When a doctor says you need a scan, the word covers very different machines. An X-ray takes a few minutes, a CT scan builds a stack of slices, and an MRI scanner is a long, loud tunnel. Each answers different questions, and each has its own practical points and safety checks. This page compares the three in plain terms. It does not suggest which one anyone should have, because that depends on the question being asked, and your doctor makes that call. For the wider family, including ultrasound and nuclear medicine, see the overview of types of medical imaging.
How each picture is made
A standard X-ray sends a small burst of radiation through the body onto a detector. RadiologyInfo.org explains that dense bone absorbs much of the radiation and appears white, soft tissue shows in shades of gray, and air appears black. Most X-ray images today are digital files stored electronically.
CT, short for computed tomography, is also an X-ray method but builds a more detailed image. The National Institute of Biomedical Imaging and Bioengineering (NIBIB) describes a motorised X-ray source rotating around a donut-shaped gantry while the patient lies on a bed that moves slowly through it. A computer turns the signals into cross-sectional "slices", which can be stacked into a three-dimensional image.
MRI does not use X-rays at all. NIBIB explains that powerful magnets align protons in the body's water, a radiofrequency pulse knocks them out of alignment, and sensors detect the energy released as they realign. Because that energy differs from one tissue to another, doctors can tell tissues apart.
What each shows best
X-ray is the familiar first look at bones. The NHS lists checking for a fracture after an accident, looking at teeth and checking the lungs among its uses, and RadiologyInfo.org says CT images of organs, bones, soft tissue and blood vessels give greater detail than traditional X-rays, especially for soft tissue and vessels. A major advantage of CT, according to the same source, is its ability to image bone, soft tissue and blood vessels all at the same time, which is part of why it is a common tool in emergencies.
MRI is the one chosen for soft tissue. NIBIB says MRI scanners are particularly well suited to the non-bony parts of the body, and that the brain, spinal cord and nerves, as well as muscles, ligaments and tendons, are seen much more clearly with MRI than with regular X-rays and CT. RadiologyInfo.org adds that MRI is often better than X-ray, CT and ultrasound at telling diseased tissue from normal tissue. In the other direction, it notes that soft-tissue details in the brain, gallbladder, internal pelvic organs and joints such as knees and shoulders can often be better evaluated with MRI than with CT.
What the visit involves
| X-ray | CT | MRI | |
|---|---|---|---|
| Energy used | X-rays | X-rays | Magnetic field and radio waves |
| Typical time | 15 to 30 minutes in all, from positioning to checking the images (RadiologyInfo.org); the NHS says the X-ray itself takes only a few minutes | Usually within 30 minutes (RadiologyInfo.org); 10 to 20 minutes (NHS) | 30 to 60 minutes (RadiologyInfo.org); 15 minutes to an hour, sometimes longer (NHS) |
| Contrast | Sometimes, for example barium | Sometimes, by mouth, by vein or, rarely, by enema | Sometimes gadolinium by vein |
| Noise | None to speak of | Slight buzzing, clicking and whirring | Loud tapping and thumping; earplugs or headphones are given |
The experience differs most in the MRI tunnel. RadiologyInfo.org says you must stay still, and that some patients find the scanner closed-in, so it advises anyone with claustrophobia or anxiety to ask their doctor about a mild sedative before the day. It describes open and wider-bore scanners as options for some exams, though not for all. CT, by contrast, is described as generally painless, fast and easy, with a gantry that is a short tunnel rather than a long one. In both, the technologist can see, hear and speak to you from the control room.
Safety questions that differ
X-ray and CT use ionizing radiation, which is why staff ask whether you might be pregnant; MRI does not. That is only one of the questions, though. Before an MRI you are screened for metal and implants because of the magnet, and RadiologyInfo.org says an MRI exam is safe for patients with metal implants in most cases, except for a few types such as some cochlear implants, certain brain aneurysm clips, some older pacemakers and defibrillators and some nerve stimulators. For CT, the same site notes that, unlike MRI, an implanted medical device of any kind will not prevent you from having a CT scan.
The details of dose, contrast and screening are covered in radiation, contrast and imaging safety. Sources agree on one point of caution: because radiation exposure is added up across a lifetime, doctors weigh whether each X-ray or CT scan is needed. When imaging of the abdomen or pelvis is needed in pregnancy, NIBIB says doctors generally prefer methods that do not use radiation, such as MRI or ultrasound, though CT may be acceptable if neither can provide the answers or there is an emergency.
How doctors tend to choose
Speed and availability matter. RadiologyInfo.org says CT is fast and is less sensitive to patient movement than MRI, which is a reason it is used in trauma and acute symptoms such as chest or abdominal pain. MRI takes longer and asks more of the patient but avoids radiation, and NIBIB calls it the imaging method of choice when frequent imaging is needed, especially in the brain, while noting that it is more expensive than X-ray or CT. For bones, joints and hands, the usual order of events is described on the page about imaging for bones, joints and hands, and for spine problems on the page about imaging for back and nerve pain.
If you are unsure why a particular scan has been chosen, you can ask. A short list of questions to ask before a scan includes whether another method could answer the same question.
Frequently asked questions
Is a CT scan the same as an X-ray?
CT is a type of X-ray scan, but a much more detailed one. NIBIB says a rotating X-ray source and detectors build cross-sectional slices that give more information than conventional X-rays.
Why is MRI so loud?
RadiologyInfo.org says the coils that generate the radio waves make loud tapping and thumping sounds when they are activated, which is why you are given earplugs or headphones.
Which scan uses no radiation?
MRI. NIBIB says it does not use the ionizing radiation of X-rays and CT. Ultrasound also uses none, and it is covered on its own page.
Can I have a CT scan if I have a pacemaker?
RadiologyInfo.org states that an implanted medical device of any kind will not prevent a CT scan, unlike MRI, where the exact device matters. Tell the staff about any implant either way.
The short version
X-ray is quick and a good first look at bones, CT adds detailed slices of bone, tissue and vessels in minutes, and MRI gives the most detail of soft tissue without radiation but takes longer and is noisier. Sources agree that the choice depends on the question being asked, and your doctor can tell you why a given scan was chosen.