|Figure 1 - MRI-safe goggles with prescription lenses|
Using them with the 32 channel head coil however, is considerably more difficult to do. It is already hard enough to fit a subject comfortably inside the coil with support padding, so attempting to include the goggles as well is simply not an option. To solve this problem we designed and machined lens holders (Figures 2 and 3) that fit inside the eye sockets of the 32 channel coil. They're easy to set up and the fit is snug so that they don't move. Sometimes we use a velcro strap to keep them together but it's not required.
|Figure 2 - 32 ch coil lens holder with velcro strap|
|Figure 3 - lens holders inserted into coil eye sockets|
Problem solved, right? Ah but what if these holders break? What if other researchers have this same issue but don't have a mechanical engineer on staff who can design and machine pieces like these? The best solution to both of these scenarios (and many more!) is to create 3D CAD models and make them available for anyone to download. So let's get started!
Recreating Parts in Tinkercad and BlenderTinkercad is a free, easy-to-use 3D CAD design tool that works in your browser and uses Boolean addition and subtraction of simple shapes to allow the creation of more elaborate models. I have to be honest, I am more comfortable with Blender, which is also free, but Tinkercad is far easier to pick up and ideal for (relatively) simple designs like these holders. I had zero experience with Tinkercad beforehand but doing their tutorials for about an hour familiarized me enough with their UI to complete the project. Using calipers to get as accurate as possible, I measured the various dimensions of one of the holders and after a few hours I came up with the following design:
|Figure 4 - Lens holder in Tinkercad|
|Figure 5 - Beveling in Blender|
After beveling, the final step in this section is to duplicate the part and mirror it about the X axis to create the second holder.
|Figure 6 - Duplicating and mirroring in Blender|
The hardest part of this project is done, though we can always make changes should the need arise (See Note #3). Now it's time to manufacture the pieces.
3D printing the HoldersIt's possible to take drawings to a machinist and have them create the parts out of nylon or some other plastic but if you've got a 3D printer handy than the best/easiest solution is to print them out yourself in just a few hours. I even printed a few versions using different material to see which one suited our needs best.
|Figure 7 - Holders printed using PLA|
|Figure 8 - Holders printed using PETG|
|Figure 9 - Holders printed using NylonX|
Although it may look like all I did was print the same thing in three different colors, there's more than meets the eye! Figures 7, 8 and 9 are images of completed lens holders printed using PLA, PETG and NylonX, respectively (See Note #4). Printing the pieces in NylonX was the best solution because of its durability and flexibility, allowing for the holders to be crammed into the eye sockets and thicker lenses to be placed inside them without fear of breakage. Below are images of a thin lens and a thick lens held securely inside the holders (See Note #5).
|Figure 10 - Top view of lens in NylonX holders|
|Figure 11 - Side view of lens with different thicknesses|
Notes#1 The merchant we purchased the interchangeable MRI-safe goggles from has since gone out of business, but a quick Google search returns other vendors with similar products.
#2 Technically you CAN create a beveling effect using some tricks but it is far more trouble than it's worth.
#3 It's easy enough to go back and alter the dimensions of the piece if need be -- that's what I did! The design was revised several times before I found the best fit. You may have to do this as well depending on the shape of the lenses you are using, just make sure you edit the areas that hold the lens and not the ones that secure the holder to the eye socket.
#4 The only 3D printer setting worth changing from whatever default you have is upping the infill to 100%. As with any print that gets handled quite often, you'll want them to be dense and sturdy so it can take a bit of abuse.
#5 If you're still having trouble with fitting thicker lenses then increase the height of the small square in the middle of the Tinkercad piece by a few millimeters until you find a value that works