Evaluation of ultrasound sensors for transcranial photoacoustic sensing and imaging

TL;DR

This study evaluates ultrasound sensors for transcranial photoacoustic imaging, demonstrating that optical sensors like PCOR outperform traditional piezoelectric sensors by providing higher sensitivity at low frequencies suitable for penetrating the skull.

Contribution

The paper introduces and validates a new optical ultrasound sensor design, PCOR, optimized for transcranial PA imaging, overcoming limitations of conventional piezoelectric sensors.

Findings

PCOR sensors have high sensitivity at low frequencies.

Transcranial PA imaging benefits from sensors with broad, flat frequency response.

Optical sensors outperform piezoelectric sensors in transcranial PA sensing.

Abstract

Biomedical photoacoustic (PA) imaging is typically used to exploit absorption-based contrast in soft tissue at depths of several centimeters. When it is applied to measuring PA waves generated in the brain, the acoustic properties of the skull bone cause not only strong attenuation but also a distortion of the wavefront, which diminishes image resolution and contrast. This effect is directly proportional to bone thickness. As a result, transcranial PA imaging in humans has been challenging to demonstrate. We measured the acoustic constraints imposed by the human skull to design an ultrasound sensor suitable for transcranial PA imaging and sensing. We imaged the phantoms using a planar Fabry-Perot sensor and employed a range of piezoelectric and optical ultrasound sensors to measure the frequency dependent acoustic transmission through human cranial bone. Transcranial PA images show typical frequency and thickness dependent attenuation and aberration effects associated with acoustic propagation through bone. The skull insertion loss measurements showed significant transmission at low frequencies. In comparison to conventional piezoelectric sensors, the performance of plano-concave optical resonator (PCOR) ultrasound sensors was found to be highly suitable for transcranial PA measurements. They possess high acoustic sensitivity at a low acoustic frequency range that coincides with the transmission window of human skull bone. PCOR sensors showed low noise equivalent pressures and flat frequency response which enabled them to outperform conventional piezoelectric transducers in transcranial PA sensing experiments. Transcranial PA sensing and imaging requires ultrasound sensors with high sensitivity at low acoustic frequencies, and a broad and ideally uniform frequency response. We designed and fabricated PCOR sensors and demonstrated their suitability for transcranial PA sensing.