X-ray production: Rad Tech

As a radiologic technologist, understanding how x-rays are produced connects what happens inside the x-ray tube to the beam used for diagnostic imaging. The production process, exposure factors, and tube design shape the emission spectrum and determine the beam’s quantity and energy. These characteristics also affect image formation and patient dose.

X-rays are produced as electrons are released from the cathode and are accelerated toward the anode. These fast-moving electrons are called projectile electrons. When they strike the anode, they interact with target atoms to produce heat and two types of x-rays: characteristic and bremsstrahlung.

Under typical diagnostic conditions, about 99% of the electron energy becomes heat, and about 1% becomes x-rays. This low efficiency makes heat management essential to safe tube operation.

So, the two types of x-rays are characteristic and bremsstrahlung. Characteristic radiation occurs when a projectile electron removes an inner-shell electron from a target atom, leaving a vacancy. An outer-shell electron then fills the vacancy, and the difference in binding energy between the two shells is released as an x-ray photon.

Because binding energies are unique to each element, characteristic x-rays are produced at specific, target-dependent energies.

For example, in tungsten, which is common in general diagnostic tubes, the atom’s innermost electron shell is the K shell. If an outer-shell electron fills a vacancy in the K shell, the resulting photon is called a K-characteristic x-ray, where the K identifies the shell being filled. Creating that vacancy requires about 69 kiloelectron volts, or keV, so the tube potential must be about 70 kilovoltage peak, or kVp, or higher. This 69-keV value is the energy needed to remove the K-shell electron. So, the photon’s energy depends on which outer-shell electron fills the vacancy. When it comes to tungsten’s characteristic x-rays, only K-characteristic x-rays are useful for general diagnostic imaging.

In contrast, bremsstrahlung radiation is produced when a projectile electron interacts with the electric field of the nucleus rather than an orbital electron. The German word means “braking radiation” and is often shortened to “brems.” As a projectile electron passes near the positively charged nucleus, it slows and changes direction. The kinetic energy it loses is emitted as a bremsstrahlung x-ray. Because the amount of energy lost can vary, bremsstrahlung X-rays span a continuous range of energies. Most x-rays in the diagnostic range are bremsstrahlung.

Together, these two types of x-rays form the x-ray emission spectrum. The emission spectrum can be displayed as a graph, the horizontal axis shows photon energy in keV, and the vertical axis shows the relative number of photons at each energy. The continuous curve represents bremsstrahlung, while narrow spikes represent characteristic x-rays.