ORIGINAL ARTICLE Bone surface topology mapping and its role in trabecular bone quality assessment using scanning confocal ultrasound Y. Xia & W. Lin & Y.-X. Qin Received: 28 June 2006 / Accepted: 27 December 2006 / Published online: 15 March 2007 # International Osteoporosis Foundation and National Osteoporosis Foundation 2007 Abstract Introduction: Quantitative ultrasound (QUS) has been used to assess non-invasively bone quality, in which ultrasound velocity (UV) is a primary acoustic property. Methods: While UV calculation requires the tissue thick- ness in the ultrasound path, a bone surface topology mapping (STM) method was developed in this study for enhancing the accuracy of the UV measurement. STM accuracy was verified by both aluminum and a QUS heel phantom, indicating that the STM can determine the phantom thickness within 0.02 mm thickness error and the aluminum calibration step within 0.1 mm thickness error. STM performance was further evaluated using 25 cadaveric human calcanei samples. Results: The UV calculations using STM had a significant better correlation to bone mineral density (BMD) (r=0.75, p<0.05), volume fraction (r=0.72, p<0.05) and modulus (r=0.69, p<0.05) than the UV with fixed thickness. The later correlation coefficients were r=0.64 for BMD, r=0.65 for volume fraction, and r=0.58 for modulus. The nBUA value determined using STM was also highly correlated to BMD (r 2 =0.74) and modulus (r 2 =0.62). This was compa- rable to the correlation result for BUA (BMD: r 2 =0.76; Modulus: r 2 =0.64). Conclusion: These results suggested that STM technique in scanning ultrasound is capable of determining calcaneus bone thickness and hence enhancing the accuracy of UV measurement. Keywords Bone mineral density . Bone quality . Osteoporosis . Speed of sound . Surface topology . Ultrasound Introduction Osteoporosis and osteopenia are major health issues that affect the aging population as well as an astronaut in a microgravity environment [1–6]. It is widely accepted that quantitative ultrasound (QUS) can provide information regarding bone structure and bone mineral density [7–11], which may prove useful in diagnosing osteoporosis and predicting fracture risk [12–16]. This is an appealing technique for both ground and space-based applications as a tool for monitoring bone strength since it emits no radiation, its ease of use, portability, it is inexpensive and is relatively accurate [4, 17]. In vivo QUS measurement can be carried out on multiple human skeleton sites, such as the calcaneus, distal radius, proximal phalanx, and mid-shaft tibia. However, the most common measurement site is the calcaneus. This site along with using the transmission technique has been suggested to be the best technique to ascertain low bone density in the axial skeleton [18, 19]. Ultrasound velocity (UV) or speed of sound (SOS) combined with broadband ultrasound attenuation (BUA) measurements at the human calcaneus has been shown to be highly correlated to the bone mineral density as well as bone microstructure, which can also predict fracture risk [20]. UV is a principal ultrasound parameter that can be used to assess bone quality because it is theoretically related to bone elastic properties [21]. In order to accurately measure UV through the calcaneus, the bone thickness is very critical [22–25]. However, calcaneal bone thickness and its topology are not directly available due to surrounding soft tissue. Measurement of UV through the calcaneus and soft tissue of unknown thickness has been shown to introduce an error that could be 3 to 20 times higher than the UV short-term precision error [23]. Furthermore, using a preset thickness, as used by many commercial QUS systems will Osteoporos Int (2007) 18:905–913 DOI 10.1007/s00198-007-0324-1 Y. Xia : W. Lin : Y.-X. Qin (*) Department of Biomedical Engineering, Stony Brook University, Psychology-A Bldg., 3rd Floor, Stony Brook, New York, NY 11794-2580, USA e-mail: yi-xian.qin@sunysb.edu