Passive acoustic mapping with absolute time-of-flight information and delay-multiply-sum beamforming

Passive acoustic mapping (PAM) is showing increasing application potential in monitoring ultrasound therapy by spatially resolving cavitation activity. PAM with the relative time-of-flight information leads to poor axial resolution when implemented with ultrasound diagnostic transducers. Through utilizing the absolute time-of-flight information preserved by the transmit-receive synchronization and applying the common delay-sum (DS) beamforming algorithm, PAM axial resolution can be greatly improved in the short-pulse excitation scenario, as with active ultrasound imaging. However, PAM with the absolute time-of-flight information (referred as AtPAM) suffers from low imaging resolution and weak interference suppression when the DS algorithm is applied.

This study aims to propose an enhanced AtPAM algorithm based on delay-multiply-sum (DMS) beamforming, to address the shortcomings of the DS-based AtPAM algorithm.

In DMS beamforming, the element signals delayed by the absolute time delays are first processed with a signed square-root operation and then multiplied in pairs and finally summed, the resulting beamformed output is further band-pass filtered. The performances of DS- and DMS-based AtPAMs are compared by experiments, in which an ultrasound diagnostic transducer (a linear array) is employed to passively sense the wire signals generated by an unfocused ultrasound transducer and the cavitation signals generated by a focused therapeutic ultrasound transducer in a flow phantom. The AtPAM image quality is assessed by main-lobe width (MLW), intensity valley value (IVV), area of pixels (AOP), signal-to-interference ratio (SIR), and signal-to-noise ratio (SNR).

The single-wire experimental results show that compared to the DS algorithm, the DMS algorithm leads to an enhanced AtPAM image with a decreased transverse MLW of 0.15 mm and an improved SIR and SNR of 31.50 and 18.77 dB. For the four-wire images, the transverse (axial) IVV is decreased by 18.37 dB (13.11 dB) and the SIR (the SNR) is increased by 26.13 dB (18.47 dB) when using the DMS algorithm. The cavitation activity is better highlighted by DMS-based AtPAM, which decreases the AOP by 0.81 mm² (-10-dB level) and 4.43 mm² (-20-dB level) and increases the SIR and SNR by 20.14 and 10.48 dB respectively. The pixel distributions of AtPAM images of both wires and cavitation activity also indicate a better suppression of the DMS algorithm in sidelobe and noise.

The experimental results illustrate that the DMS algorithm can improve the image quality of AtPAM compared to the DS algorithm. DMS-based AtPAM is beneficial for detecting cavitation activity during short-pulse ultrasound exposure with high resolution, and further for monitoring short-pulse ultrasound therapy.