growth are rather undefined.
Shown here are results analyzing the potential of OCT for imaging of a human trabecular bone sample. For validation, the OCT images are directly compared with the according µCT and histology images from the same sample position. OCT and µCT demonstrate a high correlation in visualizing trabecular architecture of fixated human bone samples. The µCT system is capable of imaging deeper lying structures, while the OCT system offers more contrast and therefore more image details than µCT of hard and soft tissue at comparable nominal resolutions (5µm x 6µm vs. 6.5µm x 6.5µm, depth x lateral). OCT uncovers marrow cell membranes and lamellar bone structures. These image details were verified with unstained and stained histology. At surface-near positions of OCT images, the lamellar structure of trabeculae seems to show slightly higher contrast than normal bone histology. This provides a detailed monitoring of bone structure and with extended periods of time also a monitoring of bone growth. The imaging of cells might offer further insights of the organism activity. Especially we suppose the possibility of monitoring the activity of osteoclasts with OCT.