How X‑ray images are made

Written by the X-Ray Atlas editorial team from the sources listed below · Medical review pending · Updated 24 October 2026 · Level: Beginner

A plain radiograph is a shadow picture. A beam of X-rays passes through the patient, and a detector on the far side records how much of the beam got through. Tissues that stop more of the beam appear white; tissues that let it pass appear black. You do not need to be a physicist to read X-rays, but a handful of principles explain almost every sign you will learn: why fluid and soft tissue look the same, why an outline vanishes when pneumonia touches the heart, and why the heart looks bigger on a portable film. This page covers those principles and nothing more.

1. Attenuation: why things are white or black

X-rays are a form of high-energy electromagnetic radiation. As the beam passes through the body, some photons are absorbed and some are scattered out of the beam. The overall reduction in the beam is called attenuation. How strongly a tissue attenuates depends mainly on two things:

• Density (how tightly packed the material is). Air in the lungs is far less dense than muscle, so almost all of the beam passes through lung.
• Atomic number of the elements present. Calcium in bone and iodine or barium in contrast agents have high atomic numbers and absorb X-rays strongly. Metal absorbs almost all of the beam.

Thickness matters too: a thick body part attenuates more than a thin one of the same tissue. Where many photons reach the detector, the image is dark (radiolucent). Where few arrive, it is white (radiopaque or radio-dense). On a conventional display, "black" therefore means "the beam got through", not "nothing is there".

2. The five radiographic densities

Plain films can separate only a small number of densities. From black to white they are:

Density	Appearance	Examples
Air (gas)	Black	Lungs, trachea, bowel gas, pneumothorax, free intraperitoneal gas
Fat	Dark grey	Subcutaneous fat, fat around the kidneys, the elbow fat pads, lipomas
Soft tissue and fluid	Mid grey	Heart, muscle, liver, blood, pus, pleural fluid, a tumour
Bone and calcium	White	Ribs, vertebrae, calcified nodes, gallstones and kidney stones that contain calcium
Metal and contrast	Brilliant white	Surgical clips, prostheses, pacemakers, barium, iodinated contrast, many foreign bodies

The most important practical point is in the middle row: fluid and soft tissue look identical on a plain film. Blood, pus, water, a solid tumour and the heart are all "soft tissue density". That is why a white area at the lung base could be consolidation, collapse, a pleural effusion or a mass, and why you must use shape, position, volume and borders to tell them apart. It is also why ultrasound or CT is often the next step.

Fat is useful precisely because it is slightly darker than soft tissue. Displaced fat pads at the elbow, for example, signal an effusion inside the joint, and loss of the fat planes around organs can suggest inflammation or fluid. Some materials sit between categories: many types of glass are partly opaque, while wood and most plastics are hard to see at all. See foreign bodies.

3. Edges: you only see a boundary between different densities

A structure is visible on a radiograph only because its density differs from whatever lies next to it. The heart border is visible because soft-tissue heart lies against air-filled lung. The outline of the diaphragm is visible because the soft-tissue diaphragm and liver lie under aerated lung.

If something of the same density moves into contact with that border, the boundary disappears. Consolidation in the right middle lobe, which touches the right atrium, erases the right heart border. Fluid at the left base erases the left hemidiaphragm. This is the silhouette sign, described by Felson, and it is the single most useful localising tool on a chest film.

The same rule explains air bronchograms. Normally the airways are invisible inside the lung, because air in the bronchus sits next to air in the alveoli. When the alveoli fill with fluid or pus, the air-filled bronchi stand out as dark branching lines against the white lung.

4. kV and mAs: controlling the beam

The radiographer sets two main exposure factors.

• kV (peak kilovoltage) sets the energy of the photons, which controls how penetrating the beam is. Higher kV gives a more penetrating beam and lower contrast: more shades of grey and less stark difference between bone and soft tissue. A PA chest is typically taken at a high kV, about 100–125 kV, so that the mediastinum and the lung behind the heart are visible through the dense central structures. Lower kV is used where high contrast is wanted, for example the bones of the hand.
• mAs (tube current × exposure time) sets the number of photons. More photons produce a less noisy image with greater overall density, at the cost of a higher dose. Too few photons give a grainy, "noisy" picture.

These factors explain the E (exposure) in the RIPE quality check. An under-penetrated chest film is too white: the area behind the heart cannot be seen and may falsely look consolidated. An over-penetrated film is too dark: small nodules fade away, and the lungs can look as if they contain too little tissue, mimicking emphysema or even pneumothorax. On a well-exposed PA chest, the thoracic vertebral bodies are just visible through the heart.

Exposure is always a balance against radiation dose. Under UK law (IR(ME)R 2017) the dose must be kept as low as reasonably practicable for the clinical purpose.

Scatter

Some photons are deflected inside the patient and strike the detector at the wrong place. This scatter adds a general grey fog that reduces contrast, and there is more of it in thick body parts such as the abdomen and pelvis. Departments reduce its effect with anti-scatter grids and careful collimation (narrowing the beam to the area of interest). Tight collimation also lowers dose, which is one reason a film may not include as much anatomy as you would like.

5. Geometry: magnification, distortion and superimposition

X-rays spread out from a small point in the tube, like light from a torch. Any object between the tube and the detector casts a shadow that is larger than the object itself. Two distances control how much larger:

• Object–detector distance. The further a structure is from the detector, the more it is magnified and the blurrier its edges.
• Source–image distance (SID). The further the tube is from the detector, the more parallel the rays and the less the magnification. A standard PA chest uses an SID of about 180 cm (72 inches).

This is why the projection matters so much. On a PA chest film the beam enters from behind and the front of the chest is against the detector, so the heart, which sits anteriorly, is close to the detector and only slightly magnified. On an AP film (portable, ward or ICU), the heart is further from the detector and the tube is usually closer, so the heart looks bigger. The cardiothoracic ratio is therefore validated only on PA films. A heart that looks large on an AP film cannot reliably be called enlarged; one that looks normal on AP is reassuring. See X-ray projections.

Superimposition

A radiograph compresses a three-dimensional patient into two dimensions. Everything along the path of each ray is stacked on top of everything else. Ribs cross over lung nodules, the heart hides the left lower lobe, and bowel overlaps the kidneys. This is the reason for the rule that one view is rarely enough: a second view at right angles (an orthogonal view) separates overlapping structures, shows depth and reveals fractures or dislocations that are invisible on the first view.

Distortion

If the beam is not perpendicular to the detector, or the patient is rotated, shapes are distorted. A rotated chest film changes the outline of the mediastinum and can make one hilum look prominent. An angled view of a long bone can foreshorten it. Some views use deliberate angulation to project a structure clear of overlapping bone, for example the angled scaphoid views of the wrist.

6. Detectors and digital images

Almost all UK departments now use digital systems. In computed radiography (CR) a phosphor plate in a cassette stores the image and is read by a laser scanner. In direct digital radiography (DR) a flat-panel detector converts the X-rays into an electronic signal straight away. Either way, the image arrives on a workstation (PACS) where you can adjust it.

• Window and level. Changing the brightness and contrast settings lets you look into the dense mediastinum or at a faint pneumothorax edge. Use them, but remember that adjusting the display cannot recover detail that was never captured.
• Magnify. Zooming in on the apices, the costophrenic angles or a cortex helps you spot small findings.
• Invert. Some readers find an inverted (black-on-white) display helps with pneumothorax lines or nodules.
• Processing. Digital systems apply software processing, such as edge enhancement, which can create thin dark halos next to bright edges. These can mimic pathology, for example a lucent line around a metal implant. See artefacts and mimics.

Digital detectors are forgiving of exposure errors on screen, because the software rescales the image. That makes it easy to over-expose without noticing. Radiographers monitor dose indicators to prevent this "dose creep".

7. Common pitfalls

• Calling heart enlargement on an AP film. Magnification alone can push an AP cardiothoracic ratio above 0.5.
• Forgetting that fluid equals soft tissue. A white hemithorax tells you something is there, not what it is. Use volume, shift and borders, then consider ultrasound or CT.
• Trusting a single view. Superimposition hides fractures, dislocations and masses. A posterior shoulder dislocation, for example, can be missed on one AP view.
• Mistaking exposure for disease. A dark, over-penetrated film can mimic emphysema; a pale, under-penetrated one can mimic consolidation behind the heart.
• Assuming a radiolucent object is absent. Wood, most plastics and uric acid stones may be invisible on plain film. A normal film does not exclude them.

Red flags: act now

What to focus on, by role

Role	Priorities
Medical student	The five densities, the idea that edges need a density difference, and why AP films magnify the heart. These three ideas underpin most exam questions.
Nurse	Understand why a portable film may look different from a departmental one, and why metal devices and tubes show up so clearly while some objects do not.
Junior doctor	Recognise when a film is too poor to answer the question. Use window and level on PACS for subtle pneumothorax and line tips. Know when CT or ultrasound is the right test.
Radiographer	Exposure factor selection, collimation, grids, SID, dose indicators and processing artefacts. Technique is where you control image quality and dose.
Advanced / FRCR	Processing artefacts, the limits of plain film density resolution, and the physics behind signs such as the silhouette and air bronchogram.

Next: X-ray projections, then image quality: RIPE. Unfamiliar terms are in the glossary.

Test your foundations →

References

1. Radiology Masterclass. Chest X-ray quality: projection. radiologymasterclass.co.uk
2. Life in the Fast Lane (LITFL). Chest X-ray interpretation. litfl.com
3. Felson B. Chest Roentgenology. Philadelphia: WB Saunders; 1973 (silhouette sign).
4. The Ionising Radiation (Medical Exposure) Regulations 2017. legislation.gov.uk
5. UK Health Security Agency. Patient dose information: guidance. gov.uk
6. Royal College of Radiologists. iRefer: making the best use of clinical radiology. irefer.org.uk

For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.