I was a bit of a lost soul after high school. I dabbled with electrical engineering for a semester but decided that it wasn’t for me – what I wouldn’t give for a do-over on that one. In my search for a way to make money, I stumbled upon radiologic technology – learning how to take X-rays. I figured it was a good way to combine my interests in medicine, electronics, and photography, so after a two-year course of study I got my Associates Degree, passed my boards, and earned the right to put “R.T.(R) (ARRT)” after my name.
That was about as far as that career went. There are certain realities of being in the health care business, and chief among them is that you really have to like dealing with the patients. I found that I liked the technology much more than the people, so I quickly moved on to bigger and better things. But the love of the technology never went away, so I thought I’d take a look at exactly what it takes to produce medical X-rays, and see how it’s changed from my time in the Radiology Department.
First things first: what are X-rays? They’re nothing more than electromagnetic waves, living on the spectrum between UV light and gamma radiation, or from about 30 petahertz to 30 exahertz. In principle, X-rays are easy to produce – all you really need is a roll of sticky tape and a vacuum chamber. That’s a trivial example, though, and relies on triboluminescence rather than through the interaction of high-energy electrons with dense metals, which is how medical X-rays are produced. Still, the principle is simple – just produce some electrons from a hot cathode and accelerate them into a target anode using high voltage.
In practice, however, it’s not that easy. While some vacuum tubes can produce X-rays incidental to their main function – the high-voltage rectifiers in old tube-type TV chassis were notorious for this – making an X-ray tube is a tricky business. Even a simple fixed-anode DIY X-ray tube requires a fair amount of skill and some specialized equipment to build, and a reliable, long-lived tube for medical X-rays is a huge engineering step beyond that.
The main problems that medical X-ray tubes – which are essentially particle accelerators – have to deal with all come from the huge energies needed to produce useful amounts of radiation. Thermal considerations are important. First, the cathode has to get hot enough to boil off electrons, which is in the 800° to 1,000°C range. And the voltage between the anode and cathode can easily exceed 100 kV; the kinetic energy of those electrons slamming into the anode can cause it to heat up to 2,500°C at the focal point.
Handling anode heating is a problem that the rotating anode tube was designed to handle. Rather than a fixed target that gets blasted by electrons repeatedly, the rotating anode tube has a disc-shaped tungsten alloy target attached to a rotating shaft. When the technician says, “Deep breath in and hold it,” and you hear a motor start turning, that’s the anode spinning up to around 10,000 RPM. The focal spot of the electron beam is still only a couple of millimeters square, but the fact that a new anode surface is rotating under that beam while it’s on spreads the thermal load out over a greater area.
Simple to say, but harder to engineer. The anode needs to be inside the vacuum tube, but the motor can’t be. That means the anode is attached to a rotor and bearings inside the tube, with the stator windings outside the tube. Bearings that can work in a vacuum under the kinds of heat loads experienced in a vacuum tube and can reliably conduct 100 kV or more are some kind of special. In some new tubes, standard ball bearings have been replaced with fluid-dynamic bearings using a thin film of a gallium-indium alloy that can conduct a couple of kilowatts of heat away from the anode.
The cathode is a pretty amazing piece of engineering too. More like the electron gun of a CRT than the filament of an audio amplifier tube, the cathode of an X-ray tube shapes and directs the beam toward the target. Tungsten filaments sit in a focusing cup that electrostatically forms the beam. Most tubes have a dual-filament cathode; the technician can select the smaller cathode for a smaller focal spot on the anode and a tighter beam that will image small structures better. Specialized exams like mammography often use tubes with a filament as small as 0.3 mm, as well as an anode made of molybdenum to get a “softer” beam that’s better at visualizing delicate structures.
While electrostatic focusing of the electron beam is pretty simple and proven, newer tubes are turning to magnetic focusing. These designs have a flat filament rather than a cup, and the beam is shaped using magnetic quadrupoles, making these X-ray tubes even more like a particle accelerator.
So what powers all this? What kind of electronics live behind those putty-gray cabinets in the radiology suite? In radiology parlance, those cabinets collectively are referred to as the generator, the job of which is to provide all the power and control for the entire X-ray suite. Obviously that means providing the current for the filaments as well as the high-voltage field to accelerate the electrons. But there’s a lot else to run in the tube – the stator windings for the rotating anode, and possibly the signals needed for magnetic focusing. There’s also the power needed to operate the physical support for the tube, the patient table controls, the operator console, and just about everything in the suite.
In the old days, the generator was a pretty simple circuit. It basically consisted of an autotransformer feeding a step-up transformer and rectifier for the anode high-voltage, and a low-voltage section to power the cathode. Timing circuitry controlled the length of exposure, and a simple console was provided for the technologist.
Now, generators are extremely sophisticated devices with embedded computers that talk Ethernet and CAN bus. Sleek touch-screen user interfaces are more likely than the old knobs and switches, and special features like automatic exposure control are built right in. But at its heart, the generator’s job is still the same – to heat the cathode and accelerate the electrons.
Even though the engineering has come a long way and the control electronics have changed, an X-ray tube in service today looks pretty much the same as the tubes I learned about 30 years ago when I was in the field. That’s pretty much due to the physics being the physics, and although there are other ways to produce medical X-rays, like Thomson scattering using terawatt lasers or undulators where electrons wiggle through alternating magnetic fields and shed energy in the form of collimated X-rays, the standard X-ray vacuum tube and generator are by far the most practical way we’ll have to deliver medical X-rays for quite some time to come.