The arc of Riolan (AOR), also known as the mesenteric meandering artery (of Moskowitz) or central anastomotic mesenteric artery, is an arterio-arterial anastomosis between the superior and inferior mesenteric arteries.
It is an inconstant artery that connects the proximal superio...
The arcuate foramen (foramen arcuale atlantis, ponticulus posticus or posterior ponticle, or Kimerle anomaly) is a frequently encountered normal variant of the atlas and is easily appreciated on a lateral plain film of the cranio-cervical junction.
It develops by calcification of the posterior ...
The arcuate ligament is part of the posterolateral ligamentous complex of the knee that is variably present, being found in ~65% (range 47.9-71%) of knees. It is a Y-shaped thickening of the posterolateral capsule, which arises from the fibular styloid and divides into two limbs:
medial limb: c...
The arcuate line is located at roughly one-third of the distance from the pubic crest to the umbilicus. It is the demarcation where the internal oblique and transversus abdominis aponeuroses of the rectus sheath start to pass anteriorly to the rectus abdominis muscle, leaving only the transversa...
The arcuate sign is often a subtle but important finding on knee x-rays and represents an avulsion fracture of the proximal fibula at the site of insertion of the arcuate ligament complex, and is usually associated with cruciate ligament injury (~90% of cases) 2. The fracture fragment is attache...
An arcuate uterus is a mildly variant shape of the uterus. It is technically one of the Müllerian duct anomalies, but is often classified as a normal variant. It is the uterine anomaly that is least commonly associated with reproductive failure. Arcuate uterus can be characterised with ultrasoun...
Areae gastricae are a normal finding on double contrast images of the stomach.
fine reticular network of barium-coated grooves between 1-5 mm islands/areas of gastric mucosa
may be seen in ~70-80% of patients if there is adequate high-density barium coating of the stomac...
The area postrema is one of the so-called circumventricular organs located on the dorsal inferior surface of the medulla oblongata at the caudal end of the fourth ventricle.
The specialised ependymal cells in the area postrema detect toxins in the blood and act as a vomit-inducing centre, along...
Argyll Robertson pupil is usually bilateral and presents as bilaterally miotic and irregular pupils, which constrict briskly with accommodation but do not react to bright light therefore displaying light-near dissociation 1.
It is a highly specific sign of late neurosyphilis, however can also ...
Arhinia refers to congenital failure of the external nose, nasal cavity, and olfactory apparatus to develop. It is an extremely rare condition that can be detected on prenatal ultrasound or MRI.
Arhinia may occur in a syndromic setting (such as ethmocephaly), but it has been repor...
A-rings are a type of distal oesophageal ring. They are above the B-ring and occur a few centimetres proximal to the gastro-oesophageal junction. They represent a physiological contraction of oesophageal smooth muscle covered by mucosa. A-rings are uncommonly symptomatic.
The arm is part of the upper limb below the pectoral girdle and above the forearm, comprising the humerus.
The elbow joint is inferior and the glenohumeral joint is superior. Arm flexion, extension, abduction, adduction, internal rotation and external rotation occur at the shoulder.
Arnold-Hilgartner classification is a plain radiograph grading system for haemophilic arthropathy of the knee 1,2:
stage 0: normal joint
stage I: no skeletal abnormalities, soft-tissue swelling is present
stage II: osteoporosis and overgrowth of the epiphysis, no cysts, no narrowing of the ca...
Arnold's nerve is the auricular branch, also known as the mastoid branch, of the vagus nerve (CN X).
Origin and course
Arnold's nerve originates from the superior ganglion of the vagus nerve and also has a small contribution from the inferior ganglion of the glossopharyngeal nerve. It ascends ...
Arrested pneumatisation of the skull base is an anatomical variant that most commonly occurs in association with the sphenoid sinus. It is known that the sphenoid bones undergo early fatty marrow conversion antecedent to normal pneumatisation. However, for unclear reasons, some individuals exper...
Arrhythmogenic right ventricular cardiomyopathy (ARVC), also referred to as arrhythmogenic right ventricular dysplasia (ARVD) or simply arrhythmogenic cardiomyopathy, is classified as a type of cardiomyopathy. It is seen particularly in young males and is one of the more common causes of sudden ...
For the diagnosis of arrhythmogenic right ventricular cardiomyopathy to be made patients must have either two major criteria, one major and two minor criteria, or four minor criteria.
global or regional dysfunction and structural alterations:
severe dilatation of the right vent...
The arrowhead sign refers to the focal caecal thickening centred on the appendiceal orifice, seen as a secondary sign in acute appendicitis. The contrast material in the caecal lumen assumes an arrowhead configuration, pointing at the appendix.
The arrowhead sign is applicable only when enter...
The peri-articular arterial anastomosis of the elbow consists of several arteries that supply the elbow joint and its supporting structures. It functions to allow blood to flow around the elbow joint no mater which position the joint is in. The arteries that contribute include:
from the brachi...
Arterial dissection occurs when blood enters the media through a tear or ulcer in the intima and tracks along the media, forming a second blood-filled channel within the wall. The normal lumen lined by intima is called the true lumen and the blood-filled channel in the media is called the false ...
In patients with a left ventricular assist device (LVAD), spectral Doppler waveforms are altered due to continuous flow provided by the pump in the device:
waveforms are monophasic with a constant antegrade flow and no flow below the baseline.
the waveform is typically parvus-tardus with a slo...
Arterial spin labelling (ASL) MR perfusion is an MR perfusion technique which does not require intravenous administration of contrast (unlike DSC perfusion and DCE perfusion). Instead it exploits the ability of MRI to magnetically label arterial blood below the imaging slab. The parameter most c...
The arteries of the head and neck are branches of the common carotid and subclavian arteries.
common carotid artery
internal carotid artery (segments)
persistent stapedial artery
The arterial supply of the lower limbs is via vessels arising from the external iliac artery.
The common femoral artery (CFA) is the direct continuation of the external iliac artery. It begins at the level of the inguinal ligament. It terminates as it gives off the profunda femoris and continu...
The arterial supply of the upper limb is derived from the subclavian artery. The right subclavian artery originates from the brachiocephalic artery, which is the first branch of the aortic arch. The left subclavian artery originates directly from the aortic arch, being the third branch.
Arterial supply to the foot can be divided into plantar and dorsal components.
Plantar arterial supply
Medial plantar artery
branch off the posterior tibial artery
smaller calibre vessel
supplies the medial side of the foot, abductor hallucis and flexor digitorum brevis.
provides the arter...
The arterial supply to the hand is comprised of a complex vascular network formed from the branches and distal continuations of the radial and ulnar arteries. This rich vascular network can be divided into palmar and dorsal components.
Palmar arterial supply
The palmar arterial supply can be d...
The arterial switch procedure (or Jatene switch) is an intervention designed to correct D-transposition of the great arteries at the level of the aorta and main pulmonary artery.
First, the left and right coronary arteries are transferred to the posterior artery (the main pulmonary artery, now ...
Arteriosclerosis is defined by thickening and loss of elasticity of the arterial walls.
There are three patterns (arteriosclerosis is used as a generic term for all patterns above):
atherosclerosis: large and medium-sized arteries
Mönckeberg medial calcific sclerosis: muscular arteries
Arteriovenous access is required for haemodialysis in renal failure patients. The upper limb is generally preferred as a site, however, lower limb access can also be obtained. Ultrasound is the preferred modality for evaluation of the vessels prior to creating an access.
An arteriovenous fistula (AVF) is an abnormal connection between an adjacent artery and vein. Unlike an arteriovenous malformation (AVM), these are frequently acquired lesions, rather than developmental abnormalities.
Arteriovenous fistulas have a number of etiologies. They can be ia...
Arteriovenous malformations (AVMs) are characterised by an abnormal leash of vessels allowing for arteriovenous shunting. They can occur anywhere in the body but have a predilection towards the head and neck. There is a direct arteriovenous communication with no intervening capillary bed. They ...
The artery of Adamkiewicz, also known as the great anterior radiculomedullary artery or arteria radicularis anterior magna, is the name given to the dominant thoracolumbar segmental artery that supplies the spinal cord.
The artery of Adamkiewicz has a variable origin but ...
The artery of Percheron is a rare variant of the posterior cerebral circulation characterised by a solitary arterial trunk that supplies blood to the paramedian thalami and the rostral midbrain bilaterally.
The term is used to refer to a solitary arterial trunk that branches from...
Artery of Percheron territory infarct is rare, on account of the relative rarity of the artery of Percheron, and presents with a variety of signs and symptoms collectively termed the paramedian thalamic syndrome. It is a type of posterior circulation infarction.
On imaging, it is classically ch...
The artery to the ductus deferens (deferential or vesiculodeferential artery) is a branch of the superior vesical artery, which in turns arises from the internal iliac artery via the umbilical artery.
origin: superior vesical artery
main branch: no named branches
Arthrofibrosis is a complication of injury or trauma to a joint. It can also be iatrogenic e.g. post knee surgeries. It consists of excessive scar tissue formation within the joint capsule, resulting in pain, stiffness, and swelling, that are greater than expected in the given clinical scenario....
Arthrogryposis (multiplex congenita) is a clinical or imaging descriptor that denotes congenital non-progressive joint contractures involving two or more body regions.
It is thought to occur in approximately 1:3000-10,000 live births 6,8.
It can result from a number o...
Articles form the encyclopaedic component of Radiopaedia.org and are collaborative efforts to create atomic reference articles for anything related to the practice of radiology. Unlike a textbook, journal publication or a written encyclopaedia, Radiopaedia.org articles allow you and other users ...
Articles pertaining conditions that affect multiple systems can often be a challenge to write, and need to be split into a number of parts:
a general overview of the condition with links to
individual system articles
Syntax and structure
The standard title syntax for system specific articles...
Article titles should be concise and accurate allowing readers to search and link to the vast volume of information on Radiopaedia.org.
The following points should be kept in mind when deciding on an article title:
UK English spelling is preferred
avoid the use of acronyms
only the first wo...
An easy to remember mnemonic for the causes of articular cartilage calcification is:
W: Wilson disease
The articularis cubiti is a muscle in the posterior compartment of the arm:
origin: posterior surface of the distal humerus
insertion: posterior surface of the elbow joint capsule
innervation: radial nerve
action: tenses the posterior elbow joint capsule during elbow extension
The articularis genu is a small flat muscle of the anterior knee. During knee extension it acts to tighten the synovial membrane superiorly thereby preventing impingement of the synovial folds between the femur and the patella.
origin: anterior distal femoral shaft
insertion: knee joi...
Artifacts in radiology refer to something seen on an image that is not present in reality but appears due to a quirk of the modality itself.
The commonest artifact seen in radiology is image noise, which is inherent to every modality and technique, and can be mitigated but never eliminated.
Artifacts that mimic breast calcification can arise from a number of sources. These include:
deodorants on skin: most practices recommend that clients for mammography do not use deodorant or perfume on the day of the study for this reason. The residue from deodorant is a very fine, dense, misty...
Artificial intelligence (AI) is the "branch of computer science dealing with the simulation of intelligent behaviour in computers" 1. It aims to either assist humans with solving a problem or solve the problem entirely by itself. The exponential increase in computational processing and memory ca...
An artificial rupture of membranes (AROM) is a procedure that can be used for the induction of labour.
A sterile, plastic, thin hook is brushed against the membranes just inside the cervix (termed a membrane sweep) This causes the fetal head to move down against the cervix, usually ...
Artificial urethral sphincters, also known as inflatable artificial sphincters and urinary control systems, are devices used for the treatment of urinary stress incontinence (e.g. due to pelvic floor dysfunction in female or prostate surgery in male) and are sometimes used in combination with a ...
The aryepiglottic folds are two ligamentomuscular structures within the supraglottic larynx that function to protect the airway when swallowing.
Each aryepiglottic fold is comprised of the superior ligamentous edge of the quadrangular membrane and covering mucous membrane 1, the ...
The arytenoid cartilages are paired hyaline cartilages that articulate with the sloping upper border of the lamina of the cricoid cartilage by the cricoarytenoid joint. This joint allows movement of the arytenoid cartilages, which is vital in approximating, tensing and relaxing the vocal folds.
Asbestos is a set of six naturally occurring silicate minerals from mixture of calcium magnesium, iron, and sodium exploited commercially for their desirable physical properties, particularly their resistance to heat and burning. They all form thin elongated fibrous crystals, and can be manufact...
An asbestos body is a histological finding in interstitial lung disease that is suggestive of significant occupational asbestos exposure. They are usually identified following a parenchymal lung biopsy 3.
Macrophage ingestion of the asbestos fibres triggers a fibrogenic response via the release...
Asbestosis refers to later development of diffuse interstitial fibrosis secondary to asbestos fibre inhalation and should not be confused with other asbestos related diseases.
Asbestosis typically occurs 10-15 years following the commencement of exposure to asbestos and is dose re...
Asbestos related benign pleural disease forms a large part of asbestos related lung changes.
The spectrum comprises of:
pleural effusions: benign-asbestos induced pleural effusions
can be associated with functional impairment
usually occur within 10 years of exposure but can also develop muc...
Asbestos related disease, in particular affecting the lung, comprise of a broad spectrum of entities related to the inhalational exposure to asbestos fibres. They can be divided into benign and malignant changes 1-3.
Benign pleural and parenchymal lung disease
asbestos related benign pleural d...
Ascariasis is due to infection with the Ascaris lumbricoides adult worm and typically presents with gastrointestinal or pulmonary symptoms, depending on the stage of development.
Ascaris lumbricoides is widely distributed in tropical and subtropical regions and in other humid ar...
Ascaris-induced pancreatitis is the most common form of parasite-induced pancreatitis.
Ascariasis in parts of India is the second most common form of pancreatitis after gallstones 1. It is rare outside of endemic regions however.
The presentation will be si...
The ascending aorta is the first part of the aorta, and begins at the aortic valve, located obliquely just to the left of the midline at the level of the the third intercostal space. It terminates as it exits the fibrous pericardium where it becomes the aortic arch, in the plane of Ludwig, a hor...
Dilatation of the ascending aorta is a common finding in the elderly but unusual in younger patients.
In adults, an ascending aortic diameter greater than 4 cm is considered to indicate dilatation 4. Aneurysmal dilatation is considered when the ascending aortic diameter reaches or ex...
Ascending aortic aneurysms are the most common subtype of thoracic aortic aneurysms and may be true or false injuries.
Ascending aortic aneurysms represent 60% of thoracic aortic aneurysms.
Typically ascending aortic aneurysms are an incidental finding an...
The ascending cervical artery is 1 of the 4 branches of the thyrocervical trunk (off the first part of the subclavian artery).
It is a small artery that ascends medial to the phrenic nerve on the prevertebral fascia. It contributes many small spinal branches into the intervertebral foramina of ...
The ascending colon is the second part of the large bowel.
The ascending colon is the continuation of the caecum superior to the ileocaecal valve. It is secondarily retroperitoneal, although it has its own mesentery in approximately 25% of patients and is 15 cm in length 1,2.
The ascending lumbar communicant vein is a communication between the left ascending lumbar vein and the left renal vein. Because of its retroperitoneal location, when dilated, it may be mistaken for a lymph node on non-contrast studies with thick collimation.
The image shows the left renal vein...
The ascending lumbar vein is a paired structure which forms a part of the venous drainage of the lumbar vertebral column.
location: near midline on the side of the vertebral column in the lumbar region
origin and termination: continuation of the lateral sacral veins; joins the subcost...
The ascending pharyngeal artery, the smallest branch of the external carotid artery, is a long, slender vessel, deeply seated in the neck, beneath the other branches of the external carotid and under the stylopharyngeus.
origin: a branch of the external carotid artery
Ascending ramus of the lateral sulcus, is located at the anterior end of the lateral sulcus (sylvian fissure), just posterior to the anterior ramus, and passes superiorly into the inferior frontal gyrus separating the pars triangularis from the pars opercularis of the frontal operculum.
Ascending transtentorial herniation is a situation where space-occupying lesions in the posterior cranial fossa cause superior displacement of superior parts of the cerebellum through the tentorial notch.
nausea and/or vomiting
rapid progression toward a decreased leve...
Ascites is defined as an abnormal amount of intraperitoneal fluid.
Patients with a large volume of ascites can present with abdominal distension (which may be painful), nausea, vomiting, dyspnoea and peripheral oedema 7, 9.
Ascitic fluid is traditionally chara...
Aseptic loosening is considered relatively common complication of hip joint replacements. It is usually considered a long-term complication and is often considered as the most common complication 3.
Aseptic loosening can occur as a result of inadequate initial fixation, mechanical lo...
Asherman syndrome, also known as uterine synechiae, is a condition characterised by the formation of intrauterine adhesions, which are usually sequela from injury to the endometrium, and is often associated with infertility.
There is a tendency for the condition to develop soon af...
The American Spinal Injury Association (ASIA) was developed by the American Spinal Injury Association in 2006, and at the time of writing (July 2016), remains the most widely used scale.
This scale is part of the ASIA spinal cord injury classification. It divides spinal cord injuries into 5 cat...
The original description of the Askin tumour (by Askin and Rosai in 1979 1), and many studies following it have led to a great deal of confusion. Until recently it has been considered a separate entity or as a type of peripheral primitive neuroectodermal tumour, usually of the chest wall.
As low as reasonably achievable (ALARA) is a principle of radioprotection stating that whenever ionizing radiation has to be applied to humans, animals or materials exposure should be as low as reasonably achievable. It is fundamental to the principles of radiation protection.
Aspergillomas are mass-like fungus balls that are typically composed of Aspergillus fumigatus and are a non-invasive form of pulmonary aspergillosis. It usually falls under the subgroup chronic pulmonary aspergillosis.
Although the term mycetoma is frequently used to describe these...
Aspergillus is a fungal genus consisting of approximately 180 species. It is a ubiquitous fungus frequently found in urban areas especially in decomposing organic matter or water damaged walls and ceilings. Only a few Aspergillus species are associated with human disease.
Aspergillus clavatus is one of the species of Aspergillus that can cause pathology in humans. It is allergenic and causes a hypersensitivity pneumonitis called malt-workers lung.
Aspergillus flavus is a fungus and one of the species of Aspergillus that is common in the environment and responsible for pathology in humans.
It is the second most common cause of pulmonary aspergillosis (after Aspergillus fumigatus) and can additionally cause corneal, otomycotic, and nasoorb...
Aspergillus fumigatus is a fungus of the genus Aspergillus, and is one of the most common Aspergillus species to cause disease in immuno-compromised individuals.
A. fumigatus is a saprotroph (an organism that gets its energy from non-living organic matter) that is widespread in nature, typicall...
Asphyxiating thoracic dysplasia, also known as Jeune syndrome, is a type of rare short limb skeletal dysplasia, which is primarily characterised by a constricted long narrow thoracic cavity, cystic renal dysplasia and characteristic skeletal features. It is also sometimes classified as one of th...
Aspiration bronchiolitis, or diffuse aspiration bronchiolitis, is a condition characterised by a chronic inflammation of bronchioles caused by recurrent aspiration of foreign particles.
The onset of aspiration bronchiolitis can be more insidious than aspiration pneumonia,...
Aspiration pneumonia is caused by a direct chemical insult due to the entry of a foreign substance, solid or liquid, into the respiratory tract.
Aspiration may be clinically silent, or it may present with dyspnoea, cough, or fever. The clinical and radiographic features d...
Asplenia refers to absence of the spleen thereby leading to deficient splenic function.
Seen in 3% of neonates with structural heart disease and in 30% of patients who die from cardiac malposition. The male-to-female ratio is 2:1.
Asplenia can be classified into two t...
Asplenia syndrome (also known as right isomerism or Ivemark syndrome) is a type of heterotaxy syndrome.
There is an increased male predilection. Asplenia syndrome is usually diagnosed in neonates.4
In contrast to polysplenia syndrome, most patients die bef...
Assessment of thyroid lesions is commonly encountered in radiological practice.
hyperplastic / colloid nodule / nodular hyperplasia: 85%
papillary: 60-80% of carcinomas
Ultrasound along with nuclear medicine, is an important modality for assessment of thyroid lesions, and it is also frequently used to guide biopsy. Diagnostic criteria for thyroid nodules continue to evolve with improving ultrasound technology.
The asterion is the junction on the side of the posteroinferior calvarium where three sutures meet:
It is located at the posterior end of the parietotemporal suture, whereas the pterion is located at the anterior end.
It is one of...
Asteroid hyalosis is a degenerative condition of the eye where there is an accumulation of calcium soaps in the vitreous chamber.
The prevalence increases with age from 0.2% in 43-54-year-olds to 2.9% in 75-86-year-olds. The overall prevalence is 1.2%. It is more commonly unilater...
Asthma is a relatively common condition that is characterised by at least partially reversible inflammation of the airways and reversible airway obstruction due to airway hyperreactivity. It can be acute, subacute or chronic.
Asthma is one of the most common chronic diseases in th...
This is a basic article for medical students and other non-radiologists
Asthma is a heterogeneous disease, usually characterised by chronic airway inflammation and airway hyperreactivity. It is defined by two main features 1:
a history of respiratory symptoms such as wheeze, shortness of breat...
Asthmatic pulmonary eosinophilia is a form of pulmonary eosinophilia which is commonly attributed to Aspergillus fumigatus. Although many cases have not shown any allergen.
Plain radiograph - patterns
hyperinflation (in acute attacks or chronic severe asthma)
Astroblastomas are rare glial tumours usually found in the cerebral hemispheres of young adults and children.
They occur at all ages range from early childhood to 6th decade but are most commonly seen in children, adolescents, and young adults with a mean age between 10-30 years ...
Astrocytes are cells of the central nervous system which act as both physical and physiological support for the neurones that are embedded between them. They are particularly abundant in the grey matter, where they are the most abundant glial cells 1.
They are highly branched and contribute to ...