This document summarizes a prototype laser chopping system that was designed and tested to be used for diffuse optical tomography imaging. The system uses a laser that is reflected by a mounted rotating mirror to sequentially direct the laser through optical fibers. The laser intensity was measured through a photomultiplier tube connected to one of the optical fibers. Testing showed that the prototype was able to successfully chop and direct the laser beam through the fiber, though motor vibrations caused noise that could be reduced with further improvements to the design. The prototype demonstrates a potential low-cost method for distributing laser light for DOT imaging applications.

1. A Prototype Laser Chopping System that can be Utilized in Diffuse
Optical Tomography (DOT) Imaging.
Ivan Felix, Rene Quiroz, Changqing Li
School of Engineering, University of California, Merced
Numerous medical imaging modalities are currently in use by
hospitals to develop a detailed three-dimensional figure of pa-
tients’ internal and external properties. Common imaging tools
used today are functional magnetic resonance imaging (fMRIs),
x-ray computed tomography (x-ray CT scans), positron emission
tomography (PET), and Single-photon emission computed tomog-
raphy (SPECT). Each device has a distinct function that can provide
us with different structural and functional information. Of course
each device also has its advantages and defects in terms of spatial
and temporal resolutions, and measurement sensitivity.
Introduction
System Setup & Overview
Figure 1: Photograph of the prototype laser instrument connected to a PMT
through an optic fiber. An aluminum mirror is used in the mounted rotating
piece. A neutral density filter is placed between the optic fiber and PMT to at-
ten uate the intensity of the laser. The voltage output from the PMT is measured
through a DAQ board connected to a PC.
The laser chopping instrument was modeled through CAD designing.
To reflect and allocate the laser, a mirror was mounted on a rotating mo-
tor. Two lens were used to converge and focus the laser beam more ac-
curately; one lens was placed between the adapter and the cover while
the other was placed between the optic fibers and the mirror.
Laser Instrument Design
Figure 2: The CAD Design of the laser instrument protoype. All pieces shown were
printed using a 3-D printer with PLA polymer material.
References
Acknowledgements
Data and Results
Conclusion
Background
Diffuse optical tomography (DOT) is a medical imaging tool that
mainly uses near-infrared light on an array of sources that are dif-
fused and scattered onto the subject [1]. The following research
explores to build a fast DOT imaging device. Furthermore it tries
to suggest a prototype that can quickly and accurately allocate a
laser to a number of optical fibers sequentially.
To examine the accuracy of the proposed laser, we used a photo-
multiplier tube (PMT), to measure the laser intensity through the optical
fiber from the instrument.The optical fibers will chop the collimated la-
ser beam and deliver the chopped beam to the PMT. We used a neutral
density (ND) filter to attenuate the laser beam before it is delivered to
the PMT.
Figure 3: PMT with adapter: There is a filter case that is inserted vertically and an optic
fiber mount that is inserted horizontally.
A voltage of 15 V is applied to the PMT by two DC power sources. Us-
ing LabView it was possible to program a data acquisition (DAQ) board.
The DAQ collected the voltage output from the PMT. The voltage value
is proportional to the light intensity delivered to the PMT. LabView pro-
gram generated a data graph from the acquired voltage.
The motor was ran at different speeds to test whether the differ-
ences were significant and to assure the device was working prop-
erly.The PMT’s sensitivity was placed at its highest possible voltage,
1 V, to acquire the best results from the laser. Because only one op-
tic fiber was in use it is expected that the graphical representation
of the voltage generated by the PMT would be pulse peaks.
1000 samples were read at a rate of 1000 Hz in a lapse of 1 second.
Placing the laser directly through the optic fiber recorded a max
voltage value of 10.7 mV. At 1.0 V the motor is revolving at about
507.6 RPMs, a number of 7 peaks resulted each plataeuing at 10.7
mV for about 6 ms.The last sequence was revolving the motor at
its highest possible RPM’s our motor could go. At 18 V, the voltage
had a range of different peaks between 1 mV to 4.6 mV. Other tests
were applied to other points in the system. All appeared identical
to the graphs above.
The laser instrument worked correctly in the end. The design
is simple and cost-effective enough to produce a pulsating laser
beam, however there are still a few presentable problems to the
instrument: A major problem is the vibration caused by the motor.
I believe that the reason the PMT records so much noise is due to
the excessive vibrations caused by the motor increasing in speed.
Reducing the vibrations will offer better results. In conclusion the
experiment was a success, the prototype laser instrument did chop
the laser appropriately to different locations. Nevertheless more
research and improvements must be done to successfully apply
this protoype to a DOT imaging.
http://store.makerbot.com/replicator2
Boas, D.A. Brooks, D.H. ; Miller, E.L. ; DiMarzio, C.A. ; Kilmer, M. ; Gaudette, R.J. ; Quan Zhang. (Nov
2001). Imaging the body with diffuse optical tomography. Signal Processing Magazine, IEEE. 18(6).
57-75. 10.1109/79.962278
http://www.hamamatsu.com/us/en/product/category/3100/3003/3045/H9306-03/index.html
Dr. Dianwen Zhu, Kun Zhang, Van Thai
Contact: Ivan Felix, email: ifelix3@ucmerced.edu
The laser instrument allocates and distributes the laser to dif-
ferent points. All points should be effectively in quick continuous
successions, so as to provide a steady light stream through the
fibers. In our case only one optical fiber was used to observe effi-
ciency. This will allow us to measure the light intensity generated
by the laser at different speeds of the motor. This information will
allow us to examine if our system is functioning correctly and if it
can be carried forward to other applications.
Figure 5: Time vs Voltage plot graph
with identical set-up as Figure 4, with
just a difference in motor speed. There is
a large voltage drop can be seen.
Figure4:TimevsVoltageplotgraphshow-
ing higher voltage as the motors spins at a
relativly low speed.
DAQ Board
PMT
Prototype:
Laser Chop-
ping System
Laser Aperture
DC Power Sup-
ply to Motor
Power Supply
to PMT
Bread Board
Connect-
ing PMT to
DAQ