Estimation of ultrasonic attenuation in a bone using coded excitation A. Nowicki a, * , J. Litniewski a , W. Secomski a , P.A. Lewin b , I. Trots a a Institute of Fundamental Technological Research, Polish Academy of Sciences, Swietokrzyska 21, 00-049 Warsaw, Poland b Drexel University, Philadelphia, USA Received 7 May 2003; received in revised form 26 June 2003; accepted 12 August 2003 Abstract This paper describes a novel approach to estimate broadband ultrasound attenuation (BUA) in a bone structure in human in vivo using coded excitation. BUA is an accepted indicator for assessment of osteoporosis. In the tested approach a coded acoustic signal is emitted and then the received echoes are compressed into brief, high amplitude pulses making use of matched filters and cor- relation receivers. In this way the acoustic peak pressure amplitude probing the tissue can be markedly decreased whereas the average transmitted intensity increases proportionally to the length of the code. This paper examines the properties of three different transmission schemes, based on Barker code, chirp and Golay code. The system designed is capable of generating 16 bits com- plementary Golay code (CGC), linear frequency modulated (LFM) chirp and 13-bit Barker code (BC) at 0.5 and 1 MHz center frequencies. Both in vivo data acquired from healthy heel bones and in vitro data obtained from human calcaneus were examined and the comparison between the results using coded excitation and two cycles sine burst is presented. It is shown that CGC system allows the effective range of frequencies employed in the measurement of broadband acoustic energy attenuation in the trabecular bone to be doubled in comparison to the standard 0.5 MHz pulse transmission. The algorithm used to calculate the pairs of Golay sequences of the different length, which provide the temporal side-lobe cancellation is also presented. Current efforts are focused on adapting the system developed for operation in pulse-echo mode; this would allow examination and diagnosis of bones with limited access such as hip bone. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Coded ultrasound; Bone examination; Broadband ultrasonic attenuation 1. Introduction This paper describes development of a robust ultra- sound system for assessment of bone osteoporosis. As evidenced below, performance of the system holds promise to be able to carry out screening of osteoporosis measurements using pulse-echo ultrasound as opposed to currently used transmission mode, where ultrasound energy is interrogating heel bone being positioned be- tween an acoustic source and receiver. The development of the pulse-echo mode is of great importance as that would allow early diagnosis of selected critical bones; non-ionizing, non-invasive diagnosis of bone such as hip is currently not available. The attenuation of the ultrasonic signals in the tissue results in echoes from deep structures being virtually buried in noise. It is desirable to maximize signal-to- noise ratio (SNR) of the echoes from highly attenuating trabecular bones. However, the SNR of the returning signals depends on the maximum acoustic pressure amplitude permitted to be launched into the examined tissue. Thus penetration depth and transmitted pulse peak pressure amplitude are the two most important considerations in ultrasound imaging and measurement systems. At present, a majority of ultrasound imaging systems uses brief pulse excitations. To limit the peak pressure amplitudes probing the tissue, while retaining or even improving the level of echoes acquired using conventional, brief pulse approach, a carefully selected pulse compression technique can be applied. In the tes- ted approach described below, a coded acoustic signal is emitted and then the received echoes are compressed into brief, high amplitude pulses through the use of matched filters and correlation receivers. In this way the acoustic peak pressure amplitude probing the tissue can be markedly decreased whereas the average transmitted * Corresponding author. Tel.: +48-22-826-6508; fax: +48-22-826- 9815. E-mail address: anowicki@ippt.gov.pl (A. Nowicki). 0041-624X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0041-624X(03)00181-1 Ultrasonics 41 (2003) 615–621 www.elsevier.com/locate/ultras