Use of Photoacoustic Imaging to Predict Viscoelastic Properties of Human Tissue

Seth Goldstein, MD, Behnoosh Tavakoli, PhD, Ralph Hruban, MD, Emad Boctor, PhD, Michael Choti, MD, MBA. Johns Hopkins Departments of Surgery and Computer Science.

Introduction: Integration of enhanced imaging modalities in the operating room offers the opportunity to improve the safety and efficiency of minimally invasive surgery. Specifically, enhanced imaging with the capability for tissue characterization can help overcome limitations related to two-dimensional imaging and absence of tactile feedback. Photoacoustic imaging is one such emerging technology based on the photoacoustic effect, by which electromagnetic energy in the form of pulsed light is absorbed by a target material and converted to a detectable wideband acoustic signal. In principle, photoacoustic signal propagation is governed by the intrinsic elastic properties of the medium and its mass density. Therefore, it has been proposed that elements of a photoacoustic wave may be affected by the viscoelastic properties of the excited sample. The objective of this study was to test the hypothesis that elements of the photoacoustic signal from biologic tissues are related to the specimens’ viscoelastic properties.

Methods: A photoacoustic imaging apparatus was constructed based on a Q-switch Nd:YAG laser generating 5 ns pulses at 1064 nm on a 10 Hz cycle. Acoustic signal was detected using a polyvinylidene fluoride hydrophone. Hydrophone signal was frequency transformed and analyzed for center bandwidth frequency and full width half maximum. Viscoelasticity validation was established with dynamic mechanical analysis at room temperature between parallel compression plates. Stress and strain data were reconstructed to calculate the viscoelastic storage and loss moduli as well as the dampening coefficient tan delta. Photoacoustic and viscoelastic testing were initially performed on a sequence of porcine gelatin tissue simulant phantoms. Subsequently, freshly resected human tissue specimens were analyzed in a similar fashion.

Results: Both gelatin phantoms and tissue specimens demonstrated viscoelastic behavior by exhibiting both in-phase and out-of-phase deformation responses to stress. The storage and loss moduli tended almost uniformly to exhibit a nadir at 40Hz, a peak at 60 Hz, and eventual leveling off at higher frequencies. Calculation of tan delta revealed a similar pattern. Tan delta at 60Hz was negatively correlated with concentration of gelatin from 4% to 10% in the phantoms linearly (R2=0.95). Calculated photoacoustic parameters were also negatively correlated with gelatin concentration (center frequency R2=0.91, full width half maximum R2=0.79). Importantly, the viscoelastic tan delta appeared to predict the photoacoustic response, (center frequency R2=0.74, full width half maximum R2=0.90). Complete signal analysis was possible for nine human tissue samples, for which tan delta predicted the full width half maximum (R2=0.86) to a greater extent than the center frequency (R2=0.34)

Conclusions: Photoacoustic signal obtained from human tissue does appear to be affected by the viscoelastic properties of the specimen. There is correlation between the dampening ratio tan delta and elements of the acoustic waveform when analyzed in the frequency domain. Further work is ongoing to accurately predict the viscoelasticity using photoacoustic signal parameters; this approach has promise as an imaging modality that can noninvasively differentiate tissue types on the basis of their mechanical properties. Incorporation of photoacoustic imaging to minimally invasive interventions offers potential for improving future clinical practice.

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