Teaching Radiography to dental hygiene students can be a daunting task. The concepts of mA and kVp seem to be among the most difficult to grasp. Many of them have a difficult time understanding how X-rays are created in the dental X-ray tube. When X-rays are created, the kinetic energy of the accelerating electrons has to be converted into electromagnetic energy. The main way this occurs is via bremsstrahlung radiation. Here’s how I explain the process:
- First, we need a free source of electrons. We do this by heating up the tungsten filament (cathode) to incandescence. I demonstrate this by turning out the lights and holding up a coiled wire as I heat it with a lighter until it starts glowing. I explain that the energy needed to heat the coil to incandescence represents the mA, which is usually in the range of 5-8 volts. As the wire heats up, a cloud of electrons will form around it.
- The next thing we need is to impart speed to the electrons and propel them toward the tungsten target (anode). We achieve this by energizing the electrons with about 60-90 kilovolts. This is the kVp. It would be like holding the wire up to a blowtorch instead of a lighter.
- At this point, the electrons accelerate toward the tungsten target with tremendous kinetic energy. For X-rays to be created, we need to convert the kinetic energy of the electrons into electromagnetic energy. Remember energy cannot be created or destroyed, but can change form.
- As the electrons slam into the tungsten target, the sudden stop converts their energy from kinetic to electromagnetic (and other energy like heat). About 1% will be converted into X-rays. Bremsstrahlung means “braking”, and describes this sudden stop.
- To help them visualize Bremsstrahlung radiation, I ask them to think about what happens if a car accelerates and crashes into a concrete wall. The sudden stop will convert the kinetic energy of motion into heat and sound waves (the crashing sound). It’s similar to what happens in an X-ray tube, just on a much smaller scale.
This explanation seems to help my students understand these difficult topics. The textbook explanations can be so difficult to understand; I always think the answer is to simplify and demystify tough concepts. Visuals and dramatic storytelling always help too. After all, we’re teachers, but we’re also actors up on stage. Give them something engaging and fun, and they will learn.