X-Ray Production

Every X-ray image in a hospital begins the same way: electrons boiled off a hot filament, hurled across a huge potential difference, and slammed into a spinning metal target. Understanding exactly how that violent collision produces X-ray photons — and how a radiographer can tune it — is the foundation for everything else in this topic.

The rotating-anode X-ray tube

The standard practical device for generating medical X-rays is the rotating-anode X-ray tube, a sealed, evacuated glass or metal envelope containing two key electrodes. At one end sits the cathode: a filament of metal wire, heated by an electric current until it is hot enough to release electrons from its surface through thermionic emission. These freed electrons are then accelerated across a large potential difference — the tube voltage, typically tens of thousands of volts — toward a metal target at the other end of the tube, called the anode.

When the fast-moving electrons slam into the anode, their kinetic energy is converted into other forms of energy. Overwhelmingly, this conversion is inefficient: well over 99% of the incoming kinetic energy ends up as heat in the target, and only a small fraction emerges as X-ray photons. This lopsided energy balance is the central engineering problem of X-ray tube design — the target has to survive an enormous, concentrated heat load without melting or degrading.

The rotating-anode design solves this problem directly. Instead of a stationary target, the anode is shaped as a bevelled disc that spins rapidly (driven by an induction motor) while the tube operates. Because the electron beam always strikes the same small spot relative to the tube housing, but the target material itself is constantly rotating beneath that spot, the intense heat load is spread around the full circumference of the disc rather than being dumped onto one fixed patch of metal. This lets the tube be run at a much higher power — producing a more intense X-ray beam — than a stationary-target tube could tolerate before the target itself would be damaged.

The continuous and characteristic spectra

The X-rays produced at the target don't all have the same photon energy. If the intensity of the emitted X-ray beam is plotted against photon energy (or equivalently wavelength), the result is a distinctive spectrum made of two separate physical processes layered on top of each other: a smooth, continuous background, and a set of sharp peaks rising out of it.

• The continuous spectrum, often called bremsstrahlung ("braking radiation"), arises because an incoming electron passing close to a target nucleus is deflected and decelerated by the nucleus's electric field. This deceleration causes the electron to radiate away some of its kinetic energy as a photon. Crucially, a decelerating electron can lose any amount of its kinetic energy in this way, from almost none up to the whole of its kinetic energy in one single interaction. Because many electrons undergo many different such interactions, losing different amounts of energy each time, the result is X-ray photons spread continuously across a whole range of energies, forming a smooth continuous spectrum.
• The characteristic spectrum appears as sharp, narrow peaks superimposed on top of this continuous background, at specific, well-defined photon energies. These arise through a different mechanism entirely: an incoming electron can collide with and eject an electron from an inner shell of a target atom, leaving a vacancy. An electron from an outer, higher-energy shell then drops down to fill that vacancy, and in doing so releases a photon whose energy is exactly equal to the difference between the two atomic energy levels involved. Since these energy levels are fixed properties of the target element's atomic structure, the resulting photon energies are fixed too — hence "characteristic" of whichever material the target is made from.

The maximum photon energy

The continuous spectrum doesn't extend to arbitrarily high photon energies — it has a sharp cut-off at a well-defined maximum. This maximum corresponds to the most extreme possible single bremsstrahlung interaction: an electron that loses all of its kinetic energy to a single photon in one interaction, rather than spreading its energy loss across several smaller interactions.

Since each electron gains kinetic energy $eV$ by being accelerated through the tube voltage $V$ (where $e$ is the electron's charge), the greatest possible photon energy obtainable from any single electron is exactly this amount:

where $h$ is the Planck constant and $f_{max}$ is the maximum photon frequency. Because photon energy and wavelength are related by $E = hc/\lambda$, this maximum energy also corresponds to a well-defined minimum wavelength in the spectrum, below which no X-ray photons are produced at all.

Controlling the beam: a radiographer's practical toolkit

Because an X-ray tube's operating settings directly shape the beam it produces, controlling those settings lets a radiographer tune the resulting image — but almost every adjustment involves a trade-off, and the most important trade-off of all runs through every single one of them: patient dose.

• Intensity (the number of X-ray photons produced per second) is controlled mainly through the tube current, which depends on how strongly the filament is heated. A hotter filament releases more electrons per second by thermionic emission, so more electrons strike the anode per second, producing more X-ray photons per second — a more intense beam — without changing the energy of any individual photon.
• Energy (how penetrating, or "hard", the beam is) is controlled by the tube voltage. A higher tube voltage gives each electron more kinetic energy before it strikes the target, shifting the whole spectrum — both the continuous background and the maximum cut-off energy — to higher photon energies, producing a more penetrating beam.
• Sharpness is improved by keeping the effective X-ray source as small as practically possible. X-rays are effectively emitted from the small spot on the anode where the focused electron beam strikes it; a larger effective source spot causes more geometric blurring at the edges of structures in the resulting image, so a smaller focal spot gives a sharper image.
• Contrast is affected by the balance of photon energies used in the beam. Choosing an appropriate tube voltage (and hence spectrum) maximises the difference in how strongly different tissue types absorb the beam, so that different structures show up with clearly distinguishable brightness in the final image.
• Patient dose runs as a trade-off through every setting above. Increasing tube current to raise intensity, or adjusting tube voltage for better penetration or contrast, generally increases the total radiation dose delivered to the patient. Real imaging always has to balance diagnostic image quality against minimising unnecessary radiation exposure.