Methodology for Air Line Extension:



Vanderbilt University

Department of Biomedical Engineering

Non-Invasive Blood Pressure Measurement Device Compatible with fMRI Imaging

Team Members:

Jose Alvarado

Benjamin Huh

Sanjeet Rangarajan

Advisor:

Andre Diedrich, M.D. PhD.

Richard Shiavi PhD.

Date of Submission:

April 26, 2005

Abstract:

Patients with baroreflex failure compose a small number of the population suffering autonomic dysfunction. fMRI imaging has recently allowed novel insights into the function of individual brain sites by providing a means of studying dynamics of the activation of specific areas with remarkable spatial and temporal resolution. Since many higher centers of the brain could have effects on controlling blood pressure associated with baroreflex failure, it would be of great investigative importance to measure blood pressure continuously during fMRI procedures. Current non-invasive, continuous blood pressure devices use the Penaz technique to monitor blood pressure, utilizing finger cuffs outfitted with air bladders, infrared light sources, and photodiodes to compose a photoplethysmograph. These cuffs, however, contain ferromagnetic materials, rendering them incapable of operating within an fMRI’s highly powerful magnetic fields. Our group sought to retrofit the finger cuffs of such a device, the Finometer, by removing ferromagnetic components, and using pneumatic/hydraulic and optical transmission systems to extend the cuff from the device by 12 feet. Achieving an extension length would allow the Finometer to be placed a safe difference from an fMRI’s magnetic field, facilitating the development of new clinical options and studies to study baroreflex failure. Our method of preserving air bladder pressure over 12 feet, a hydraulic system was only moderately successful, allowing the Finometer to operate for only a short while before the device reported an error. Our optical transmission system was capable of delivering 1.81 mW of infrared light from the diode to the finger, 37% of the LED’s total output. In order to obtain a fully functioning design, we have recommended several potential solutions that could be used to enhance or improve the current design of the device.

Introduction:

The Vanderbilt Autonomic Dysfunction Center was established in 1978 as the first international center for patient care, research, and training focusing exclusively on disorders of the autonomic nervous system1. One of the numerous disorders being studied today, baroreflex failure, affects 11 out of every patients with sever autonomic dysfunction, causing them to have severely volatile high blood pressure. Efferent autonomic activity controlling blood pressure is determined at the level of medullary brainstem nuclei, which receive multiple inputs from the baroreflexes located in the carotid arteries and aortic arch1. These inputs determine the amount of sympathetic and parasympathetic stimulation needed to control blood pressure in the body. By properly integrating information from the baroreflexes, arterial blood pressure may be maintained within a certain acceptable range. Additionally, it is possible that influence of higher centers of brainstem and cortical structures will have potentiating effects on changes in autonomic outflow1. Patients with baroreflex failure typically have major physiological complications as a result of their disorder, and the goal of the Autonomic Dysfunction Center is to find better ways to treat such individuals.

Functional Magnetic-Resonance Imaging (fMRI) has recently allowed novel insights into the function of individual brain sites by providing a means of studying dynamics of the activation of specific areas with remarkable spatial and temporal resolution. Since many higher centers of the brain could have effects on controlling blood pressure associated with baroreflex failure, it would be of great investigative importance to measure blood pressure continuously during fMRI procedures in order to study the mechanism of interaction in these patients.

Devices are currently available to measure blood pressure, in a non-invasive manner, on a continuous basis5. The problem with current devices and our desired application is their incompatibility with fMRI machines. Current devices and their attachments contain ferromagnetic materials which interfere with the powerful magnetic fields created by the fMRI hardware.

Current non-invasive continuous blood pressure measuring devices, such as the Finometer by Finapres Medical Systems, use the Penaz measurement technique as the basis for their function6. This technique involves using an inflatable bladder within a finger cuff to clamp blood volume in the finger. An infrared LED shines through the finger and a photodiode measures the transmitted light, which is proportional to the clamped blood volume, to form a photoplethysmogram. When the cuff pressure equals arterial pressure, the arterial walls are unloaded and the photoplethysmogram is constant, creating a set point for further action by the device. Increasing or decreasing arterial pressure leads to a corresponding change in the photoplethysmogram and the system increases or decreases cuff pressure, so that cuff pressure equals arterial pressure on a continuous basis.

The pressure to the finger cuff is controlled by a fast-acting servo loop that is able to regulate the pressure to the finger cuff on a continuous basis. The set point (unloaded blood volume) used for the servo loop is determined in two ways. Initially, the set point is found using the servo start-up adjustment method, which gradually increases cuff pressure in steps until the variations in the photoplethysmogram become maximal6. Variations in the photoplethysmogram become maximal when cuff pressure is between systolic and diastolic pressure. The second method the Finometer uses to define the set point is the servo self-adjustment method. This method provides a fine-tuning of the set point and corrects for slowly changing physiologic conditions in the finger. Please see the Appendix for a diagram of the finger cuff, servo loop and associated controllers.

Goals of this project include designing a device capable of continuously and non-invasively measuring blood pressure during electromagnetically sensitive procedures, such as fMRI studies. This device must be compatible with commercially available devices such as the Finometer by Finapres Medical Systems, so that the technology will be easier and more likely to be adopted by researchers. In order to prevent interference with the fMRI magnet, all ferromagnetic components within the finger cuff (the LED and the photodiode) must be removed, and optical signals must be extended far from the body before being interpreted by the device’s semiconductors. Ideally, the finger cuff should be extended to a distance of at least 12 feet away from the Finometer. This extension includes extending the air lines that supply the inflating cuff with air pressure to clamp the finger’s blood vessels. Achieving an extension length would allow the main Finometer device to be placed in a shielded observation room, away from the magnet of 3.0T fMRI scanner at the Vanderbilt University Medical Center, facilitating the development of new clinical options and studies to study baroreflex failure.

Methodology:

The project advisor, Dr. Andre Diedrich, had previously attempted a similar project and advised our group on where to begin. He allowed us use of his lab, which included the Finometer, along with any supplies we could find, such as cardiac monitor line, various connectors, and adaptors for air line connections. The first task that needed to be completed to define our project was to visit the 3.0T fMRI scanner in the Vanderbilt University Institute for Imaging Sciences and determine how far the system needed to be extended to avoid interference. The distance from the fMRI machine to the wall was measured to be a distance of 10 feet 8 inches. Ideally, the Finometer would place on the opposite side of the wall in the observation room. In order to properly shield the procedure room the walls are 16 inches thick. This makes the total distance that the system needs to be extended 12 feet long. If extension were not a possibility, another solution could be to leave the Finometer inside the procedure room, and attempt to shield it from magnetic fields. This would allow for a shorter extension length, but it not the solution that is desired by the customer, Dr. Diedrich.

Using Innovation Workbench®, a software solution developed by Ideation International, our group was able to use TRIZ, the Theory of Inventive Problem Solving, to break the problem down into small pieces, which proved useful in determining the goals for the project. The Directions for Innovation reported by Innovation Workbench are included in the Appendix of this report.

With the extension length identified, our group began working on extending the air lines to the required distance. The simplest and most straightforward approach was to extend the air line from the finger cuff using piping of equal diameter the small amount of air piping already present on the cuff. The results section will discuss the success of extending the air lines pneumatically and also will discuss why the design was changed from using pneumatic extension to hydraulic extensions.

In addition to ensuring that the air bladder portion of the cuff was able to sustain a pressure after being extended 12 feet, our group was required to determine how to avoid interference caused by the infrared LED and photodiode used to measure blood volume in the finger artery. To achieve our goal of creating a completely fMRI-compatible system, our group proposed to create a fiber optic transmission system to extend the LED and photodiode away from the patient, leaving the fMRI scanning area devoid of ferromagnetic materials. Hence, we developed a method to achieve this by using an optical fiber to deliver infrared light to the patient’s finger, while using another fiber to capture light transmitted through the finger and deliver it to the Finometer device.

Results for the Air Line Extension:

This first attempts to extend the distance from the Finometer device to the finger cuff involved simple pneumatic extension. For our first attempt, cardiac monitor line was used to extend the finger cuff an additional 5 feet from the device. The cardiac monitor line was chosen first because it was the same diameter as the tubing used to attach the finger cuff to the Finometer. Additionally, there were readily available valves and fittings that would allow for easy, leak proof extensions. At an extension length of 5 feet there was no detectable loss of function by the Finometer. The finger cuff inflated properly and the servo-valve was able to properly regulate the pressure that was delivered to the cuff.

An extension length of 5 feet was still well short of our goal of a 12-foot extension length, so another 5 foot section of cardiac monitor line was attached to the end of the previous extension. This connection was done using the fittings that come already attached to each end of the monitor line. Including the 6 inches of length that is part of the finger cuff itself the total extension length was 10.5 feet. However, when tested with the Finometer, this extension system failed completely. Very little pressure was being delivered to the cuff, and the Finometer would automatically stop measurements due to a detectable lack of pressure. The connections were checked and no leakage was occurring. Hence, we made the conclusion that the proper pressure was not being delivered to the bladder because of loss of pressure within the tubing itself. The monitor line tubing has a very narrow diameter, which is the major factor for determining the resistance of the tube. A large resistance means that there will be a large pressure loss is the tube. Knowing that a 10 foot extension using the monitor line leads to a high resistance, shorter extension lengths were attempted, and the maximal extension length using the monitor lines was approximately 6 feet.

Since the resistance with the small diameter tubing was too large, it was decided that larger diameter tubing was needed to decrease the resistance. New 0.5” diameter tubing was purchased to overcome the resistance problem. This new diameter tubing led to an unexpected result. We expected the larger diameter would decrease the resistance, which would allow extensions of greater than 6 feet. The result was that with the larger diameter it was only possible to make an extension of about 5-6 feet before the Finometer loses functionality. The most likely explanation for this result was that there was too much air volume present, damping the pressure preserved within the tubing.

Since extending the air line pneumatically did not seem to be working, we decided to pursue a hydraulic system. A hydraulic system can carry pressures over long distances without substantial loss. A hydraulic system could also help decrease the total air volume, which would lead to a decrease in the damping. The initial design was to have two small reservoirs connected by a tube of smaller diameter. There would be no pistons, but the water would be driven by increased air pressure in the reservoir, see figure 1. The first hydraulic system was constructed from easily assessable laboratory materials in the medical center in order to save on cost. The reservoirs were constructed from Fisherbrand 50mL centrifuge tubes and again the connecting lines were constructed from cardiac monitor line. To assemble the device a hole was drilled in both reservoirs at equal levels in order that an adapter from the monitor line could fit in tightly, connecting the two reservoirs. For sake of space a 2 foot section of monitor line was used. The adapter was sealed in place using epoxy and silicon. Both caps of the centrifuge tubes were drilled through. A short length of monitor line was inserted directly through the hole, then sealed with epoxy and silicon. The monitor lines coming from the caps were attached to three-way valves. One three way valve was connected to the finger cuff; the other was connected to the Finometer. The three way valves were necessary in order to allow a way to release pressure from the system after each test without disassembling the apparatus. In the case that there was too much fluid in the system and some of the fluid started to enter the cuff, the valve could have been closed to prevent more water from entering. Another preventative measure that was added to prevent moisture from entering either the Finometer or the finger cuff was the addition of a small wad of cotton to act as a slight membrane.

Needless to say, our first prototype failed. The Finometer was not able to complete the servo start-up adjustment. After some analysis the conclusion was that the monitor line was again too small in diameter to allow for proper flow of the water between reservoirs. With such high resistance due to the small diameter the Finometer would have to generate very high pressures to provide sufficient flow. Since the pressures were so high many of the seals began to leak even after being thoroughly sealed with epoxy and silicon.

The second prototype tried to improve on some of the shortcomings of the first prototype. Instead of monitor line, a large diameter respiratory line was used to connect the two reservoirs. This large diameter tubing was attached directly to the bottom of each reservoir by cutting off the bottom the reservoir and fitting on the connecting tubing. The expectation was that with larger diameter tubing there would be very little resistance to flow between the tubes and water would be able to move from one reservoir to the next with relative ease and speed.

The second prototype did not provide a functional extension from the Finometer either. The expected result was that the hydraulic fluid, water, would move between reservoirs with speed and ease, but what was observed was a more sluggish response. What was happening was that not enough thought was put into the materials used to construct the system. The connecting respiratory tubing was ribbed, so when the Finometer began to pressurize the reservoirs the hydraulic system transmits that force in all directions. Since the walls of the connecting tube were not rigid, some of the pressure was lost in the expansion of the tubing walls. This tubing was not rigid there was a constantly changing volume of the hydraulic system, which is desirable. Also since the diameter of the connecting tubing was so large there was an excessive amount of water volume in the system that the pump had to push around, which was a problem given the small nature of the pump.

The third prototype attempted to fix the problem of a changing system volume by using connecting tubing made of rigid 0.5” diameter polyurethane. This tubing was attached to the reservoirs by cutting of a small portion of the bottom and fitting the tubing into the hole. The tubing was sealed in place with epoxy and silicon. This prototype proved very difficult to seal properly. Many thick layers of epoxy and silicon were needed to ensure a leak-proof seal. Over the course of the trials that we put the prototype through, the seals between the reservoirs and connecting tubes would fatigue and break. Another source for leakage was the cap of the reservoirs. They were not designed to handle high pressures, and therefore would leak air slowly in high-pressure situations such as our experiment. Leaks in the system were found by submerging the whole system underwater and pressurizing it to 200mmHg using a small hand pump and gauge. Air bubbles could be easily spotted, confirming leaks in the system. Leakage from the caps meant that there would be pressure loss in the system, which would ultimately lead to a failure of the system. When tested, the loss of pressure could be observed by the slight decreases in water levels during the Servo Start-up adjustment method. This leakage caused the Finometer to have an error and its display would indicate a need to check the device’s pressure connection.

Leakage in the system posed a significant problem. Instead of continuing to use the laboratory supplies to make the prototypes, it was decided that PVC tubing would provide for more rigid and leak proof connections. The first PVC prototype was constructed of 0.5” diameter piping. The reservoirs and the connecting tubing were both the same diameter. PVC cement was used to make the connection between different fittings, and threaded caps were used to make removable seals at the top of each reservoir. Testing of this prototype showed that the pressure was being properly transmitted across the system. The servo start-up adjustment method showed clear steps and achieved a high end pressure of 190mmHg. The system failed, however, after the servo start-up adjustment method once the Finometer tried to rapidly adjust the pressure with the servo valve. This result indicates that there is no issue with the pressure being transmitted over the system. The problem arises when trying to make rapid adjustments.

Thinking that the reason for the failure of the fourth prototype was because of the amount of water in the system, a fifth prototype was constructed. This prototype again involved PVC piping, but also included rigid 0.5” diameter polyurethane tubing for connecting the reservoirs. In order to ensure that there was no leaking from the caps Teflon tape was used a precautionary measure.

When tested the fifth prototype came the closest to extending the Finometer’s air lines using a hydraulic system, but still failed. The Finometer was able to complete the servo start-up adjustment method to identify the first set-point, but failed shortly thereafter when the fast servo valve adjustment began. The hydraulic system’s response time could not keep up with the demands of the servo valve. It is clear from these results that the hydraulic system does indeed transmit the pressure over a large distance; however, for complete functionality of the Finometer it is necessary to find a way to improve the response time of the hydraulic system.

We believe that the response time is slow because the weight of the water being moved vertically in the tubes is sufficient to cause a build up of momentum. Since the servo valve tries to fine tune the pressure in the cuff at a very high rate the velocity of the water moving up and down in the reservoirs will be very high. Due to the small time between the servo adjustments there is no time for gravity to slow the momentum of the water.

Results for the Optical Transmission System:

The first task our group needed to accomplish on the optical transmission system was to extract the infrared diode from the existing system, verify that it could be operated correctly outside the cuff, and modify it so that it could be used in our system. We extracted the diode from the finger cuff by severing a working finger cuff from its external connections, removing the diode’s shielding, and dissecting the electrical components. A two-pin, clear colored infrared LED was extracted from the cuff and cleaned to remove adhesive residue from the outer surface of the electronics. Wires were soldered to the cathode and anode sides of the diode so it could be introduced into our breadboard circuit for further characterization.

Our group then attempted to power the standalone diode using a simply prepared breadboard circuit. The circuit was developed using a 100Ω resistor, and powered using a variable power source. By manipulating the voltage across the circuit, we were able to demonstrate that the diode possessed a minimum turn on voltage of 1.4 V, and remained constant even when increasing the voltage across the system even further. Various voltages were introduced across the circuit when measuring the transmission of infrared light, resulting in different amounts of current flow, and hence, infrared light output. When taking measurements, 8 V was applied to the circuit, which in turn created a current flow of approximately 5.56 mA. We chose 8 V because we were eventually able to gain the greatest amount of optical transmission at this voltage while still remaining below 6 mA in current flow, a safe value for LEDs. It was important that we be careful not to drive too much current through the LED, as this is the primary reason why LEDs burn out over time in other systems. When powering the system with 8V, the power output from the diode was measured to be 4.88 mW using an optical power meter calibrated for infrared light.

After confirming that the harvested LED was indeed operational outside of the cuff, our group began working on methods of transmitting the output light through optical fibers. 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. In order to simplify our early experiments and calculations, we decided to utilize single fibers instead of bundles to demonstrate that IR light from the diode could be transmitted through the fiber medium and characterized by a power meter. The single fibers we obtained needed to be cut and polished before they could be used, as the terminal junction of a fiber needs to be perfectly polished to allow for maximum light entry. The fibers were cut with razor blades and polished using the bench top polisher in the Department of Biomedical Engineering’s Optics lab. Proper polishing was confirmed by checking the fiber surfaces under both light and surgical microscopes. It was later observed that unpolished fibers transmitted far less light than their polished counterparts.

Our first prototype utilized a single 4 ft long optical fiber manufactured by 3M Corporation with a core diameter of 600 µm. We polished the fiber until its ends looked satisfactory under the lab’s microscopes and then attempted to gather infrared light from the diode. Our initial attempts with this fiber lacked effective fixtures or connectors to align the fiber with the diode. Instead, transmission of infrared light was achieved through manual manipulation of the fiber at different locations and orientations along the surface of the diode. By holding one end of the fiber very close to the LED, and the other end to the photo-detecting portion of the power meter, small power output readings were recorded. By placing the power meter directly on top of the infrared LED, we were able to estimate its total power output to be 4.8 mW. When holding one end of the 600 µm fiber next to the LED and the other end at the power meter, a meager 0.31 mW was recorded. When attempting to transmit light from the fiber through the finger, no light was detected on the other side of the finger, indicating that insufficient light was being delivered to the fiber. Given that our light transmission from the LED through the fiber was approximately 6.5%, and that propagation through the finger was non-existent, our group sought to create a more sophisticated solution. We determined that one way to increase the efficiency of our optical transmission system would be to develop better ways of aligning our infrared light source with the fiber instead of simply holding it arbitrarily. Our second prototype was centered on the idea of aligning the fiber correctly with its source and destination.

Our second prototype (see Figure 14) utilized a 2 ft long optical fiber manufactured by Fiberguide Industries with a core diameter of 400µm (Numerical Aperture = 0.22). SMA connectors and rail mounts were used to ensure proper alignment of the fiber with the source and destination of the infrared light. By properly situating the diode so that the maximum amount of light will enter the fiber, we expected transmission rates to increase in spite of the smaller diameter of the fiber. This approach proved to be far more successful. Using the second prototype (see Figure 14) of our proposed optical transmission system, we were able to significantly better optical power transmission data than with the 600 µm fiber. From this experimental design, we recorded 1.81 mW of light passing out of the fiber, an almost 37% transmission rate through the fiber with proper connectors and alignments. We were also able to confirm transmission of 0.09 mW of infrared light through the finger when using the 400µm fiber, which proves conceptually that our proposed solution will work if we can increase the amount of light that enters the fiber from the LED.

Safety Issues:

Since Finometer and our proposed extension system is non-invasive, there are only a few safety concerns pertaining to the device. In order to better assess the possible hazards, we utilized Designsafe to identify all known possible hazards and took into consideration potential risk reduction methods. Some of the more significant hazards determined for the patients and those administering the continuous blood pressure measurement during the fMRI study included situations with wet, electromagnetic susceptibility of the electronics, a hydraulics rupture, fluid leakage and ejection, and magnetic attraction to surrounding devices. Utilizing a hydraulic system with water, while connected to an electronic device can cause a potentially hazardous situation. In order to prevent damage to the device, de-ionized water was used. Routine checks for system leaks were made to avoid potential problems with wet locations. Another potential hazard in using our hydraulic system is rupture, which has a low probability of occurring, but can be eliminated by following a standard procedure of checks procedures. Due to the functions of the device, changes in the pneumatic line design can lead to unsafe pressure levels, potentially leading to fluid leakage and ejection. In order to alleviate these possibilities, precautions were taken to use powerful sealants and the device operators can learn to turn off the device if a leak is suspected. Concerning the hazards of magnetic attractions, placing the Finometer outside 3.0T fMRI lab would fully eliminate the hazard since the procedure room is designed to shield the outer room from the magnetic effects. Designsafe was a very helpful and effective method in determining the possible hazards and potential solutions in minimizing the hazards. Our group believes that our device design is not potentially hazardous when used as indicated and will not create any significant hazards after the changes and system checks have been made. The complete Designsafe report can be found in the Appendix.

Economic/Market Issues

The original budget produced for the device can be found below.

|Item |Quantity |Cost per Item |Total Cost (Dollars) |

|Fiber Optic Cable |15 Feet |$1 per Foot |$15 |

|LED's |4 |$5-10 |$40 |

|Air Piping |15 Feet |$0.50 per foot |$7.50 |

|Photodetector |1 |$25 |$25 |

|Rubber Insulation |2 sq Feet |$3 per sq. foot |$6 |

|Inflation Bag for Cuff |1 |$5 |$5 |

|  |  |Total Cost |$98.50 |

The budget was split up to create an extended fully pneumatic system with a newly created cuff and a simple fiber optic cable transmission section. This estimated total cost came to a value of approximately $98.50. Unfortunately due to the unforeseen consequences of the pneumatic system not meeting our needs, the budget had to be changed in order to incorporate materials for the hydraulic system. The items purchased including PVC tubing and connectors, rigid piping, and sealants as well as adhesives. After these changes in the budget, the estimated cost of the device rose to approximately $118.

This device can be made and will be affordable due to the reusability of the device for several patients, helping to defray the cost. When considering the developmental cost analysis, it would take 2 students at $15/hr approximately 20 hours to produce the device, which comes to a total of $600 for labor with an additional $118 for parts. Therefore, the total cost will be approximately $718 and the cost of maintenance will be minimal.

There exists a fairly significant market potential for a continuous blood pressure measurement device which is compatible with fMRI imaging. This is due to the increasing popularity of the use of MRI as a means of research and patient diagnosis, and the fact that the device we design does not exist elsewhere on the market. The device would be mainly used in research and hospital settings, where fMRI and autonomic disorder research is being conducted, but there is also potential for using the device in procedures which are electromagnetically sensitive such as image-guided surgery. Because our device is very specific and requires current commercial systems to be relevant, the most lucrative way to produce a profit from our device would be to sell the design and accompanying rights to companies such as TNO BMI or GE Healthcare, who have a commanding presence in the biomedical instrumentation and diagnostics industry.

Conclusions

The results of our designs show that it is possible to retrofit the Finometer blood pressure measurement device with materials allowing for operation within fMRI settings. For the optical transmission system we were able to obtain a transmission of 37% of IR light through the finger and have introduced recommendations on how to possibly gain a higher transmission rate. For the hydraulic systems, we felt that despite the multiple prototypes, we were unable to produce a system that met our needs. The group believes that even though we had attempted numerous possibilities and subsequently ran over schedule, we did not exhaust all of our options. Acting on one of the several potential solutions provided in the recommendations section may result in a workable product.

Recommendations

The completion of a workable product is very feasible. In order to obtain a fully functioning design, we have recommended several potential solutions that could be used to enhance or improve the current design of the device. On matters concerning the pneumatic/hydraulic extensions, the first and simplest solution would be to couple our pneumatic-only prototype of 6 feet of monitor line with our optical transmission system and then to effectively shield and position the device outside the effective range of the fMRI fringe field. Unfortunately, the downfalls of this solution would be due to the possible distortions due to the proximity of the device to the MRI device as well as the unlikely nature of the lab technicians even allowing the metallic device so close to the sensitive equipment. This would not be an ideal solution. Another problem with the hydraulic system was possibly with the strength of the pump not being large enough to overcome the dampening effects. A potential solution would be to replace the pneumatic pump within the Finometer with a stronger pump that might increase the effective distance over which pressure measurements could be taken. On the other hand, replacing the pump in the device may lead to software calibration issues that could become difficult to deal with. A final potential solution to the hydraulic system was to eliminate the pneumatic to hydraulic to pneumatic design by fully filling the monitor line all the way into the cuff with de-ionized water while blocking the pneumatic to hydraulic switch with a semi-permeable membrane. Eliminating the reversion from hydraulic to pneumatic may induce a faster response time in the system. Possible pitfalls with this solution would be finding a suitable semi-permeable membrane that would allow for the quick transport of air through the system, and for water being in the cuff without causing damage or calibration problems.

There exist methods to increase the transmissions of the current design of the optical transmission system as well. The first possibility would be to replace the current IR diode with either a stronger diode or a laser diode system. Replacing the current IR diode with a stronger one will allow for a larger power output from the diode which would allow for a larger amount of power to be transmitted through the finger while replacing the current IR diode with a laser diode system would possibly increase the amount of power transmitted as well as decrease the amount of light divergence. Potential problems will be that of software calibration issues due to a change in the components of the device. The final optical transmission system solution recommended is to utilize a lens system in order to focus divergent light from the diode. The lens if created and positioned properly will increase the amount of light collected in the fiber. The only setback for this solution would be to ensure the proper shapes and positioning of the lens in order to optimally improve the amount of light transmitted through the finger.

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2 Imholz BP, van Montfrans GA, Settels JJ, van der Hoeven GM, Karemaker JM, Wieling W. Continuous non-invasive blood pressure monitoring: reliability of Finapres device during the Valsalva manoeuvre. Cardiovasc Res. 1988 Jun;22(6):390-7.

3 Silke B, McAuley D. Accuracy and precision of blood pressure determination with the Finapres: an overview using re-sampling statistics. J Hum Hypertens. 1998 Jun;12(6):403-9.

4 McAuley, D., Silke, B., & Farrell, S. (1997). Reliability of blood pressure determination with the Finapres with altered physiological states or pharmacodynamic conditions. Clinical Autonomic Research, 7, 179[pic]184

5Finapres Manual

6 Penaz, J. (1973) Photoelectric measurement of blood pressure, volume and flow in the finger. Digest. 10th Int. Conf.

Appendices:

Appendix A – Innovation Workbench Processes

Appendix B – Designsafe Report

Appendix C – Finger Cuff Schematic

Appendix A - Innovation Workbench Ideation Process

Innovation Situation Questionnaire

1. Brief description of the problem

The problem is that researchers currently can not use today's non-invasive blood pressure measuring devices during an MRI or fMRI procedure because the electrical components of current devices interfere with magnetic field of the MRI scanner and also the magnetic field causes inaccurate readings of a patients blood pressure.

2. Information about the system

2.1 System name

Non-invasive Blood Pressure Device

2.2 System structure

In its non-operating state the finger cuff contains a plastic inflatable bag, a series of LEDs, a photodetector, and a rubber casing which acts to shield the unit from ambient light. There is a connection to the plastic inflateable bag that is attached the Servo pump and is a 1cm diameter plastic tube. There are also electrical cables that run from the LEDs to the processing device as well as another electrical cable from the photodetector to the the processing device and hardware.

2.3 Functioning of the system

The function of a Non-Invasive Blood Pressure measuring device is to measure a person's blood pressure on a continuous basis and produce a graph of the blood pressure function so that it may be analyzed by a doctor or technician.

The device uses light absorption and the variablity of light absorption through the cardiac cycle to measure the blood pressure within the vessels. The purpose of the device is to monitor changes in blood pressure.

The device achieves its primary function by using LED's to shine light through the finger. As light passes through the finger some of the light is absorbed and some of the light is transmitted. The transmitted light is then sensed by the photodetector which lies directly on the other side of the finger from the LEDs. The photodetector produces a voltage based on the amount of light that is absorbed. This voltage is fed back to the hardware and processing device where the voltage reading is converted to a pressure measurement. The signal is then displayed on a graph on a continuous basis.

2.4 System environment

The enviornment for such a blood pressure measuring device in the case of our study would be an MRI lab. In the MRI lab there exists a magnetic field that is emitted by the MRI scanner. The cuff will be located on the finger which has a temperature between body temperature and the exterior temperature. Since the cuff will be wrapped tightly around the finger there could be extra moisture present.

3. Information about the problem situation

3.1 Problem that should be resolved

Current non-invasive blood pressure measuring devices are not compatible with MRI machines because they contain electrical components that interfere with the magnetic field produced by the MRI machine.

3.2 Mechanism causing the problem

The problem is due to the magnetic field that is produced by the MRI scanner. This field interferes with the signal that is transmitted back to the processing unit. Also the cables that run from the cuff cause a magnetic field themselves which cause distortion in the MRI images.

3.3 Undesired consequences of unresolved problem

The interfering magnetic field of the MRI unit causes distortion and inaccuracy in the blood pressure measurement. The electrical cables used in the cuff cause image distortion of the MRI.

3.4 History of the problem

The problem has existed for as long as current devices have existed. To date no company has a product that is compatible with MRI machines because they all use electrical components to transmit the signals. Our advisor Dr. Deidrich has attempted previous solutions to the problem but has failed in substantially increasing the length that the pump could be extended away from the actual cuff. With out getting the pump sufficiently far from the cuff there can be no solution to the problem.

Dr. Deidrich has also attempted to extend the pump just a short distance away from the cuff and use a shielding box to shield the pump and the processing unit from the magnetic fields, but had limited success because the unit was located in a box which made the blood pressure graphs unreadable.

3.5 Other systems in which a similar problem exists

This problem would occur in any electrical device that you would want to use during an MRI such as and EKG. However to date there have be minimal advances in making current electrical devices compatible with MRI machines.

3.6 Other problems to be solved

It is not necessary to completly redesign a new system to fix the problem. What is needed is simply to somehow move all the electrical components away from the MRI scanner (a distance of 5 meters) so there will be minimal or no interference from the magnetic field. If the electrical components can not be moved to this distance then another solution may be to shield these components from the magnetic field to minimize the amount of distortion.

4. Ideal vision of solution

The magnetic fields of the MRI scanner do not interfere with the signal needed by the processing unit to display an accurate reading of the subject's blood pressure.

5. Available resources

Substance Resources:

Rubber Casing

LEDs

Photodetector

Coax Cables

Inflatable Plastic Bag

Servo Pump

Plastic Tubing

Velcro

Carbon Fiber Tubing

Fiber Optic Cables

Field Resources:

Electrical Energy

Light Energy

Magnetic Energy

Space Resources:

Size of the finger

Size of the LEDs and Photodetector

Time Resources:

Length of the MRI experiment

Informational Resources:

Fields irradiated from the system: noise, magnetic, light

Materials Emitted from the system: Air pressure

Characteristics of the System:

6. Allowable changes to the system

Small Changes in the system are allowed in order to keep functionality.

The LED wavelenths should not change because they have been optimized for light transmission through the finger. The pressure that is needed from the Servo pump can not be changed. The length of the tubing from the pump to the cuff can be changed but is limited by the maximum pressure output of the pump. The transmission of the light signal from the photodetecor and to the LEDs can be changed from electrical to optical.

7. Criteria for selecting solution concepts

The output blood pressure reading must not be distorted.

The MRI images must not be distorted.

The output of the blood pressure must be continuous.

The cuff must be comfortable for the patient during the lengthy MRI sessions.

8. Company business environment

The financial funding for the project will come from Dr. Deidrich and also from Dr. King and the grant money that has been set aside specifically for senior projects. The human resources that are available are the three project group members: Jose Alvarado, Ben Huh, Sanjeet Rangarajan. We also have three advisors: Dr. Andre Deidrich, Dr. John Gore, and Dr. Richard Shiavi. We also have use of the Clinical Research Center at the Vanderbilt University Medical Center with will provide us with the space and machines to do the design.

9. Project data

Project Name: Group 15 of BME Senior Design at Vanderbilt University

Project Objectives:

Design a new cuff for non-invasive blood pressure measuring that will be compatible with MRI studies.

Project Timeline:

Due date of April 30th.

Project Team:

Jose Alvarado

Ben Huh

Sanjeet Rangarajan

Project Advisors:

Dr. Andre Deidrich

Dr. John Gore

Dr. Richard Shiavi

Contact Info:

jose.e.alvarado@vanderbilt.edu

615.491-6104

Problem Formulation

1. Build the Diagram

[pic]

2. Directions for Innovation

1. Find a way to eliminate, reduce, or prevent [the] (can't measure blood pressure) under the conditions of [the] (interferes with electrical components).

2. Find a way to eliminate, reduce, or prevent [the] (magnetic fields) in order to avoid [the] (interferes with electrical components), under the conditions of [the] (MRI machine), then think how to provide [the] (creates images).

3. Try to resolve the following contradiction: The harmful factor [the] (magnetic fields) should not exist in order to avoid [the] (interferes with electrical components), and should be in place in order to provide or enhance [the] (creates images).

4. Find an alternative way to obtain [the] (MRI machine) that does not cause [the] (magnetic fields).

5. Try to resolve the following contradiction: The useful factor [the] (MRI machine) should be in place in order to fulfill useful purpose and should not exist in order to avoid [the] (magnetic fields).

6. Find a way to eliminate, reduce, or prevent [the] (interferes with electrical components) in order to avoid [the] (can't measure blood pressure) and (inaccurate blood pressure measurements), under the conditions of [the] (magnetic fields).

7. Find an alternative way to obtain [the] (creates images) that offers the following: does not require [the] (magnetic fields), is not influenced by [the] (interferes with electrical components).

8. Consider transitioning to the next generation of the system that will provide [the] (creates images) in a more effective way and/or will be free of existing problems.

9. Find a way to eliminate, reduce, or prevent [the] (inaccurate blood pressure measurements) under the conditions of [the] (interferes with electrical components).

Prioritize Directions

1. Directions selected for further consideration

First Priority

Utilize stiffer tubing in order to keep the air pressure to an acceptable level

Long Term

Determine whether to use carbon fiber or fiber optics in place of the electrical components and then to replace the electrical components with these materials

Out of Scope

Creating a new monitor for blood pressure measurement that is highly shielded

2. List and categorize all preliminary ideas

PNEUMATIC PUMP

-Test tubing of different radii and stiffness in order to determine which might be better for the pneumatic pump

-Research the possibility of using a stiff unbendable tube

-Lengthen the tube used

ELECTRICAL COMPONENT REPLACEMENT

-Research the use of fiber optic cables

-Research the use of carbon fiber cables

-Increase the length of the cables to at least 5 meters away

ALTERNATIVE POSSIBILITIES

-Research shielding possibilities

Appendix B – Designsafe Report

Appendix C – Finger Cuff Schematic

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Figure 5: Diagram showing the basic design of the hydraulic system and it connections to the finger cuff and Finometer

Figure 6: First Hydraulic Prototype

Figure 7: Second Hydraulic System Prototype.

Figure 8: Third Hydraulic Prototype

Figure 9: Fourth Hydraulic Prototype

Figure 10: Fifth Hydraulic Prototype

Figure 11: Finometer graph from the final hydraulic system showing completion of the servo start-up self adjustment, but failure to make rapid adjustments due to slow response time.

Figure 3: Diagram of the 3.0T fMRI scanner at Vanderbilt.

Figure 1: Blood Pressure Waveform from the Finometer Display

Figure 2: Finometer Device

Figure 4: Design diagram reported by Innovation Workbench

Figure 12: Finger Cuff LED

Figure 13: Proposed Optical Transmission System

Figure 14: Second Prototype

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