Effect of Transducer Distance on Ultrasonic Backscatter Measurements of Cancellous Bone

Ultrasonic backscatter measurements may be used to detect changes in bone caused by osteoporosis and other diseases. Backscatter measurements are commonly performed at peripheral skeletal sites such as the heel bone (calcaneus). The transducer is usually placed in direct contact with the body which locates the interrogated region of bone tissue in the acoustic near-field of the transducer. The goal of the study was to determine how the distance between the transducer and bone affects backscatter measurements of bone. A rigid, open cell polymer foam specimen was used to simulate cancellous bone tissue. Ultrasonic measurements were performed in a water tank using a planar 2.25 MHz single element transducer. Backscatter measurements were performed by propagating ultrasonic pulses into the specimen and receiving the returned, backscattered signal. Signals were acquired for five transducer-specimen distances: 15.125 mm, 30.250 mm, 45.375 mm, 60.500 mm, and 75.625 mm corresponding to N/4, N/2, 3N/4, N and 5N/4 where N is the near field distance. Seven backscatter parameters were measured: apparent integrated backscatter (AIB), frequency slope of apparent backscatter (FSAB), frequency intercept of apparent backscatter (FIAB), normalized mean of the backscatter difference (nMBD), normalized slope of the backscatter difference (nSBD), normalized intercept of the backscatter difference (nIBD), and backscatter amplitude decay constant (BADC). FSAB was most sensitive to changes in the transducer-specimen distance, decreasing by approximately 400% as the distance was increased. In contrast, BADC changed by approximately 3%. The results indicate that transducer-specimen distance may have a strong effect on backscatter measurements of cancellous bone, depending on the backscatter parameter measured.