Radiology glossary

180 X-ray terms, signs and eponyms, defined in plain language.

A

Acetabular index
Angle between Hilgenreiner's line and a line drawn along the acetabular roof on an infant's AP pelvis. A raised value suggests a shallow, dysplastic hip; normal values fall with age. Learn more →
Acro-osteolysis
Resorption of the bony tufts at the ends of the fingers or toes. Causes include scleroderma, psoriatic arthritis, hyperparathyroidism and frostbite. Learn more →
Air bronchogram
Dark, branching air-filled bronchi seen against surrounding white, airless lung. It shows that an opacity lies within the lung itself, as in consolidation. Learn more →
Air crescent sign
A crescent of gas between a mass and the wall of the lung cavity containing it, classically with an aspergilloma or recovering invasive aspergillosis.
Air–fluid level
A straight horizontal line where gas sits on top of fluid. It is only seen when the X-ray beam is horizontal, as on an erect or decubitus film. Learn more →
ALARP / ALARA
The principle that radiation doses should be kept as low as reasonably practicable (UK term) or achievable (US term) while still answering the clinical question. Learn more →
Angulation
The angle between the two main fragments of a fracture, described by the direction in which the apex points or the distal fragment tilts. Learn more →
Anterior humeral line
On a true lateral elbow, a line drawn down the front of the humerus that should cross the middle third of the capitellum. If it passes in front, suspect a supracondylar fracture. Learn more →
AP (anteroposterior)
A projection in which the beam enters the front of the patient and exits the back. On a chest film it magnifies the heart. Learn more →
AP window (aortopulmonary window)
The normally concave gap between the aortic knob and the left pulmonary artery on a frontal chest film. Filling it in suggests lymph nodes or a mass. Learn more →
Apical cap
A rim of pleural opacity over the lung apex. Usually harmless scarring, but after trauma it can be blood tracking from an aortic injury. Learn more →
Atelectasis
Partial or complete loss of air from part of the lung, so that it collapses and loses volume. Learn more →
Attenuation
The weakening of the X-ray beam as tissue absorbs or scatters it. Dense tissues attenuate more and appear whiter. Learn more →
Avascular necrosis (osteonecrosis)
Death of bone after its blood supply is lost, most often in the femoral head. It progresses from sclerosis to subchondral fracture and collapse. Learn more →
Avulsion fracture
A fracture in which a tendon or ligament pulls a small piece of bone away from its attachment. Learn more →
Azygos fissure/lobe
A normal variant in which the azygos vein passes through the right upper lobe inside a fold of pleura, producing a curved line near the right apex. Learn more →

B

Bamboo spine
Fusion of the spine by thin bony bridges (syndesmophytes) between the vertebral bodies, giving a smooth, bamboo-like outline in advanced ankylosing spondylitis. Learn more →
Bat's wing (butterfly) opacity
Hazy shadowing spreading out from both hila in a symmetrical pattern, typical of alveolar pulmonary oedema. Learn more →
Baumann's angle
An angle measured on the AP elbow between the humeral shaft and the lateral growth plate, used to judge alignment of a supracondylar fracture. Compare with the other side. Learn more →
Boutonnière deformity
A finger posture with the middle (PIP) joint bent and the end (DIP) joint over-straightened, seen in rheumatoid arthritis and after tendon injury. Learn more →
Bucket-handle fracture
A classic metaphyseal lesion seen at an angle, so the detached rim of metaphysis looks like a curved handle. In infants it is strongly associated with abuse. Learn more →
Buckle (torus) fracture
A childhood fracture in which the cortex bulges or crumples under compression without breaking through. Most common in the distal radius. Learn more →
Bulla
A thin-walled air space in the lung larger than 1 cm across, often associated with emphysema. Learn more →
Burst fracture
A vertebral body fracture from axial loading that breaks the posterior wall, often pushing bone fragments back into the spinal canal. Learn more →
Böhler's angle
An angle on the lateral calcaneus formed by lines across its upper surface; normally about 20–40°. A reduced angle suggests a depressed calcaneal fracture. Learn more →

C

Callus
New bone laid down around a fracture as it heals. Its appearance helps estimate how long ago a fracture occurred. Learn more →
Cannonball metastases
Multiple large, round, well-defined lung metastases scattered through both lungs. Learn more →
Cardiothoracic ratio (CTR)
The widest diameter of the heart divided by the widest internal diameter of the chest. Above 0.5 on an adult PA film suggests cardiomegaly; it is unreliable on AP films. Learn more →
Carina
The point where the trachea divides into the right and left main bronchi. It is a landmark for endotracheal tube and central line tips. Learn more →
Cavitation
A gas-filled space inside a lung lesion, formed when dead central tissue is coughed up. Seen in TB, abscesses and some cancers. Learn more →
Cavo-atrial junction
The point where the superior vena cava enters the right atrium, a common target for central venous catheter tips. Learn more →
Cervicothoracic sign
A rule for locating a mass at the top of the chest: if its edge stays sharp above the clavicles it lies posteriorly; if the edge fades at the clavicles it lies anteriorly. Learn more →
Chance fracture
A horizontal flexion-distraction fracture running through the vertebral body and posterior elements, often caused by a lap seatbelt. Learn more →
Chilaiditi sign
Bowel lying between the liver and the right hemidiaphragm. Its haustral folds distinguish it from free gas, which it can mimic. Learn more →
Chondrocalcinosis
Calcification within cartilage, such as the knee menisci or wrist fibrocartilage. It is the radiographic hallmark of CPPD. Learn more →
Chondroid matrix
Calcification in rings, arcs or popcorn shapes produced by cartilage-forming tumours such as enchondroma and chondrosarcoma. Learn more →
Classic metaphyseal lesion (CML)
A fracture running across the metaphysis of an infant's long bone, close to the growth plate. It is highly specific for physical abuse. Learn more →
Codman triangle
A triangle of new bone where the periosteum is lifted off at the edge of a fast-growing bone lesion. It indicates an aggressive process. Learn more →
Coffee bean sign
A massively dilated, inverted U-shaped sigmoid loop resembling a coffee bean, seen in sigmoid volvulus. Learn more →
Comminuted fracture
A fracture with more than two fragments. Learn more →
Consolidation
Filling of the alveolar air spaces with fluid, pus, blood or cells, producing a white area of lung, often with air bronchograms. Learn more →
Continuous diaphragm sign
Gas outlining the central part of the diaphragm beneath the heart so that both hemidiaphragms appear joined. It suggests pneumomediastinum. Learn more →
Cortex
The dense outer layer of a bone, seen as a white line on X-ray. Tracing it is the key step in finding fractures. Learn more →
Costophrenic angle
The sharp angle where the hemidiaphragm meets the chest wall on a chest film. Blunting is an early sign of pleural fluid. Learn more →
Crescent sign (bone)
A thin lucent line just under the joint surface representing a subchondral fracture, seen in avascular necrosis and Perthes disease. Learn more →
CRITOE
Mnemonic for the usual order in which the elbow ossification centres appear: capitellum, radial head, internal epicondyle, trochlea, olecranon, external epicondyle. Learn more →
Cubitus varus
An elbow deformity in which the forearm angles inwards ("gunstock" deformity), usually after a malunited supracondylar fracture. Learn more →

D

Decubitus view
A radiograph taken with the patient lying on one side and a horizontal beam, used to show free gas or mobile fluid. Learn more →
Deep sulcus sign
An abnormally deep and dark costophrenic angle on a supine chest film, caused by a pneumothorax collecting at the front and bottom of the chest. Learn more →
Density (radiographic)
How white a structure appears on the image, which depends on how much it attenuates the beam. Learn more →
Diaphysis
The shaft of a long bone, between the two metaphyses. Learn more →
Dislocation
Complete loss of contact between the joint surfaces of a joint. Learn more →
Displacement
Movement of a fracture fragment away from its normal position, described by the direction and amount of shift. Learn more →
Distraction
Separation of fracture fragments along the long axis of the bone, so that a gap opens between them. Learn more →
Double bubble sign
Two gas-filled structures in the upper abdomen, the stomach and duodenal cap, with no gas beyond. It indicates duodenal obstruction such as duodenal atresia. Learn more →
Double-wall sign
Another name for Rigler's sign: both sides of the bowel wall visible because gas lies on both sides of it. Learn more →

E

Effusion
An abnormal collection of fluid within a body space, such as the pleural space, pericardium or a joint. Learn more →
Enchondroma
A benign cartilage tumour within the medullary cavity, often in the hand, with ring-and-arc calcification. Learn more →
Epiphysis
The end of a long bone on the far side of the growth plate, forming part of the joint. Learn more →
Erosion
A focal defect in bone at or near a joint, caused by inflamed synovium, crystal deposits or infection. Learn more →
Exposure
The combination of tube voltage (kV) and tube current-time (mAs) that determines how dark and how contrasty the image is. Learn more →
Extrapleural sign
A lesion with a smooth edge that meets the chest wall at obtuse angles and tapers off, indicating that it arises from the pleura or chest wall rather than the lung.

F

Fallen fragment sign
A piece of broken cortex lying in the lowest part of a fluid-filled simple bone cyst after a fracture through it. Learn more →
Fat pad sign
Displacement of the fat pads at the elbow by fluid in the joint. A visible posterior fat pad on a true lateral is always abnormal and suggests an occult fracture after trauma. Learn more →
Felson's silhouette sign
See Silhouette sign. Learn more →
Fissure
A thin pleural line separating lobes of the lung: the horizontal (minor) fissure on the right and the oblique (major) fissures on both sides. Learn more →
Flail chest
A segment of chest wall that moves independently because several adjacent ribs are each broken in more than one place. Definitions of how many ribs vary. Learn more →
Fleck sign
A small bone fragment between the bases of the first and second metatarsals, indicating a Lisfranc ligament avulsion. Learn more →
Football sign
A large oval lucency over the abdomen on a supine film, caused by a large volume of free gas, usually in infants. Learn more →
Frog-leg lateral
A hip view taken with the hips bent and turned outwards. It is more sensitive than the AP for a slipped upper femoral epiphysis. Learn more →

G

Galeazzi fracture
A fracture of the radial shaft combined with dislocation of the distal radioulnar joint. Learn more →
Garden classification
A four-stage grading of intracapsular femoral neck fractures by displacement, from incomplete (I) to fully displaced (IV). Learn more →
Geographic lesion
A bone lesion with a well-defined edge and a narrow transition to normal bone, usually indicating slow growth. Learn more →
Ghon complex
The combination of a primary tuberculous focus in the lung and an affected hilar lymph node. Learn more →
Gilula's arcs
Three smooth curves traced along the carpal rows on a PA wrist film. A break in an arc suggests a carpal fracture or dislocation. Learn more →
Golden S sign
An S-shaped edge formed by a collapsed right upper lobe wrapping around a central mass. It should prompt a search for a tumour. Learn more →
Greenstick fracture
An incomplete childhood fracture in which the cortex breaks on the stretched (convex) side while the other side bends. Learn more →
Ground glass
A hazy increase in lung density through which vessels can still be seen. Learn more →

H

Hampton hump
A wedge-shaped opacity based on the pleura, caused by a pulmonary infarct after embolism.
Haustra
Folds of the colonic wall that extend only partway across the bowel, helping distinguish large bowel from small bowel. Learn more →
Hilar convergence sign
Pulmonary vessels running into an enlarged hilum, showing that the enlargement is a dilated pulmonary artery rather than a mass. Learn more →
Hill-Sachs lesion
A dent in the back and outer side of the humeral head caused by impaction on the glenoid during an anterior shoulder dislocation. Learn more →
Hilum overlay sign
Hilar vessels remain visible through an apparent hilar mass, showing that the mass lies in front of or behind the hilum rather than within it. Learn more →
Honeycombing
Clusters of small, thick-walled air cysts at the lung periphery, indicating end-stage pulmonary fibrosis. Learn more →
Hydropneumothorax
Air and fluid together in the pleural space, producing a straight air–fluid level on an erect film. Learn more →
Hyperinflation
Overexpanded lungs with flattened diaphragms and more ribs visible than usual, as in COPD or asthma. Learn more →

I

Ileus
Dilated bowel caused by reduced bowel movement rather than a mechanical blockage. Learn more →
Impacted fracture
A fracture in which one fragment has been driven into the other. Learn more →
Insall-Salvati ratio
The length of the patellar tendon divided by the length of the patella on a lateral knee film. It identifies a high-riding (alta) or low-lying (baja) patella. Learn more →
Interstitial pattern
Lung shadowing caused by thickening of the supporting tissue of the lung, appearing as fine lines (reticular), dots (nodular) or both. Learn more →
Intra-articular
Within a joint. An intra-articular fracture extends to the joint surface. Learn more →
Involucrum
A shell of new living bone formed around dead bone in chronic osteomyelitis. Learn more →
IR(ME)R
The Ionising Radiation (Medical Exposure) Regulations, the UK law governing who may justify and carry out medical radiation exposures. Learn more →

K

Kerley lines
Thin lines caused by thickened interlobular septa. Kerley B lines are short horizontal lines at the lung bases reaching the pleura, typical of interstitial oedema. Learn more →
Klein's line
A line drawn along the upper edge of the femoral neck on an AP hip film. It should pass through part of the femoral epiphysis; if it does not, suspect a slipped upper femoral epiphysis. Learn more →
kV (kilovoltage)
The tube voltage setting that controls the energy of the X-ray beam and therefore its penetration and the image contrast. Learn more →

L

Lateral decubitus
See Decubitus view. Learn more →
Lead-pipe colon
A narrowed, shortened colon that has lost its haustral folds, seen in long-standing ulcerative colitis. Learn more →
Lightbulb sign
A rounded, symmetrical humeral head on the AP shoulder film because the arm is fixed in internal rotation, suggesting posterior dislocation. Learn more →
Lipohaemarthrosis
A layer of fat floating on blood in a joint, seen on a horizontal-beam film. It indicates a fracture into the joint. Learn more →
Lisfranc injury
Disruption of the joints and ligaments between the tarsal bones and the metatarsal bases in the midfoot. Learn more →
Looser zone
A pseudofracture in osteomalacia: a lucent band running across the cortex, often on both sides symmetrically, for example in the medial femoral neck. Learn more →
LOSS
Mnemonic for the X-ray signs of osteoarthritis: loss of joint space, osteophytes, subchondral sclerosis and subchondral cysts. Learn more →
Lucency (radiolucent)
A dark area on the image where X-rays pass through easily, such as gas or fat. Learn more →
Luftsichel sign
A crescent of lucent lung beside the aortic arch in left upper lobe collapse, where the over-expanded lower lobe moves up next to the aorta. Learn more →

M

Mach band
An optical illusion of a thin dark or bright line along an edge, which can mimic a pneumothorax or fracture. Learn more →
Maisonneuve fracture
A fracture of the upper fibula combined with injury to the ankle syndesmosis and medial ankle structures. Learn more →
mAs (milliampere-seconds)
The tube current multiplied by exposure time. It controls the number of photons and so the overall image density and noise. Learn more →
Mediastinal shift
Movement of the heart and mediastinum towards a side that has lost volume, or away from a side that is expanded, for example by a tension pneumothorax. Learn more →
Meniscus sign
The curved upper edge of a pleural effusion on an erect film, higher at the side than in the middle. Learn more →
Metaphysis
The flared part of a long bone next to the growth plate, where many childhood fractures and bone tumours occur. Learn more →
Miliary pattern
Countless tiny, uniform nodules (about 1–3 mm) spread evenly through both lungs, classically in miliary TB. Learn more →
Monteggia fracture
A fracture of the ulna combined with dislocation of the radial head at the elbow. Learn more →
Moth-eaten
A pattern of multiple small, irregular holes in bone that merge together. It suggests an aggressive lesion. Learn more →

N

Nidus
The small lucent centre of an osteoid osteoma, often surrounded by dense sclerotic bone. Learn more →
Nodule
A rounded opacity in the lung measuring up to 3 cm across. Larger lesions are called masses. Learn more →

O

Oblique fracture
A fracture whose line runs at an angle across the long axis of the bone. Learn more →
Occipitomental (OM) view
A facial X-ray taken with the chin raised so that the facial bones are projected clear of the skull base; 15° and 30° versions are used. Learn more →
Oligaemia
Reduced blood vessel markings in part of the lung, making it look darker.
Onion-skin periosteal reaction
Layers of new bone laid down by the periosteum one after another, giving a lamellated appearance. Seen in Ewing sarcoma and osteomyelitis. Learn more →
Opacity (radiopaque)
A white area on the image where X-rays are blocked, such as bone, metal or fluid-filled lung. Learn more →
Osteopenia
Bone that appears less dense than normal on a radiograph. It is a descriptive term, not a diagnosis of osteoporosis. Learn more →
Osteophyte
A bony spur that grows at the edge of a joint, typical of osteoarthritis. Learn more →
Ottawa rules
Validated clinical decision rules that help decide whether an injured ankle, foot or knee needs an X-ray. Learn more →

P

PA (posteroanterior)
A projection in which the beam enters the back and exits the front, with the detector against the chest. It is the standard projection for chest films. Learn more →
Pathological fracture
A fracture through bone weakened by disease, such as a tumour, cyst or metabolic disorder, often after minor trauma. Learn more →
Pencil-in-cup
A joint in which one bone end is worn to a point and sits inside a cupped neighbouring bone, typical of psoriatic arthritis. Learn more →
Penetration
Whether the beam has passed adequately through the body. On a PA chest film the vertebrae should just be visible behind the heart. Learn more →
Periosteal reaction
New bone produced by the periosteum in response to injury, infection or tumour. Its pattern helps indicate how aggressive a process is. Learn more →
Permeative pattern
Many tiny, poorly defined holes in bone with no clear edge to the lesion. It is the most aggressive pattern of bone destruction. Learn more →
Physis
The growth plate: the cartilage disc between the epiphysis and metaphysis where a bone grows in length. Learn more →
Pleural plaque
A localised area of pleural thickening, often calcified, that marks previous asbestos exposure.
Pneumatosis intestinalis
Gas within the wall of the bowel. In premature infants it is the hallmark of necrotising enterocolitis. Learn more →
Pneumobilia
Gas within the bile ducts, often after surgery or sphincterotomy, or in gallstone ileus. Learn more →
Pneumomediastinum
Gas within the mediastinum, outlining structures such as the heart, aorta and trachea. Learn more →
Pneumoperitoneum
Free gas inside the peritoneal cavity, most often from a perforated bowel or after surgery. Learn more →
Pneumothorax
Air in the pleural space, causing the lung to separate from the chest wall. Learn more →
Popcorn calcification
Clumps of irregular calcification resembling popcorn, seen in pulmonary hamartomas and cartilage tumours. Learn more →
Portal venous gas
Gas in the portal venous system, seen as branching lucencies extending to the edge of the liver. It can indicate severe bowel ischaemia or necrotising enterocolitis. Learn more →
Predental (atlanto-dental) interval
The gap between the front of the dens and the back of the anterior arch of C1 on a lateral neck film. Widening suggests ligament injury or rheumatoid instability. Learn more →
Prevertebral soft tissue
The soft tissue in front of the cervical spine on a lateral film. Swelling can indicate injury, haematoma or infection. Learn more →
Pseudosubluxation
An apparent forward slip of C2 on C3 that is a normal finding in young children. Learn more →

R

Radiocapitellar line
A line along the long axis of the radius that should pass through the capitellum on every elbow view. If it does not, the radial head is dislocated. Learn more →
Retrocardiac
Behind the heart on a frontal chest film; a common place for collapse, consolidation and masses to be missed. Learn more →
Retropulsion
Backward displacement of vertebral body fragments into the spinal canal, as in a burst fracture. Learn more →
Rib notching
Erosions along the lower edges of the ribs, classically caused by enlarged collateral arteries in coarctation of the aorta.
Rigler's sign
Both the inner and outer walls of a bowel loop are visible because gas lies inside and outside it. It indicates pneumoperitoneum. Learn more →
Rigler's triad
Small bowel obstruction, gas in the biliary tree and an ectopic gallstone together, indicating gallstone ileus. Learn more →
RIPE
A checklist for chest film quality: rotation, inspiration, projection and exposure. Learn more →
Rotation (film)
Turning of the patient away from a true frontal position, judged on a chest film by comparing the distance of each clavicle head from the spinous process. Learn more →
Rotation (fracture)
Twisting of one fracture fragment relative to another around the long axis of the bone. Learn more →

S

Sail sign (chest)
A triangular density behind the heart from left lower lobe collapse; in infants the same term describes the normal thymus. Learn more →
Sail sign (elbow)
The anterior elbow fat pad lifted away from the humerus into a triangle by a joint effusion. Learn more →
Salter-Harris classification
A five-type system describing growth plate fractures according to which parts of the bone the fracture crosses. Learn more →
Sclerosis
An area of increased bone density, appearing whiter than surrounding bone. Learn more →
Segond fracture
A small avulsion fracture from the lateral tibial plateau, strongly associated with anterior cruciate ligament tears. Learn more →
Sentinel loop
A single dilated loop of bowel lying next to an inflamed organ, such as the pancreas or appendix. Learn more →
Sequestrum
A fragment of dead bone that has separated from living bone in osteomyelitis. It appears denser than the bone around it. Learn more →
Shenton's line
A smooth curve from the medial femoral neck to the lower edge of the superior pubic ramus. A break suggests a hip fracture, dislocation or dysplasia. Learn more →
Silhouette sign
Loss of the normal edge of a structure where an abnormality of the same density lies directly against it. It helps locate lung disease. Learn more →
Spine sign
On a lateral chest film the lower thoracic vertebrae normally get darker towards the diaphragm; if they get whiter, there is disease in a lower lobe. Learn more →
Spiral fracture
A fracture that winds around the bone in a helix, caused by a twisting force. Learn more →
Spondylolisthesis
Forward slippage of one vertebra over the one below. Learn more →
Spondylolysis
A defect or stress fracture in the pars interarticularis, the bony bridge between the facet joints of a vertebra. Learn more →
Steeple sign
Symmetrical narrowing of the trachea just below the vocal cords on a frontal neck film, seen in croup.
Subchondral
Lying just beneath the articular cartilage of a joint. Learn more →
Sunburst periosteal reaction
Spikes of new bone radiating outwards from a bone lesion, typical of osteosarcoma. Learn more →
Swan-neck deformity
A finger posture with the middle (PIP) joint over-straightened and the end (DIP) joint bent, typical of rheumatoid arthritis. Learn more →
Syndesmophyte
A vertical bony bridge joining the edges of adjacent vertebral bodies, seen in ankylosing spondylitis and other spondyloarthropathies. Learn more →
Syndesmosis (ankle)
The fibrous joint binding the lower tibia and fibula together just above the ankle joint. Learn more →

T

Target sign (intussusception)
Concentric rings of soft tissue and fat produced by bowel telescoped within bowel, seen on ultrasound and sometimes on an abdominal film. Learn more →
Tension pneumothorax
A pneumothorax in which air enters the pleural space but cannot leave, pushing the mediastinum away and compromising the circulation. It is a clinical emergency. Learn more →
Terry-Thomas sign
A widened gap between the scaphoid and lunate on a PA wrist film, indicating scapholunate ligament injury. Learn more →
Thumbprinting
Thickened, swollen folds of the colon wall indenting the gas column like thumbprints, seen in colitis and bowel ischaemia. Learn more →
Tram-track opacities
Pairs of parallel lines formed by thickened bronchial walls, typical of bronchiectasis. Learn more →
Translation
Sideways shift of one fracture fragment relative to the other, usually given as a percentage of the bone's width. Learn more →
Tree-in-bud
Small branching and nodular opacities resembling a budding tree, caused by plugged small airways. It is a CT sign, usually of infection.

U

Upper lobe diversion
Upper zone vessels on an erect chest film that are as large as or larger than lower zone vessels, a sign of raised pulmonary venous pressure. Learn more →

V

Valvulae conniventes
The mucosal folds of the small bowel, which cross the full width of the bowel and help identify dilated small bowel. Learn more →

W

Weber classification
A three-group system for ankle fractures based on where the fibular fracture lies relative to the syndesmosis: below (A), at (B) or above (C). Learn more →