Effects of ultrasound frequency, temporal sampling frequency, and spatial sampling step on the quantitative ultrasound parameters of articular cartilage.

Quantitative ultrasound imaging may provide a technique for diagnosing initial signs of osteoarthritis (OA), such as surface fibrillation of articular cartilage. Because subchondral sclerosis and osteophyte formation occur in OA as well, ultrasonic analysis of subchondral bone could yield useful diagnostic information. In this study, we investigated whether low-frequency (5 MHz) ultrasound, typically used in bone diagnostics, would be feasible for evaluating the integrity of the surface of the cartilage. The reflection parameters in the time and frequency domains, the ultrasound roughness index, and the wavelet-based parameters were evaluated using ultrasound transducers operating at 5, 10, and 50 MHz frequencies. The effects of variable size of spatial sampling steps and of temporal sampling frequencies were also investigated. Custom-made phantoms and cartilage samples with various surface characteristics were analyzed. The reflection parameters detected the surface degradation with all ultrasound frequencies. The roughness of the surface could only be evaluated reliably with the 50 MHz-focused transducer. In conclusion, simultaneous analysis of the reflection parameters of the cartilage and the subchondral bone is feasible at low (5 MHz) ultrasound frequencies. However, reliable evaluation of the microtopography of the cartilage requires use of a higher ultrasound frequency.