Transcript Final Poster - Research

Background
Non-Invasive Blood Pressure Measurement Device
Compatible with fMRI Imaging
Jose Alvarado1,
 Efferent autonomic activity controlling blood pressure is
determined at the level of medullary brainstem nuclei, where efferent
parasympathetic and sympathetic activity is received and integrated.
 An essential portion of the input information comes from the
baroreflexes, whose function is to maintain arterial pressure within a
narrow physiologically appropriate range.
 The baroreflex system can potentially be damaged at any site (e.g.
aortic arch, carotid arteries). Failure of the baroreflex at any point
produces a volatile blood pressure and heart rate.
 The incidence rate of this disorder is 0.2% for patients with
autonomic disorders.
 Monitoring blood pressure continuously in conjunction with
Functional Magnetic-Resonance Imaging (fMRI) provides a means of
understanding this disorder by studying the activation of specific
brain sites with remarkable spatial and temporal resolution.
Figure 1: Diagram of the finger cuff system and
associated servo controls. (Finapres Manual)
 The Finometer utilizes the Penaz technique to measure blood pressure
using a finger cuff fitted with a pneumatic bladder, infrared LED, and
photodetector to measure finger artery size and to apply external
pressure to the artery. This technique of optically measuring volume
changes in parts of the body is called photoplethysmography.
 The bladder pressure is varied using a servo-valve which allows cuff
pressure to equal arterial pressure throughout each cycle. If an
externally applied pressure is equal to arterial pressure at all times, the
arterial walls are unloaded and the photoplethysmogram will be
constant.
Figure 4: (Top)First generation
prototype using monitor line. (Middle)
Second generation prototype using
larger diameter tubing to decrease
resistance. (Bottom) Third generation
prototype using rigid tubing to decrease
water volume and resistance.
Figure 2: Finometer noninvasive blood pressure
measuring device.
Goals
Design a device capable of continuously
measuring blood pressure, non-invasively,
during an electromagnetically sensitive
procedure, such as an fMRI study:
Figure 3: Diagram of the Philips
Intera Achieva 3.0T fMRI
scanner at Vanderbilt University
showing the required distance
needed to extend the system.
Retrofit finger cuff blood pressure devices to use
optical transmission techniques instead of
electrical transmission techniques.
 Extend length between cuff and electrical
components without losing pneumatic function to
the desired distance of 12 feet away from the
fMRI machine, which will allow all the electrical
components to be outside of the procedure room.
Figure 6: (Top) Diagram of proposed
optical transmission system (Middle)
Interaction of infrared light with layers
of the finger, showing the relatively little
light that is transmitted to the
photodetector (Bottom) Propagation of
light through an optical fiber (D.
Jansen – Vanderbilt Optics)
 The first attempts to extend the air pressure lines to the cuff
were pneumatic-only extensions using cardiac monitor lines.
 Plastic tubes of varying diameters were used to extend the air
lines, but the system lost functionality after extending the lines
past 6 feet.
 After extending the air lines, dampening caused by expansion o
the tubing walls caused a loss of function in the device.
 When the pneumatic system could not be extended to the proper
length, we pursued a hydraulic system to avoid the dampening
problems.
 Five generations of hydraulic prototypes were constructed and
tested at an extension length of 2ft.
 Leakage was a problem encountered at many points in the
development process. The pressure built up within the canisters
caused many of the epoxy/silicon seals to crack over time.
 The weight of our chosen fluid, water, also proved difficult to
work with. In order for the system to work correctly it must be
able to change water levels at a very fast rate. The weight of the
water caused a build up of momentum which limited the response
time of the hydraulic system.
Optical Transmission System
 Like existing systems, our solution utilizes optical
photoplethysmography to measure volume changes as a result of
localized blood flow in the finger.
 By passing infrared light from an LED harvested from an
existing finger cuff through an optical fiber, the finger was bathed
with light of a suitable wavelength. Light transmitted through the
finger was collected in another optical fiber and delivered to a
power meter to measure relative transmission percentages. In a
working system, the transmitted light would be delivered to a
silicon photodiode for interpretation by the Finometer.
 To power the diode, a breadboard circuit was developed using a
100Ω resistor, and powered using a variable power source.
During initial experiments, the constant voltage drop across the
diode was found to be 1.4 V. When measuring the transmission of
infrared light, 8V was applied to the circuit, creating a current
flow of approximately 5.56mA.
 Our first prototype utilized a 4 ft long optical fiber (3M) with a
core diameter of 600µm without effective fixtures or connectors
to align the fiber with the diode. Transmission of infrared light
was achieved through manual manipulation of the fiber at
different locations along the surface of the diode.
 Early in the testing of our first prototype, we confirmed that the
greatest difficulty with our approach would be maximizing the
amount of divergent IR light entering the fiber core from the
diode due to the narrow acceptance angle of the fiber material.
 Our second prototype (see Figure 7) utilized a 2 ft long optical
fiber (Fiberguide Industries) with a core diameter of 400µm (NA
= 0.22) with SMA connectors and rail mounts to ensure proper
alignment. This approach was far more successful.
 Future attempts will utilize a convex lens to focus the divergent
LED light at a proper acceptance angle for propagation into the
fiber.
Results
 Originally we were able to extend the pneumatic system to a
distance of 6 feet.
 Several prototypes were made for the hydraulic system, but
we were unable to produce a system which met our needs.
 Using the second prototype (see Figure 7) of our proposed
optical transmission system, we were able to obtain the
following optical power transmission data.
 Power Output from Diode: 4.88 mW
 Power Output Passed Through Fiber: 1.81 mW
 Power Output Passed Through Fiber & Finger: 0.09 mW
 From these results, we were able to achieve 37%
transmission through the fiber with proper connectors and
alignments, and were able to confirm transmission of 0.09 mW
of infrared light through the finger. The acceptance angle
calculated for the fiber (~12.71°) explains the relative difficulty
of collecting light in the fiber.
Pneumatic and Hydraulic Extensions
The Problem With Current Devices
 Current commercial devices are not able to
continuously measure blood pressure in the
presence of magnetic fields.
 Ferromagnetic materials in the Finometer
system and the highly sensitive 3.0T magnet in
the fMRI scanner interfere with each other,
leading to distortion in both sets of data.
 Some components of the cuff are made up of
ferromagnetic materials and can be dangerous
when placed in close proximity to an fMRI
machine.
and Sanjeet
Rangarajan1
Advisors: Dr. Andre Diedrich, M.D. PhD2; Sachin Paranjape2; Dr. Richard Shiavi, PhD1
1Department of Biomedical Engineering, Vanderbilt University, Nashville, TN USA
2Clinical Research Center, Vanderbilt University Medical Center, Nashville, TN USA
Current Devices
 The Penaz technique is used
by current devices to
continuously measure blood
pressure and is based on the
principle that the force
exerted by a body may be
measured by determining the
counterforce needed to stop
the original force from
causing disruption.
Benjamin
Huh1,
Market Potential
Figure 5: (Top) Fourth generation
prototype using PVC piping to eliminate
leaks. (Middle) Fifth generation
prototype using PVC piping and rigid
polyurethane tubing to eliminate excess
water volume. (Bottom) Hydraulic
system extension between device and
finger cuff..
 Based on our research, this is the first known attempt to create
an fMRI-compatible device capable of measuring a continuous
blood pressure waveform.
 Our device will be primarily used in research hospital settings
where fMRI and autonomic disorder research is being
conducted.
 There exists significant market potential for our device
because of the explosion of new studies being commissioned in
MRI/fMRI and brain activity research.
 An optical, continuous, non-invasive blood pressure measuring
device could be used during many different kinds of procedures
where electromagnetic sensitivity is an issue, such as some
forms of image-guided surgery.
 The most lucrative way to turn profits from our device would
be to sell the design and accompanying rights to a medical
device manufacturer already in the bioinstrumentation and
diagnostics market, such as TNO BMI or GE Healthcare.
Potential Solutions
 The first and simplest solution would be to couple our
pneumatic-only prototype (6 ft) with our optical transmission
system and shield/position the device outside the effective range
of the fMRI French field, but this solution is not ideal.
 Replacing the pneumatic pump within the Finometer with a
stronger one would likely increase the effective distance over
which pressure measurements can be taken, but may introduce
software calibration issues.
Using a semi-permeable membrane and fully filling the
monitor line including the finger cuff bladder with water may
induce faster response times in the system.
 Replacing the IR diode with a stronger one, or using a laser
diode system would significantly increase the rate of optical
transmission in the system. This solution may introduce
software calibration issues as well.
 Using a lens system to focus divergent light from the diode
would also increase the amount of light collected in the fiber.
Figure 7: (Top) Modified infrared diode
extracted from Finometer finger cuff
(Middle) Polished 400µm fiber
mounted inside an SMA connector
(Bottom) Mounted experimental setup
showing the position of both optical
connections, and locations of the power
meter and proposed biconvex lens.
Acknowledgements: Dr. Andre Diedrich, Dr. Paul King, Dr. Richard Shiavi, Dr. Duco Jansen, Dr. John Gore, Sachin Paranjape, and Yuti Dalal
Conclusions
 In conclusion, our group has shown that it is possible to
retrofit the Finometer blood pressure measurement device with
materials allowing for operation within fMRI settings. Acting
on one of the several potential solutions provided may result in
a workable product.