An X-ray is a type of high-energy light that passes through soft tissue but is absorbed by dense materials like bone, producing an image of structures inside your body. It’s the most common and oldest form of medical imaging, first discovered by physicist Wilhelm Conrad Roentgen on November 8, 1895. A standard X-ray is fast, painless, and exposes you to very small amounts of radiation.
How X-Rays Create an Image
X-rays are a form of electromagnetic radiation, the same family that includes visible light and radio waves, but with much shorter wavelengths (between 0.03 and 3 nanometers) and far more energy than ultraviolet light. That energy is what allows them to pass through your body.
The basic principle is simple: different tissues absorb X-rays at different rates. Dense materials like bone, metal implants, and certain contrast agents absorb most of the X-ray beam, so they appear white on the resulting image. Air absorbs almost none, which is why your lungs look black on a chest X-ray. Fat, muscle, and water fall somewhere in between, appearing in shades of gray. An X-ray detector on the other side of your body captures the rays that made it through, and those varying levels of absorption create the image your doctor reads.
This is why X-rays are excellent at revealing broken bones, dental cavities, and objects like swallowed coins or surgical hardware, but less useful for distinguishing between two types of soft tissue that absorb radiation at similar rates.
Types of X-Ray Imaging
The basic X-ray (also called a radiograph) is just one version of X-ray-based imaging. Several other techniques use the same underlying technology in different ways.
- Standard radiography produces a single, still image. It’s what most people picture when they hear “X-ray” and is used for fractures, chest infections, and dental exams.
- Fluoroscopy passes a continuous X-ray beam through the body, transmitting a live, moving image to a monitor. This lets doctors watch things in real time, like how you swallow, how a joint moves, or where a catheter is positioned during a procedure.
- CT scans (computed tomography) use specialized X-ray equipment and computers to take many images from different angles, then assemble them into detailed cross-sectional slices of the body. A CT can reveal soft tissue detail that a standard X-ray cannot, but it delivers a significantly higher radiation dose.
What Happens During an X-Ray
For a standard X-ray, the process is quick. You’ll be asked to wear a hospital gown and remove all jewelry, watches, eyeglasses, and anything else made of metal, since these show up on the image and can obscure the area your doctor needs to see. A technologist will position you against the X-ray detector (a flat panel or digital plate) and ask you to hold still, sometimes holding your breath for a second or two. The actual exposure takes a fraction of a second.
Fluoroscopy takes longer because it captures moving images. Depending on the procedure, it can last anywhere from 30 minutes to two hours, and a radiologist or assistant will be in the room giving you instructions throughout.
Some X-ray exams require a contrast agent to make certain structures visible. If your doctor needs to see your digestive tract, you may drink a barium solution or receive an iodine-based contrast through an IV. These substances are highly effective at blocking X-rays, so organs and blood vessels that temporarily contain them stand out clearly on the image. Barium is typically used for the esophagus, stomach, and intestines, while iodine-based contrast highlights blood vessels and other soft tissue structures.
Radiation Dose and Safety
All X-ray imaging involves ionizing radiation, which is the main safety consideration. The dose varies enormously depending on the type of exam. A dental X-ray delivers about 0.005 millisieverts (mSv), roughly equivalent to a few hours of natural background radiation from the environment. A chest X-ray delivers about 0.1 mSv. A CT scan of the abdomen and pelvis, by comparison, delivers around 7.7 mSv, which is nearly 80 times the dose of a chest X-ray.
To put those numbers in context, the average person absorbs about 3 mSv per year just from natural sources like radon gas, cosmic rays, and minerals in the soil. A single chest X-ray adds a negligible amount on top of that. CT scans add more, which is why doctors weigh the diagnostic benefit against the exposure before ordering one.
The risk from a single diagnostic X-ray is extremely small for most people. Radiation exposure is cumulative over a lifetime, though, so unnecessary imaging is generally avoided. For children, who are more sensitive to radiation, doctors typically use the lowest dose that still produces a useful image.
X-Rays During Pregnancy
Pregnancy adds an extra layer of consideration, but it doesn’t automatically rule out X-ray imaging. The American College of Radiology distinguishes between exams that don’t directly expose the pelvis or uterus, like a chest X-ray, an extremity X-ray, or head and neck imaging, and those that do, like a CT scan of the abdomen or pelvic fluoroscopy. Exams in the first category generally don’t require pregnancy verification at all, since the radiation beam isn’t directed anywhere near the developing fetus.
For exams that do involve pelvic exposure, the benefit of the imaging is weighed against the risk. Patients of childbearing potential are typically asked about pregnancy status before the exam. If a procedure is expected to deliver a high dose to the area near the fetus, a pregnancy test within 72 hours before the procedure is recommended. But when the imaging is medically necessary, even during pregnancy, it can often still be performed safely. The goal is informed decision-making, not blanket avoidance.
What X-Rays Can and Cannot Show
X-rays are best at imaging structures with high contrast differences. Bones, teeth, and metal objects show up clearly because they absorb far more radiation than the tissue around them. Chest X-rays are effective at spotting pneumonia, lung tumors, and heart enlargement because the air-filled lungs provide strong contrast against denser structures.
Where standard X-rays fall short is in distinguishing between soft tissues of similar density. A torn ligament, a brain tumor, or early-stage organ disease won’t show up well on a plain X-ray. For those situations, doctors turn to CT scans for detailed cross-sectional views, MRI for soft tissue and brain imaging (which uses magnets instead of radiation), or ultrasound for real-time imaging without any radiation at all. Each tool has a role, and the X-ray’s role is as the fast, low-cost, low-dose first look that answers straightforward questions: is it broken, is there fluid in the lungs, has the tooth decayed.