抄録
組織に音響放射圧を加えることでひずみを発生させ,組織の機械的特性を計測する試みが多くの研究者により報告されている.しかし対象物の弾性率が周囲媒質の弾性率に比べ大きい場合,1つの音響放射圧を対象物に照射すると位置変化が生じてしまい,ひずみを効率的に発生させることができない.そのため,本研究では2つの周期的に変動する音響放射圧を,筋肉を模擬した対象物の両側から印加する方法を提案する.音響放射圧を両側から印加することで対象物は水平方向に圧縮され,対象物の非圧縮性により,上下に膨張する.その結果生じる変位分布を別の超音波を用いて計測する.わずかに異なる周波数1MHzと(1M+5)Hzの連続正弦波を重ね合わせた信号を加振用トランスジューサに印加することにより超音波の焦域において差の周波数5Hzで変動する音響放射圧が発生する.本報告では,このような音響放射圧を2つ用いて対象物の加振を行い,対象物内部に生じる変位を超音波を用いて計測することで,対象物内部にひずみを発生させ,さらにその変位を超音波計測できることを確認する.
Many studies have been conducted to measure mechanical properties of tissues by applying an ultra-sound-induced acoustic radiation force. To assess mechanical properties, strain of an object must be generated. However, one radiation force is not effective because it also generates translational motion when the object is much harder than surrounding medium. In this study, two cyclic radiation forces are applied to a muscle phantom from two opposite horizontal directions so that the object is cyclically compressed in the horizontal direction. By the horizontal compression, the object is vertically expanded due to the incompressibility. The resultant vertical displacement is measured using ultrasound. Two ultrasonic transducers for actuation were both driven by sums of two continuous sinusoidal signals at two slightly different frequencies (1MHz and (1M+5) Hz). The displacement of several micrometers in amplitude, which fluctuated at 5Hz, were measured by the ultrasonic phased tracking method. Upward displacements at the surface were found when acoustic radiation forces increased. Such displacements correspond vertical expansion due to the horizontal compression.
