Perhaps the biggest constraint in modern neurosurgery is the need to see inside the brain during an operation. An X-ray fluoroscope mounted on C-arm can be wheeled in for quick checkups, but they expose the patient to significant radiation, and don’t give the full picture. What surgeons really need is an MRI on wheels. Faced with the impossibility of bringing a machine into the operating room that transforms gas cylinders into missiles, researchers at Vanderbilt are now proposing to do the only other thing left — bring the surgery into the MRI.
Vanderbilt engineers seem to have a way with fluid power. A few years ago they built a rocket-powered exoskeleton arm that was actuated by the explosive decomposition of hydrogen peroxide. As one might imagine, neither Home Depot, or for that matter anyone else, stocks hydraulic valves that would be capable of precision limb control under the extreme range of conditions such an application would demand. Their solution was to machine their own valves to exacting tolerances which could maintain smooth operation and sealing as their parts expanded and contracted with temperature, or deformed under load.
The requirements for a device to perform surgery inside an MRI scanner’s 10-Tesla magnet are a little different than for rocket arms, but the point is that the can-do philosophy here is identical. Case in point: another group at Vanderbilt has already built a steerable robot arm to go inside the brain and bust clots. This device consists of a series of retracting and rotating tubes which can be maneuvered through the twisting vessels of the brain under guidance of CT (computed tomography). To translate this kind of device into the ferrous-free world of an MRI scanner, you generally need to ditch the electomagnetics (the motors) and get familiar with pneumatic circuit symbols and nickel-titanium shape-memory alloy (an alloy that, when heated, returns to its original shape).
MRI + robots = very tidy through-the-cheek surgery indeed
To detect and track seizures a through-the-cheek approach has previously been used to implant electrodes near the hippocampus — a frequent problem area in epilepsy. These electrodes are not put right on top of the hippocampus as that would require some fancy twisting and turning. To actually destroy the offending cells once they are identified, the neuron police generally need to go in through the skull with a straight needle, and deliver their assault of choice. We recently described a straight-through approach (using futuristic lasers and fancy robotics) that enters from the back of the head to target the hippocampus. While an improvement, this technique still has a significant trauma component, and is largely a pre-planned ballistic affair with little room for on-the-fly adjustments.
Seeing the big picture, Vanderbilt surgeons decided to try to use the through-the-cheek approach to the hippocampus. Their twist is that instead of just placing electrodes, they will try to perform a complete MRI-guided surgery using a seizure-busting robot. Their device is now in the beginning stages and they note that it could take up to 10 years to be perfected for the OR. Right now they can position a nickel-titanium probe about 1 mm wide to an accuracy of roughly the same dimension. They can mechanically advance it in millimeter steps like a mechanical pencil, and can also turn it by actuating the shape-memory hardware.
The surgical-technological zeitgeist
If there was ever a surgical-technological zeitgeist, a spirit of the times, we might say the time is now and the spirit the MRI. Tuesday we described new transparent graphene based implants that will permit simultaneous electrical and optical recording or control. The secret bonus is that the same properties that permit transparency, namely the lack of metal, also make them MRI compatible. As long as the materials are not ferrous, and not significantly magnetizable like nonferrous aluminum, then we should be good. Being able to see exactly where one is placing deep brain electrodes in real time, rather than going in blind using coordinates from a previous scan on an ever-changing brain (particularly in cases like hydrocephalus or other swelling), is key.
Perhaps an even more extreme potential application for surgical MRI, is in a new technology we described on Wednesday that has now enabled a paralyzed man to walk. The trick here was to transplant olfactory ensheathing cells (OECs) from his olfactory bulb into his spine. There is some controversy at the moment as to exactly which elements of the larger surgery were responsible for his dramatic recovery. What we didn’t mention in that post, is that the best way to place these cells and later verify their connections may be to label them with biocompatible iron oxide nanoparticles –superparamagnetic iron oxide particles encapsulated with inert hydrophilic polyethylene glycol, to give some more detail — and track them with MRI. The kicker is that others have already succeeded in performing this tricky kind of labeling in cells — and the cells they did it in to most notable effect are precisely these OECs.
One last thing to consider is that while routine MRIs can be creepy enough for some, several kinds of neurological procedures are now done best when the patient is alert and able to communicate. To say the least, when combining the two — awake brain surgery inside a magnet — we might only hope there is time for the patient to have a dry run beforehand to better prepare themselves.