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Two Photon Microscope Support Structure

Ramzi Doudar, Alex Hung, Chase Ogden, Giovanni Rosales

Sponsor: Dr. Matthew Shtrahman

Background

Brain imaging has long been a very involved process to properly execute, generally requiring large and expensive machinery and extensive patient cooperation to obtain a viable image.

Advances in Calcium imaging pioneered by the late UCSD professor Dr. Roger Tsien resulted in the development of more sophisticated brain imaging techniques. This advanced brain imaging is carried out with a two photon microscope which essentially uses a series of lasers and mirrors to obtain a brain image. This technology has been used nearly exclusively in a lab setting with animal test subjects and has rarely been adapted for use in a clinical setting due to patient safety concerns. Recent advances in lasers have rendered the lasers used in two photon microscopy light and small enough to introduce to the operating room. Having a tool such as this in an operating room would be monumental for medicine and research in neurology as it allows the user to get real time brain imaging done with single neuron resolution.

Concept photo illustrating the desired degrees of freedom.

Courtesy of Shtrahman Lab.

Two Photon Microscopy and its Importance

Two Photon Microscopy (TPM) is an imaging technique utilizing the fluorescence of ions to image tissue up to 1 mm in depth. Some sort of indicator such as OG488 BAPTA-1, an organic molecule that fluoresces under the right circumstances, is introduced to the tissue of interest. When an ion (in this case Ca2+) is subsequently introduced, the ion binds to the indicator and the result of this reaction is a green fluorescence that allows the relevant cells to be imaged. TPM captures the desired images by exciting the necessary molecules through photons. The excitation of these fluorescent molecules results in the emission of a fluorescent photon, which is how the image is able to be captured.

TPM differs from more common clinical imaging techniques such as MRIs, PETs, and CTs in the sense that it offers a more localized image at a much higher resolution. The increased resolution is attractive to medical and research personnel since TPM is capable of resolving images of individual neurons. Having this technology in the arsenal of medical tools would assist medical and research professionals in the diagnosis and treatment of brain diseases as well as advancing the study of human brain function.

Objectives

Our goal is to create a supporting structure that can make this brain-imaging laser easily maneuverable in the operating room. This brings on several challenging project parameters:

  • Support the 25 kg two photon microscope.

  • 4 degree of freedom (DoF) motion (see above image).

    • Requirements: x,y translation and rotation.

  • Lock the DoFs into place.

  • Isolate vibrations to ensure a clear image is captured.

  • Engineered fail safe safety features.

Final Design Solution

This video illustrates the degrees of freedom of the support structure.

CAD renderings and a photo of the final design solution can be seen below. This design makes use of low friction linear slides to seamlessly traverse the hefty microscope approximately 45 cm in the x & y directions. The two degree of freedom gimbal, dubbed the "double rotisserie", provides the desired roll and pitch DoFs. The double rotisserie was deliberately designed such that both rotary axes pass through the center of mass of the two photon microscope assembly. This made for a naturally balanced tilting mechanism, which allows the appropriate personnel to easily tilt the microscope into the desired position and lock it into place without the need for an external counterbalancing mechanism. Considering the very fine resolution these microscopes are capable of, the team elected to utilize four Newport vibration isolators to isolate external vibrations and ensure that clear images could be captured. Without some sort of vibration isolation method, vibrations transmitted to the microscope from disturbances such as footsteps, mechanical building equipment, and vehicular traffic have the potential to produce a shaky image. Each wheeled base features two 15.24 cm (6") pneumatic swivel casters for easy transportation. Using the large pneumatic casters allows for easy transportation and minimizes the chances of getting stuck in small gaps or on power cables.

Trimetric view CAD rendering highlighting major components Front view CAD rendering highlighting major components

Considering the inherent safety risks this design poses during typical use, care was taken to implement a fail safe safety mechanism to virtually eliminate a catastrophic event due to operator misuse. Operator misuse was deemed highest risk at the double rotisserie, where failure to lock the mechanism could result in the double rotisserie tipping over, causing a direct impact with the patient's brain. A mechanical collar lock was implemented but this was not enough as the operator could easily forget to engage the lock. The electromagnetic brake seen in the front view was implemented in such a way that it restricts rotation of the double rotisserie until the operator presses a button. When the operator releases the button the rotisserie is automatically locked without the need for further operator intervention.