Final Design

The final assembly consists of five main components: the RF coil assembly, hammock, head positioning, fiber optic positioning, and air puff/feed tube delivery system. Most of the components slide in the X-axis along a T-shaped track. In addition, there is a third support ring for the larger bore diameter near the edge of the fMRI machine. The adjustable parts are held in place using 6-32 Nylon Knurled Thumb Screws. All 3D printed parts were printed out of polycarbonate at Stratasys Direct Manufacturing.

Radio frequency (RF) Coil Assembly

RF Coil Assembly

RF Coil Mount

The RF coil assembly consists of 4 main components: the RF coil mount, the adjustment shaft, the shaft support, and the shaft support ring. The mount is designed to position two RF coils concentrically above the mouse's head with the bottom coil located at the center of the bore. The remote adjustability of the top RF coil is achieved through a rack and pinion set up which operates through a 12.7 mm (0.5 in.) diameter polycarbonate rod that extended out 775 mm (30.5 in.) to the back of the MRI machine. In addition, the remote adjustment shaft is easily detachable for transportation and storage. Since this is a long length, a solid foam support ring is used. A shaft support closer to the shaft coupling, which connects the gear with the rod, was also created for extra support.

Hammock Assembly

Hammock Assembly

The hammock assembly is used for the body positioning of the mice. It implements a similar design to the prototype used for photomicroscopy where the hammock slips onto the two extended prongs. One change was the ability to rotate the arms, which allows for width adjustability for varying mice sizes. Two slots on the tower is used for vertical adjustment with a square slot in between to prevent misalignment during adjustment.

Head Positioning Assembly

Head Positioning Assembly

The head positioning assembly is used to position and fix the mice's head at the isocenter of the bore, or right under the lower RF coil. It is also similar to the previous design used for photomicroscopy, except T-slots legs were created for adjustment in the X-axis to assimilate it for this cradle. There is a total of four degrees of freedom to ensure the mouse’s head is positioned as desired.

Fiber Optic Assembly

Fiber Optic Assembly

This assembly is used to position the optic fiber directly above the mouse's head, through the middle of the RF coils. The T-shaped railing design is used for the X-direction movement. The lead screw is for the Z-direction movement due to its high precision adjustability and ease of use. Since the optic fiber is less than 1 mm thick in diameter, it can be clamped between two plates and with the Y-direction position hand adjusted. A slot design is implemented for the secondary support tower, which allows the movement of the horizontal bar without the cocking issue that would appear with two lead screw support towers.

Air Puff/Feed Tube Assembly

Air Puff/Feed Tube Assembly

Air Puff Assembly

Feed Tube Assembly

The air puff assembly is used to deliver an air puff to the back of the whiskers of the mice. The clamp is angled at 45 deg. to make sure that it will hit the whiskers rather than the ear. The feed tube assembly is used to deliver sugar water to the mice, with a clamp for small plastic tube that runs to the outside of the machine. This will allow the experimenter to pump the sugar water to the mice every 15 minutes as a reward for staying still during the experiment. The horizontal slider is also angled downward, which provided more Z-axis adjustment room for the feed tube clamp since it was situated under the lower RF coil. The tower slides have a T-slot railing to allow movement in the X-direction. Rectangular slots are used for Y-direction adjustment for the feed tube clamp, and Z-direction adjustment for both the feed tube clamp and the air puff clamp.

Third Support Ring

Third Support Ring

The third support ring is used to better support the base and further minimize the vibration effects from the fMRI machine, as suggested by the team that created the previous design for the anesthetized mice. The ring design allows a inflatable rubber ring to fit on it, which is similar to what is used for the two smaller support rings. Since the ring sits closer to the edge of the fMRI machine, it has to fit the larger diameter bore of 190 mm.

Performance

The final product was able to fit in the mock bore replica of the fMRI machine, which we used as our test bed. However, our main concerns were focused on the tolerance of the Stratasys Dimension 1200es 3D printing machine we used for the prototypes and the clearance required when designing a part to offset the tolerance effect. Below is a table of the observed tolerance of some parts compared to the dimensions of their CAD model counterparts.

Observed Tolerance Comparison

It may appear at first glance that the Dimension 1200es machine did not perform to its claimed tolerance of 0.01” but this was not exactly the case as revealed by closer analysis. In general, the machine added material to each surface due to the thickness of each filament layer. This additional material could be as thick as 0.01”(the tolerance of the machine) but when it was added to two inner surfaces of a dimension, the error would propagate, and it could cause that dimension to be well outside the expected tolerance. As a result, the dimensions of exterior parts such as the side tower, were slightly larger than expected due to added layer thickness. Similarly, inner dimensions such as those on the horizontal slider, were smaller than their CAD models. This caused very tight fits or impossible assembly in some cases on the first 3D printing iteration. By the 2nd and 3rd iterations however, the pattern described above was consistently observed and verified, which allowed optimal clearances to be made in the CAD models for future parts with sliding features. Although the final parts are not be printed with this printer, the printer used by Stratasys Direct Manufacturing is very similar and has the same tolerance rating.