Table Of ContentHOUSTON, TEXAS
April 2016
ABSTRACT
Light-Guide Snapshot Imaging Spectrometer for Biomedical
Applications
by
Ye Wang
Fiber-based snapshot imaging spectrometer has found increasing potential
of applications in the field of biomedical imaging these years. However, currently
the technique’s spatial and spectral sampling still need improvement for most
requirements in biomedical applications. In this thesis, I propose a strategy design
and proof-of-principle prototype system of fiber-based snapshot imaging
spectrometer to provide a solution for increasing the spatial and spectral sampling.
Through a custom fabricated fiber bundle, the object image is collected with an 81 x
96 spatial sampling, then divided into 3 x 96 spatial groups with gaps in between for
dispersion, and finally captured by a CCD camera. To extract the (x, y, λ) datacube
from the raw image, a spectral calibration algorithm is implemented to locate each
wavelength and obtain point spectrum. Then a phase-shifting spatial calibration
procedure is performed to remap the fibers and reconstruct single channel images.
The prototype system is designed for visible range from 400 nm to 700 nm and is
able to record 71 spectral samples within the range. Preliminary results of oxygen-
saturation in occluded finger are presented to show the system spectral and spatial
resolving ability. The fibers are packed in an efficient way and the system could be
scalable to larger formats with higher spatial sampling. The gaps between fiber
groups are designed to be tunable to enable high spectral sampling which has
advantage in medical devices such as optical coherence tomography (OCT) in the
future.
Acknowledgments
Firstly I would like to express my appreciation to my advisor, Dr. Tomasz
Tkaczyk, for his mentorship, guidance and encouragement along the way for the
past two years, which taught me how to become a good researcher. I would also like
to acknowledge Dr. Michal Pawlowski for his careful and detailed instruction in
laboratory operation, as well as helpful discussions in overcoming technical hurdles.
I would like to thank Dr. Thuc-Uyen Nguyen, for her preliminary exploration in fiber
bundle fabrication process. I would also like to thank Jason Dwight who helped me in
reconstruction codes in Matlab. Moreover, I would like to acknowledge all the members
in Tkaczyk lab for all the helpful discussions and assistance. I would thank Dr. Michael
Diehl, Dr. Erzsébet Merényi and Dr. Richard G. Baraniuk for being part of my committee
for this thesis.
Contents
Acknowledgments ..................................................................................................... iv
Contents .................................................................................................................... v
List of Figures ............................................................................................................ vi
List of Equations ....................................................................................................... viii
Nomenclature ........................................................................................................... ix
Introduction ............................................................................................................... 1
1.1. Hyperspectral Imaging (HSI) ..................................................................................... 1
1.2. Biomedical Applications of Hyperspectral Imaging ................................................. 3
1.3. Fiber-Based Snapshot Imaging Spectrometer .......................................................... 5
Materials and Methods .............................................................................................. 8
2.1. System Design .......................................................................................................... 8
2.2. Prototype System Setup ......................................................................................... 11
2.3. Fiber Bundle Fabrication ........................................................................................ 13
2.3.1. Fiber Assembly with Molds and Adhesive ....................................................... 14
2.3.2. Laser Cutting .................................................................................................... 15
2.3.3. Assembly of a Complete Fiber Bundle ............................................................. 16
2.3.4. Polishing Fiber Ends ......................................................................................... 16
2.4. System Calibration.................................................................................................. 20
2.4.1. Spectral Calibration ......................................................................................... 21
2.4.2. Spatial Calibration ............................................................................................ 22
Results ..................................................................................................................... 27
3.1. Hemoglobin Spectrum Measurement .................................................................... 27
Conclusions and Future Work ................................................................................... 34
4.1. Conclusions ............................................................................................................. 34
4.2. Discussions and Future Works ............................................................................... 35
References ............................................................................................................... 38
List of Figures
Figure 1-1 –Three-dimentional datacube and data acquisition mode in
hyperspectral imaging [1] ..................................................................................................... 2
Figure 1-2 a retina imaged by a fiber bundle snapshot spectrometer, ......... Error!
Bookmark not defined.
Figure 1-3 snapshot hyperspectral imaging using a maneuverable coherence
fiber bundle [3] ......................................................................................................................... 5
Figure 1-4 Design of a fiber-based snapshot hyperspectral imaging system ..... 7
Figure 2-1 design of the image collection fiber bundle .............................................. 9
Figure 2-2 Configuration of spectral lines in fiber bundle for group of 4, 3 and
2 rows ......................................................................................................................................... 10
Figure 2-3 dispersion rotation angle calculations ..................................................... 11
Figure 2-4 Prototype system setup .................................................................................. 12
Figure 2-5 Extension tubes connecting the Canon camera body and the
focusing lens to control the built-in adjustable stop. In this way the chromatic
aberration was reduced. ...................................................................................................... 13
Figure 2-6 Process of fiber bundle fabrication (a) multimode 12-fiber ribbon.
(b) 3 pieces of ribbon glued together in the mold as a small bundle. (c) 8 small
bundles glued together in a mold as a large bundle. (d) Assembly of a complete
fiber bundle (e) separated output ends with spacers in between (f) Polishing
the ends of bundle .................................................................................................................. 14
Figure 2-7 Assembly of a 3-layer small-group with 12 x 3 fibers using Teflon
mold and epoxy. ...................................................................................................................... 15
Figure 2-8 Microscope photo of fiber bundle end before polishing (b)
microscope photo of fiber bundle end after polishing, (c) light transmittance
image before polishing (d) light transmittance image after polishing ............... 17
Figure 2-9 white light interferometer image showing one fiber end shape after
polishing .................................................................................................................................... 18
vii
Figure 2-10 Comparison of the output ends of fiber groups in the bundle
before (a) and after (b) polishing ..................................................................................... 19
Figure 2-11 Comparation of light transmittance after manual and machine
polishing .................................................................................................................................... 19
Figure 2-12 Flat-field image illuminated with a white LED .................................... 20
Figure 2-13 Spectral calibration procedures on raw image. Green dots:
positions of 3 single-wavelengths: 488nm, 514.5nm, 632.8nm. Red dots:
positions of the interpolated wavelength positions from 450nm to 650nm. ... 22
Figure 2-14 Phase-shiftting calibration system setup .............................................. 23
Figure 2-15 Six phase-shiftted sinusoidal patterns projected on the fiber
bundle input end .................................................................................................................... 24
Figure 2-16 Reconstructed channel images for USAF resolution target ............ 25
Figure 2-17 Comparison of different periods of pattern results ........................... 26
Figure 3-1 Finger occlusion experiment ........................................................................ 29
Figure 3-2 Averaged spectrum before and after occlusion removal ................... 32
Figure 3-3 Finger oxygen saturation changes during the occlusion experiment
...................................................................................................................................................... 33
Figure 4-1 Output shape change after tunning for current fiber bundle ........... 37
viii
List of Equations
Equation 1 Calculation of fiber bundle rotation angle .............................................. 10
Equation 2 Linear unmixing of deoxy-hemoglobin and oxy-hemoglobin
spectrum where X is the measured spectrum and A1, A2 are the ratio of deoxy-
hemoglobin spectrum 𝑯𝒃𝑶𝟐 and oxy-hemoglobin spectrum 𝑯𝒃 ......................... 33
Nomenclature
HSI Hyperspectral Imaging
OCT Optical Coherence Tomography
CCD Charge-Coupled Device
LED Light-Emitting Diode
RGB Red, Green, Blue
NA Numerical Aperture
WD Working Distance
FOV Field Of View
PTFE Polytetrafluoroethylene
Chapter 1
Introduction
1.1. Hyperspectral Imaging (HSI)
Spectral imaging, also called imaging spectrometry, is an imaging technology
which captures the object’s channel image together with the spectrum of each pixel
in the image. The obtained result is typically called a three-dimensional datacube (x,
y, λ) which includes the optical intensity in two spatial (x, y) and one spectral (λ)
coordinate’s sampling. Spectral imaging could be divided as multispectral imaging
and hyperspectral imaging. According to the recent literature [1], multispectral
imaging usually has a spectral resolution lower than 10 nm, spectral bands from 3 to
tens and normally spaced spectral band. While hyperspectral imaging usually has a
spectral resolution higher than 10 nm, spectral bands from tens to hundreds and
continuous spaced spectral band. So HSI usually provides higher spectral resolution
and more details in spectral signatures which has found broad applications in fields
1
