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The project entails redesigning a component of a MRI-compatible small animal ventilator system using a stepper-motor controlled piston. The design will deliver small volumes of an oxygen and hyperpolarized helium mix in a study using Helium imaging.
This project was continued from last semester. To see its website, please click Animal Ventilator Spring 2006.
From left to right:
Matt Smith (Biomedical Web Implimenting Group Representative)
Micah Brown (Biomedical Student Advisory Committee Representative)
Ashley Anderson (Communications)
Chris Wegener (Leader)
We have worked hard over the summer to calibrate our prototype. We analyzed the data from experiments to find an equation to define the output given the input. Having trouble with this lead to engineering issues with the valve timing, which we took on as one of our goals this semester. We have also come up with three possible designs for an new improved model which we plan to start building within a few weeks. Pictures of these ideas can be seen below.
Background
Magnetic Resonance Imaging (MRI) is commonly done by tuning the machine to read information from hydrogen in the body. Recently, a new process in MR imaging has emerged as an excellent way to image airways. Instead of tuning the machine to hydrogen, it is tuned to helium. Therefore, if the patient inhales a breath of helium, and a scan is done, the resulting image will be of their airways, both brachii and lungs. Below are two examples of the image quality achieved with He MR imaging.
(Left) Image acquired using a three-dimensional (3D) imaging sequence. The spatial resolution is exemplified by the surface-rendered volume of the 3D data. The ribs and tracheal rings are visible, as is a ventilation defect in the left lung (arrow). (Right) 3He (hyperpolarized Helium) MRI in a healthy volunteer. Branching to the fifth generation is visible. Recall, these are not blood vessels, these are airways that contain 3He at the time of the scan. These visible "bronchioles" reveal the shape and efficiency of the lungs.
Past Setup Used by Client
Below are pictures of the past setup. The stepper motor allows hyperpolarized air to be drawn into the syringe from a holding bag. The air is then injected into the gas tubes leading to the small animal. The setup with the stepper motor is unnecessarily too big. Our goal is to create a smaller motor driven syringe gas delivery system that mixes Oxygen and 3He at the precise time it is inhaled by the small animal. This will cut down scan time from 8 min to possibly 2 min. (place cursor over picture for caption)
Current Prototype Designed by Our Team
The following pictures represent the prototype that we have built to satisfy the clients needs.
Summer Work
We performed trials over a range of motor steps, from 250-400 steps. We attached the output of the device to one end of a manometer (tubing and a meter stick) and recorded the beginning water level. After one "breath" or movement of the specified number of steps, we recorded the end water level. Calculating the difference in water level and dividing by the number of steps gave us "inches/step". We first converted inches to mL volume by using the cross sectional area of the inner diameter of the tube and the displacement height of the water (calculated by hand to be 0.201101 mL/inch). Inverting this ratio we got our desired step/mL. We then calculated the mL output given by the specified number of steps.
Problems that Occurred:
When we found this equation that described the output with a given input, and entered it into the appropriate vi file, we then attempted to verify that indeed a desired output volume was actually outputed. However, this was not the case. Everytime we performed a verification trial, the output was consistently off by no more than .3 mL. Although precise, the actual output is not yet accurate. Other trials were done, and equations tried, with no better accuracy. Further tests will need to be done to find a more accurate calibration equation. The calibration procedure may need to be modified to achieve better accuracy. Also, a new model with significant structural and mechanical changes may significantly improve our results.
This is where the semester began...
This semester we focused on two main goals: 1) fix valve timing and 2) develop new improved model.
Goal 1: Valve Timing
During testing this summer, we used a monometer to watch and measure the output volume of gas that the device produced given a specified input. While we were watching the water level breath after breath, we noticed that every fourth breath (when the software switched to our device for the Helium breath) there seemed to be an overshoot of gas. We spent hours trying to pinpoint why this error occurred. It turned out that our device was performing as expected but the pneumatic valves that our client was using were at fault. After determining the order of valve firing, opening and closing, we diagnosed the problem occurs in the software. One of the valves opens too early and causes an "oxygen chaser" as we’ve learned to call it. This oxygen chaser increases the output every fourth breath. This was a problem that has been occurring before our group began the project. Our client was pleased that we found and diagnosed this problem, something he didn’t even know was present.
The valves function off separate counters in LabView, so it was a matter of changing the timing of the counter so that the valve will open at the appropriate time.
Goal 2: New Improved Model
There were several improvements we wanted to make from the original prototype. These will be highlighted in reference to the Solidworks pictures we used to aid in the designing process.
Size
The original model had 5 inches of travel for the sliders, and thus for the syringe plunger as well. However, the syringes currently being used in the study have only a 3 inch travel space for the plunger, so we decreased the size of the overall design since the length was not needed. The sliders and aluminum rods are also smaller because the extra size of the original was not needed.
Syringe Placement
Since our client wishes to exchange the large syringes out for small ones when imaging mice, we needed a better, more accessible position for the syringes. We decided to position the syringes off to the side of the sliders (see pictures below). This gives more accessibility and lead to an improvement of syringe attachment.
Syringe Attachment
Our original mechanism of attaching the slider motion to the syringe plunger was beginning to show signs of wear and tear. Therefore we wanted to focus on a new method of attaching the syringes to the sliders. We designed a plate that will be attached to the back of the sliders and will pinch the back of the syringe’s plunger (see picture below). Screws will maintain the force needed to maintain the connection.
Below are more pictures of the new improved model we’ve designed this semester:
| Week | Reporting Period Beginning | Activities |
|---|---|---|
| 1 | September 8 | Decide roles of new semester, plan semester’s goals |
| 2 | September 15 | Finish first device calibration |
| 3 | September 22 | Research and design development of second gen. device |
| 4 | September 29 | Give proposed designs to client for his decision |
| 5 | October 6 | Finalization of new design |
| 6 | October 13 | Diagnose valve timing issue and what needs to be done |
| 7 | October 20 | Improve new prototype design |
| 8 | October 27 | Midsemester presentation |
| 9 | November 3 | Gather parts list and finalize Solidworks design for machining |
| 10 | November 10 | Begin 2nd prototype contruction |
| 11 | November 17 | Prototype contruction |
| 12 | December 1 | Collect any relevant calibration data if construction is done |
| 13 | December 8 | Start poster, gather data and pictures |
| 14 | December 15 | Create poster and finalize website and paper |
| 15 | December 22 | Final presentation |
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Final Paper (last semester) (Oct 26 2006, 305 kb) |
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Product Design Specification (Oct 27 2006, 13 kb) |
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Midsemester Presentation (Oct 27 2006, 5379 kb) |
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Final Poster (.pdf) (Dec 10 2006, 1443 kb) |
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Final Paper (Fall 2006) (Dec 13 2006, 183 kb) |
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SolidWorks Animation of Design Mechanisms (.avi) (Dec 13 2006, 4602 kb) |
