Attenuation coefficient and speed of sound in immature and mature rat cartilage: a study in the 30-70 MHz frequency range

In order to determine the influence of cartilage structural changes on high frequency ultrasound propagation, speed of sound and the frequency variation of attenuation in the 28-70 MHz bandwidth were estimated in immature and mature cartilage of rat patellae. Frozen patellae were cut in the sagittal direction perpendicular to the articular surface and the different layers of the cartilage sections were explored using a 55 MHz backscatter ultrasound microscope. At each scan position, the speed of sound and attenuation coefficient were estimated using substitution method. For immature cartilage, acoustic parameters decreased significantly (p<0.05) from the articular surface to the chondral/bone interface. The parameter mean values averaged over the entire depth of the tissue were significantly lower than those measured in mature cartilage (p<0.05). Correlation with histologic images showed that acoustic parameters are markedly influenced by changes in the collagen fibril density