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Spring 2015 MAE 156B Project 7
Spring 2015 MAE 156B Project 7
MRI Test Sample Positioning System
Sponsored by Robert Bussell of the CFMRI
BACKGROUND
The Center for Functional MRI at the University of California, San Diego houses three MRI systems: two 3 tesla GE Discovery MR 750 short-bore scanners for human studies; and one 7 tesla Bruker Biospec 70/20 USR scanner for imaging small animals. To image a sample, MRI scanners produce a rapidly changing magnetic field which aligns the hydrogen nuclei in the sample body. The hydrogen nuclei, after absorbing energy from an applied radio frequency (RF) pulse,
Functional Requirements:
Retain existing system's positioning tolerance of ±0.5 mm
Repeatable positioning within desired tolerance range
Little to no noise or interference with MRI scanning capabilities
Mobile stand for supporting positioning system next to machine
Ability to level positioning system with respect to scanner bore
Tooling plate's vertical position can be adjusted 150 mm with 5 mm accuracy in increments decided on by the different system tooling
Cable management solution to prevent pinching of wires or tubes
Controller inside of the scanner room for adjusting sample position
Deliverables:
In-room positioning controller system
Three buttons: Set landmark, Move to isocenter, Move to landmark
Mobile positioning system stand with leveling ability and 150 mm of vertical height adjustment
3 degrees of freedom: roll, pitch and vertical (z) adjustment
FINAL DESIGN
FINAL DESIGN
OBJECTIVE
create a brief, faint electromagnetic field signals that the RF coils in the MRI system detect. Samples are positioned at the bore’s isocenter, where the strictly homogeneous magnetic field creates the optimal conditions for producing high-resolution images.
Our sponsor, Robert Bussell, is an associate development engineer at the CFMRI who specializes in the 7 Tesla Bruker Biospec scanner. The current positioning system used at the center comprises of tooling manually positioned along a dovetail shaped guide. Our goal is to modify an automated positioning system built by a team last year by adding a position controller in the scanner room and by building an adjustable stand that is compatible with the dimensions of the Bruker Biospec scanner.
There are two main components to the final system design: the in-room position controller, and the positioning system stand.
Positioning System Stand
The 150 mm (6 inches) of vertical travel was accomplished through the design and fabrication of a traveling-nut linear actuator. A large lead screw was machined on a lathe from an aluminum rod. To accomplish the 150 mm of travel, ACME threads were cut into the rod at a rate of four threads per inch, a number that was chosen as it would allow the table to be raised the full 150 mm distance in only 24 turns while preserving the ability to make fine adjustments to the height. To turn the lead screw a wheel was fabricated from aluminum stock. This provides the user with a mechanical advantage making it possible to raise and lower the table with very little effort. The lead screw was threaded into a Delrin lead nut which in turn was attached to the table top. Since the lead screw was securely fastened to the table's lower shelf, when the screw is turned, the lead nut travels up and down raising and lowering the table. The Table Lifting Animation below demonstrates how the user can raise and lower the tabletop.
The positioning system stand is designed to support the positioning system, a controller box, and various shelved items. The design was able to minimize the use of magnetic components, with only the casters and the thrust bearing are made of ferromagnetic metals. This results in a system that is strong, durable and compatible with the MRI environment.
Leveling was accomplished through the use of four leveling casters. Extending the casters' leveling pads allows for the height of each corner of the table to be adjusted until the table is level. This allows for the system to be compatible with floors that are sloped or otherwise uneven. When the leveling pads are raised completely off the ground, the nylon wheels allow for the table to be easily rolled across a variety of floor types.
Leveling system and Height adjustment
The leveling of the positioning system is a crucial functional requirement. Without a leveling system in place, the tooling can hit the sides of the bore cavity, damaging the MRI equipment or the samples. The positioning system is leveled by a set of casters with built in leveling pads. The pads and raised and lowered by turning the thumb wheel while the leveling of the system is checked against a bubble level.
The height adjustment system is comprised of a lead nut, lead screw, turning handle, and thrust bearing assembly. The entire lifting mechanism was machined in house due to the availability of non-magnetic components. The lead screw and nut are threaded with ACME threads and a turning handle is attached to to lead screw with three set screws. The assembly sits on a steel thrust bearing which is housed in a cuff that ensures minimal movement of the bearing.
In-Room Position Controller
Interfaces remotely with existing LabVIEW control system
utilizes existing electronics and PCB (printer circuit board) FOSR(fiber optic send/receive) board along with newly designed PCB to communicate from scanner room to control room
push buttons trigger existing controller functionality to maintain existing repeat-ability and accuracy
PERFORMANCE RESULTS
PERFORMANCE RESULTS
Lifting Mechanism
To test the performance of the lead screw/nut lifting system a spring force gauge was used. The image below shows the technique used to test the performance. During the test the lead screw required a torque of 1.2 N·m (0.9 ft·lbf) when the table was unloaded. When the table was loaded with 13.5 kg (30 pounds), the lead screw required a torque of 2.3 N·m (1.7 ft·lbf). The results of lifting the table were qualitatively acceptable with the fact that the table is physically easy to raise. The results of this test show quantitatively that the table requires less force to lift than the worst case analysis done in the report, which was still considered an acceptable required load.
In-Room Position Controller
The final performance test for the controller was performed at the Center for Functional MRI in the actual MRI machine that the system will be used. A test sample, which was a Lego block submersed in a tube of water, was attached to the test sample tooling. The test sample was ran through the process that an actual sample may undergo, such as, be placed at a "Landmark" position, be sent to the "Isocenter" position of the bore, have fine adjustments administered to it, be brought out of the bore for observation and adjustments, and then sent back to the finely adjusted position. The images below show what the test sample looked like and the results of the run. As you can see in figure 11, the top left image is when the sample was first sent to "Isocenter", which is compared to the top right image that is the position after the fine adjustments. The bottom left image is again the finely adjusted position and is compared to the bottom right image, which the finely adjusted position after a repeat-ability run has been completed. The repeat-ability consisted of taking the sample all the way out of the bore and then sent back to the saved fine position.
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