See Right Through You

So, I think getting an X-ray is pretty cool. Since you're usually getting an X-ray because something broke, I would imagine that machine mechanics is not on the list of things to think about. So hold on to your fabulous hospital gown because next time you get an X-ray (standard dental exam of course) you'll know exactly what's going on.

^{Image Credit:interbent.com}

What are X-ray Sources?

All X-ray machines have what's called an X-ray source. These are vacuum tubes that use an electrostatic field to produce X-rays. This is done by accelerating electrons to a high velocity and then suddenly stopping them.

Typically, the tubes used by X-ray sources are made of glass, surrounded by metal, and sealed by a vacuum. The cathode is located in the dome-shaped part of the tube.

How X-ray Sources Work

I know most of you are disappointed (or in my case, relieved) that X-ray glasses do not exist. Currently, X-ray tubes have several basic components. These include: a source of electrons such as a filament with a heating source, a system capable of accelerating electrons across a space where there is nothing to impede them, i.e., a vacuum glass tube with high tension (HT) transformer, and a target structure where the electrons strike, also known as the anode.

^{Image Credit: http://comicbooksrevisited.blogspot.com}

The X-ray is produced by applying a high voltage so the X-ray tube releases electrons from the filament cathode. These electrons then race towards and collide into the anode. The high-speed collision of electrons produces X-ray photons. The electrons continue towards a beryllium foil that absorbs the scattered electrons in order to allow the X-rays to pass through the tube. The passing of the electrons from the cathode to the anode establishes a flow of electrical current, known as a beam, through the tube.

Types of X-ray Source Tubes

The GlobalSpec SpecSearch database provides information about several types of X-ray tubes. The Coolidge Tube, also known as a hot cathode tube, is the most popular X-ray source. The characteristics of this device are its high vacuum and its use of a heated filament as the source of the electrons.

^{Image Credit: Oak Ridge Associated Universities}

In a rotating anode tube, the anode target disc rotates on a highly specialized ball bearing system. The target is subjected to a focused stream of electrons emitting from the cathode and accelerated by a high potential difference between the target disc and the cathode. When the electron beam hits the anode, it produces the X-ray beam. The advantage of using a rotating anode tube is it permits selection of higher electrical load without the risk of overheating. It can be used in almost every radiography application.

Then there are the application specific X-ray tubes which include:

o Radiotherapy tubes

o Stereographic X-ray tubes

o Mammography X-ray tube

o Tubes for computed tomography (CT)

o Field emission X-ray tubes (also called cold cathode tubes)

If you're planning on buying one of these tubes you should know about some common variables in X-ray tube design and selection: operating heat produced, operating heat capacity, target angle, focal spot size, duty cycle, operating wattage, application for which the tube will be used, physical dimension considerations, and many others. The full X-ray source selection guide gives comprehensive information and formulas to help you to figure this out.

X-ray tubes aren't just used by dentist, doctors, and TSA agents; they are used in a variety of applications, including some cool ones such as:

Image Credit:Nick Veasey

• Security control systems and equipment
• Non-destructive testing (NDT)
• Production inspection systems
• Metal treating/metallurgical
• Metrology
• Laboratory analytical instruments
• Fresh fruit and vegetable inspection