doi:10.1016/j.ultrasmedbio.2008.09.022 ● Original Contribution IMAGING OF GAPS IN DIGITAL JOINTS BY MEASUREMENT OF ULTRASOUND TRANSMISSION USING A LINEAR ARRAY HIDEYUKI HASEGAWA, #, *MICHIKO MATSUURA, † HIROSHI SATO, † TERUKO YAMAMOTO † and HIROSHI KANAI ,# # Graduate School of Biomedical Engineering; *Graduate School of Engineering; and † Graduate School of Dentistry, Tohoku University, Sendai, Japan (Received 14 December 2007; revised 18 August 2008; in final form 15 September 2008) Abstract—In orthodontic dentistry for young subjects, it is important to assess the degree of growth of the jaw bones to determine the optimum time for treatment. The structure of the digital joint changes with age, with such changes correlating to the degree of bone growth (including jaw bones). There are two gaps in the digital joint of a young subject, one of which disappears with aging. In the present study, a method for noninvasive assessment of such change in the structure of a digital joint was examined, in which continuous-wave ultrasound is radiated to a digital joint by a single-element ultrasonic transducer. This continuous ultrasound, which passes through the digital joint, is received by a linear array ultrasonic probe situated opposite the transducer. The probe simultaneously realizes pulse– echo imaging and imaging of transmission ultrasound, which passes through the joint. Using this experimental apparatus, the existence and position of a gap can be detected clearly by imaging the transmission ultrasound on a pulse– echo image. In basic experiments, continuous-wave ultrasound generated by a planar or focused transducer was radiated to a gap between two acrylic bars, which simulated that in a digital joint; transmission ultrasound, which passed through the gap, was measured with a linear array probe. The basic experimental results showed that a gap with a width >0.4 mm is detectable and that the width at half maximum of the amplitude profile of the received transmission ultrasound that passed through the gap correlated with the width of the gap. Furthermore, in the preliminary in vivo experiments, transmission ultrasound that passed through two gaps in the case of a child was clearly imaged by the proposed method, and that which passed through only one gap in the case of an adult was also imaged. These results show the possibility for the use of the proposed method to noninvasively assess the change in the structure of a joint as a result of aging. (E-mail: hasegawa@ecei.tohoku.ac.jp) © 2009 World Federation for Ultrasound in Medicine & Biology. Key Words: Bone age, Digital joint, Continuous-wave ultrasound, Ultrasound transmission, Linear array. INTRODUCTION In orthodontic dentistry for young subjects, it is impor- tant to assess the degree of growth of the jaw bones to determine the optimum time for treatment. The ossifica- tion of bones in a hand is often used for assessment of bone age, such ossification being evaluated by x-ray imaging of a hand and wrist (Tanner et al. 1975; Rucci et al. 1995; Wit et al. 2005; Jones and Ma 2005). In addi- tion, Khal et al. (2008) have recently proposed a method by which the degree of bone growth is assessed based on the maturation of the cervical spine. The bone maturity assessed by this method was found to correlate well with that assessed by hand radiography. This method would be useful because the cervical spine is always seen on the lateral cephalometric radiograph. Although these meth- ods give an accurate estimate of bone age, exposure of healthy subjects to x-rays is inevitable. Because ultrasound is useful for noninvasive diag- nosis of bone, many studies on ultrasonic measurement of bone have been conducted. Wells (1975) reported the speed of sound and ultrasonic absorption in bone, whereas Williams (1991) predicted the propagation of slow and fast waves propagating through bone by Biot’s theory (Biot 1962) and also experimentally measured these waves. Otani and colleagues extensively investi- gated slow and fast waves (Hosokawa and Otani 1997, 1998; Hosokawa et al. 1997; Otani 2005), which propa- gate in bone marrow and the contained hard structure, respectively. Therefore, the amplitude of a slow-wave Address correspondence to: Hideyuki Hasegawa, Department of Biomedical Engineering, Graduate School of Biomedical Engineering, Tohoku University, 6-6-05 Aramaki-aza-Aoba, Aoba-ku, Sendai 980- 8579, Japan. E-mail: hasegawa@ecei.tohoku.ac.jp Ultrasound in Med. & Biol., Vol. 35, No. 3, pp. 382–394, 2009 Copyright © 2009 World Federation for Ultrasound in Medicine & Biology Printed in the USA. All rights reserved 0301-5629/09/$–see front matter 382