Biomedical Photoacoustic Imaging Technology Based On Microfiber Fabry-perot Interferometer

Biomedical engineering is an application of natural science and engineering in principles and methods to the researches on the microscopic structure, functionality, and other phenomena for the living organisms, especially those of the human body. Biomedical imaging technology, one of the most important subdisiplines of biomedical enginnering, can obtain the spatial distribution of human internal tissues, taking advantage of optical microscopy techniques, magnetic resonance and other available tools. As a result, useful information can be extracted out of the image and offers reliable references for early diagnosis of major diseases, which is of great importance for human health. Optoacoustic imaging technology, one of the newest imaging approaches, has received great interests since it presents the advantages of high contrast and high penetration depth. This approach, based on the optoacoustic effect and the detection of ultrasonic signals, has developed as a highly effective non-destructive means for biomedical imaging. The main challenge for this imaging technique lies in how to simultaneously realize high sensitivity, high contrast, and high spatial resolution. Detection of ultrasond signal is the critical process in optoacoustic imaging. Conventiona methods relying piezo-electrical sensors present relatively low sensitivities and narrow response bandwidth. In contrast, fiber optic ultrasound sensors can present higher sensitivity and intrinsic electromegnatic immunity. In order to realize high sensitivity, high spatial resolution optoacoustic imaging, this thesis propose and demonstrat the detection of ultrasound signals by use of a Fabry-Perot interferometer fabricated in a optical microfiber. Compared to the existing fiber-based techniques, the detector has a cross sectional diameter of several microns and enables higher spatial resolution and high contrast. In addition, the sensitivity to ultrasound pressures is greatly enhanced as a result of the strong evanescent field interaction and successful inscription of Bragg gratings.The content of the thesis include:A. The study on the fabrication, spectral characteristics, and sensitivity properties of the proposed detector—Fabry Perot cavity based on microfiber Bragg gratings. This devices is fabricated by inscribing two wavelength-matched Bragg gratings in a single microfiber. By use of a multimode optical fiber as the preform of the microifber, the overlap between the grating and the optical modes can be greatly improived and Bragg gratings can be formed with enhanced efficienty. The spectral characteristics has been studied based on finite-element-method. The dependance of its tranmission spectrum on parameters including fiber diameter and reflectivities of the gratings has been investigated. The response characteristics of the device temperature and refractive index of fundamental mode and higher-order mode are difference, which can deal with the cross sensitivity problem based on temperature.B. The working principle and sensitivity of the proposed ultrasound detector. We first investigate its response to ultrasound signals theoretically and figured out the critical role of the evanescent-field interaction. In the experiment, the measured sensitivity is 1.845mV/kPa with a detector with a diameter of 5.2μm. In contrast, a F-P detector fabricated in conventional optical fiber present a sensitivity of only 0.184mV/kPa, which suggests that the effect of evanescent field can improve the sensitivity by about 10 times. We further measured the electronic spectrum, the directionality and the frequency spectrum of the detector, and propose two methods to obtain higher sensitivity: Increasing the reflectivities of the gratings for higher finesses of the interferometric fringes, or reducing the fiber diameter for stronger evanescent field interaction.C. Realization of optoacoustic imaging. We have studied the excitaion of optoacoustic signals and the propagation of ultrasound signals, based on which the method for reconstruction of the ultrasound source is illustrated. In the experiment, optoacoustic imaging is realized for a piece of hair from a adult human being. The imaging is implemented based on the scanning detection of the laser induced ultrasound waves by use of the F-P cavity, and the reconstruction of the spatial profile of the ultrasound source. The spatial resolution of the imaging reaches 95μm. We then studied the dependance of the imaging quality on the number of measuring positions, and found that higher contrast and spatial resolution can be achieved by increasing the potisoning numbers. However, the dependance is not a linear variation. In practice, the factors including the contrast, the spatial resolution, and the scanning speed should be comprehensively taken into consideration to optimize the imaging result.