Photoacoustic Tensile Imaging

Photoacoustic (PA) imaging combines the high optical absorption contrast of optical imaging with the deep tissue penetration of ultrasound detection, offering great potential for functional imaging and disease diagnosis. However, current PA imaging methods mainly explore optical absorption properties of biological tissue. To the best of our knowledge, tensile measurement based on PA effect is still an untapped area to be explored. In this work, we propose photoacoustic tensile imaging (PATI), a new PA imaging modality enabling quantitative assessment of tensile stress in biological samples. PATI exploits the nonlinear PA response induced by dual-pulse laser excitation to establish a mapping between the applied tension and the increment of the nonlinear PA signal. By varying the temporal delay between the heating and detecting laser pulses, the relationship between tensile force and nonlinear PA characteristics is quantitatively analyzed. Phantom experiments demonstrate a strong correlation between the nonlinear PA signal intensity and the applied tensile force. These results confirm the feasibility of the proposed approach for tensile force monitoring, which holds potential in biomedical applications, such as vascular pressure monitoring.

💡 Research Summary

The manuscript introduces Photoacoustic Tensile Imaging (PATI), a novel photoacoustic (PA) modality that quantifies tensile stress in biological samples by exploiting the nonlinear PA response generated through dual‑pulse laser excitation. Conventional PA imaging primarily maps optical absorption, providing structural and functional information but lacking sensitivity to mechanical load. PATI fills this gap by delivering a heating pulse followed, after a short controllable delay Δt, by a detecting pulse. The heating pulse raises the local temperature, transiently increasing the Grüneisen parameter (Γ). Because the second pulse interacts with this elevated temperature field, the resulting PA signal is amplified relative to a single‑pulse excitation. The magnitude of this amplification depends on the residual temperature (i.e., on Δt) and on the mechanical state of the material, which influences both acoustic (sound speed) and thermal (thermal conductivity, diffusivity) properties.

The authors develop a theoretical framework linking tensile force f (or stress σ = f/A) to two key measurable parameters of the nonlinear PA signal: the amplitude coefficient V_ts and the thermal‑relaxation rate b. Assuming small strains, they model sound speed and thermal conductivity as linear functions of stress (v_s²(σ) ≈ v_s0²(1+ξσ), k(σ) ≈ k0(1+κσ)). Consequently, Γ(σ) and the thermal diffusivity α(σ) inherit the same linear dependence, leading to V_ts(σ) ∝ (1+ξσ) and b(σ) ∝ (1+κσ). Within the elastic regime, V_ts varies linearly with applied force: V_ts(f) = V_ts0 + S·f, where S is a calibration constant.

Experimentally, the system uses two Q‑switched 532 nm lasers (≈8 ns pulse width, 10 Hz repetition). Laser‑1 provides the heating pulse (2.23 mJ), Laser‑2 the detecting pulse (0.86 mJ). A programmable delay generator sets Δt from 0 to 200 ns in 2 ns steps. A tungsten wire (100 µm diameter) serves as the test specimen; tensile forces ranging from 0.29 N to 3.87 N are applied via a mechanical loading rig equipped with a force gauge. Two detection configurations are employed: (i) a single‑element 5 MHz ultrasound transducer for point‑wise measurements, and (ii) a 128‑element linear array (7.5 MHz) for photoacoustic computed tomography (PACT).

Point measurements reveal that the nonlinear PA increment ΔV₂(Δt) decays with increasing Δt, confirming the thermal‑confinement principle. Importantly, higher tensile forces produce larger ΔV₂ across the entire Δt range, and the decay curve shifts toward shorter delays, indicating faster thermal relaxation under tension. Fitting the data to the proposed model yields a strong linear correlation between V_ts and tensile force (R² = 0.9913). The relaxation rate b also correlates with force but less robustly (R² = 0.8127), while offset and generation‑rate parameters show minimal sensitivity.

PACT imaging demonstrates that nonlinear PA images obtained at Δt = 0 ns have markedly higher contrast than conventional linear PA images. As Δt increases, contrast diminishes, mirroring the reduced nonlinear contribution. At fixed delays of 100 ns and 200 ns, increasing tensile force enhances target visibility and contrast, consistent with the point‑measurement findings. Dynamic imaging, where the tensile load is varied continuously while Δt is held at 100 ns, produces real‑time PA maps that track the applied force, illustrating the feasibility of continuous mechanical monitoring.

The work’s significance lies in (1) establishing a quantitative link between mechanical tension and a PA‑derived metric, (2) demonstrating that dual‑pulse nonlinear PA can serve as a contrast mechanism for stress imaging, and (3) providing a calibration protocol that could be adapted for in‑vivo applications such as vascular pressure monitoring, aneurysm wall‑stress assessment, or tissue‑damage detection.

Nevertheless, several limitations are acknowledged. The use of a metal wire, with thermal and elastic properties far from soft tissue, raises questions about translatability; tissue‑specific calibration of ξ and κ will be required. The chosen Δt range (≤200 ns) matches the thermal relaxation time of the metal but may be suboptimal for biological tissues where diffusivity is lower, potentially demanding longer delays. The laser fluences employed are relatively high, posing a risk of photothermal damage in vivo; future systems must operate within safety limits while preserving sufficient signal‑to‑noise ratio. Finally, the paper provides limited detail on real‑time signal extraction and reconstruction algorithms, which could be computationally intensive for clinical deployment.

Future directions suggested include (i) validation on tissue‑mimicking phantoms and ex‑vivo/in‑vivo models, (ii) development of low‑fluence, high‑sensitivity detection schemes, (iii) implementation of fast, possibly GPU‑accelerated, nonlinear PA signal demodulation for real‑time imaging, and (iv) integration with existing ultrasound platforms to enable multimodal mechanical and functional imaging. If these challenges are addressed, PATI could become a valuable, non‑invasive tool for monitoring biomechanical states in health and disease.