Challenges that X-ray Tube Engineers Face
X-ray tubes are essentially simple devices (see sidebar on page 17). “Anybody can make a rudimentary X-ray tube,” said Dennis Runnoe, vice president of research and development for Varian’s X-Ray Products business. “The challenge is making a safe, efficient, quiet-operating, long-lasting, cost effective tube that yields superior image quality.” That means:
Making a tube that can run at very high power levels, consistently, without failing.
Optimizing beam quality, so that the image is the best it can be.
Designing a tube that can take the stress in a CT scanner, when spinning around a patient at a rate of up to three times per second.
Finding ways to reduce the noise of operation, even though tubes contain components that spin fast on steel ball bearings.
“The technical challenges are endless—and they’re always changing, as imaging technologies improve,” Runnoe said.

Handling Heat—A Varian Specialty
The biggest problem in generating X-rays is dealing with the extreme heat that is created in the process. An X-ray tube generates temperatures as high as 2,000 degrees centigrade. Early tubes burned out quickly, or had to be shut off frequently to cool down. Imaging systems could not be used while they were cooling. There was always the danger of damaging the sensitive imaging equipment, if the tube ran too hot.
         Varian engineer Rob Treseder pioneered a novel approach for dealing with excess heat. He received the first patent for a CT scanning tube with a “grounded anode.” In this tube, electrons that strike the anode and bounce off aren’t drawn back to the anode to generate additional heat. Treseder and his colleagues also added a special electron “collector” to their new tube. “Our collector gathers electrons that would otherwise keep striking the anode and heating up the tube,” Treseder said. “The tube runs cooler, so we can put more power through it. That means images can be generated more quickly, and the tube lasts a lot longer.”
         “Doctors want tubes with more power so they can reduce examination time, and someone in pain doesn’t have to hold still for long,” said Runnoe. “That means dealing with more heat. A faster, longer-lasting X-ray tube means that diagnostic imaging equipment runs more reliably, with less downtime. With this kind of improved technology, doctors can scan more patients more quickly and serve more patients per hour.”

Higher resolution—
improving the “signal to noise” ratio
The electron collector in the anode-grounded CT tube adds another important benefit: higher resolution images that give doctors more detail. Resolution is a function of the “signal to noise” ratio. Electrons bouncing around a tube create “noise” in the form of unwanted radiation that muddies the picture. When a critical number of these electrons are collected out of the way, the amount of “noise” is decreased and the tube emits a clear beam for a much sharper image.
         “Better-quality images facilitate quicker, more accurate patient diagnosis,” Runnoe said. “Our tubes today are essential components in equipment that generates images that are so good, we can take a snapshot of heart motion and see small irregularities.”