Aberrant left pulmonary artery, also known as pulmonary sling, represents an anatomical variant characterized by the left pulmonary artery arising from the right pulmonary artery and passing above the right main bronchus and in between the trachea and esophagus to reach the left lung. It may lea...
Aberrant right subclavian arteries (ARSA), also known as arteria lusoria, are among the commonest aortic arch anomalies.
The estimated incidence is 0.5-2%.
They are often asymptomatic, but around 10% of people may complain of tracheo-esophageal symptoms, a...
An absent azygos vein is a very uncommon variant in which the azygos vein fails to develop. In cases of agenesis of the azygos vein, the hemiazygos and accessory hemiazygos veins play an important role in venous drainage, accounting for drainage of both the right and left intercostal veins 1-3.
Accessory fissures of the lung usually occur at the borders of bronchopulmonary segments. They are common normal variants but are less commonly seen on imaging.
Some of the more common accessory fissure include 1:
azygos fissure: most commonly seen accessory fissure
inferior accessory fissur...
The accessory (or superior) hemiazygos vein forms part of the azygos system and along with the hemiazygos vein, it is partially analogous to the right-sided azygos vein. It drains the left superior hemithorax.
Spelling it "hemiazygous" when referring to the vein is incorrect, rega...
The accessory phrenic nerve is an anatomical variant seen in a little over one third of patients (36%). It most commonly arises from the ansa cervicalis, or slightly less commonly, the subclavian nerve. It is unknown as to how much the accessory phrenic nerve contributes to diaphragmatic functio...
The aerodigestive tract is a non-TA descriptive collective term for the respiratory tract and proximal portion of the digestive tract. As it is a non-standard term, its precise components vary somewhat with the context in which the term is being employed.
Definitions of what precis...
The biological/medical term agenesis (plural: ageneses) refers to failure of an organ to grow or develop during the embryological period.
corpus callosum agenesis
dental agenesis (anodontia)
The alveoli (singular: alveolus) are tiny hollow air sacs that comprise the basic unit of respiration.
Alveoli are found within the lung parenchyma and are found at the terminal ends of the respiratory tree, clustered around alveolar sacs and alveolar ducts. Each alveolus is app...
The anatomy curriculum is one of our curriculum articles and aims to be a collection of articles that represent the core anatomy knowledge for radiologists and imaging specialists.
Head and neck anatomy
Abdominal and pelvic anatomy
The anterior junction (or junctional) line is a feature of frontal chest radiographs and chest CTs. It is a result of the parietal and visceral pleura meeting anteromedially. It normally contains a small amount of fat but can form a stripe of variable thickness if there is a lot of fat present o...
The anterior mediastinum is the portion of the mediastinum anterior to the pericardium and below the thoracic plane.
It forms the anterior part of the inferior mediastinum, and contains the thymus, lymph nodes, and may contain the portions of a retrosternal thyroid.
The aortic arch represents the direct continuation of the ascending aorta and represents a key area for a review of normal variant anatomy and a wide range of pathological processes that range from congenital anomalies to traumatic injury.
origin: continuation of the ascending aorta at...
The aortic hiatus is one of the three major apertures through the diaphragm and lies at the level of T12. Strictly speaking, it is not a real aperture in the diaphragm, but an osseoaponeurotic opening between it and the vertebral column.
The hiatus is situated slightly to the left of the midli...
The aortic-pulmonary stripe is an uncommon feature of frontal chest radiographs and was first described by Keats in 1972 1.
It is formed by the interface of the pleural surface of the anterior segment of the left upper lobe contacting the mediastinal fat that is anterolateral to the pulmonary t...
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 ...
Owing to their features, the first, eleventh and twelfth ribs are considered atypical ribs. Some authors however describe the second, tenth and eleventh ribs as atypical ribs also.
Of all ribs, the first is the strongest, broadest and most curved. Ribs eleven and twelve are unique, among other ...
The azygo-esophageal recess (also known as the line/interface) (AER) is a prevertebral space formed by the interface of the posteromedial segments of the right lower lobe and the azygos vein and esophagus 1-3. The AER extends from the azygos arch to the aortic hiatus and has the following border...
An azygos lobe is a normal variant that develops when a laterally displaced azygos vein creates a deep pleural fissure into the apical segment of the right upper lobe during embryological development. It is not a true accessory lobe as it does not have its own bronchus or corresponding specific ...
The azygos vein is a unilateral vessel that ascends in the thorax to the right of the vertebral column, carrying deoxygenated blood from the posterior chest and abdominal walls. It forms part of the azygos venous system.
The spelling azygous when referring to the vein is incorrect,...
The azygos venous system , also known as just the azygos system, is a collective term given to the H-shaped configuration of the azygos, hemiazygos, accessory hemiazygos and left superior intercostal veins.
It is responsible for draining the thoracic wall and upper lumbar region via the lumbar ...
Bifid or forked or bifurcated rib is a congenital skeletal abnormality of the rib cage with the cleaved sternal end into two. They are thought to occur in ~0.2% of the population and there may be a female as well as right-sided predilection 2.
Bifid ribs can be seen ...
Bovine arch is the most common variant of the aortic arch and occurs when the brachiocephalic (innominate) artery shares a common origin with the left common carotid artery.
A bovine arch is apparent in ~15% (range 8-25%) of the population and is more common in individuals of African descent. ...
The Boyden classification of bronchi refers to the standard nomenclature used to describe bronchopulmonary segmental anatomy.
Each lung has 10 segments, however, on the left, the first two segments share a common trunk and are hence B1/2. Also given the shared trunk on the left of the lower lob...
The brachiocephalic trunk (BCT) (also known as the brachiocephalic artery, and previously as the innominate artery) is the first branch of the aortic arch and supplies the head, neck and right arm.
Although sometimes described as such, the brachiocephalic trunk is not one of the gr...
The brachiocephalic veins, previously known as the innominate veins, are large paired valveless asymmetric central veins that drain the head, neck, upper limbs and part of the thorax and mediastinum.
In the root of the neck, the internal jugular (IJV) and subclavian vein...
Useful mnemonics to remember the four branches of the thoracoacromial artery are:
CAlifornia Police Department
Cadavers Are Dead People
B: breast (pectoral)
CAlifornia Police Department
A bridging bronchus is a rare congenital bronchial anomaly where there is an anomalous bronchus to the right lung arising from the left main bronchus. It has a high association with right upper lobe bronchus (pig bronchus) and congenital cardiac and vascular malformations, particularly a left pu...
The bronchial arteries are the major supply of high-pressure oxygenated blood to the supporting structures of the lung, including the pulmonary arteries, yet they are responsible for only 1% of the lung blood flow overall.
Bronchial artery anatomy is variable, most commonly classified according...
The bronchial veins are counterparts to the bronchial arteries and drain the bronchi, hilar structures and the mid-portion of the esophagus.
There is typically a single bronchial vein at each hilum, formed from the superficial bronchial veins with deep bronchial veins draining in...
Bronchioles are the branches of the tracheobronchial tree that by definition, are lacking in submucosal hyaline cartilage.
The bronchioles typically begin beyond the tertiary segmental bronchi and are described as conducting bronchioles. Following the tertiary segmental bronchi,...
The bronchomediastinal trunks (a.k.a. bronchomediastinal lymphatic trunks) are lymphatic trunks, one on each side of the body. On the left, the bronchomediastinal trunk is a tributary of the thoracic duct, and on the right, it is a tributary of the right lymphatic duct. Although, in some individ...
Bronchopulmonary segmental anatomy describes the division of the lungs into segments based on the tertiary or segmental bronchi.
The trachea divides at the carina forming the left and right main stem bronchi which enter the lung substance to divide further. This initial division ...
Mnemonics to remember the bronchopulmonary segments are:
A PALM Seed Makes Another Little Palm (right lung)
ASIA ALPS (left lung)
'A PALM Seed Makes Another Little Palm'
right upper lobe
A: apical segment
P: posterior segment
A: anterior segment
L: lateral segment...
The bronchus intermedius is one of the two bronchi which the right main bronchus bifurcates into, the other being the right upper lobe bronchus.
The bronchus intermedius runs distal to the right upper lobe bifurcation and follows the trajectory of the right main bronchus 1. Its m...
The canals of Lambert are microscopic collateral airways between the distal bronchiolar tree and adjacent alveoli. They are poorly formed in children, and along with poorly formed pores of Kohn, are thought to be responsible for the high frequency of round pneumonia in that age group.
A cardiac bronchus (or sometimes termed accessory cardiac bronchus (ACB)) is a rare anatomic variant of the tracheobronchial tree, arising from the medial aspect of the bronchus intermedius.
This anomaly is rare and is reported in ~0.3% (range 0.09-0.5%) of individuals 3-5. There ...
The cardiac plexus is a plexus of nerves situated at the base of the heart. It is formed by cardiac branches derived from both the sympathetic and parasympathetic nervous systems.
Sympathetic cardiac nerves are derived from T1 to T4 segments and partly from the T5 segment of the ...
The carina is the sagittally-oriented cartilaginous ridge at the bifurcation of the trachea and is an important reference point in chest imaging.
The carina represents the inferior termination of the trachea into the right and left main bronchi.
The carina usually sits at the le...
Cartilage or cartilaginous tissue is a resilient and type of connective tissue of mesodermal origin that forms an integral part within the musculoskeletal system and as a structural component in other organs.
Cartilage can be generally classified into the following main types:
A number of cell groups in the reticular formation of the pons and medulla are responsible for the central control of the respiratory cycle:
inspiratory center (a.k.a. dorsal respiratory group) - bilateral groups of cells in the region of the nucleus of the tractus solitarius in the dorsum of t...
The muscle fibers of the diaphragm converge and attach to the central tendon of the diaphragm. It is a thin but strong layer of aponeurosis which forms an intergral part of respiration.
The central tendon of the diaphragm is located near the center of the diaphragmatic muscle bu...
The centrilobular region, in context of the lungs and HRCT, refers to the central portion of the secondary pulmonary lobule, around the central pulmonary artery and bronchiole.
Cervical aortic arches are a rare aortic arch anomaly characterized by an elongated, high-lying aortic arch extending at or above the level of the medial ends of the clavicles.
Patients with cervical aortic arch are usually asymptomatic. Symptomatic patients may present w...
Cervical ribs are supernumerary or accessory ribs arising from the seventh cervical vertebra. They occur in ~0.5% of the population, are usually bilateral, but often asymmetric 2, and are more common in females.
Although cervical ribs are usually asymptomatic, they are the mo...
Chest x-ray lines and stripes are important to recognize on chest radiographs.
Lines are usually less than 1 mm in width and are comprised of tissue outlined on either side by air and typically represent pleural-covered structures within the middle and superior mediastinum 1,2:
Congenital pulmonary venolobar syndrome is a condition comprising a rare group of cardiac and pulmonary congenital abnormalities occurring variably in combination. The abnormalities include:
anomalous pulmonary venous drainage
particularly scimitar syndrome with hypogenetic right lung
The coracoclavicular joint is a normal variant of the pectoral girdle, where the conoid tubercle of the clavicle appears enlarged or elongated, with a flattened inferior surface where it approximates the coracoid process of the scapula to form an articulation.
More common in Asia...
The costal cartilages form part of the thoracic cage and anterior chest wall. There are ten costal cartilages bilaterally, one for each of the corresponding 1st to 10th ribs, and each of the first seven ribs forms one of the seven costochondral joints.
Costal cartilages 1-7 articulate with the ...
The costocervical trunk is one of the branches of the second part of the subclavian artery. It arises from the posterior wall of the subclavian artery, posterior or medial to the anterior scalene muscle and courses posterosuperiorly across the suprapleural membrane where it divides into 2 branc...
The costochondral joints are the joints between each rib and its costal cartilage. They are primary cartilaginous joints. These joints represent the demarcation of the unossified and ossified part of the rib 1. The joint is held together by periosteum, with the lateral aspect of the costal carti...
The costoclavicular ligament or rhomboid ligament (a.k.a. Halsted's ligament 2) is the major stabilizing factor of the sternoclavicular joint and is the axis of movement of the joint.
The costoclavicular ligament binds the inferior medial clavicle (via the rhomboid fossa) to the ...
The costoclavicular space is the anterior portion of the superior thoracic aperture, between the clavicle and first rib. The subclavian vessels and brachial plexus pass though the space related to the scalene muscles. Proximally, the plexus passes through the scalene triangle, and distally throu...
The costovertebral joint is the articulations between the ribs and the vertebral column.
The ribs articulate with the thoracic vertebrae via two distinctly different joints:
costovertebral joint - articulation between the head of the rib and the vertebral body
The costoxiphoid ligaments, also known as the chondroxiphoid ligaments, are inconstant fibrous structures joining the anterior and posterior surfaces of the xiphoid to the respective surfaces of the adjacent seventh and, occasionally, sixth costal cartilages.
A crus (plural: crura) is an anatomical term used for a structure which resembles a leg.
crus (internal capsule)
crus (semicircular duct)
The diaphragm is the dome-shaped muscle that separates the thoracic cavity from the abdominal cavity, enclosing the inferior thoracic aperture.
On chest imaging, in particular chest radiography, an imaginary anteroposterior halfway line divides the diaphragm into two, forming the l...
The diaphragmatic apertures are a series of apertures that permit the passage of structures between the thoracic and abdominal cavities. There are three main apertures:
aortic hiatus (T12) (not a true aperture)
esophageal hiatus (T10)
vena caval foramen (T8)
The vertebral levels of these ape...
Two useful mnemonics to remember the thoracic spinal levels at which the three major structures pass through the diaphragmatic apertures is:
I 8 10 eggs at 12 - where 8 is a homophone for 'ate' and the 'e' in eggs is for the US spelling of esophagus
the number of letters per structure
Double aortic arches are the most common symptomatic type of the aortic arch variant. It may account for up to 50-60% of vascular rings.
Double aortic arch is mostly diagnosed in childhood due to symptoms related to esophageal and/or tracheal obstruction. Respiratory symp...
The ductus arteriosum (DA) (or arteriosus) is the thick short conduit for blood to bypass the non-ventilated lungs in the fetus. It is located between and connects the proximal left pulmonary artery and the undersurface of the aortic arch distal to the origin of the last branch of the arch, at t...
Aortic ductus diverticulum is a developmental outpouching of the thoracic aorta which may be mistaken for an acute aortic injury.
It is usually seen at the anteromedial aspect of the aorta at site of the aortic isthmus, where the ligamentum arteriosum attaches. It is also the sit...
The atypical 11th rib is one of two floating ribs.
The 11th rib has a single facet on its head for articulation with the T11 vertebra. It has a short neck and no tubercle. The angle is slight. Its costal groove is shallow. The internal surface of this rib faces slightl...
The eparterial bronchus is a synonymous term for the right superior lobar bronchus. Its name is derived from the bronchus being the only one originating superior to the level of the pulmonary artery. Conversely, all other bronchi can be referred to by their anatomical relationship to the pulmona...
Episternal (or suprasternal) ossicles are accessory bones and a normal variant of the sternum. They result from supernumerary ossification centers and are seen in ~4% (range 1-7%) of the population.
Episternal ossicles are usually located posterior or superior to the superior bor...
The external (or outermost) intercostal muscles are important muscles of respiration. They number eleven on each side and are located in the intercostal space, expanding the transverse dimension of the thoracic cavity during inspiration.
The external intercostal muscles are the o...
The first rib is the most superior of the twelve ribs. It is an atypical rib and is an important anatomical landmark and is one of the borders of the superior thoracic aperture.
Compared to a typical rib, the first rib is short and thick and it has a single articular f...
The foramina of Morgagni, also known as the sternocostal triangles, are small defects in the posterior aspect of the anterior thoracic wall between the sternal and costal attachments of the diaphragm. The internal thoracic vessels descend through these foramina to become the superior epigastric ...
The Haller index (HI), also known as the pectus index, is a simple mathematical way to assess and describe the chest cage on CT of the thorax and is used in the detection of pectus excavatum, as well as preoperative and postoperative assessment 1,5.
The Haller index is calculated by ...
The heart is a hollow, muscular organ of the middle mediastinum, designed to pump oxygenated blood around the systemic circulation and deoxygenated blood around the pulmonary circulation.
The heart has a somewhat conical form and is enclosed by the pericardium. It is positioned p...
There are four heart chambers, the right atrium, left atrium, right ventricle and left ventricle. These receive blood from the body and lungs and contract to transmit blood to the lungs for oxygenation and to the body for use in metabolism.
It is best to list the four chambers in order of the s...
The hemiazygos vein is the asymmetric counterpart to the azygos vein and forms part of the azygos venous system.
The spelling hemiazygous when referring to the vein is incorrect, regardless of whether British or American English is used 7. In the context of anatomy, hemiazygos vei...
The hilar point on chest radiographs is formed by the outer margins of the superior pulmonary vein and the descending pulmonary artery as they cross past each other.
Honeycomb sterna are considered as a rare developmental variant of the sternum, resulting from unfused lateral ossification centers of the sternebrae, which gives a honeycomb configuration of the mesosternum. Usually asymptomatic and discovered incidentally during a routine exam of the chest.
The horizontal fissure (also called the minor fissure) is a unilateral structure in the right lung that separates the right middle lobe from the right upper lobe.
The horizontal fissure arises from the right oblique fissure and follows the 4th intercostal space from the sternum u...
Horseshoe lung is one of the rare congenital anomalies of the lung. A band of pulmonary parenchyma is formed extending between the right and left lungs. The pulmonary tissue can be seen either anterior to the aorta or posterior to the pericardium at its caudal aspect.
Several normal anatomical structures and rare organ variants have been described as being horseshoe-shaped.
horseshoe pancreas 1
A hyparterial bronchus is any bronchus originating inferior to the level of the pulmonary artery. Conversely, the right superior lobar bronchus can be referred to by its anatomical relationship to the pulmonary arteries as being eparterial.
The term may be encountered in the classification of ...
Incomplete double aortic arch is a rare vascular ring anomaly wherein a segment of the minor aortic arch, usually the left, is atretic.
As in the case of other vascular rings, this anomaly can cause 1:
Some patients may reach adulthood with...
The inferior accessory fissure of the lung, also known as Twining's line, divides the medial basal bronchopulmonary segment from the rest of the lower lobe.
This accessory fissure is present in around 12% of people when examined with CT and is visible on 5-8% of frontal chest x-ra...
The inferior cavoatrial junction (ICAJ) is the term given to the point at which the inferior vena cava (IVC) enters the right atrium. It is less commonly used/seen, in contradistinction to the superior cavoatrial junction.
Accurate localization of the inferior cavoatrial junction is of practic...
The inferior mediastinum is the box-shaped space in the mediastinum below the transthoracic plane of Ludwig between the wedge-shaped superior mediastinum above and the diaphragm and inferior thoracic aperture below. There are no physical structures that divide the superior and inferior mediastin...
The inferior pulmonary ligament (or just the pulmonary ligament) is a normal anatomical structure that is often seen on chest x-ray and CT chest.
The inferior pulmonary ligament is a fused triangular-shaped sheet of parietal and visceral pleura that extends from the hilum to the...
The inferior thoracic aperture connects the thorax with the abdomen.
The inferior thoracic aperture is irregular in shape and is more oblique and much larger than the superior thoracic aperture. The diaphragm occupies and closes the inferior thoracic aperture, thereby separating ...
The innermost intercostal muscles are muscles of respiration. They are the deepest intercostal muscles located in the intercostal spaces, and contract along with the internal intercostal muscles to reduce the transverse dimension of the thoracic cavity during expiration.
The interchondral joints are small articulations between the apposed costal cartilages of the ribs 7-10.
On each side are three diminutive synovial joints between the surfaces of the 6th and 7th costal cartilages, 7th and 8th costal cartilages and 8th and 9th costal cartilages. The 9th and 10th...
The intercostal muscles are an important group of muscles in the intercostal spaces (between the ribs) that contract during respiration. Three muscles are classically described, from superficial to deep:
external intercostal muscles
internal intercostal muscles
innermost intercostal muscles
The intercostal nerves are the somatic nerves that arise from the anterior divisions of the thoracic spinal nerves from T1 to T11. These nerves in addition to supplying the thoracic wall also supply the pleura and peritoneum.
Intercostal nerves can be divided into atypical and ty...
The intercostal spaces, also known as interspaces, are the space between the ribs. There are 11 spaces on each side and they are numbered according to the rib which is the superior border of the space.
The intercostal spaces contain three layers of muscle: the external, internal...
The interlobular septa (singular: interlobular septum) are located between the secondary pulmonary lobules and are continuous with both the subpleural interstitium (peripheral connective tissue) and the peribronchovascular interstitium (axial connective tissue) as well as the more delicate intra...
The internal intercostal muscles are important muscles of respiration. They number eleven on each side and are located in the intercostal spaces, reducing the transverse dimension of the thoracic cavity during expiration.
The internal intercostal muscles are the middle muscle of ...
The internal thoracic artery (previously called the internal mammary artery) supplies the anterior body wall and its associated structures from the clavicles to the umbilicus.
The internal thoracic artery arises from the first part of the subclavian artery in the base of ...
The intralobular septa (sing: septum) are delicate strands of connective tissue separating adjacent pulmonary acini and primary pulmonary lobules. They are continuous with the interlobular septa which surround and define the secondary pulmonary lobules.