Calibrated tomographic schlieren system for characterization of medical probes

Ultrasound (US) probes in the MHz range used in the medical field must comply with specific rules. Traditionally, calibrated hydrophones measure the pressure of the US field within a given volume but this procedure is time consuming. More recently, the Schlieren system has been demonstrated a fast and accurate technique for the quantification of the ultrasonic field characteristics (beam width and average acoustic intensity in function of the gray level). However, a relationship between the video gray level and the peak pressure was not determined, and only a small number of medical probes could be studied in real time due to a limited field of view. In this paper, the realization of an experimental set-up based on the Schlieren effect, allowing real time imaging, and a quantitative tomographic reconstruction of the US field of a wide range of medical probes, driven with continuous waves, is reported. We also investigated the relationship between pressure measurements and tomographic luminance values, in order to calibrate our Schlieren system. The results demonstrate that this new Schlieren system of compact dimension, 40 cm x 100 cm x 40 cm, is a time efficient method to characterize several medical probes in the frequency range 1-40 MHz. A non-linear relationship between the gray level of the tomographic reconstruction and the peak pressure was experimentally found. Once the system has been calibrated, it allows the quantitative reconstruction of tomographic views at any depth. Our future work is to quantify the video of the US field in real time and to obtain a 3D visualization.