In vitro studies and safety assessment of Doppler ultrasound as a diagnostic tool in rhinosinusitis.

We have previously proposed the use of Doppler ultrasound to noninvasively stage a sinus infection. In this study, we first investigated the acoustic properties of nonpurulent and mucopurulent sinus secretions. The density, viscosity, speed of sound and attenuation of 18 samples of sinus fluid were examined. We then assessed the safety of the method by determining the temperature increase when ultrasound is transmitted through a bone sample of the same thickness as the anterior wall of the maxillary sinus. As a measure of the probability to generate acoustic streaming, we determined the ratio of sound attenuation over the viscosity of the sinus fluid and compared this with the value obtained from acoustic streaming measurements on a model system. The results indicated that detectable levels of acoustic streaming can be generated in serous sinus fluid, which has a low viscosity, but is very unlikely in mucopurulent secretions. The attenuation of the mucopurulent sinus fluid was 10 times higher than that of the serous cyst fluid, but the viscosity of the mucopurulent secretion was a thousand times higher than that of serous fluid. The safety experiments gave a temperature increase of the bone of <1.5°C at I(spta) of 640 mW/cm(2), below the temperature increase considered to be harmful by the World Federation for Ultrasound in Medicine and Biology.

Feasibility of measuring acoustic streaming for improved diagnosis of rhinosinusitis.

No noninvasive methods exist currently with the capability of distinguishing between various stages of a sinus infection. We studied a method based on induced acoustic streaming in the accumulated fluid within the maxillary sinuses. The hypothesis was that acoustic streaming will not be induced at clinically acceptable intensity levels in infectious mucous fluid because of its high viscosity, whereas detected acoustic streaming is a strong indication that the sinus content is a noninfectious serous fluid. As a model, an anthropomorphic sinus phantom with bovine cortical bone to mimic the bone surrounding the maxillary sinus was constructed. Milk (1.5% fat content) was used as model fluid. From fluid and bone attenuation measurements, an ultrasound frequency of about 5 MHz was estimated to produce the highest acoustic streaming in the sinus phantom. Simulations of the acoustic streaming in a sealed cavity also showed that the width of the ultrasound beam should be about half the size of the cavity to optimize the streaming velocity. With a 4.9-MHz continuous-wave transducer operating at a spatial peak temporal average intensity (I(spta)) of 640 mW/cm(2), an acoustic streaming velocity of 0.19 cm/s was generated and detected in the sinus phantom.