Photoacoustic viscoelasticity imaging dedicated to mechanical characterization of biological tissues

Since changes in mechanical properties of biological tissues are often closely related to pathology,the viscoelastic properties are important physical parameters for medical diagnosis.A photoacoustic(PA)phase-resolved method for noninvasively characterizing the biological tissue viscoelasticity has been proposed by Gao et al.[G.Gao,S.Yang,D.Xing,\Viscoelasticity imaging of biological tissues with phase-resolved photoacoustic measurement,"Opt.Lett.36,3341–3343(2011)].The mathematical relationship between the PA phase delay and the viscosity–elasticity ratio has been theoretically deduced.Moreover,systems of PA viscoelasticity(PAVE)imaging including PAVE microscopy and PAVE endoscopy were developed,and high-PA-phase contrast images re°ecting the tissue viscoelasticity information have been successfully achieved.The PAVE method has been developed in tumor detection,atherosclerosis characterization and related vascular endoscopy.We reviewed the development of the PAVE technique and its applications in biomedical¯elds.It is believed that PAVE imaging is of great potential in both biomedical applications and clinical studies.

Optically-excited simultaneous photoacoustic and ultrasound imaging based on aflexible gold-PDMS film

We constructed a flexible gold-polydimethylsiloxane(gold-PDMS)nanocomposites film with controllable thickness and light transmittance,to realize optically-excited simultaneous photo-acoustic(PA)and ultrasound(US)imaging under a single laser pulse irradiation.Benefiting from the excellent thermoelastic properties,the gold-PDMS film absorbs part of the incident laser energy and produces a high-intensity US,which is used to realize US imaging.Meanwhile,the partly transmitted light is used to excite samples for PA imaging.By controlling the thickness of the gold-PDMS,we can control the center frequency in the US imaging.We experimentally analyzed the frequency of the produced US signal by the gold-PDMSfilm and compared it with the finite element analysis(FEA)method,where the experiments agree with the FEA results.This method is demonstrated by the experiments on phantoms and a mouse model.Our work provides a cost-effective methodology for simultaneous PA and US imaging.

Establishment of probabilistic model for Salmonella Enteritidis growth and inactivation under acid and osmotic pressure

The growth and survival characteristic of Salmonella Enteritidis under acidic and osmotic conditions were studied.Meanwhile,a probabilistic model based on the theory of cell division and mortality was established to predict the growth or inactivation of S.Enteritidis.The experimental results demonstrated that the growth curves of planktonic and detached cells showed a significant difference(p<0.05)under four conditions,including pH5.0+0.0%NaCl,pH7.0+4.0%NaCl,pH6.0+4.0%NaCl,and pH5.0+4.0%NaCl.And the established primary and secondary models could describe the growth of S.enteritis well by estimating four mathematics evaluation indexes,including determination coefficient(R2),root mean square error(RMSE),accuracy factor(Af)and bias factor(Bf).Moreover,sequential treatment of 15%NaCl stress followed by pH 4.5 stress was the best condition to inactivate S.Enteritidis in 10 h at 25◦C.The probabilistic model with Logistical or Weibullian form could also predict the inactivation of S.Enteritidis well,thus realize the unification of predictive model to some extent or generalization of inactivation model.Furthermore,the primary 4-parameter probabilistic model or generalized inactivation model had slightly higher applicability and reliability to describe the growth or inactivation of S.Enteritidis than Baranyi model or exponential inactivation model within the experimental range in this study.

Collagen fiber anisotropy characterization by polarized photoacoustic imaging for just-in-time quantitative evaluation of burn severity

Just-in-time burn severity assessment plays a vital role in burn treatment and care.However,it is still difficult to quantitatively and promptly evaluate burn severity by existing medical imaging methods via initial burn depth measurement since burn wounds are usually dynamically developed.As an elastic skeleton of skin,the degree of conformational changes of collagen fibers caused by overheating can reflect the burn severity in a timelier manner.Herein,the polarized photoacoustic technique(PPAT)for just-in-time quantitative evaluation of burn severity via collagen fiber anisotropy assessment is proposed.First,phantom experiments demonstrate the ability of PPAT for deep imaging in a transport mean free path and accurately quantify changes in microstructural order by thermal damage.Then,the Pearson correlation coefficient of the PPAT in assessing burn severity is shown to be up to 0.95,validated by burn skin samples.The PPAT provides a just-in-time quantitative strategy for burn severity evaluation.

Thermally confined shell coating amplifies the photoacoustic conversion efficiency of nanoprobes

Efficient probes/contrast agents are highly desirable for good-performance photoacoustic （PA） imaging, where the PA signal amplitude of a probe is dominated by both its optical absorption and the conversion efficiency from absorbed laser energy to acoustic waves. Nanoprobes have a unique micro- mechanism of PA energy conversion due to the size effect, which, however, has not been quantitatively demonstrated and effectively utilized. Here, we present quantitative simulations of the PA signal production process for plasmon- mediated nanoprobes based on the finite element analysis method, which were performed to provide a deep understanding of their PA conversion micromechanism. Moreover, we propose a method to amplify the PA conversion efficiency of nanoprobes through the use of thermally confined shell coating, which allows the active control of the conversion efficiency beyond that of conventional probes. Additionally, we deduced the dependence of the conversion efficiency on the shell properties. Gold-nanoparticles/polydimethylsiloxane nanocomposites were experimentally synthesized in the form of gel and microfilms to verify our idea and the simulation results agreed with the experiments. Our work paves the way for the rational design and optimization of nanoprobes with improved conversion efficiency.

New insight into photoacoustic conversion efficiency by plasmon-mediated nanocavitation： Implications for precision theranostics

The probe-assisted integration of imaging and therapy into a single modality offers significant advantages in bio-applications. As a newly developed photoacoustic （PA） mechanism, plasmon-mediated nanocavitation, whereby photons are effectively converted into PA shockwaves, has excellent advantages for image-guided therapy. In this study, by simulating the laser absorption, temperature field, and nanobubble dynamics using both finite-element analysis and computational fluid dynamics, we quantified the cavitation-induced PA conversion efficiency of a water-immersed gold nanosphere, revealing new insights. Interestingly, sequential multi-bubble emission accompanied by high PA signal production occur under a single high-dose pulse of laser irradiation, enabling a cavitation-induced PA conversion efficiency up to 2%, which is -50 times higher than that due to thermal expansion. The cavitation-induced PA signal has unique frequency characteristics, which may be useful for a new approach for in vivo nanoparticle tracking. Our work offers theoretical guidance for accurate diagnosis and controllable therapy based on plasmon-mediated nanocavitation.

Vacancy-defect-dipole amplifies the thermoacoustic conversion efficiency of carbon nanoprobes

The immense potential of carbon nanoprobes(CNPs)for using as contrast agents has propelled much recent research and development in the field of thermoacoustic(TA)molecular imaging,while the proper engineering and design of such materials with required high TA conversion efficiency is still a highly challenging task.In this work,we proposed a controllable strategy to amplify the TA conversion efficiency of the CNPs by constructing vacancy defect(VD)dipoles,and systematically demonstrated the amplification mechanism through theoretical and experimental investigations.First-principles calculation results indicate that,when a carbon atom is removed from the CNPs by chemical approach,owing to local electron density redistribution,the VDs are formed at the positions of the original carbon atoms and act as the structural origin of permanent electric dipoles with the dipole moment several orders higher than that of non-defect sites.Under pulsed microwave irradiation,the VD dipoles are polarized repeatedly and significantly contribute to the conversion efficiency from absorbed electromagnetic waves to ultrasound through enhanced dielectric relaxation losses.We experimentally synthesized graphene samples with different VD densities and VD types to demonstrate the efficiency of the proposed strategy,and results coincide well with the theoretical proposition.This work offers feasible guidance to the systematic development and rational design of new high-conversion-efficiency TA CNPs via VD engineering.

Nonlinear photoacoustic imaging dedicated to thermal-nonlinearity characterization

We proposed a nonlinear photoacoustic(PA) technique as a new imaging contrast mechanism for tissue thermalnonlinearity characterization.When a sine-modulated Gaussian temperature field is introduced by a laser beam, in view of the temperature dependence of the thermal diffusivity, the nonlinear PA effect occurs, which leads to the production of second-harmonic PA(SHPA) signals.By extracting the fundamental frequency PA and SHPA signal amplitudes of samples through the lock-in technique, a parameter that only reflects nonlinear thermal-diffusivity characteristics of the sample then can be obtained.The feasibility of the technique for thermal-nonlinearity characterization has been studied on phantom samples.In vitro biological tissues have been studied by this method to demonstrate its medical imaging capability,prefiguring great potential of this new method in medical imaging applications.