Temperature measurements provide 3-D ultrasound maps rapidly

There are several techniques for visualizing ultrasound fields. One involves scanning a hydrophone, an underwater microphone, through a set of points. Another approach relies on acousto-optical effects, which are based on a change in the optical refractive index by pressure (i.e., sound) waves. Other techniques such as Doppler velocimetry have also been used, but all of these methods are slow or require specialized optical setups.

In an article by Melde et al., the authors describe a new method for producing 3-D maps of an ultrasound field using thermography. The measurement device consists of an open cylindrical cup that has a thin membrane across the bottom surface. A thermal camera mounted above the cup scans through various positions with an automated stage.

The membrane is made of a sound-absorbing material; the authors investigated two different types of membranes. Both membrane types convert absorbed sound waves to heat, which leads to a local rise in temperature. The camera records the detected temperature distribution across the membrane. Calibration of the membrane and camera allows conversion from temperature to sound pressure amplitude in the water tank below the cup.

Initial measurements show good agreement between experimental results and a simple model for membrane heating, and the data show the expected quadratic dependence of temperature on sound pressure. While the results appear to be not as sensitive or accurate as those gathered through hydrophone measurements, the data are collected much more quickly. A full 3-D image of a sound profile in a 54-cubic-centimeter volume was produced in just 33 seconds with this set-up – this is orders of magnitude faster than a hydrophone could achieve.

Source: “Fast spatial scanning of 3D ultrasound fields via thermography,” by K. Melde, T. Qiu, and P. Fischer, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5046834 .