Practical considerations in the calibration of array probes for quantitative cavitation imaging: Lessons learned from 16 attempts

Passive Acoustic Mapping (PAM) is a leading approach for the quantification of cavitation energy and can inform the safe and effective use of cavitation in medicine. To achieve quantitative measurements, the sensitivity and directivity of each element in a PAM array must be calibrated, and any issues in the calibration can propagate into poor resolution, incorrect magnitudes, and location errors in all subsequent PAM images. Accurate array calibrations are therefore crucial in the pursuit of a “cavitation dose” to correlate with bioeffects. The most commonly used calibration method, reported by Gray and Coussios in 2018, is a substitution approach using a fine wire scatterer to approximate a point source. Despite its apparent simplicity, this experiment features a number of complicating factors that can prohibit successful measurements, particularly for newer acousticians. Here, we describe the calibration experiment in detail, along with the impacts, causes, and solutions of a range of issues identified through the authors’ hard-won experience. Special consideration will be given to the selection of sources and hydrophones, alignment, management of signal-to-noise ratio, and data processing. We will also discuss common assumptions and their validity, measurement uncertainty, comparing results to theory, and future directions for this area of ultrasound metrology.