Quick answer: both use x-rays, but a regular x-ray takes one flat picture, while a CT scan rotates around you and builds detailed cross-sectional images, like slices of bread. An x-ray is quick, cheap and uses a very small dose (a chest x-ray is about 0.1 mSv, roughly 10 days of natural background radiation). A CT shows much more detail, especially of soft tissues, organs and blood vessels, but uses more radiation (a chest CT is about 6.1 mSv, roughly 2 years of background). X-ray is often the first test; CT is used when more detail is needed.
I do both every day, and patients are often surprised to learn that a CT is also an x-ray test. Here is how they differ and why doctors often start with one and then order the other.
How each one works
- X-ray (radiograph): a small amount of radiation passes through the body to a detector. Bones absorb more x-rays and appear white; soft tissues appear in shades of gray; air appears black. The result is a single flat image in which structures overlap.
- CT scan (computed tomography): x-ray beams and detectors rotate around you while the table moves. A computer processes the data into thin cross-sectional slices that can be combined into 3D views, so structures no longer overlap.
X-ray vs CT at a glance
| X-ray | CT scan | |
|---|---|---|
| Image | One flat, 2D picture per view | Many cross-sectional slices; 3D views possible |
| Detail | Good for bones, lungs and some joints | Much more detail of organs, soft tissues, blood vessels and bones |
| Radiation | Very low to low | Higher, varies by exam |
| Time | Minutes | Scanning takes seconds; exam usually within 30 minutes |
| Contrast | Usually none | Sometimes oral and/or IV contrast |
| Equipment | X-ray tube and detector plate; you stand, sit or lie | Donut-shaped scanner; you lie on a moving table |
| Cost | Lower | Higher |
| Availability | Very widely available | Widely available, especially in hospitals |
Radiation doses compared
These are approximate effective doses for an average-sized adult, from the American College of Radiology and RSNA. The average person in the U.S. receives about 3 mSv per year from natural background radiation.
| Exam | Approximate dose | Same as natural background for about |
|---|---|---|
| Extremity x-ray (hand, foot) | Less than 0.001 mSv | Less than 3 hours |
| Chest x-ray | 0.1 mSv | 10 days |
| Lumbar spine x-ray | 1.4 mSv | 6 months |
| CT of the head and neck | 1.2 mSv | 5 months |
| CT of the brain | 1.6 mSv | 7 months |
| Low-dose CT for lung cancer screening | 1.5 mSv | 6 months |
| CT of the chest | 6.1 mSv | 2 years |
| CT of the abdomen and pelvis | 7.7 mSv | 2.6 years |
| CT of the spine | 8.8 mSv | 3 years |
Actual doses vary with body size, the reason for the scan and the equipment. More in [link: How Much Radiation Is in a CT Scan?].
When doctors usually choose an x-ray
- Suspected broken bones in the arms, legs, hands and feet
- Chest problems such as pneumonia, a collapsed lung or heart size
- Joint wear such as arthritis
- Dental problems
- Checking tubes and devices placed in the body
- As a quick first look before deciding whether more imaging is needed
When doctors usually choose CT
- The x-ray was normal or unclear but the problem persists, for example a suspected fracture that does not show on x-ray
- Complex fractures and injuries that need detailed planning, such as in the spine, pelvis or face
- Trauma, to find internal injuries and bleeding quickly
- Abdominal pain, for causes such as appendicitis, diverticulitis or kidney stones
- Lung nodules, cancers and blood clots in the lungs
- Head injuries and suspected bleeding in the brain
- Guiding biopsies and other needle procedures
See [link: What to Expect During a CT Scan] and [link: Abdominal CT Scan: Preparation and What It Shows].
From the scanner room: [TU EXPERIENCIA: un ejemplo típico (sin datos identificables) de paciente que llega con una radiografía normal o dudosa y termina en el TAC, o al revés. Qué diferencia notan los pacientes entre las dos pruebas.]
Why you might get both
It is very common to start with an x-ray because it is fast, inexpensive, low-dose and available almost everywhere. If the x-ray answers the question, that may be the only test you need. If it does not, or if it shows something that needs a closer look, your doctor may order a CT. Sometimes MRI or ultrasound is a better next step, especially for soft tissues or when radiation should be avoided. See [link: CT Scan vs MRI].
Safety: what to know
- No radiation remains in your body after an x-ray or CT.
- Imaging centers follow the ALARA principle (As Low As Reasonably Achievable), using the lowest dose that gives a useful image.
- Tell the team if you might be pregnant before any x-ray or CT.
- Children are more sensitive to radiation, so their scans use doses adjusted to their size.
- The benefit of an accurate diagnosis usually outweighs the small risk. If you have concerns, ask your doctor whether the test is necessary and whether there is an alternative without radiation.
Tips from the technologist
[TU EXPERIENCIA: 2-3 consejos propios. Ideas: quitar el metal de la zona, avisar de posible embarazo, preguntar si una radiografía previa evita repetirla.]
Frequently asked questions
Is a CT scan just a more detailed x-ray?
In a way, yes. Both use x-rays, but CT takes many images from different angles and a computer turns them into cross-sectional slices, which show far more detail.
Which has more radiation, an x-ray or a CT?
CT. A chest CT, for example, is about 6.1 mSv, roughly 60 times a chest x-ray at 0.1 mSv.
Can an x-ray miss a fracture that a CT would find?
Yes, sometimes. Small or complex fractures may not be visible on x-ray, which is one reason doctors order a CT or MRI when pain persists.
Is a CT scan better than an x-ray?
Not always. CT shows more detail, but an x-ray is often enough, faster, cheaper and lower in radiation. The right test depends on the question.
Do I need contrast for an x-ray?
Most standard x-rays need no contrast. Some special x-ray studies use contrast, and CT often does.
This article is for general information and does not replace advice from your doctor or imaging team.