| Photoacoustic imaging,as a new type of biomedical imaging technology,has developed rapidly over past decades.Compared with optical and acoustic imaging,photoacoustic imaging technology is more advantageous in terms of imaging depth and contrast.However,most photoacoustic microscopic systems use piezoelectric ultrasonic transducers for photoacoustic signal detection.The detection bandwidth of the transducer is usually around tens of megahertz,due to the inherent physical properties of piezoelectric materials,resulting in inaccurate response to the short-pulse photoacoustic signals.As a result,the imaging system has poor axial resolution of>20?m,which makes it difficult to accurately estimate the depth of the absorbers.Additionally,a huge difference between the axial and lateral directions occurs in the optical-resolution photoacoustic microscopy system,seriously degrading the image reconstruction of the three-dimensional morphological structure of the tissue.Surface plasmon resonance(SPR)features fast time response and high-sensitivity refractive index sensing,which thus potentially detects transient changes in the refractive index of the medium with a sensitivity at the order of 10-8.Therefore,broadband photoacoustic detection can be achieved using SPR sensing technology.In this work,SPR sensing technology is developed to detect photoacoustic pressure transients.Moreover,an acoustic microcavity is designed for improving the pressure detection sensitivity of our SPR sensor.By integrating the SPR sensor and the acoustic microcavity,we establish a photoacoustic microscopy with an enhanced detection sensitivity and broad bandwidth.Our work is detailed as follows:First of all,to overcome the shortcomings of the limited bandwidth in the piezoelectric transducers,the SPR sensing technology is developed for improving the frequency response.Through FDTD simulation,we determine the Au film at 50 nm in thickness and the incident light with an incident angle of 71.5°when applying for the incident laser with the wavelength of 632.8 nm,allowing for the optimum performance of the sensor.With a traditional piezoelectric transducer as the ultrasonic source,the sensor can accurately respond to the excitation of ultrasonic waves with an estimated pressure detection sensitivity of 477Pa.Our SPR sensor operates at single-point flat-field detection modality.Thus,it is highly desired to enhance the acoustic detection sensitivity for various biomedical PA investigations.Further,we develop an acoustic microcavity to improve the pressure detection sensitivity of the system.According to the characteristics of photoacoustic wave propagation and the spatial distribution of the sound field,a metal acoustic microcavity with an ellipsoidal inner surface is designed.By configuring the spatial structure of the SPR sensor and the acoustic microcavity,the spherical photoacoustic waves emitting backwards from the sample surface are converged to the detection position of the SPR sensor,allowing high-sensitivity ultrasonic detection with an estimated sensitivity of?160 Pa.A reflection-mode photoacoustic microscopy is built by incorporating the SPR sensor and the acoustic microcavity as the ultrasonic detector.The system possesses an acoustic spectral response as broad as 98.3 MHz.The lateral resolution and axial resolution is determined at?4.2?m and?10.5?m,respectively.Finally,we use the photoacoustic microscopy to image the vascular network of living mice.Utilizing the intrinsic strong optical absorption of blood at the visible light spectrum,the system obtains a three-dimensional image of a microvascular network label-freely.Relying on the system's high ultrasonic detection sensitivity and broad bandwidth,we acquire in vivo vascular images from both the thin ear(a thickness of hundreds of microns)and the thick forelimb skin with strong optical scattering.These results suggest that our novel photoacoustic microscopy can potentially benefit various biomedical studies,for example imaging tumor vasculation. |
PDF Full Text Request