X-rays are produced when high-speed electrons slam into a metal target, converting their kinetic energy into electromagnetic radiation. This happens inside a specialized vacuum tube where electrons are generated at one end, accelerated by a high voltage, and then abruptly stopped by a dense metal target at the other end. The process is remarkably inefficient: roughly 99% of the electron energy becomes heat, with only about 1% at diagnostic energy levels (and up to 5% at very high voltages) actually producing usable X-rays.
Inside the X-Ray Tube
An X-ray tube is a sealed glass envelope with the air pumped out to create a vacuum. That vacuum is essential because electrons need a clear path to travel without bumping into air molecules. At one end sits the cathode (the negative side), and at the other end sits the anode (the positive side). A high voltage applied between these two electrodes pulls electrons from the cathode toward the anode at tremendous speed.
The cathode contains a thin tungsten wire filament, only about 0.2 mm thick. When electric current heats this filament to extreme temperatures, electrons “boil off” its surface in a process called thermionic emission. The freed electrons form a small cloud near the filament, held briefly in place by their mutual repulsion. A molybdenum focusing cup surrounds the filament and carries a negative charge that narrows the electron stream into a tight beam aimed at the anode.
The anode’s target is also made of tungsten, chosen for good reason. Tungsten has a high atomic number (74), which makes it efficient at producing X-rays. It also has an extraordinarily high melting point of about 3,400°C, which matters because the target absorbs enormous amounts of heat. A small percentage of rhenium (roughly 10%) is alloyed into the tungsten to prevent the target from cracking under repeated thermal stress.
Stationary vs. Rotating Anodes
In simpler or low-power tubes, the anode target stays fixed in place. But most modern diagnostic tubes use a rotating anode, which spins the tungsten disk at high speed during exposure. The rotation spreads the electron bombardment across a much larger area of the target surface rather than concentrating all that energy on a single spot. This lets the tube handle the higher power levels needed for most medical imaging without melting the target.
Two Ways X-Rays Are Created
When the accelerated electrons hit the tungsten target, X-rays are generated through two distinct physical processes. Both happen simultaneously, but they produce different types of radiation.
Bremsstrahlung (Braking Radiation)
Most X-rays in a diagnostic beam are produced this way. When a fast-moving electron passes close to the nucleus of a tungsten atom, the strong positive charge of that nucleus pulls on the electron and deflects it from its path. This sudden deceleration forces the electron to shed some of its kinetic energy, which is released as an X-ray photon. The German word “bremsstrahlung” literally means “braking radiation.”
The key detail is that each electron loses a different amount of energy depending on how close it passes to a nucleus. An electron that barely grazes the edge of a nuclear field loses only a small fraction of its energy and produces a low-energy photon. One that collides nearly head-on can lose almost all its energy in a single interaction, producing a high-energy photon. Because these interactions vary widely, bremsstrahlung creates a continuous spectrum of X-ray energies ranging from near zero up to the maximum voltage applied across the tube.
Characteristic Radiation
The second process produces X-rays at very specific, predictable energies. If an incoming electron has enough energy, it can knock an inner-shell electron completely out of a tungsten atom, leaving a vacancy. That vacancy makes the atom unstable. An electron from a higher energy shell quickly drops down to fill the gap, and the energy difference between the two shells is released as an X-ray photon. Because these energy differences are fixed properties of tungsten’s atomic structure, the resulting X-rays always come out at the same discrete energies. They are, in effect, the “fingerprint” of the target material.
What Controls the X-Ray Beam
Operators adjust two main settings to control the X-ray output. The tube voltage, measured in kilovolts peak (kVp), determines how fast the electrons travel and therefore how much energy the X-rays can carry. Higher voltage means more penetrating X-rays. A chest X-ray typically uses a different kVp than a dental image or a spinal exam. For abdominal and pelvic imaging, optimal tube voltages fall around 70 kVp, while lumbar spine imaging works best around 80 to 100 kVp.
The tube current, measured in milliamperes (mA), controls how many electrons flow from cathode to anode per second. More electrons hitting the target means more X-ray photons produced, which translates to a brighter image. Adjusting the exposure time alongside the current lets technologists balance image quality against the radiation dose delivered to the patient.
Filtering Out Unwanted Radiation
The raw beam coming off the target contains a wide range of photon energies, including many low-energy X-rays that would be absorbed by the patient’s body without ever reaching the detector. These photons add to the radiation dose without contributing to the image. To remove them, thin sheets of aluminum (and sometimes copper) are placed in the beam’s path. This filtration hardens the beam by stripping out the weakest photons and letting only the more useful, penetrating ones through. Federal regulations require that a minimum level of filtration is permanently built into every diagnostic X-ray system.
Why the Focal Spot Matters
The point on the anode where electrons strike is called the focal spot, and its size directly affects image sharpness. A smaller focal spot produces sharper images because the X-rays appear to come from a more precise point, similar to how a small flashlight creates crisper shadows than a large floodlight. The trade-off is heat: concentrating all that electron energy onto a tiny area can overheat the target. Larger focal spots spread the thermal load but reduce spatial resolution. The anode face is angled, typically between 5 and 15 degrees, which makes the effective focal spot appear smaller from the patient’s perspective than it actually is on the target surface. This trick, called the line focus principle, gives the benefits of a small focal spot for image quality while maintaining a larger actual target area to handle the heat.
Heat: The Dominant Byproduct
The overwhelming majority of energy deposited into the target becomes heat, not X-rays. Even at 1 million volts hitting a tungsten target, only about 5% of the electron energy converts to X-ray photons. At the lower voltages used in diagnostic imaging, the efficiency is even smaller. This is why anode cooling is one of the biggest engineering challenges in X-ray tube design. Rotating anodes, oil-bath cooling systems, and heat-resistant alloys all exist to manage the thermal load that is an unavoidable consequence of how X-rays are made.