  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.
