Abstract
Shockwaves have been used routinely for the treatment of urolisthesis for decades. Strong evidencessuggest that cavitation bubble activity plays an important role on the stone breakage. Recently,shockwaves were also shown to be effective in the treatment of certain soft tissue disorders such astendinosis. However, whether or not cavitation occurs in tight tissue and contributes to tendon healingas well as pain relief remain unknown. In order to understand the possible physical mechanism of theshockwave therapy, we focused on searching the evidences of cavitation in tissue during the shockwavetreatment, and also the determining factors of bubble generation. It was hypothesized that theshock-wave induced cavitation was constrained by the space for bubble expansion, and thus occurredmore easily in vessels of larger diameters than capillaries, intercellular space and inside cells. To proveour hypothesis, a series of studies was performed to detect the presence of inertial cavitation in softtissue blocks of various densities ex-vivo and elastic tissue phantoms embedded with vessels ofdifferent size in-vitro. We found that shock-wave induced bubble generation highly depended on the‘porosity’ (equivalent to the perfusion status) of the ex-vivo tissue blocks. The bubble generation inmuscle was substantially less than that in liver, and was undetectable in tendon. For vessel phantoms,cavitation occurred easily in vessels with diameters greater than or equal to 0.7 mm when tap waterwas pumped through. However, no trace of bubble signal was found in smaller vessels under thesame acoustic conditions. Size threshold existed. After adding low concentrations of contrast agent(Definity®) in the flow-through fluid, cavitation threshold dropped substantially. For a vessel of 0.32mm in diameter, bubble and signal enhancement could be easily obtained even after a single shot ofshockwave. Furthermore, the presence of cavitation-induced bubbles significantly enhanced thesignal-to-noise ratio of vessels in the B-mode image, comparing with the use of contrast agent alone.Concentration as low as 0.005% (v/v) could elevate the B-mode’s averaged gray level in the target areaup to 76 levels, much higher than the enhancement contributed by contrast agent alone (about 2 to 3levels in this concentration)