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MRI Fine Positioning Mouse Cradle

Overview

Optogenetics is the combination of genetics and optics to control specific event in targeted cells of living tissue in mammals. This is applied through the insertion of light-sensitive genes into certain cells and delivering light through fiber-optic tools as seen in Fig. 1. Functional Magnetic Resonance Imaging (fMRI) is a non-invasive imaging method that measures brain activity by detecting the change in magnetization between oxygen-rich and oxygen-poor blood cells, which relates to energy used by brain cells. At University of California San Diego, Dr. Anna Devor and the Neurovascular Imaging Laboratory, hopes to further the understanding of optogenetics by combining these two methods to examine the effect of optical stimulus on brain activity of mice. This is achieved by taking fMRI images of the mouse’s brain at the exact moment that a light from an optical fiber placed near the mouse’s head is triggered.

Source: Scientific American. <http://www.nature.com/scientificamerican/journal/v303/n5/box/scientificamerican1110-48_BX1.html>

Our sponsor performs MRI experiments using the Bruker 7T Biospec fMRI machine on genetically modified mice where neuronal firing can be controlled by light to investigate their brain activity. Originally, these experiments were performed on anesthetized mice so they could be left immobile on a hard surface during the experiment. Three years ago, another MAE 156B team created a design for that purpose, featuring a bite bar and ear pin assembly, RF coil assembly, and optic fiber assembly. However, the sponsor wishes to start experimenting on awake mice, who need a more comfortable surface to lie on to stay still. A prototype used for experiments involving photomicroscopy was created that could comfortably support and position an awake mouse, but since the assembly was made of metal, it could not be used in the MRI scanner. Thus, the goal of this project is to design, build, and test a MRI-compatible device that would allow adjustable positioning of the awake mouse, as well as other components that will be used in the experiment.

Previous Design for Anesthetized Mice

Photomicroscopy Design for Awake Mice

Objective

The primary objective of this project is to design and fabricate a MRI compatible assembly small enough to fit inside the 112 diameter bore of a Bruker 7T Biospec fMRI machine. This assembly will consist of multiple parts that each require independent adjustability to allow for optimal positioning of the mice, and be enabled for the use for all shapes and sizes.

Primary Requirements:

- Design and manufacture a MRI compatible assembly that will fit within bore dimensions, including:
- An adjustable positioning assembly for stabilizing the head such that the top will be in the isocenter of the MRI machine (~1mm below the center of the bore)
- An adjustable hammock assembly to support the mouse body in a comfortable position
- An adjustable mechanical assembly to position an optical fiber for optogenetic applications
- An adjustable mechanical assembly for a glass tube to deliver air puffs to the mouse whiskers on one side
- An adjustable tube assembly to feed sugar water to the mouse every 15 minutes while in the fMRI scanner

Secondary Requirements:

- Incorporate a third support ring and rubber ring proposed by previous team for better support of the base and further minimize vibration effects from the MRI machine
- Include a radio frequency (RF) coil mount

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. 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 positions two RF coils concentrically above the mouse's head with the bottom coil located at the center of the bore. It also has a rack and pinion setup operated by an adjustment shaft to provide the ability for Z-axis adjustment of the top coil in millimeter increments up to 1 cm from outside the fMRI machine. Since the extending distance of the rod is expected to be 775 mm, a shaft support ring is used to support the rod, with a shaft support closer to the mount for extra support. In addition, the remote adjustment shaft is easily detachable for transportation and storage.

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 shown earlier where the hammock slips onto the two extended arms. 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 Results

The final product was able to fit in the mock bore replica of the fMRI machine, which we used as our test bed. 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 summary of the observed tolerance of some parts compared to the dimensions of its CAD model counterpart. 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.

Observed Tolerance Comparison

Horizontal Slider

Side Tower

Track

Third Ring

Part