Yi-Chun Chen

Prof. Yi-Chun Chen’s research in the field of Biomedical Imaging includes the instrumentation of nuclear medicine imaging and surface plasmon resonance based biosensors.

Nuclear Medicine Imaging Instrumentation

Micro-SPECT imaging along with small-animal models of human diseases is widely used in biomedical research to study disease mechanisms and investigate potential therapies.  These imaging systems require high spatial resolution and high sensitivity to ensure the accuracy of imaging results.  From August 2010, a joint project between this lab and Dr. Mei-Ling Jan’s lab in Institute of Nuclear Energy Research, Atomic Energy Council, Taiwan was initiated.  The project is in the development of a rapid construction method of pinhole SPECT imaging system matrices.  A simplified grid-scan experiment was utilized to measure the image responses of selected elements in the system matrix.  A complete matrix was constructed by using the relations between the Gaussian coefficients and the geometric parameters of the imaging system.  The preliminary results of the proposed algorithm have been presented in IEEE Medical Imaging Conference 2011.  In order to facilitate the validation of the proposed method within this lab, a gamma camera module is under construction with a symmetric charge division circuit, mean-detector-response-function measurements, and a maximum-likelihood position estimator.  We anticipate applying the method for single-pinhole SPECT, multi-pinhole SPECT and dual-isotope SPECT with appropriate modifications.

Figure 1. Schematic of the simplified grid-scan calibration.  Shown are the FOV in the middle and two rings of cameras at both sides.  The simplified grid pattern contains the boundary surface and six inner planes of the cylindrical FOV.

Figure 2. The line profiles across one slice of the Derenzo phantom reconstructed with the measured and the interpolated system matrices.

Surface Plasmon Resonance Biosensor – Development and Applications

A dynamic Surface Plasmon Resonance (SPR) imaging sensor was developed to realize high-resolution high-throughput applications.  The SPR device consisted of a half-cylinder prism, 45nm-thick gold thin film and a custom-designed flow cell to construct the Kretschmann configuration.  A cylindrical lens pair in conjunction with the half-cylinder prism was used to simplify the optical alignment procedure and to ensure plane-wave propagation inside the prism.  Phase-shifting interferometry was implemented by using a piezoelectric transducer (PZT) driven by a triangular voltage waveform.  A CCD camera was employed to acquire the sequential interference patterns required for phase calculations.  A reference signal obtained from a photodiode before the SPR device was used to compensate the system instability from the laser intensity, environmental disturbances, and mechanical vibrations from the PZT.  Integrating-bucket data acquisition was realized with the synchronization between the photodiode and the CCD camera to preserve the dynamic capability of the SPR sensor.  System evaluations were performed by salt-water mixture measurements and gold-spot array imaging.  The achieved phase-measurement stability was 0.40 degrees and the system sensitivity was 5.14×10⁴ degree/RIU (refractive index unit).  The corresponding system resolution was 7.8×10^-6 RIU.  The main research results have been presented and published in SPIE Optics + Photonics (2009).  This SPR imager is currently being optimized with random phase-shifting interferometry and a temperature-control unit, and is commissioned in studying biomolecular interactions.

Figure 3. Optical configuration of the dynamic SPR imaging sensor.

Figure 4.  Performance evaluations of the dynamic SPR imaging sensor. (a) Phase-stability measurement with DI water; (b) The relation between the phase differences and the refractive indices of salt-water mixtures with various concentrations.

Szu-Yu Chen

Prof. Szu-Yu Chen’s research in the field of Biomedical Imaging emphasizes on the development of super-resolution structured illumination microscopy (SIM) and two-photon hyperspectral microscopy.

Development of Super-resolution Optical Microscopy

Due to the diffraction nature of light, the spatial resolution of traditional optical microscopy is limited by diffraction limit. To overcome this limitation, recently, many modalities of super-resolution optical microscopy are developed to resolve sub-hundred-nanometer structures in bio-tissues, including structured illumination microscopy (SIM). With the support from National Science Council since November 2010, a two-photon SIM system (TP-SIM) is now being built in this lab. This system combines both two-photon fluorescence (TPF) microscopy and SIM in order to achieve super-resolution optical sectioning in thick tissues. Different from wide-field SIM, this system is based on a point-scanning geometry due to the much higher excitation intensity required by TPF. A grid illumination pattern is produced by temporally modulating the excitation laser intensity and spatially scanning the focal spot. Under this structured illumination, the higher spatial frequency components of tissues in Fourier space can be shifted into the region of system optical transfer function (OTF), and the spatial resolution can thus be increased beyond the diffraction limit. Besides, since TPF possesses the optical sectioning power due to its nonlinearity, combining SIM with TPF can further improve the axial resolution of imaging and achieve the super-resolution optical sectioning in thick tissues possible.

Figure 5. Schematic of point-scanning-based two-photon structured illumination microscopy. An acoustic-optical modulator (AOM) is used for temporally intensity modulation, while a pair of galvanometer mirror is used for 2D point scanning.

Figure 6. Simulated (a) TPF and (b) TP-SIM image of 20-nm-diameter fluorescence beads. (c) The intensity profiles versus position along the yellow lines in (a) and (b). Analyzing the FWHM of the profiles, the resolution of TPF (blue) is ~343 nm, while that of TP-SIM (green) is ~164 nm.

Development of Two-Photon Hyperspectral Microscopy

Molecular imaging has become a very important topic of biomedical researches. However, since the molecular composition of bio-tissues is quite complicated and the fluorescence emission spectrum of different molecules may seriously overlaps, using beamsplitters and filters to separate fluorescence signals is not enough to obtain molecular images with high accuracy. To solve the crosstalk problems, hyperspectral microscopy recording both spatial and spectral information simultaneously should be applied. Through the spectrum analysis, the fluorescence signals from different molecules can be clearly unmixed without crosstalk. With the support from National Central University’s Plan to Develop First-class Universities, Top-level Research Centers Grants since April 2011, a joint project among this lab, Dr. Chao-Wen Liang, Sheng-Hui Chen, Te-Yung Chung, and Yen-Hung Chen’s lab was initiated to build a point-scanning two-photon hyperspectral microscopy system. In this project, free-form optics design, advanced thin-film coating, and multi-wavelength excitation will be combined with this system to further improve the image quality, spatial resolution, and the accuracy of spectral analysis. Based on the optical sectioning power in thick tissue, lower photodamage, and higher penetrability provided by two-photon fluorescence microscopy, this system can be expected to be applied to in vivo or clinical applications and provide more valuable molecular information for biomedical researches and clinical diagnosis.

Figure 7. Basic concept of point-scanning hyperspectral microscopy.

Figure 8. Schematic of point-scanning two-photon hyperspectral microscopy system. The scanning in X direction (fast axis) and Y direction (slow axis) are accomplished by a galvanometer mirror and a translation stage, respectively. The signals of the fast scanning line are diffracted by a reflective grating and then recorded by a 2D CCD camera.