Download A Low Cost Optical Coherence Tomography Machine

A Low Cost Optical Coherence Tomography Machine
Kevin White
[email protected]
EBME 105: Introduction to Biomedical Engineering
Tim Buetler
[email protected]
5/4/2017
Introduction:
Optical Coherence Tomography (OCT) is an emerging non-invasive imaging technique
with state of the art designs reaching sub micrometer resolutions (1). While technology is
beginning to become widely accepted and used in the developed world, it is out of reach for many
third-world countries because it is too expensive for the poorer nations of the world. The goal of
this project is to vastly reduce the cost of this technology for glaucoma screening in poorer
nations. This project aims to reduce the cost of this OCT machine to $4,000-$6,000 while
designing the machine to be relatively small and portable.
OCT is based on the principles of low-coherence interferometry and often employs a
Michelson interferometer (2). An interferometer is a device which utilizes lights phase properties
to compare the light that returns from two mirrors; one mirror a known distance from the receptor
and another mirror an unknown distance. The unknown distance can be calculated by comparing
the phases of the light from the unknown length and the light at a known length (2).
In a basic OCT set-up a light source emits a broadband spectrum of light which is carried
over fiber optic cable to an evanescent mode coupler. The coupler divides the light evenly, with
half of the light directed toward the reference arm and the other half directed to the sample which
is to be scanned. The light is reflected back from both the reference arm and the scanning sample
and is recorded by a photoreceptor. The data is then translated in a digital signal, processed,
displayed by a computer (2).
Since OCT has become a more popular imaging modality, different engineers have tried
to eliminate some of the drawback typically associated with OCT such as low scan speed and lack
of portability. These different designs have altered some of the basic principle on which the
technology was originally based and has led to a few significantly different types of OCT
machines. The main difference between the separate designs is the reference arm and how the
light that is recorded is processed. The original design for OCT is called Time Domain OCT
because the intensity of the light which is recorded is graphed and processed as a function of time.
For a functional image, the known length of the reference arm must be changed over the length of
one scan to accommodate structures which are at different lengths. In the first OCT machines,
this problem was dealt with by simply moving the mirror in the reference arm back and forth.
This is a straightforward method, but unfortunately for this project, the piezoelectric motor which
would be required proves to be too expensive (3).
All of the other designs for OCT that were considered for this project employ a Fourier
transformation which changes the data into a function of time (4). Utilizing this Fourier
transformation, engineers have designed other types of OCT. Swept-source OCT was only
considered briefly for this project. In swept source OCT, a tunable light source is used so that,
instead of changing the location of the reference mirror, the wavelength of the light source is
changed (4). It was quickly realized that the price for the tunable light sources were much higher
than the budget allowed. Spectral OCT is a popular method of OCT because of its very quick
acquisition time. In spectral OCT, a spectrometer is used to read all of the different wavelengths
at once instead of both moving the reference mirror and slowly scanning through the separate
wavelengths (5). The biggest drawback for the spectral OCT for our project was the cost of the
spectrometer. The method that was chosen for this project is called rotational OCT. In this
technology, the source light is shown through a diffraction grating which separates the different
wavelengths of the light. The light is then reflected off a rotating array of mirrors. The light hits
the mirrors at an angle allowing the light of different wavelengths to travel different lengths.
When the light is recorded, a Fourier transformation is performed, and the intensity of light can
be processed as a function of time (6).
The biggest limiting factor as to which OCT design was used for this project was the
price. There are only several components which drive the cost of this technology up, but the
prices were significant enough to eliminate several designs from contention. The parts in OCT
which drive the price up are the light source, the reference arm, and the photoreceptor (3).
Additionally, the price of the processing power that is needed for changing the received light into
a usable image is relatively expensive, but competition for the technology has driven the price
around as low as it will go.
Time Domain
Time domain OCT was the format first used in early OCT machines. The concepts
behind the technology are relatively straightforward and it is an easy technology to fabricate. As
described above, the most basic component of time domain OCT is the interferometer. This
device splits the light and sends it down both the reference arm and the sample arm. The
reference arm is a known length from the photoreceptor while the sample is an unknown length
from the photoreceptor. As the image is acquired, the reference arm must be moved either closer
to the photoreceptor or away from the photoreceptor. This is required so that the phases of the
light returned from the sample arm can be compared with multiple known lengths from the
reference arm (7).
There are several advantages to using time domain OCT as the principle design of the
low cost OCT system. First, time domain is easy to understand; it is important for all project
members to have a complete grasp of the technology involved. The processing involved in time
domain OCT is also much simpler than the processing of the other designs. Time domain does
not require a Fourier transformation because the data collected is collected as a function of time.
The lower processing requirement reduces the cost of the processing equipment involved. The
biggest drawback to time domain OCT for our project is the movement of the mirror. Moving the
reference mirror quickly is technically challenging. The arm needs to move very quickly and
precisely each time it is used. The mirror which is moved cannot vibrate excessively or twist so
that the light is reflected away from the detector. The image quality greatly depends on the
quality of the reference arm construction. One of the most important factors for time domain
OCT is the speed which the in the reference arm moves. The slower the motor moves, the lower
the signal to noise ratio and the longer acquisition time that is required (3). The speeds which are
required for time domain OCT are too high for the cheap and easily accessible AC and DC
motors. Instead, time domain requires the more complex and expensive piezoelectric motor.
A piezoelectric motor uses a material which changes shape when it is exposed to a
voltage differential. A voltage is applied to the material and it is forced to change shape. This
changing shape forces the motor to spin very rapidly (8). This motor would work very well for
our OCT project because it is commercially available and easily attached to the device. The one
drawback associated with piezoelectric motors is their cost. The cost of piezoelectric motors
makes them out of reach for our low budget model.
Many of the features of time domain make it an attractive option for our low cost project.
The design plan is very straightforward and the principles behind the system are easy to
comprehend. While the time domain design for OCT theoretically fits the needs of our project
very well, the practical limits, namely the cost of the system, make time domain infeasible.
Swept Source
Swept Source OCT works in a very different manner than time domain. The data that is
collected from a swept source OCT machine undergoes a Fourier transformation and is therefore
considered as an example of Fourier domain OCT (9). In time domain OCT, the reference mirror
is moved back and forth, whereas in swept source the reference is stationary. To achieve scans of
different depths, swept source OCT uses a tunable laser to vary the frequency which the light
source emits. While most OCT machines require a light source with a very broad range of
frequencies, swept source demands the opposite (4). A narrower and therefore more specific light
source is preferred The varied frequencies of the light source have different penetrating depths
and scattering properties which, when processed with a Fourier transformation and other
processing techniques, result in an image which has a high resolution and accurately represents
the sample which was imaged. The basic setup for swept source OCT is very similar to that of
time domain OCT. The light source is connected via fiber optic cables to a coupler which splits
the intensity of the light. Half of the light goes down the reference arm while the other half goes
to the sample which is to be imaged. The light is reflected back from both of the arms and a
photoreceptor detects and records the light which hits it. The procedural difference between
swept source OCT and time domain OCT is that in swept source the frequency of the laser is
tuned and in time domain the reference mirror is moved (9). The signal processing is
significantly more involved for swept source OCT however. When the data is collected in swept
source OCT, it cannot be manipulated in its raw form to display a coherent image. Instead, a
Fourier transformation must be used. This Fourier transformation changes the data into intensity
as a function of time which can then be manipulated to form a recognizable image (4).
As with all of the different methods of OCT, there are several advantages to using swept
source OCT. Swept source has the advantage of utilizing a Fourier transformation which is
inherently faster than time domain (4). This increased speed decreases the scanning time, which
is good for image quality because of decreased motion artifacts. Also, swept source OCT has a
better signal to noise ratio than time domain OCT (3). While image quality is not a paramount
concern of this project, the images need to be clear enough to be understandable for glaucoma
diagnosis. Since cost is the biggest factor for this project, if a higher image quality can be
achieved with swept source OCT, the mirrors and lenses can be downgraded to help lower the
cost of the machine without bringing the image quality below the acceptable standard.
The main factor which limits the use of swept source OCT is once again the price. While
the cost of the set-up is fairly inexpensive, the light source is very expensive. Swept source OCT
does not require the use of an expensive CCD camera, however the cost of the tunable laser puts
the cost of the system way above our price range. It is very likely that in the future, the prices of
lasers in general will decrease, but, until that happens, swept source OCT costs too much to be
implemented into the low cost system.
Swept source OCT has many important features which make it an attractive option for
designing a low cost OCT system. It has a straightforward system design and is technically
simple to build. The image quality is very good and the acquisition time is significantly shorter
than that of time domain. However, all of the important positives which are displayed by swept
source OCT are overshadowed and rendered useless because of the expensive light source which
is required. The cost of the tunable light source makes swept source OCT too expensive for a low
cost system. The unrealistic price completely overshadowed all of the benefits offered by a swept
source design (9).
Spectral OCT
Another commonly used OCT Fourier domain design is known as spectral domain OCT.
Spectral OCT works on the same principle as swept source OCT, but achieves its results in a
slightly different manner. In the swept source OCT design, a narrowband light source which
could be quickly tuned to different frequencies is desired. In spectral OCT, the opposite is what
is required. A very broadband and light source with a low coherence length is what is needed for
spectral OCT (5). Instead of the multiple frequencies being scanned at separate times like with
swept source, in spectral OCT the light source is shown through fiber optic cables to a coupler
and down both the reference and sample arms. When the light returns, instead of passing straight
through to a photoreceptor, it must first pass through a spectrometer which separates the light into
its spectrum. Because the light is separated into a much broader spectrum, a normal
photoreceptor cannot simultaneously record such a broad spectrum. Therefore, a CCD, or
Charged-Coupled Device, camera must be used. This camera is able to simultaneously record the
entire spectrum of the light that is returned from both the reference and the sample arm. Once the
light has returned from both arms, has travelled through the spectrum, and has been recorded by
the CCD camera, it is processed by the computer and can eventually be displayed as an image (3).
The processing which is required for Spectral domain OCT is similar to that of swept source OCT
and requires a Fourier transformation. This Fourier transformation changes the data into intensity
as a function of time. Spectral OCT works because the different frequencies of light can all
penetrate different lengths into the tissue. When the frequencies returned from the sample arm
are separated in the spectrometer, their interference patterns with the light from the reference arm
can be used to calculate the distances the light has travelled and can then be constructed into a
clear picture (5).
Because both spectral domain OCT and swept source OCT are both example of Fourier
domain OCT, they have many of the same advantages and drawbacks. For spectral OCT the
acquisition time is much quicker than that of both swept source and time domain (2). Because the
spectrometer splits the light into its component frequencies in one step, the machine does not
need to wait as the light source is tuned across a spectrum of frequencies like it does in swept
source or for the reference mirror to move like it does in time domain. This shorter scanning time
is most important for applications which have a very large scanning volume. With glaucoma
screening however, the area that needs to be visualized is relatively small. The advantages of the
quicker scanning times are still evident; they help with reducing the involuntary eye movements
in patients, but it is a much less important factor than the cost. Additionally, spectral OCT
benefits from a much improved signal to noise ratio when compared with time domain (5). As
explained earlier, this improved sensitivity would leads cheaper lenses and other components
which would ultimately bring down the price of the unit.
As with all of the OCT designs, a major drawback to spectral OCT is the price.
Depending on the OCT design, there are usually two components which make up a majority of
the price. For time domain, they were the light source and piezoelectric motor. In swept source it
was just the very expensive tunable laser. In spectral OCT however, there are three exceedingly
expensive components. The first expensive item is the light source. Regardless of the design that
is chosen for this project, the light source will be a significant portion of the budget. It is no
different for spectral OCT, the light source itself is not too expensive for the project, but, when
combined with the other two excessive expenses, the price becomes unmanageable. The second
expensive component is the spectrometer. Commercially available spectrometers are just out of
our monetary range, and it is very likely that, within a few years, they will be within the budget.
However, for now, the spectrometers remain too expensive for this project. The last expensive
component is the CCD camera. There are reasonably priced CCD cameras on the market, but the
resolution and sensitivity that are required for OCT make them another very expensive
component. None of these components alone is enough to break the budget, however, when they
are all combined, they are too expensive for this low cost project.
Spectral OCT is a very attractive design plan for the low cost OCT project. It nearly
made the cut, bit the problems associated with this technology are the slightly too expensive
major components. The advantages of spectral OCT are very similar to those of swept source,
but they can be achieved at a lower price. The quicker acquisition time is not as critical in this
project, but it is still a notable advantage. The better signal to noise ratio of spectral OCT helps
lower the price and make the system easier to fit in our budget, but it just is not enough. The high
cost of the main components renders the spectral OCT difficult to fit in our budget. It is very
likely that all or some of these components will go down in price in the future and will make
spectral OCT a very good option for a low cost OCT machine.
Rotational Fourier Domain
Rotational Fourier domain is a novel OCT design where the principles of both time
domain and spectral OCT are combined. The front half of the OCT system is similar to the other
OCT designs. The light leaves the broadband light source and travels through fiber optic cable.
A coupler splits the light between the reference arm and the sample arm. It is in the reference
arm where the rotational design radically differs from the other designs. Instead of having one
flat mirror in the reference arm, rotational OCT has a polygonal arrangement of multiple mirrors
(6). The mirrors are arranged in a circular orientation and are attached to a DC motor. The motor
spins the mirrors around when the scanning is in progress. The rotational OCT works by shining
the source light onto a diffraction grating to physically separate the different frequencies of light
before they hit the mirror. The light hits the spinning mirrors at angle, allowing the different
frequencies of light to travel different lengths. The light is then deflected back to the detector and
processed (6). The data once again must go through a Fourier transformation as it is processed
before it can manipulated into an image. The theory behind rotational Fourier domain OCT is
that the different frequencies in the reference arm travel different distances and after a Fourier
transformation has been performed, the data will be in the same format as a simple time domain
OCT design (3). The data processing that is required is slightly more involved than that of the
other designs because the angle of the mirror is not a constant 90 degrees. The light does not
travel a uniform distance in each scan as well, which needs to be accounted for in the processing
(6). The equations to adjust for these differences are well documented, however, and would not
be exceedingly hard to implement.
Rotational OCT has many advantages over the other current designs for our application.
As a low cost implementation of OCT, rotational OCT works very well. Many of the expensive
components required in other designs are not present in the rotational OCT design. Instead of an
expensive piezoelectric motor for example, rotational OCT only requires a very common DC
motor. An initial concern which was associated with rotational OCT was the cost of the
multifaceted mirror. This concern proved unnecessary because many barcode scanners contain
similarly designed mirrors so they are cheap and easy to procure. Additionally, rotational OCT
has those previously mentioned advantages which are associated with Fourier domain OCT. The
scans are much faster than those done with time domain and there is bigger signal to noise ratio
(3).
While rotational OCT is cheaper to build, it is also more technically challenging. The
reference mirror is not stationary and rotates at a considerable speed. Also, the refraction grating
needs to be properly aligned and, because the light needs to be physically split, some of the
reference arm needs to allow the light to travel through free space. Therefore, the background
light must be eliminated.
The main goal of this project is to demonstrate the possibility of lowering the cost of
OCT so that it can become more widespread in third-world countries. With this goal in mind,
rotational domain OCT is the best choice for lowering the cost. The lack of many of the
components of the other designs makes it an ideal choice for our project.
Summary
When choosing an appropriate design for this low cost OCT project, many aspects of the
design’s performance must be taken into account. The many applications for OCT have
generated great interest in the technology so most of the designs are very well developed and
offer nearly equivalent images. With all of the designs offering such equivalent images, the main
parameter which was used to differentiate between designs was the price. While time domain is
the easiest to understand and requires the least amount of processing power, the expensive
piezoelectric motor which is necessary to quickly move the reference mirror removed the design
from contention. Swept source was the first Fourier domain design considered and offered
considerable advantages to time domain OCT. Once again it was one expensive component, the
tunable laser in this case, which quickly ruled out swept source OCT. Spectral OCT was the
hardest to eliminate from contention because there was not one very expensive component. The
three main components in spectral OCT were slightly too expensive when their prices were added
together. The only design which could be built to give a reasonable image with such a restrictive
budget is the rotational domain OCT. This technology uses commonly available parts to keep the
price low. The drawback is that the technology is much less explored and documented. It also
requires the most processing. With such a tight budget there is no current design which fits the
project perfectly. The challenge that this project presents is one of minimalism. The current
designs need to be completely understood so that they can be stripped to their bare essentials, yet
still provide a functional image for diagnosing glaucoma.
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