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When the first group of American astronauts started training for space flight in the 1950s, Air Force doctors put them through a number of wrenching trials. In one, they had to endure many multiples of the force of gravity we experience at sea level — or G-force. John Glenn experienced 7.9 Gs during his first orbital flight, and others briefly went as high as 32 Gs on Houston’s infamous G Machine. “You couldn’t lift your arm out of the couch above about 6 or 7 Gs,” Glenn told a historian. “Beyond that you were just supported there.”
But all of those numbers pale in comparison with a new kind of G Machine— GE’s computed tomography scanner called Revolution CT. Rather than preparing humans for exploring the universe, doctors use it to look inward and probe the mysteries of the body. One of its key imaging components — which weighs about 100 pounds — routinely reaches 70 Gs as it circles around patients lying inside the machine’s gantry up to five times per second.
The machine is currently on display at annual meeting of the Radiological Society of North America (RSNA) in Chicago, a “Grand Slam” trade show and gathering for radiologists. It is expected to draw 60,000 visitors and exhibitors this year.
All that speed—combines with “intelligent” motion correction—allows doctors to image the heart in just one heartbeat, among other things. But it also makes the spinning part — which includes an X-ray tube, a detector and a high-voltage transformer supplying them with power — accelerate so fast that it weighs the equivalent of 3.5 tons. “We designed the parts to survive much larger acceleration than astronauts experience in space,” says Hervé Blanc, a GE Healthcare engineer managing the team that designed the CT scanner’s high-voltage generators.
We talked to Blanc about the design during a recent visit to his office in Buc, France. Here’s the edited version of our conversation.
GE Reports: Why does the X-ray tube move so fast?
Herve Blanc: You have to complete one full rotation on the gantry around the body in one heartbeat in order to get a clear and detailed image of the heart. That’s about five rotations per second. CT images the body one slice after another. At this speed and power, we can obtain 512 slices per second. As you can imagine, more slices generally mean a better image quality.
GER: What is a gantry?
HB: The gantry is the donut-like shape when you look at the machine. The gantry of the Revolution CT machine has a diameter of about 80 centimeters. As I said, the X-ray tube and the other parts attached to the gantry ride on it and can complete five rotations per second. When you do the math, you get into the space program territory.
GER: What kind of materials do you use?
HB: We work with aerospace-grade aluminum, but the material is just one element of the design. There is a system of springs that absorb the acceleration and make sure parts like the electronic board inside the detector do not move. We also included fail-safe design features. The 100-pound box that rotates around the gantry is attached with screws and also a dovetail lock. It’s a double system of safety. In case the screws break, the rail will hold it back.
GER: How do you test 70 Gs?
HB: We use a centrifuge. Actually, 70 Gs is what you experience in normal operations. In testing, we go much higher than that. We want the parts survive much higher accelerations.
GER: How do you supply power to a machine that’s zipping five times a second a around a 3-foot donut?
HB: Old-school CT scanners used brushes to transmit power. But when you increase the speed, you run into wear and reliability issues. We use contactless transmission — induction — to send power to the machine and collect the imaging signal from it.
GER: How much power?
HB: A basic CT uses 24 kilowatts and generates eight image slices per second. Revolution CT uses 100 kilowatts or more and gives us 512 slices. This allows us to image moving parts like the heart.
GER: What else can it do?
HB: The design allows us to change the energy of the imaging spectrum during a scan. When we then process the images with a special algorithm, we can see the composition of the body.
GER: How? Can you explain that?
HB: It is as if you were looking at a scenery with a pair of red glasses and then a pair of green glasses. By combining the information from both you can calculate the colors in the image. During CT imaging, soft tissues that contain water do not behave the same way as bones. By changing the energy spectrum — the color of your glasses, if you will — you are able to add another dimension to your pictures that describes the material content of the body. The software knows exactly with what energy you acquired the image. It has a physical model of energy absorption and is able to recognize the material it’s looking at.
GER: What are you working on right now?
HB: The Revolution CT has already allowed us to increase the quality of the images and reduce the amount of radiation at the same time. This is obviously good for the patients and helpful for doctors. We are now trying to develop the ultimate generator that would let us see exactly what we want in terms of the quantity and quality per image, and reduce radiation even more compared with today’s already good performance.