X-ray, CT and MRI: how each image is made

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

An X-ray tube produces a beam that crosses the patient and lands on a detector, either a film cassette or a digital panel. Every structure in the path removes part of the beam, in proportion to its density and to the atomic number of the atoms it contains. Calcium in bone removes a good deal; water in muscle and fat removes less; air removes very little. The detector records what is left of the beam, and the result is a flat map of everything the beam met along the way.

The shadow comparison is not decorative. Hold a hand between a lamp and a wall and you see a dark outline with no depth: you cannot tell whether the finger is nearer the lamp or nearer the wall. An X-ray image has the same property. A rib and a lung nodule can sit at different depths and still overlap on the same picture. Radiographers therefore take at least two views, usually from the front and from the side, so that the position of a finding can be worked out from two projections. The X-ray image, shadow comparison is the one most often used when explaining this to a patient, because it accounts for both the strength of the method, its speed and low dose, and its main weakness, the loss of depth.

How does a CT scanner create a computed slice?

A CT scanner rotates an X-ray tube and a detector ring around the patient. Instead of one projection, it collects a few hundred or a few thousand projections in a single pass, each from a different angle. A computer then solves the geometry: for every small volume of tissue in the scanned region, it finds the density value that would produce the measurements actually recorded. The output is a grid of numbers, and each number becomes a shade of grey in the displayed image.

The word computed is literal. Nothing in the machine produces a slice directly; the slice exists only after the calculation. This is why CT images can be reformatted after the examination, for example into coronal or sagittal views, or into three-dimensional models for surgical planning, without exposing the patient again. It also explains the dose: because many projections are needed, a CT examination delivers more ionising radiation than a single plain radiograph, which is why the clinical question and the justification for the examination are checked before it goes ahead.

Why does an MRI scan not use ionising radiation?

MRI relies on the magnetic behaviour of hydrogen nuclei, which are abundant in the body because of water and fat. Inside the scanner, a strong static magnetic field aligns these nuclei. A radiofrequency pulse then tips them out of alignment, and as they relax back, they emit a weak signal that coils around the patient pick up. The radio waves used sit in the same part of the spectrum as broadcast and mobile signals, far below the energy needed to remove an electron from an atom.

Ionising radiation is defined by its ability to ionise, that is, to knock electrons off atoms and thereby damage molecules including DNA. X-rays carry enough energy to do this; radio waves do not. That is the whole reason MRI is described as non-ionising, and it is why the method is used freely for brain, spinal, joint and soft-tissue examinations where repeated or follow-up imaging may be needed. It does not follow that MRI is risk-free in every circumstance. The strong magnetic field attracts ferromagnetic objects, so implants, pacemakers and loose metal must be declared before entering the scanner room, and the examination is noisy and longer than a plain radiograph.

Ultrasound: a fourth way to see inside

Ultrasound sends short pulses of high-frequency sound into the body and listens for the echoes that return from boundaries between tissues. The time taken by each echo gives depth, and the strength of the echo gives a shade of grey. There is no ionising radiation here either, which is why obstetric imaging relies on it. Its limits are different from those of CT and MRI: sound does not travel well through bone or gas, so the method is less useful for the lungs and for most of the skull.

What the three methods share

All of these techniques produce a picture of the inside of the body without opening it, and all of them depend on a clinician's written request stating the question to be answered. Before an examination is performed, the imaging department checks that it is justified, that the right method has been chosen, and that the dose, where dose applies, is as low as reasonably practicable. The report that comes back is written by a radiologist, while the examination itself is usually carried out by a radiographer, or by a sonographer in the case of ultrasound.

For a patient, the practical differences are easier to hold on to than the physics. A plain radiograph takes seconds and shows overlap. A CT scan takes minutes, produces cross-sections, and uses ionising radiation. An MRI scan takes longer, uses no ionising radiation, and requires a check for metal before entering the room. An ultrasound examination uses sound, is often done in real time, and is limited by bone and gas. Knowing which of these applies to the appointment letter in your hand is usually enough to make the day less opaque.

Questions to ask before the appointment

Which method has been requested, and what question is it meant to answer? Is there any preparation, such as fasting or a full bladder? Will a contrast agent be injected, and has any allergy or kidney problem been reported? Is there any metal in the body, and if so, has it been declared? When and how will the report reach the referring clinician? These are ordinary questions, and the imaging department can answer them before the day of the examination rather than during it.

Source: nhs.uk.

The other half of the same habit, reading what a prescription actually says, is set out in how a drug label is organised.

Source: nhs.uk.