Dear All,
My name is Duane Kaufman, and I am one of the researchers looking into using photoacoustic methods of bone assessment.
I am analysing follow-on data to the 2020 BME paper "Functional Photoacoustic and Ultrasonic Assessment of Osteoporosis: A Clinical Feasibility Study" (https://pmc.ncbi.nlm.nih.gov/articles/PMC10521673/)
A quick outline of the experimental setup:
A subject's foot is placed in a body-temperature waterbath, with an ultrasound transducer on one side of the heel (500 kHz center, 250 kHz BW), and a laser fiberguide on the opposite side of the heel
The heel is illuminated with a 5 ns laser pulse, whose wavelength can be changed (690-950nm)
Laser power illuminating the heel is characterized for each wavelength
The ultrasound signal produced usually looks similar to that outlined in the paper:
The first ultrasound to reach the transducer seems to be coming from within the bone tissue region of the heel (using SOS and time-of-arrival), and usually exhibits an ultrasound burst envelope (which grows in amplitude then subsides) as depicted. We have taken ultrasound images of the heel as well to establish tissue widths on either side of the bone, and bone thickness.
I am trying to understand the generation and evolution of the ultrasound being produced by the laser pulse.
We have measured over 130 subjects, both left and right heels, both illuminated laterally and medially (4 datasets per subject), each dataset containing data from 27 different wavelengths.
General observations we have seen:
1) at wavelengths with more light absorption in the tissues (melanin, lipids, blood, bone, water) the energy in the ultrasound coming from the bone region decreases.
2) at wavelengths with higher light absorption the ultrasound coming from the bone region is delayed more than at wavelengths with lower absorption.
I have tried a simple k-wave model incorporating only bone as a material (light absorption and ultrasound attenuation), using the Beer–Lambert law to compute the light intensity decay, and the local absorbed energy (∝ μₐ·I) is converted to an initial pressure (using a Grüneisen parameter Γ) to establish an initial pressure field.
My model produces no oscillatory output, much like what I would expect from a uniformly-illuminated region of tissue, so I believe I am missing something that is actually happening in the subject's heel.
Your guidance is appreciated.
Sincerely,
Duane Kaufman