Gaseous emboli detection through subharmonic and ultraharmonic emissions

Since several years an increased interest is devoted to the detection and characterization of gas embolism within a variety of clinical fields like carotid or cardiac surgery. To select the appropriate treatment and reduce the risk of embolism, it is essential to first detect, classify (particulate or gaseous matters) and ultimately size these emboli. Current detection and classification techniques are based on Doppler processing but their accuracy and clinical significance have not yet been scientifically established. We propose in this study an approach based on the nonlinear scattering properties of gaseous emboli. The focus was primarily directed to subharmonic (SH) and ultraharmonic (UH) generation. Gaseous emboli were produced by generating uniform air bubbles with diameters ranging from 40 μm to 150 μm. This range of sizes is comparable to emboli sizes encountered in the clinic. Acoustic measurements were carried out using a scanning frequency of 130 kHz and a wide receive frequency band with a needle hydrophone. SH and first UH components were investigated as a function of applied acoustic pressure (80 kPa up to 400 kPa) and bubble sizes. For the scanning frequency and the applied acoustic pressures used in this study, only bubbles ranging from 58 μm up to 110 μm were capable of generating subharmonic and ultraharmonic frequency components. However, gas emboli outside this range behave differently: bigger bubbles behave only linearly while smaller bubbles could generate higher harmonic components (2, 3, etc.). In conclusions, such an approach can be used to provide information needed to classify and size emboli.