"Scan" is a loose word. It can mean a two-minute X-ray of a wrist, a half-hour trip into a magnet, a wand moved over the abdomen or an injection followed by hours of waiting. These are different methods, each making its picture from a different kind of energy, and the differences explain most of what you are asked to do, how long it takes and which safety questions come up. This page is the map of the whole subject. It describes the main types in general terms, using public health and radiology sources, and links to the shorter guides on each. It cannot say which scan is right for anyone, which is a decision for your doctor.
One idea behind every scan
Every method works by sending some kind of energy into the body, or into the body's own chemistry, and recording what comes back or passes through. Tissues respond differently, so the record can be turned into a picture. What changes from method to method is the energy and, with it, what the picture is good at. RadiologyInfo.org, the patient site of the Radiological Society of North America and the American College of Radiology, describes X-rays as a form of energy like light and radio waves that is strong enough to pass through the body and, on the way, passes through bones, tissues and organs differently, which lets a radiologist create images.
The doctor who reads the pictures is a radiologist. The person who runs the scanner is a technologist or radiographer, as described on the page about what a radiologic technologist does. Their report goes to the doctor who asked for the scan, as explained in reading a radiology report.
X-ray
The oldest and most widely used method, the X-ray sends a small burst of radiation through the body onto a detector. The National Institute of Biomedical Imaging and Bioengineering (NIBIB) says the image represents "shadows" formed by objects inside the body, and lists the problems it can reveal: broken bones, bone changes, scoliosis and other spine curvatures, osteoarthritis, pneumonia and other lung infections, kidney stones, foreign objects and dental problems. A series of X-rays can also be used to show the inside of the body in real time, which doctors use to guide procedures such as placing a stent or catheter; this is called fluoroscopy.
The exam is quick. RadiologyInfo.org says an X-ray exam, from positioning to checking the images, is usually completed within 15 to 30 minutes, and that most need no special preparation. Its limits are at the soft tissues, where X-rays give little information about muscles, tendons or joints. The comparison with the next two methods is on the page about X-ray, CT and MRI compared, and the common bone uses are on imaging for bones, joints and hands.
CT
Computed tomography, or CT, is an X-ray method that builds cross-sectional "slices". NIBIB describes an X-ray source rotating around the patient inside a donut-shaped gantry, with detectors opposite, and a computer turning the signals into slices that can be stacked into a three-dimensional image. It says CT can be used to identify disease or injury in many regions of the body, from looking for tumours in the abdomen to locating head injuries, clots leading to stroke and bleeding, and revealing lung problems, and that it is particularly useful for complex bone fractures.
RadiologyInfo.org calls CT one of the fastest and most accurate tools for examining the chest, abdomen and pelvis and notes that in emergencies it can reveal internal injuries and bleeding quickly. It also describes the practical side: the exam is usually completed within 30 minutes, and some CT exams use contrast material, which can be swallowed, injected into a vein or, rarely, given by enema. Because CT uses X-rays it involves ionizing radiation, a topic covered on the page about radiation, contrast and imaging safety.
MRI
Magnetic resonance imaging uses no X-rays. NIBIB explains that powerful magnets align protons in the body, a radiofrequency current is pulsed through the patient, and sensors detect the energy released as the protons realign; the time that takes and the energy released differ with the type of tissue. MRI is described as particularly well suited to soft tissue: the brain, spinal cord and nerves, as well as muscles, ligaments and tendons, are seen much more clearly than with regular X-rays and CT, which is why it is used for knee and shoulder injuries.
The trade-offs are in the experience. RadiologyInfo.org says an MRI exam usually takes 30 to 60 minutes, is loud, requires you to keep still and may make people feel closed in, and that metal and some implants must be screened. A contrast agent containing gadolinium is sometimes injected. These points are covered under preparing for a scan and your results.
Ultrasound
Ultrasound makes pictures from sound waves too high to hear. A handheld transducer sends them in and listens for echoes, and a gel on the skin helps the sound travel. NIBIB says ultrasound can image internal organs non-invasively, is widely used in pregnancy to monitor growth and development of the fetus, and also images the heart, blood vessels, thyroid, breast and abdominal organs; Doppler techniques show blood flow. It is not good for imaging bone or tissue that contains air, such as the lungs. NIBIB describes diagnostic ultrasound as generally regarded as safe and as not producing ionizing radiation. More is on the page about ultrasound basics.
Nuclear medicine
Nuclear medicine works in a different direction. NIBIB describes it as a specialty that uses injected radioactive molecules, called tracers, to assess bodily functions and to diagnose and treat disease, with specially designed cameras tracking where the tracers go. RadiologyInfo.org says it determines how the body is functioning at a cellular level and lists uses such as looking at blood flow in the heart, checking bones for fractures, infection and arthritis, and measuring thyroid function. Tracers can be injected, swallowed or inhaled. The patient is exposed to radiation from the tracer. A fuller account is on the page about nuclear medicine in plain terms.
Mammography
A mammogram is not a separate kind of energy but a specialised use of one. RadiologyInfo.org describes mammography as imaging that uses a low-dose X-ray system to see inside the breasts, used for screening people without symptoms and for investigating symptoms. NIBIB notes that it works by compressing the breast against a flat digital detector with a parallel plate. Screening guidance differs between countries, as explained on the page about mammography and screening basics.
Beyond the picture: contrast, fusion and guidance
Several of these methods can be extended in the same visit. One extension is contrast material. RadiologyInfo.org explains that contrast agents are substances that temporarily change the way X-rays or other imaging tools interact with the body, making selected organs, vessels or tissues look different from their surroundings; they are not dyes that permanently discolour anything. Iodine-based and barium compounds are used for X-ray and CT, gadolinium for MRI, and tiny gas microbubbles for some ultrasound exams. The safety questions around them are covered on the safety page.
Another is fusion. RadiologyInfo.org says many centres combine nuclear medicine images with CT or MRI so that the doctor sees information from two different exams in one image, which it says leads to more precise information and a more exact diagnosis. A third is guidance. Because ultrasound and CT show what is happening in real time, both are used to steer a needle during a biopsy, and ultrasound is also used to follow a catheter inside a blood vessel.
Finally, the same physical principles that make pictures are also used for treatment, which is a separate subject from diagnosis. NIBIB notes that X-rays at much higher levels can be used to destroy cancer cells and that the dose used for treatment is much higher than the dose used for diagnostic imaging. Therapeutic ultrasound and nuclear medicine therapy likewise use higher outputs for treatment than for imaging. Everything on this site concerns diagnostic imaging unless it says otherwise.
The methods side by side
| Method | Picture made from | Ionizing radiation | Typical exam time |
|---|---|---|---|
| X-ray | X-rays through the body onto a detector | Yes | 15 to 30 minutes in all |
| CT | X-rays from a rotating source, built into slices | Yes | Usually within 30 minutes |
| MRI | Magnetic field and radio waves | No | 30 to 60 minutes, sometimes longer |
| Ultrasound | High-frequency sound and its echoes | No | About 30 minutes, up to an hour for fuller exams |
| Nuclear medicine | A radioactive tracer followed by a camera | Yes | Imaging 20 minutes to several hours, sometimes over several days |
| Mammography | Low-dose X-rays of the compressed breast | Yes | About 30 minutes |
How the choice is made
No method is best at everything, and the sources repeatedly say so. RadiologyInfo.org's page on paediatric CT says ultrasound or MRI can sometimes give pictures as good as or better than CT, and that the doctor and the radiologist decide together which examination is best. The same page says radiation exposure in children can be limited by avoiding scans that are not clearly needed and by considering other tests, such as MRI or ultrasound, that might give the same information. Issues of cost, availability, speed, the part of the body, implants and pregnancy all feed in. If you want to understand why a certain scan was chosen, the list of questions to ask before a scan is a place to start, and the guides on back and nerve pain and children and pregnancy show how the choice plays out in particular situations.
Frequently asked questions
What are the main types of medical imaging?
The main types are X-ray (including CT and mammography), MRI, ultrasound and nuclear medicine. Each uses a different kind of energy: X-rays, magnetic fields and radio waves, sound waves, or a radioactive tracer.
Which imaging tests do not use radiation?
NIBIB says MRI does not use the ionizing radiation of X-rays and diagnostic ultrasound does not produce ionizing radiation. X-ray, CT and nuclear medicine do involve radiation.
What is the difference between a CT scan and an MRI?
CT builds slices from X-rays and is fast, while MRI uses magnets and radio waves, takes longer and shows soft tissue in detail. The comparison page explains what each is best at.
Who decides which scan I have?
Your doctor, often with a radiologist, chooses the scan best placed to answer the medical question. If you are unsure why a scan was chosen, ask.
The short version
Medical imaging is a family of methods that use different energy to picture the body: X-rays for bones and quick looks, CT for detailed slices, MRI for soft tissue without radiation, ultrasound for live pictures from sound, nuclear medicine for how organs work and mammography for the breast. Which one is used depends on the question being asked, and your doctor can tell you why.