Hiroshi Kuroki, Yasuaki Nakagawa, Koji Mori
Arthritis Research & Therapy
This letter is the response to the concern shown by Zheng  regarding our article . Because our ultrasound system obtains indirect information on intrinsic physical characteristics of living human articular cartilage in vivo settings, we consider that the signal intensity obtains information including the intrinsic physical characteristics. We do not disregard to measure the intrinsic physical characteristics. Indeed, the signal intensity correlated with the aggregate modulus of articular cartilage significantly . We mentioned the equations of Young modulus, indicating speed of sound, density of a material, and acoustic impedance of a material  and presented Gabor function as the mother wavelet and equations . The signal intensity did not depend on the surface curvature, in radiuses of more than 40-mm, and mainly reflects the condition from surface of cartilage to one wavelength depth . The tip of the probe with an ultrasonic transducer is designed to achieve uniform distance between the transducer and cartilage surface. Our ultimate goal is not to measure the intrinsic physical characteristics but to improve the diagnostic use of an arthroscopic ultrasound system and the method to detect early stage of degeneration of human articular cartilage. It is not easy to calculate the intrinsic physical characteristics from ultrasonic echo obtained under arthroscopy. True it is ideal that the intrinsic physical characteristics of cartilage are measured accurately, but it is still difficult in clinical settings using existing devices. We consider that weakness to measure the intrinsic physical characteristics accurately does not interfere with our final purpose, i.e. quantitative evaluation of human cartilage in clinical settings such as arthroscopy. Information on the signal intensity is valuable for clinicians who want to know mechanism of degeneration of cartilage without performing a tissue biopsy. We believe that the ultrasound information that our technique provides will help clinicians to understand degeneration of articular cartilage, even if the information is obtained from an indirect measurement of the intrinsic physical characteristics.