Head Coil-Mounted Vision Correction Device for Magnetic Resonance Imaging

This paper introduces a 3D-printable, MRI-compatible head coil-mounted device designed to economically and ergonomically correct spherical refractive errors during fMRI visual experiments by overcoming the limitations of previous goggles and coil-mounted designs through features like easy lens switching and adjustable inter-pupillary distance.

The Gist

Imagine you are trying to take a high-definition photo of your own brain while lying inside a giant, noisy donut (that's an MRI machine). But there's a catch: you can't wear your regular glasses because the metal in the frames would turn into a projectile in the magnetic field. And if you wear contact lenses, well, not everyone can handle them.

For years, scientists have had to strap bulky goggles to people's faces or use fixed lenses on the MRI machine itself. But these solutions have problems: the goggles feel like a vice grip on your head, and the fixed lenses don't account for the fact that everyone's eyes are spaced differently apart.

Enter the "Head Coil Glasses."

This paper introduces a clever, 3D-printed device that solves these headaches. Think of it as a custom-built, adjustable eyeglass frame that snaps directly onto the MRI machine's headrest, rather than onto your face.

Here is the breakdown of how it works, using some everyday analogies:

1. The Problem: The "One-Size-Fits-None" Dilemma

• The Goggles: Imagine wearing swim goggles that are too tight. After an hour, your face hurts. That's what happens with traditional MRI goggles during long brain scans.
• The Old Machine-Mounted Lenses: Imagine a pair of glasses where the distance between the lenses is glued in place. If you have wide-set eyes, the lenses are too close together; if you have narrow-set eyes, they are too far apart. It's like trying to wear someone else's glasses that are permanently fused to the bridge of your nose.

2. The Solution: The "Lego" Lens Holder

The authors built a device that acts like a universal adapter for the MRI machine.

• The Mount: It's a plastic bracket that snaps securely onto the front of the MRI head coil (the part that holds your head). It uses the machine's own "eye slots" to lock into place, like a key in a lock.
• The Adjustable Lenses: This is the magic part. The device has two separate holders for the lenses. You can slide them left or right, just like adjusting the nose pads on a pair of reading glasses. This allows the lenses to match the exact distance between your pupils (Inter-Pupillary Distance), ensuring the vision correction is sharp and accurate.
• The "Flip-Top" Design: Each lens holder has a hinged lid. You don't have to unscrew anything to change the lenses. You just lift the lid, drop in a new lens (like swapping a battery), and snap it shut.

3. How It's Made: The "LEGO" of Science

• 3D Printing: The entire device is printed using a standard 3D printer and a plastic called PLA (the same kind used for biodegradable cutlery). It's cheap (about $25 to make) and safe for the MRI environment because it contains no metal.
• The "Gel Bumpers": To stop the device from rattling or vibrating during the loud scanning process (which sounds like a jackhammer), the designers added little gel dots to the bottom. Think of these as the shock absorbers on a car, keeping the device steady and quiet.

4. How to Use It: The "Pre-Game" Routine

Before you even go into the MRI room:

1. Test Your Eyes: A technician checks your vision with a standard eye chart and picks the right lenses.
2. Snap and Slide: They pop the lenses into the device and slide them until they feel perfect for your eyes.
3. Lock It Down: They tighten the screws just enough to hold the position.
4. Scan Time: The whole assembly is placed on the MRI head coil. The participant lies down, and the lenses are right in front of their eyes, perfectly aligned, with nothing touching their face.

Why Does This Matter?

This device is like giving the MRI machine a customizable, comfortable, and affordable pair of glasses.

• Comfort: No more face-strap pressure.
• Precision: It fixes vision errors perfectly for your specific face shape, not a generic average.
• Accessibility: Because it's 3D printed and open-source (the plans are free online), any hospital can make one for a fraction of the cost of buying a commercial system.

In short, this paper describes a simple, smart hack that makes brain scans more comfortable and accurate for people who need to see clearly, proving that sometimes the best medical tech doesn't need to be high-tech—it just needs to be well-designed.

Technical Summary

1. Problem Statement

Functional Magnetic Resonance Imaging (fMRI) experiments involving visual stimuli require participants to have corrected vision to ensure data accuracy. However, standard vision correction methods present significant challenges in the MRI environment:

• Personal Contact Lenses: While ideal for comfort and accuracy, not all participants can wear them or have access to them.
• Head-Mounted Goggles: These often cause ergonomic discomfort due to elastic straps pressing against the head during long scans. Furthermore, in participants with larger heads, the goggles can press against the anterior element of the head coil, causing facial pressure.
• Existing Coil-Mounted Devices: While these avoid contact with the participant's face, current commercial and open-source designs (e.g., practiCal fMRI or Cambridge Research Systems) suffer from a critical limitation: they lack Inter-Pupillary Distance (IPD) adjustment. Fixed lens positions reduce correction accuracy for participants with varying eye spacing. Additionally, many existing designs do not support cylindrical (astigmatic) corrections or higher-order errors.

2. Methodology

The authors developed a custom, 3D-printable vision correction device designed to mount directly onto the anterior element of a commercial head coil. The development process involved:

• Design Software: The device was modeled using SolidWorks Education Edition 2023.
• Geometry Acquisition: To ensure a precise fit for the widely used Siemens 32-channel head coil, the authors imported orthographic photographs of the coil into SolidWorks using the "Sketch Picture Tool" to trace the geometry directly.
• Mechanical Design:
  • Head Coil Mount: Features large vertical slots and protruding guide features to lock securely into the coil's eye slots without contacting the participant.
  • Lens Holders: Designed with a hinged cover for easy lens insertion/removal. Three versions accommodate different lens thicknesses (corresponding to diopter strength).
  • IPD Adjustment: The core innovation is the ability to slide lens holders horizontally along the mount, allowing for independent adjustment of the inter-pupillary distance.
  • Vibration Damping: Gel dots were added to the design to minimize acoustic vibration effects during scanning.
• Fabrication:
  • Material: Printed using Polylactic Acid (PLA), an MRI-compatible plastic.
  • Hardware: Fasteners (nuts, bolts, washers) are made of non-metallic materials (nylon or 3D printed) to ensure MRI safety.
  • Printing Parameters: Printed on a Bambu Lab X1 Carbon with 0.08mm–0.20mm layer heights, 5 wall loops, and 30% cross-hatch infill for durability.
• Assembly: The device is assembled by attaching gel bumpers to the mount, inserting bolts through the mount and lens holders, and loosely tightening them to allow for IPD adjustment before final tightening.

3. Key Contributions

• IPD Adjustability: Unlike prior coil-mounted solutions, this device allows for continuous horizontal adjustment of lens holders to match the specific IPD of the participant, significantly improving visual correction accuracy.
• Ergonomic Improvement: By mounting on the coil rather than the head, it eliminates strap discomfort and facial pressure issues associated with goggles.
• Customizability & Modularity: The design is modular, supporting various lens geometries and thicknesses. It can be adapted for other head coil manufacturers (e.g., GE, Philips) by modifying the mount geometry.
• Cost-Effectiveness: The total hardware cost is approximately $25, making it highly accessible compared to commercial alternatives.
• Open Source: All CAD files, STL files, and assembly instructions are released under a CC BY-NC-SA 4.0 license on GitHub.

4. Results and Specifications

• Compatibility: Successfully designed and tested for the Siemens 32-channel head coil, with adaptability for other systems.
• Performance: The device supports spherical corrections from -5 to +5 diopters and, crucially, allows for the integration of cylindrical (astigmatic) lenses, which previous coil-mounted devices could not accommodate.
• Visual Acuity: When properly adjusted, the system achieves corrected visual acuity of 20/30 to 20/20, meeting standard experimental requirements.
• Artifact Mitigation: The design includes features to reduce lens reflections (recommending anti-reflective coatings) and acoustic vibrations (gel dots).

5. Significance

This work addresses a critical gap in fMRI methodology by providing a low-cost, open-source solution that balances participant comfort with optical precision. By enabling IPD adjustment and astigmatic correction in a coil-mounted form factor, the device expands the pool of eligible participants for visual fMRI studies (including those who cannot wear contacts or tolerate goggles) and improves the quality of visual data collected. The open-source nature of the project encourages the broader scientific community to adapt the design for different hardware configurations and experimental needs, such as occluding vision in specific eyes for binocular rivalry studies.