Download PhotoAcoustic Schlieren Elastography II

PhASE: PhotoAcoustic Schlieren Elastography II
Design Team
Taryn Connor*, Sarah Janiszewski*,
Emmanuel Llado∞, Stephanie Reimer∞, Tristan Swedish*
*ECE Dept.
∞
MIE Dept.
Design Advisors
Prof. Charles A. DiMarzio, ECE, MIE
Prof. Gregory Kowalski, MIE
Abstract
The ability to measure the mechanical properties of the cornea accurately and noninvasively
benefits society by providing clinical practice with the potential to diagnose diseases of the
cornea earlier than current technology. The objective of this capstone project continues the work
of the PhotoAcoustic Schlieren Elastography, PhASE system, by benchmarking it against
phantoms of known mechanical properties. The PhASE system operates by utilizing several
biomedical optics techniques. First, a pulse generator is used to control a heating laser directed at
the cornea to produce an acoustic wave. This acoustic wave changes the index of refraction in a
localized region of the tissue as it propagates. The schlieren imaging system is used to track this
pressure wave by stroboscopically imaging the induced gradient in the index of refraction of the
tissue. Several steps have been laid out in order to bring PhASE II closer to the clinical setting.
By focusing on the problems of the original system, it has been deemed necessary to automate the
system, while also utilizing a more realistic cornea phantom that has been completely
mechanically characterized.
LED
Phantom
Laser
Camera
Knife-Edge
For more information, please contact [email protected] or
[email protected]
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The Need for Project
The PhASE system seeks to
Over 35 million Americans are estimated to have corneal diseases,
create an in-vivo method of which can lead to vision loss. The current diagnostic methods often
measuring the mechanical detect these diseases only after irreversible damage has been done.
properties of the cornea for Advances are needed in research, modeling, and instrumentation to
diagnosing diseased states. provide earlier detection of corneal tissue damage. PhASE II focuses on
research of more effective diagnostic methods and computer modeling
of the system’s resulting images. More research is necessary before
moving from tissue phantoms to excised corneas and in-vivo corneas.
The Design Project Objectives and Requirements
The objective is to design a Design Objectives
system capable of stimulating
The objective of this project is to provide researchers with a
and observing a mechanical method of characterizing the mechanical properties of the cornea. The
response in a cornea-like PhASE system will allow researchers to study the corneal mechanics in
phantom, and use the elastic various disease states. PhASE II seeks to create a system that can
property data to benchmark the measure the shear modulus and pressure wave modulus of an acoustic
system. pressure wave in a phantom and confirm these moduli values with
traditional mechanical analysis. The combination of schlieren imaging
and computational algorithms should yield the shear modulus and
elastic modulus of the sample by finding the velocity of the pressure
wave. Once two of the six elastic moduli are known, the remaining can
be calculated to fully characterize the phantom. Since the PhASE I
system is conceptually accurate, PhASE II seeks to make
improvements on the system—such as better signal processing, a fully
designed phantom, and an automated measurement process—that will
help make this project more credible as a research tool.
Design Requirements
As this technology is still fairly new, the PhASE system needs to
Elastic moduli relations for
PhASE and Traditional
Mechanical Analysis
be benchmarked against a material of known mechanical properties.
Phantoms made of silicone were used to calibrate the system and tested
mechanically for shear and elastic moduli. Other requirements that
must be met for PhASE II to be considered successful include better
signal processing and the implementation of a fully automated
measurement process.
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Design Concepts Considered
Several non-contact methods
Photoacoustic stimulation of waves was identified as the most
of mechanical stimulation were reliable non-invasive method. The creation of a photoacoustic wave
investigated while focusing on requires heating a small area of material quicker than the heat can
optical setups because they dissipate through conduction. A thermal stress confinement induces the
require minimal contact with propagation of pressure and shear waves, whose velocities are related
the sample and have high to the P-wave and shear modulus. The pulse width of the laser needs to
resolution. be short enough to cause a thermal stress confinement within the
medium, while conversely it cannot exceed levels of heating which
could damage the corneal tissue.
The acoustic wave front will travel at the speed of sound in the
tissue (approx. 1500 m/s) across the 1.5 cm diameter of the cornea. The
detection system must have temporal resolution of about 100
nanoseconds (ns) and a spatial resolution of at least 100 micrometers
(m) to capture data at distinct instances of the wave propagation.
Schlieren imaging is capable of detecting the pressure wave
without contacting the surface of the eye. Transmissive-mode schlieren
imaging is the easiest to implement but requires a light source and a
detector on opposite sides of the tissue; a reflective-mode system can
remain on one side of the sample, but requires costly components.
The general schlieren system needs to achieve temporal resolution
on the order of 100 nanoseconds. High-speed cameras are potentially
capable of meeting the required sampling rate, but are extremely costly.
A strobing method was used in which waves are produced and detected
at discrete time steps. Wavefront propagation is captured by changing
the offset between the initial laser pulse and the image capture.
High frequency triggering of the illumination source is attractive in
its simplicity, but also requires a rapid illumination rise time. LEDs are
capable of high speed triggering and do not have problems with
diffraction.
Tissue phantoms would be used to validate the system. Hydrogels,
PhASE Proof of Concept
similar to the material of contact lenses, are most similar to the cornea,
but have the same problems in precise measurement: they require
constant hydration and are ill-suited to traditional mechanical analysis
(TMA). Silicone phantoms are less optically and mechanically similar
to the cornea than hydrogels, but don’t degrade and can be tested
intensively with TMA. It is important to use a stable material of known
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mechanical properties to benchmark the system.
Recommended Design Concept
The proof of concept design Design Description
uses a rapidly pulsed heating
The heating laser produces 10 Watts (W) at the 850 nm
laser to induce mechanical wavelength and is integrated with a high-speed pulse generator to pulse
waves in a silicone phantom. A for 50 ns durations. The abstract figure shows the layout of the PhASE
strobed-source, transmissive system. The light source of the schlieren system is a high power, 2000+
schlieren imaging system is lumen LED with a 10 ns rise time. A 100 ns output signal from the
used to observe the waves and pulse generator is amplified to 40 W and controls the LED strobing.
a post-processing algorithm The output is offset from the heating pulse by 100 ns. This strobed
results in quantitative source beam is focused onto a high precision slit, and then collimated
measurements of elastic by a lens into a circular beam. A sample is placed in the beam path
moduli. between the first and second lens. After the second lens, a knife-edge is
placed at the focal point to block any un-modulated light propagation.
Light refracted by the wave propagation is focused by a third lens onto
a high-resolution detector with an infrared filter that ensures that only
visible light from the source is imaged.
Phantoms were made with Sylgard 184, a clear two-part silicone.
Although it is not a perfect mimic, Sylgard 184 is able to maintain its
mechanical properties and is capable of TMA due to its durability. To
produce a clearer pressure wave, a near-infrared dye will be added to
the phantom to increase the level of laser absorption. It is vital that the
phantom absorbs the 850 nm wavelength of the heating laser.
The image analysis algorithm produces a curve of the pixel
intensity with respect to pixel location across a predefined radial vector
on an image. The average velocity of each wave is calculated by
finding the distance between the wavefronts of two subsequent images,
converting this pixel displacement to physical distance, and dividing
this value by the time step between the two images.
Experimental Investigations
An LED was capable of capturing schlieren data in pulsed
operation. An optical enclosure for the system was used to improve the
signal-to-noise ratio of the image data. The pulse circuits for strobing
were tested using an oscilloscope and confirmed the laser/LED signal
pulse widths, timing offsets, voltage levels, and amplification gains.
Phantoms were fabricated according to a design of experiments to
find the combination of variables that yield the most detectable
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pressure wave. The three variables of experimentation are: base-tocuring agent ratio, concentration of dye in solvent, and optical pulse
rate. The use of a near-infrared dye in the phantom will increase the
level of laser absorption in the phantom, creating a clearer acoustic
wave. The final designed phantom will be mechanically evaluated
through tensile testing and 3-point bending to identify its elastic
modulus and shear modulus.
Key Advantages of Recommended Concept
The main advantages of the PhASE system are that two elastic
moduli can be found, which allows for complete mechanical
characterization of the tissue. Current technology can only measure one
modulus, varies in accuracy, and requires excision of the cornea.
Silicone phantoms are stable enough to benchmark the system against
something of known mechanical properties.
Financial Issues
The proof of concept’s total
Most of the equipment in the PhASE system was donated by
lifetime cost is $850, while a Professor DiMarzio’s Optical Science Laboratory. PhASE I spent
second-generation, optimized approximately $550 in additional equipment. PhASE II spent
system would cost at least approximately $300 for new phantom materials. The total cost of the
$25,000. project in a second-generation model with new and optimized
components would cost between $25k and $35k. New components
would include a high-speed camera for image collection, a pulsed light
source with higher luminous intensity, and an optimized wavelength
laser diode for photoacoustic stimulation.
Recommended Improvements
Before being considered a
The PhASE system has several required modifications before it
diagnostic tool, PhASE will can be considered a non-invasive diagnostic tool. A phase sensitive
need to implement a reflective reflectance mode detection scheme would allow in-vivo use by having
schlieren system, a higher the detector and light source on the same side of the eye. The laser
wavelength laser, a more diode currently used to generate a pressure wave is not safe for human
powerful illumination source, use. Infrared radiation from a 1550 nm laser diode would be mostly
and a higher speed camera. absorbed by the cornea, meaning less power would be required for
heating and almost no harmful light would reach the retina. A more
powerful illumination source would also provide higher contrast
imaging. The use of a higher speed and more sensitive detector would
allow the measurements to be taken more quickly.
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