Broad Band Ultrasound Attenuation (BUA) is the widely used ultrasound parameter used in bone properties assessment. However, its application is restricted by its own definition, i.e. there must be a linear relationship between the ultrasound attenuation and frequency so that the slope of the linear fitting curve can be reliably obtained. BUA works quite well in regular shaped bone samples such as calcaneus but not on other bones. Frequency modulated ultrasound can focus ultrasound energy in a designated frequency band and is an ideal acoustic signal to study the frequency response of bone to ultrasound. Two separated studies have demonstrated the new signal is promising to characterize bone properties.
The first study [1] was to investigate the efficacy of modulated ultrasound signals compared with BUA. Twenty-one trabecular bone cubes were harvested from sheep femoral condyles and the cube axes corresponded to the anatomic superior-inferior (SI), anterio-posterior (AP), and medial-lateral orientations (ML). Micro-CT measurements were made on those samples to obtain bone volume fraction (BV/TV), trabecular thickness (Tb.Th) and trabecular separation (Tb.Sp). Ultrasound tests were performed in the three orthogonal orientations using pulse and frequency modulated ultrasound. The comparison of the frequency modulated attenuation (FMA) with the broadband ultrasound attenuation (BUA) was made within the frequency band between 300 kHz and 700 kHz. Results showed that FMA demonstrated higher correlations to the trabecular structure properties in the SI orientation (R2=0.84 for BV/TV, R2=0.77 for Tb.Th, R2=0.7 for Tb.Sp) than BUA (R2=0.30 for BV/TV, R2=0.27 for Tb.Th, R2=0.33 for Tb.Sp). In the AP orientation, FMA had higher correlation to Tr.Sp (R2=0.64) than BUA (R2=0.48), and relatively lower correlation to BV/TV (R2=0.48) and Tb.Th (R2=0.31) than BUA (R2=0.64 for BV/TV and R2=0.58 for Tb.Th). The results suggested that FMA could be a new ultrasound index for bone properties assessment.
The second study [2] investigated the efficacy of frequency modulated ultrasound attenuation in the assessment of the trabecular structural properties in four frequency bands centered at 500 kHz, 900 kHz, 1.3 MHz and 1.7 MHz with the bandwidth of 400 kHz. Five one-centimeter trabecular cubes were harvested from fresh bovine distal femur. The cubes underwent four steps of demineralization process to expand the sample size to twenty five with the greater variations of the structural properties for the better correlation study. Pearson correlation study was performed between the ultrasound attenuation in four frequency bands and the trabecular structural properties. The results showed that correlations of frequency modulated ultrasound attenuation to the trabecular structural properties are dependent on frequency bands. The attenuation in proximal-distal orientation had the highest correlation to BV/TV (R2=0.73, p<0.001) and trabecular thickness (R2=0.50, p<0.001) at the frequency band centered at 1.7 MHz. It was equivalent in the four frequency bands in correlation to the trabecular number (average R2=0.80, p<0.001) and to the trabecular separation (average R2 =0.83, p<0.001). The attenuation in anterio-posterial orientation had the highest correlation to BV/TV (R2=0.80, p<0.001) and trabecular thickness (R2=0.71, p<0.001) at the frequency band centered at 1.3 MHz. The attenuation in the first frequency band was the most sensitive to the trabecular number (R2=0.71, p<0.001) and trabecular separation (R2=0.80, p<0.001). No significant correlation was observed for the attenuation in medial-lateral orientation across the four frequency bands.
- Lin W., Xia Y, Qin Y, Characterization of the trabecular bone structure using frequency modulated ultrasound pulse, Journal of Acoustic Society of America, 125(6), 4071-4077, June 2009
- Lin, W., Serra-Hsu, F., Cheng, J, Frequency Specific Ultrasound Attenuation Is Sensitive to Trabecular Bone Structure, Ultrasound in Medicine and Biology, 38, No. 12, p2198-2207, 2012