1.Institute of Acoustics, School of Physical Science and Engineering, Tongji University, Shanghai 200092, China 2.Department of Electronic Engineering, Fudan University, Shanghai 200433, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11874289, 11827808, 11804056, 11525416) and the Fundamental Research Funds for the Central Universities, China (Grant No. 02302150002).
Received Date:23 April 2019
Accepted Date:20 June 2019
Available Online:01 September 2019
Published Online:20 September 2019
Abstract:Ultrasonic backscatter has been gradually applied to the assessment and diagnosis of bone disease. The heavy frequency-dependent attenuation of ultrasound results in weak ultrasonic signals with poor signal-to-noise ratio and serious wave distortions during propagation in cancellous bone. Ultrasonic attenuation measured with the through-transmission method is an averaged result of ultrasonically interrogated tissues (including the soft tissue, cortical bone and cancellous bone). Therefore, the through-transmission measurements can not accurately provide ultrasonic attenuation of cancellous bone of interest. The purpose of this study is to estimate ultrasonic frequency-dependent attenuation with ultrasonic backscatter measurements and to compensate for the frequency-dependent attenuation in an ultrasonic backscatter signal from cancellous bone. In-vitro ultrasonic backscatter and through-transmission measurements are performed on 16 cancellous bone specimens by using 1.0-MHz transducers. Spatial scans are performed in a 10 mm × 10 mm scanned region with a spatial interval of 0.5 mm for each bone specimen. The frequency slope of ultrasonic attenuation is measured with the ultrasonic through-transmission signals serving as a standard value. Four different algorithms (the spectral shift method, the spectral difference method, the spectral log difference method, and the hybrid method) are used to estimate the frequency slope of ultrasonic attenuation coefficient from ultrasonic backscatter signal. The results show that the frequency-dependent attenuation coefficient ranges from 2.3 dB/mm/MHz to 6.2 dB/mm/MHz for the bovine bone specimens. The through-transmission measured frequency slope of ultrasonic attenuation coefficient is (4.14 ± 1.14) dB/mm/MHz (mean ± standard deviation), and frequency slopes of ultrasonic attenuation coefficient are estimated by four backscattering methods to be (3.88 ± 1.15) dB/mm/MHz, (4.00 ± 0.98) dB/mm/MHz, (3.77 ± 0.84) dB/mm/MHz, and (4.05 ± 0.85) dB/mm/MHz, respectively. The estimated frequency-dependent attenuation is significantly correlated with the standard attenuation value (R = 0.78-0.92, p < 0.01), in which the spectral difference method (R = 0.91, p < 0.01) and the hybrid method (R = 0.92, p < 0.01) are more accurate with an estimated error less than 20%. The results prove that it is feasible to measure the frequency-dependent attenuation from ultrasonic backscatter signal of cancellous bone. Based on Fourier transform-inverse Fourier transform, the frequency-dependent attenuation can be compensated.The compensated ultrasonic signals are with significantly improved signal intensity and improved signal-to-noise ratio. This study is conducive to the subsequent ultrasonic backscatter measurement and ultrasonic imaging of cancellous bone. Keywords:ultrasonic backscatter/ bone evaluation/ frequency-dependent attenuation/ attenuation compensation
不同于谱差法使用感兴趣区域内的所有时间窗, 谱对数差法只使用感兴趣区域内近端(图2中W1)和远端(图2中W7)的功率谱. 用近端和远端时间窗的功率谱分别除以参考模型的功率谱, 计算其自然对数比然后相减得到[24]: 图 2 松质骨样本的超声背散射信号(ROI, 感兴趣区域) Figure2. Backscatter signal of cancellous bone sample (ROI, region of interest).
图3所示为四种背散射方法测量的频散衰减系数值与透射法频散衰减标准值的关系. 结果表明: 超声背散射方法测量的频散衰减系数值与标准值具有较高的相关性(相关系数R = 0.78—0.92, p < 0.01), 其中谱差法(R = 0.91, p < 0.01)和混合法(R = 0.92, p < 0.01)的测量结果与标准值的相关性更高, 测量结果更为稳定、准确. 图 3 频散衰减系数测量值与透射法频散衰减标准值的关系 (a)谱移法; (b)谱差法; (c)谱对数差法; (d)混合法 Figure3. Relationship between the measured frequency-dependent attenuation and the standard frequency-dependent attenuation: (a) the spectral shift method; (b) the spectral difference method; (c) the spectral log difference method; (d) the hybrid method.