by Caroline Rusk
Nick Gidmark has x-ray vision. He’s no superhero, but he can see what’s happening under skin and muscle.
Scientists have studied how animals move from watching them on the outside. But they have never been able to see how the bones move in 3-D motion on the inside – until now. They can do this using a new technology called XROMM, or X-Ray Reconstruction of Moving Morphology. It combines x-ray video, laser scans, and 3-D animation to show how skeletons move. And someday, it might be a tool in doctors’ offices, just like a stethoscope.
Nick Gidmark, a graduate student at Brown University, wants to know more about how a certain group of fish moves. This group, called Teleosts, includes many familiar bony fish, such as herring, minnows, and trout. These fish have very movable skulls. Humans only have two major bones in their heads, the skull and the jaw. This makes chewing very simple, because the jaw can only move in so many directions. Teleost fish, however, have many bones in their heads, so that they can move more flexibly. For these fish, chewing is a little more complicated.
One fish Gidmark studies is the carp. A carp has more than 40 bones, just in its head. When a carp eats, it extends its mouth out towards its food, almost like us puckering our lips to drink from a straw. But instead of just the carp’s lips moving, its bones move too, pushing its entire mouth out to grab its next meal. With all of the bones moving, some of them deep in the fish’s head, it’s hard to tell what is going on.
All that is visible when the carp eats and swims is a smooth covering of scales. It is easier to see what bones are moving by dissecting the head. Then the bones, joints, and muscles are visible, but not in motion. Another way to see the bones is to watch them move in an x-ray screen. But x-rays can give fuzzy or unclear images, and sometimes bones are hidden behind others. These two options are not always enough to understand the movement of the skeleton, simply because neither gives a clear picture of all of the bones in motion.
Nick Gidmark studies one particular bone in the carp’s head. Neither x-rays nor dissections can help him to understand the bone because the tiny bone sits deep in the middle of the carp’s head. And it isn’t attached to any other bones or muscles – it is woven into a basket of ligaments. This makes it hard to see and to understand. It is called a kinethmoid. Nobody is quite sure what it does, and that’s exactly what Gidmark wants to figure out. “I think fish are cool,” he says, “I want to learn how this works. I’m curious.”
From observing the fish normally, anatomists think that the kinethmoid rotates as the carp sticks its mouth out. But Gidmark wants to know more. If it does rotate, he wants to know what makes it do so and how that is related to the carp stretching its mouth out to eat. XROMM is just the tool to show him the answers.
To use XROMM, Gidmark puts 3 metal beads in each of the four bones he wants to watch. Then he takes x-ray videos of the carp eating from two different angles. In the x-ray, the images of the metal beads appear dark and sharp against the gray transparent bones, which is exactly what he wants. He also gets the exact shape of the bone using laser scans of the carp. Gidmark brings all of these elements together in an animation program called Maya, the same one that Pixar used to bring Woody and Buzz to life in Toy Story. The finished product looks, as Gidmark says, “as if you had x-ray vision and you were watching some bones move while the fish was eating.” The video is in 3-D and looks like a fish head skeleton from a museum brought to life. And what excites Gidmark is that it clearly shows the kinethmoid rotate as the carp extends its mouth. The 3-D nature of the image makes the bone visible among the others.
Gidmark has concluded that there is a very close connection between the kinethmoid rotating and the fish extending it mouth to eat. This means it probably plays some sort of cause-and-effect role in the chain reaction to move the mouth. But its exact function is still unclear. Gidmark explains that he plans to put off learning more for the moment and use XROMM to study other things. But he will probably return to the kinethmoid in the future.
Carp aren’t the only animals whose moving skeletons XROMM has revealed. Researchers have also made 3-D x-ray videos of a pig foot, a pig head, a duck head, an iguana’s ribcage, a frog’s ankle, and the wings of pigeons and chukar partridges. But beyond just watching animals move, this technology has some other practical applications.
Brooke Flammang-Lockyer, a teaching fellow and Ph.D. candidate at Harvard, studies how fish move when they swim. For her post-doctoral research next year, she plans to build a robotic pectoral fin. Pectoral fins are the arm-like fins on the fish’s sides. XROMM is going to help build the robot.
The Navy is interested in Flammang-Lockyer’s robotic fin because it wants to invent underwater vehicles that are easier to control and turn than the submarines they have now. A robotic fin may just be the answer. The Navy is in luck, because there are already hundreds of fin designs for them to choose from – someone just has to figure out how to get them off of the fish and onto a vehicle. That is Flammang-Lockyer’s job.
“Bio-inspired design is becoming a very large field now,” she says. It makes sense: nature usually equips animals for their environment better than we can equip ourselves. The plan is to build a fin that a submarine can use the same way a fish uses its own fins. Flammang-Lockyer explains that many researchers have studied the pectoral fin, but there has been no work done on the shoulder bones underneath the fish’s skin that control the fin. She needs to understand how they work to design the best robotic fin.
This is where XROMM comes in. Flammang-Lockyer has long been aware of the x-ray video system because her lab at Harvard often works with the Gidmark’s lab at Brown. She hopes to create a 3-D animated video of the movement of the fish’s shoulder bones so that she can replicate the natural fin movement.
Besides propelling Navy submarines, Flammang-Lockyer sees a future for a robotic fin in research. The robot would provide a more reliable test subject than a live fish. Flammang-Lockyer says, “Fish are often hard to get to repeat things when you want them to, but if you have a good robotic model, then you can use that to do different tests.”
Even though Nick Gidmark is mainly interested in fish, he thinks that XROMM is going to contribute to human medicine. He explains that stemming from his lab’s interest in the way animals move, “we get these totally unexpected applications that are really useful for human anatomy and clinical science that nobody really thought of until a couple years ago.”
Gidmark thinks XROMM has the potential to change human orthopedics. He imagines a day when people with knee problems will be able to walk into the doctor’s office, get a CT scan of their knee, and walk on a treadmill next to a video camera. Then the doctor will make an XROMM video and tell what the problem with the joint is. It will work for any joint: shoulder, jaw, hip. A surgeon could look at the 3-D video and see exactly how to fix the problem: shave a little bone here, stick a little cartilage there, and voilà, that joint that used to bother you when you ran or played basketball wouldn’t hurt anymore.
But before any of this can happen, there needs to be a less invasive way to track movement than inserting metal markers into the bone. Right now, Gidmark explains, computer programmers are developing a way for a computer to make the bone’s digital image follow its motion exactly. But until the program is finished, researchers would have to spend months to create a full XROMM video without markers. Gidmark predicts that the computer-run technique will be available for use in about five years.
Brooke Flammang-Lockyer has faith that humans will soon have access to XROMM for their own use. “Coming up with the technology was the hard part, but adapting it for use in humans is much easier in comparison.”
And that may just translate into taking the pain out of people’s everyday lives. Who says scientists can’t be superheroes?