Characterization of center frequency and bandwidth of broadband ultrasound reflected by the articular cartilage to subchondral bone interface

Osteoarthritis (OA) produces degenerative changes both in articular cartilage and subchondral bone. During OA, reflection of high frequency ultrasound from the cartilage-bone interface is affected by both changes in attenuation of the cartilage layer and acoustic properties of the interface. The objective of this study was to experimentally investigate the spectral content of ultrasound reflection from the cartilage-bone interface. Specifically, we analyzed the center frequency and -6 dB bandwidth of the broadband high-frequency (40 MHz) ultrasound signal. Intact bovine articular cartilage samples with and without the underlying subchondral bone (n = 6) were measured in vitro using a commercial high-frequency ultrasound scanner. Furthermore, the diagnostic potential of the measurement of center frequency and bandwidth for OA was studied with another series of bovine articular cartilage samples (n = 40) after enzymatic degradations of tissue proteoglycans and collagen. Compared with the reference spectrum at the same depth from a perfect reflector, a major downshift (>51%) of the center frequency and a reduction (>42%) of the bandwidth were observed in both sample groups when analyzing the ultrasound reflection from the cartilage-bone interface. The results suggest that attenuation in the cartilage layer primarily controls the observed downshift of the center frequency and acoustic properties of the subchondral bone play only a minor role in affecting the spectrum of the cartilage-bone interface. Changes in the ultrasound bandwidth of the cartilage-bone interface signals, compared with reference signals, were found to vary more than those in the center frequency in both cartilage sample groups. Compared with pretreatment values, a significant downshift in center frequency (p < 0.01) and a minor reduction in bandwidth of spectra from the cartilage-bone interface were recorded after chemical degradation of proteoglycans with trypsin. In contrast, center frequency and bandwidth of the echoes from the cartilage-bone interface did not change after the chemical degradation of cartilage collagen fibrils. The results suggest that proteoglycan loss, typical to OA, may be detected via the changes in the center frequency of the ultrasound reflected from the cartilage-bone interface. (E-mail: ypzheng@ieee.org, simo.saarakkala@oulu.fi)