X-ray machines work by firing a beam of high-energy light through your body and capturing what comes out the other side. Dense structures like bone absorb most of the beam and appear white on the image, while softer tissues let more of it pass through and appear darker. The entire process, from beam generation to image capture, takes a fraction of a second.
Inside the X-Ray Tube
The heart of every X-ray machine is a sealed glass vacuum tube containing two key components: a cathode (negative side) and an anode (positive side). When the machine is switched on, an electrical current heats a small wire filament inside the cathode to extreme temperatures. This intense heat causes electrons to boil off the filament’s surface and form a cloud of charged particles, a process called thermionic emission.
Those electrons just hover near the filament until the operator triggers the exposure. At that point, the machine applies a massive voltage difference between the cathode and anode, sometimes tens of thousands of volts. The electrons slam across the vacuum tube toward the positively charged anode target, typically made of tungsten, at tremendous speed.
When the fast-moving electrons hit the tungsten target, two things happen. Most of them pass close to the nuclei of tungsten atoms and get sharply deflected, converting their energy into X-ray photons. This “braking radiation” (called bremsstrahlung in physics) produces a broad spectrum of X-ray energies and accounts for the majority of useful X-rays. A smaller number of incoming electrons knock orbital electrons out of tungsten atoms entirely. When other electrons drop down to fill those vacancies, they release photons at very specific energy levels unique to tungsten. Together, these two processes generate the X-ray beam that exits through a small window in the tube housing.
How the Beam Creates an Image
X-rays lose energy as they pass through material, and different tissues absorb different amounts. Ten centimeters of bone absorbs far more X-ray energy than ten centimeters of water or soft tissue. This difference in absorption is what creates contrast in the final image. Areas where the beam was heavily absorbed (bone, metal implants) show up bright white. Areas where the beam passed through easily (lungs full of air, fat) appear dark. Soft tissues like muscle and organs fall somewhere in between.
This makes X-ray imaging especially good at revealing fractures, changes in bone density from conditions like osteoporosis, and foreign objects. It’s less ideal for distinguishing between two types of soft tissue that absorb X-rays at similar rates, which is why other imaging methods like MRI or ultrasound are better for certain problems.
Controlling the Beam
Technologists adjust two main settings to get a useful image while keeping your radiation exposure low. The first is voltage, measured in kilovolts (kV). Higher voltage makes the X-ray beam more penetrating, which is necessary for imaging thick or dense body parts like the pelvis. At 120 kV, a chest image might need only 6 milliampere-seconds (mAs) of exposure, while the same image at 60 kV would require 141 mAs. Higher voltage always reduces patient dose, but it also increases scatter, which can degrade image sharpness.
The second setting is current multiplied by time (mAs), which controls how many X-ray photons the tube produces during the exposure. More photons mean a brighter, less grainy image, but also more radiation. The technologist balances these two dials to get a diagnostic-quality image with the least radiation necessary.
A device called a collimator also sits between the tube and the patient. It’s essentially a set of adjustable lead shutters that narrow the beam to cover only the body region being examined. Collimating tightly reduces the amount of tissue exposed to radiation and improves display contrast by eliminating scatter from surrounding areas.
From Beam to Picture
Older X-ray systems used film coated with silver bromide crystals, similar in concept to camera film. When X-ray photons hit the crystals, they converted silver bromide into black metallic silver. The more photons that reached a spot on the film, the darker it appeared. Film worked but had a narrow useful range: about 40 to 1 between the brightest and darkest intensities it could capture. If the exposure was slightly off, the image could be too dark or too washed out to read, and there was no way to fix it after the fact.
Computed radiography, introduced in the 1980s, replaced film with reusable phosphor plates. After exposure, a laser scans the plate, causing it to release stored energy as light, which gets converted into a digital signal. These plates are two to four times more sensitive than film and can be erased and reused. More importantly, because the image is digital, it can be adjusted for brightness and contrast after the exposure is taken.
Modern digital radiography skips the intermediate step entirely. Flat-panel detectors built from semiconductor materials convert X-ray energy directly into electrical signals. These detectors have a dynamic range of 1,000 to 1 or more, compared to film’s 40 to 1. That wider range means the system can capture detail in both very dense and very transparent parts of the body in a single exposure. It also delivers better image quality at a lower radiation dose than either film or computed radiography.
Real-Time X-Ray: Fluoroscopy
Standard X-ray machines produce a single still image. Fluoroscopy uses the same basic physics but captures a continuous stream of images displayed on a monitor in real time, essentially an X-ray video. This lets doctors watch movement: a contrast dye flowing through blood vessels, a catheter being guided into position, or the motion of a joint. The tradeoff is a higher cumulative radiation dose, since the beam stays on for seconds or minutes rather than a fraction of a second.
How Much Radiation You Actually Get
A standard chest X-ray delivers about 0.1 millisieverts of radiation. For context, the average person in the United States absorbs roughly 3.1 millisieverts per year just from natural background sources like radon in soil, cosmic rays, and trace radioactive elements in food. A single chest X-ray adds the equivalent of about one day’s worth of natural background exposure.
Radiation safety in medical imaging follows a principle called ALARA: as low as reasonably achievable. In practice, this comes down to three strategies. Minimize the time the beam is on. Maximize distance between the X-ray source and anyone not being imaged. Use shielding where appropriate. Lead aprons, the heavy vests you may be asked to wear, are typically lined with 0.5 millimeters of lead or its equivalent and block about 90% or more of scatter radiation. Staff working in X-ray rooms wear these routinely, along with thyroid shields, and stand behind lead-lined barriers during exposures.
A Discovery That Changed Medicine
Wilhelm Conrad Röntgen is credited with discovering X-rays in November 1895 while experimenting with vacuum tubes and electrical discharges. The finding was so unexpected that he named the rays “X” for unknown. What’s less well known is that five years earlier, in 1890, physicist Arthur Goodspeed and photographer William Jennings at the University of Pennsylvania inadvertently created the first X-ray image when coins sitting on photographic plates were exposed during experiments with discharge tubes. They didn’t realize what they had until Röntgen’s announcement made it obvious. Within months of that announcement, doctors were using X-rays to locate broken bones and foreign objects, and the technology has been a cornerstone of medical imaging ever since.