Imaging methods: X-ray, CT, and MRI basics

How an X-ray forms a shadow, how a CT scanner builds a computed slice, and why MRI avoids ionising radiation, explained for patients and families.

An X-ray image is a two-dimensional projection, a shadow cast by the parts of the body that absorb more of the beam. A CT scanner takes many such projections around the body and computes cross-sectional slices from them. An MRI scan uses a strong magnetic field and radio waves instead of X-rays, so it does not involve ionising radiation.

These three sentences answer the most common questions people ask when a clinician has requested an imaging exam. The detail behind them matters, because it explains why one method is chosen over another, what the appointment will involve, and what the resulting report can and cannot show.

What is an X-ray image and why is it like a shadow?

An X-ray tube produces a beam that passes through the body and strikes a detector on the other side. Dense tissues such as bone absorb more of the beam, while air in the lungs absorbs very little. The detector records the pattern of what got through, and that pattern is displayed as shades of grey: white where little radiation reached the detector, dark where most of it did.

The result is a flat projection, not a slice. Everything along the path of the beam is superimposed on the same image, which is why a chest X-ray can show ribs, heart outline, and lung fields at once. The shadow comparison is useful because it captures both the mechanism and the limitation: a shadow tells you something is there, but not always how deep it sits. Radiographers often take a second view from a different angle to give the reporting clinician a sense of depth.

For a patient, the practical points are simple. The examination is usually quick, the dose is low compared with other X-ray based methods, and no contrast agent is needed unless the clinician has asked for a specific study. A page such as X-ray image, shadow explains the same projection principle in plain language, alongside the other imaging methods covered on that site.

How does a CT scanner create a computed slice?

A CT scanner places the X-ray tube and the detectors on a rotating ring around the patient. As the ring turns, the system collects hundreds of projections from many angles. A computer then reconstructs those projections into cross-sectional images, which is where the word computed comes from. Each slice represents a thin band of the body, and the stack of slices can be viewed from the front, the side, or any plane the reporting clinician chooses.

Because the data is digital and three-dimensional, CT can separate structures that overlap on a plain X-ray. That is why it is often used when a clinician needs to see bones, blood vessels, or organs in relation to one another. The trade-off is dose: a CT examination generally delivers more ionising radiation than a single plain radiograph, which is why the request is reviewed against the clinical question before the appointment is booked.

Some CT studies use an iodine-based contrast agent, given by mouth, by injection, or both. The contrast changes how certain tissues appear on the images and can help distinguish normal from abnormal. Patients are usually asked about kidney function, allergies, and pregnancy before contrast is given, and the radiology team will explain what to expect during the scan itself.

Why does an MRI scan not use ionising radiation?

MRI stands for magnetic resonance imaging. It uses a strong magnetic field and pulses of radio waves to interact with hydrogen nuclei, mostly in water and fat, throughout the body. When the pulses stop, the nuclei release energy that the scanner detects, and a computer turns that signal into images. No X-ray tube is involved, so there is no ionising radiation dose.

That difference matters for repeated examinations, for younger patients, and for anyone who has already had several CT scans. It does not make MRI risk-free or suitable for everyone. The strong magnetic field means that certain implants, pacemakers, and metallic foreign bodies need to be declared and checked before entering the scanner room. The examination is also longer and noisier than a plain X-ray, and some people find the enclosed space difficult.

MRI is particularly good at showing soft tissues such as the brain, spinal cord, joints, and pelvic organs. A clinician may choose it when the question is about those structures rather than about bone detail. As with CT, a contrast agent is sometimes used, and the radiology team will confirm beforehand whether it is needed.

How is the right method chosen for a given question?

The choice is not a ranking of methods. It is a match between the clinical question and the physical properties of each technique. A suspected fracture in a limb is often assessed with plain X-ray first. A suspected problem in the abdomen or chest may need CT. A suspected issue in the brain, spine, or a joint may need MRI. Ultrasound, which uses sound waves rather than X-rays, is common for soft tissues and for guiding certain procedures.

In the United Kingdom, the request usually comes as a written referral from the clinician who has examined the patient. The radiology department checks that the request is justified for the individual, that the examination is optimised to answer the question, and that the dose is kept as low as reasonably practicable. These principles are set out in the Ionising Radiation (Medical Exposure) Regulations 2017, often shortened to IR(ME)R 2017, which place duties on the referrer, the practitioner, and the operator.

Patients do not need to memorise the regulations. It helps, however, to know that the system is designed so that someone other than the referrer reviews the request, and so that the person performing the examination is trained and accountable. If a patient is unsure why a particular method was chosen, the radiology department is the right place to ask.

What happens during the appointment and after?

On the day, a radiographer or, for ultrasound, a sonographer, will confirm the patient's identity and the examination to be performed. They will explain the position, the breathing instructions if any, and how long the scan is expected to take. For CT and MRI, the patient lies on a table that moves through a ring-shaped scanner. For plain X-ray, the patient may stand, sit, or lie down depending on the body part.

After the images are taken, they are reviewed by a radiologist, a doctor who specialises in interpreting imaging. The radiologist writes a report that describes the findings and answers the clinical question. That report goes back to the referring clinician, who discusses the result with the patient and decides on any next steps. Patients who want a copy of their report can usually request it through the hospital's records process.

Understanding the method behind the image helps a patient follow that conversation. A shadow on a plain film, a computed slice from a CT scanner, and a magnetic resonance signal from an MRI are three different ways of looking inside the body, each with its own strengths and its own limits. Knowing which one was used, and why, makes the report easier to place in context.

Imaging and measurement questions come back in other rooms. Checking electromagnetic exposure at home applies the same limits, and what to bring to a new clinic covers the appointment around the scan.

Change note: this page is part of the August 24, 2026 offline build. Source ownership and review dates are visible so future checks can be made without guessing.