Ultrasonic interference analysis and elimination for LED-based photoacoustic imaging systems

Photoacoustic imaging (PAI) enables visualization of optical absorption contrast through ultrasonic detection, and light emitting diode (LED) based PAI systems have attracted attention due to their high repetition rates, portability, and cost-effectiveness. However, these systems typically require high driving currents and rapid switching speeds to activate the LEDs, which generates significant electromagnetic interference (EMI). In this study, we identified that this EMI induces currents within the ultrasound probe circuitry, driving the probe to passively emit ultrasonic waves and resulting in ultrasonic noise. To address this issue, we analyzed the amplitude spectrum of the EMI signals and discovered that modulating the LED pulse width shifts its spectral nulls. A pulse width adjustment algorithm was proposed based on the ultrasound probe bandwidth. By aligning the spectral nulls with the center frequency of the probe, EMI coupling is minimized, thereby reducing ultrasonic noise. Additionally, we propose a residual reconstruction algorithm based on an optical shutter. Since the optical shutter modulates the generation of photoacoustic signals while leaving the ultrasonic noise unaffected, computing the residual between images reconstructed in the open-shutter and closed-shutter states allows for the elimination of ultrasonic noise. Validated on phantoms and in vivo human vessels, pulse width adjustment improved the signal-to-noise ratio by 3.4 dB, while residual reconstruction yielded an enhancement of 13.3 dB. This study provides an in-depth analysis of the noise issues in LED-based PAI systems, which might contribute to enhanced image quality and the development of low-cost photoacoustic systems, thereby facilitating the broader clinical application of LED-based photoacoustic imaging.