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13,609 results found

Abrodil

Abrodil (marketed as Skiodan in the US) was a water-based iodinated contrast medium introduced for clinical use in 1930 3. It was developed by Bronner, Hecht and Schueller in Germany. Chemically, Abrodil is the sodium salt of monoiodomethanesulfonic acid. It was initially employed for intraveno...

Abscess

Abscesses are focal confined collections of suppurative inflammatory material and can be thought of as having three components 1: a central core consisting of necrotic inflammatory cells and local tissue peripheral halo of viable neutrophils surrounded by a 'capsule' with dilated blood vessel...

Absent bow tie sign (knee)

The absent bow tie sign represents the loss of the normal appearance of the menisci on parasagittal MRI images and is suggestive of meniscal injury. Normally the medial and lateral menisci appear as low signal bow-tie-shaped structures between the femoral condyles and tibial plateaux. As the no...

Absent ductus venosus

Agenesis of the ductus venosus (ADV) is a rare fetal vascular anomaly. Epidemiology According to the data obtained from the screening tests performed at 11-14 weeks of pregnancy, its incidence is reported to be 1/2500 12. Pathology In a large case series, Acherman et al. 13 defined five drai...

Absent fetal stomach on ultrasound (differential)

Non-visualization of the fetal stomach on ultrasound can occur with various physiological as well as pathological processes. It becomes a significant sonographic observation >14 weeks of gestation (about the time the fetus begins to swallow). Causes include: physiological emptying: transient ...

Absent infrarenal inferior vena cava

An absent infrarenal inferior vena cava can be congenital, due to the failure of development of the posterior cardinal and supracardinal veins, or acquired, as a result of intrauterine or perinatal inferior vena cava thrombosis. Epidemiology It is an extremely rare anomaly. Clinical presentat...

Absent nasal bone

In a fetal sonographic assessment, an absent nasal bone is a feature which can sometimes be used as a surrogate marker for fetal aneuploidy. Radiographic assessment Antenatal ultrasound It is assessed on a midline sagittal view. In this section, the nasal bone is often seen as a bright echoge...

Absent patella

An absent patella is a rare finding and can be found with an equally rare set of associations: surgical removal of patella (patellectomy) nail patella syndrome 2 popliteal pterygium syndrome proximal focal femoral deficiency (PFFD) Meier-Gorlin syndrome 3

Absent septum pellucidum

An absent septum pellucidum may rarely be an isolated finding, or more commonly be seen in association with a variety of conditions. Epidemiology The septum pellucidum is partly or entirely absent in 2 or 3 individuals per 100,000 in the general population. Pathology An absent septum pelluc...

Absent thumb

An absent thumb can have many associations. They include: Fanconi anemia (pancytopenia-dysmelia syndrome) Franceschetti syndrome Holt-Oram syndrome phocomelia (e.g. thalidomide embryopathy) Poland syndrome (pectoral muscle aplasia and syndactyly) Rothmund-Thomson syndrome Seckel syndrome ...

Absent umbilical arterial end-diastolic flow

Absent end-diastolic flow (AEDF) in an umbilical artery Doppler assessment is a useful feature which indicates underlying fetal vascular stress if detected in mid or late pregnancy. It is often classified as Class II in severity in abnormal umbilical arterial Dopplers 9. Pathology The presence...

Absent yolk sac

Absence of the yolk sac in the presence of an embryo on a transvaginal ultrasound is considered abnormal, and in general is associated with subsequent embryonic death. See also yolk sac

Absorbed dose

Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation. It is equal to the energy deposited per unit mass of a medium, and so has the unit joules (J) per kilogram (kg), with the adopted name of gray (Gy) where 1 Gy = 1 J.kg-1. The absorbed dose is not a good indica...

Absorption (ultrasound)

In ultrasound, absorption is the reduction in intensity of the sound waves as it passes through tissue. Most of the energy lost is in the form of heat.

Abusive head trauma

Abusive head trauma is a term that is used to related to inflicted head injury that has occurred by either shaking, impact head trauma, or both. Pathology Intracranial injuries Subdural hemorrhage in a child should be viewed with suspicion. Most often, the subdural hemorrhages will demonstrat...

Aceruloplasminemia

Aceruloplasminemia is an autosomal recessive type of neurodegeneration with brain iron accumulation and disorder of iron metabolism caused by a mutation in the ceruloplasmin (CP) gene resulting in the production of dysfunctional ceruloplasmin. Epidemiology Aceruloplasminemia is a very rare dis...

Acalculia

Acalculia (or acquired dyscalculia) represents the acquired loss of the ability to perform simple arithmetic tasks secondary to CNS pathology. It is not to be confused with developmental dyscalculia, which is a different entity and represents developmental disturbance of computational ability. ...

Acardiac twin

An acardiac twin, also known a recipient twin, refers to the haemodynamically disadvantaged twin of a twin-pair in the setting of twin reversed arterial perfusion (TRAP) sequence. The acardiac twin undergoes secondary atrophy and is non-viable. Epidemiology Acardiac twinning is thought to affe...

Acardius acormus

Acardius acormus is a morphologic subtype of an acardiac twin in a twin reversed arterial perfusion (TRAP) sequence. This entity is the least common of the four subtypes, comprising of cephalic structures only. It is seen in approximately 5% of cases.

Acardius amorphus

Acardius amorphus is a morphologic subtype of the acardiac twin in a twin reversed arterial perfusion (TRAP) sequence. This entity is the least well differentiated of the four subtypes, comprising of an amorphus mass with no recognizable structures. It is seen in approximately 20% of cases.

Acardius anceps

Acardius anceps is a morphologic subtype of the acardiac twin in a twin reversed arterial perfusion (TRAP) sequence. This entity is the most well differentiated of the four subtypes, with a partly formed head, thorax, and abdomen. It is seen in approximately 10% of cases.

Acardius anephus

Acardius anephus is a morphologic subtype of an acardiac twin in the twin reversed arterial perfusion (TRAP) sequence. In this entity, no cephalic or structures are present, but the lower limbs are preserved. This is the most common subtype, seen in 60-75% of cases1.

ACC/AHA classification of coronary lesions

ACC/AHA classification of coronary lesions is a system use to classify coronary arterial calcific plaque burden. It is classified as type A discrete (< 10 mm) concentric nonangulated segment < 45º smooth contour little or no calcification less than totally occlusive not ostial in locati...

Accepted abbreviations

There are a number of accepted abbreviations that we use on Radiopaedia.org. We would like the site to be as standardized as possible and we have therefore chosen our accepted abbreviations and would ask that where possible these are used: a.k.a. not aka (short for "also known as") cf. not c.f...

Accessory abductor digiti minimi muscle (hand)

An accessory abductor digiti minimi (ADM) muscle is the commonest accessory muscle of the hypothenar eminence, found in 24% individuals. When present it is one of the intrinsic muscles of the hand. Summary origin: antebrachial fascia passing anteriorly to Guyon canal occasionally arises from...

Accessory anterior inferior tibiofibular ligament

The accessory anterior inferior tibiofibular ligament (accessory AITFL), also known as Bassett's ligament, is an anatomical variant present in many ankles. Pathological thickening of the accessory ligament is seen in the setting of inversion injury that causing the pain due to mild anterior inst...

Accessory appendicular artery

The accessory appendicular artery, also known as the artery of Seshachalam, is a branch of the posterior cecal artery. It arises from the ileocolic artery, and runs in the mesoappendix. The exact prevalence of this accessory artery and its impact upon the risk of appendicitis varies among studi...

Accessory breast tissue

Accessory breast tissue is a relatively common congenital condition in which abnormal accessory breast tissue is seen in addition to the presence of normal breast tissue. This normal variant can present as a mass anywhere along the course of the embryologic mammary streak (axilla to the inguinal...

Accessory fissures of the lung

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...

Accessory flexor digitorum longus muscle

The accessory flexor digitorum longus muscle is an accessory muscle in the deep posterior compartment of the leg with a reported prevalence of 6-8%. Unilateral muscles are more common although bilateral cases have been reported. Summary origin: variable; either the medial margin of the tibia a...

Accessory gallbladder

Accessory gallbladders are a rare anatomical variant occurring in 0.03% of cases (approximately 1 in 3000 people). They can arise from either the left or right hepatic ducts or both. Accessory gallbladders arise from a bifid diverticulum of the hepatic duct in the 5th or 6th week of development ...

Accessory hemiazygos vein

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. Terminology Spelling it "hemiazygous" when referring to the vein is incorrect, rega...

Accessory left atrial appendage

An accessory left atrial appendage is a frequent fortuitous finding in cardiac imaging, encountered in ~10% of patients. They are more often seen as a small diverticular structure projecting from the right upper side of the left atrial wall. Differential diagnosis it must not be confused with ...

Accessory maxillary ostium

Accessory maxillary ostia are a common anatomic variant, and are usually found incidentally on CT scans of the paranasal sinuses. Accessory ostia of the maxillary sinus are common, occurring in up to 40% patients 1. No significant association has been found between the presence of accessory osti...

Accessory meningeal artery

The accessory meningeal artery is a branch of the maxillary artery but can also branch from the middle meningeal artery. The artery passes upwards through the foramen ovale to supply the trigeminal ganglion and the dura mater of Meckel cave and the middle cranial fossa. It also usually supplies...

Accessory middle cerebral artery

The accessory middle cerebral artery is a variant of the middle cerebral artery (MCA) that arises from the anterior cerebral artery (ACA). It is different from a duplicated middle cerebral artery, in which the duplicated vessel originates also from the distal end of the internal carotid artery (...

Accessory muscle

Accessory muscles are a form of anatomic variation that refers to supplementary discrete muscles that are found alongside the normal expected musculature. They have been described in the neck, pelvis, upper and lower limbs.

Accessory navicular

An accessory navicular is a large accessory ossicle that can be present adjacent to the medial side of the navicular bone. The tibialis posterior tendon often inserts with a broad attachment into the ossicle. Most cases are asymptomatic but in a small proportion it may cause painful tendinosis d...

Accessory navicular syndrome

Accessory navicular syndrome occurs when a type II accessory navicular becomes painful due to movement across the pseudo-joint between the ossicle and the navicular bone. Radiographic features Ultrasound It can be inferred on musculoskeletal ultrasound if a patient's pain is located at a type...

Accessory ossicle of the anterior arch of the atlas

The accessory ossicle of the anterior arch of the atlas is a normal variant and is best appreciated on a lateral cervical/sagittal study. It is observed as a circular and corticated osseous density that articulates with the inferior aspect of the anterior arch of the atlas. It is not associate...

Accessory ossicles

Accessory ossicles are secondary ossification centers that remain separate from the adjacent bone. They are usually round or ovoid in shape, occur in typical locations and have well defined smooth cortical margins on all sides. In most cases, they are congenital in origin, although they may occ...

Accessory ossicles of the foot

Accessory ossicles of the feet are common developmental variants with almost 40 having been described. Some of the more common include 1-4: os peroneum os subfibulare os subtibiale os tibiale externum (accessory navicular) os trigonum os calcaneus secundaris os intermetatarseum pars pero...

Accessory ossicles of the lower limb

There are numerous named and unnamed accessory ossicles of the lower limb. These include: ossicles of the hip os acetabuli ossicles of the knee os fabella cyamella ossicles of the foot os peroneum os subfibulare os subtibiale os tibiale externum os trigonum os calcaneus secundaris o...

Accessory ossicles of the wrist

Accessory ossicles of the wrist are commonly seen on plain radiographs of the wrist and associated cross-sectional imaging. Over 20 were originally described 2, although the more common include 1: lunula: between TFCC and triquetrum os styloideum (carpal boss): on dorsal surface of 2nd or 3rd ...

Accessory ossicles of the wrist (mnemonic)

Accessory ossicles of the wrist can be easily recalled with the mnemonic: LOTTEO 1 HOTELS Mnemonic LOTTEO L: lunula O: os styloideum (carpal boss) T: (os) triangulare T: (os) trapezium secondarium E: (os) epilunate O: os hamuli proprium HOTELS H: (os) hamuli proprium O: os triangula...

Accessory parotid gland

Accessory parotid glands are a normal variant and represent ectopic salivary tissue separate from, but usually in close proximity to, the main parotid glands. Epidemiology Accessory parotid glands are commonly picked up incidentally on ultrasound; seen in ~20% of the general population 2. Gro...

Accessory peroneal muscles

Accessory peroneal muscles are a group of accessory muscles that can occur in the foot region as a normal variant in some individuals. The peroneal compartment is known as the lateral compartment of the leg. Peroneus quartus muscle Originally, several accessory muscles were distinguished in th...

Accessory phrenic nerve

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...

Accessory renal artery

Accessory renal arteries are a common variant and are present in ~25% (range 20-30%) and are bilateral in ~10% of the population 7. Their proper identification is of utmost importance for surgical planning prior to live donor transplantation 2,3 and renal artery embolization for various reasons ...

Accessory right inferior hepatic vein

An accessory right inferior hepatic vein is the most common variation of the hepatic veins. It is present in up to 48% of the population and drains the posterior part of the right lobe (mainly segments 6 and 7) directly into the inferior vena cava. Variations in hepatic vascular anatomy are pa...

Accessory sacroiliac joint

Accessory sacroiliac joints are a common finding, present on ~15% (range 13-17.5%) of CT studies, and may be unilateral or bilateral. They are an articulation between the medial aspect of the posterior superior iliac spine and the sacrum just lateral to the second dorsal sacral foramen. They may...

Accessory semimembranosus muscle

The accessory semimembranosus muscle is a rare accessory muscle of the posterior compartment of the thigh. It arises from the distal aspect of the semimembranosus muscle belly and courses through the popliteal fossa between it and the semitendinosus muscle medially and the biceps femoris lateral...

Accessory soleus muscle

The accessory soleus muscle is an anatomical variant characterized by an additional distinct muscle encountered along a normal soleus muscle. It is uncommon with a prevalence of ~3% (range 0.7-5.5%). Summary origin: fibula, soleal line of the tibia, or the anterior surface of the soleus muscle...

Accessory superior acetabular notch

An accessory superior acetabular notch is a normal variant of the acetabulum, which can be seen on radiographs. It may lead to diagnostic confusion, especially in younger patients. Radiographic features MRI appear as bilateral symmetric fluid-filled pits in the roof of the acetabulum with sh...

Accessory sutures

The parietal and occipital bones in particular are common regions for accessory sutures because of their multiple ossification centers. It is important to know these anatomic variations, mainly on the head trauma image studies in children, where it could be difficult to differentiate non-depres...

Accordion sign (colon)

The accordion sign (also known as concertina sign) is seen on CT examinations of the abdomen and refers to the similarity between the thickened edematous wall of pseudomembranous colitis and the folds of an accordion. This appearance is the result of hyperemic enhancing mucosa stretched over mar...

Ace-of-spades sign (heart)

Ace-of-spades sign refers to the pathognomonic configuration of the left ventricle as seen in apical hypertrophic cardiomyopathy 1-3. It consists of marked ventricular wall thickening at the apex resulting in cavity narrowing at the apex with a relatively normal appearance of the mid-ventricula...

Acetabular angle

The acetabular angle is a radiographic measurement used when evaluating potential developmental dysplasia of the hip (DDH). It is most useful in patients who have started to ossify the epiphysis since ossification diminishes the usefulness of ultrasound. The angle is formed by a horizontal lin...

Acetabular foramen

The acetabular foramen is formed by the bony margins of the acetabular notch and completed by the transverse ligament of the hip. From its margins (both transverse ligament and acetabular notch) arises the ligamentum teres. Through it pass nutrient vessels to the femoral head epiphysis.

Acetabular fossa

The acetabular fossa, also known as the cotyloid fossa, is the central aspect of the medial wall of the acetabulum that hosts the ligamentum teres and the fibrofatty pulvinar. It is the nonarticular portion inside the U-shaped labrum that extends to the acetabular notch 1. The acetabular fossa i...

Acetabular fracture

Acetabular fractures are a pelvic fracture, which may also involve the ilium, ischium, and/or pubis depending on fracture configuration. Epidemiology Acetabular fractures are uncommon. The reported incidence is approximately 3 per 100,000 per year. This study reported a 63% to 37% male to fema...

Acetabular labral calcification

Acetabular labral calcification in the hip can be a finding detected on imaging studies. It has an associated with osteoarthritis of the hip and may account for higher pain levels in individuals with a high degrees of calcification independent of age and histological degeneration 2. Differentia...

Acetabular labral sulcus

An acetabular labral sulcus is type of normal variant seen as cleft like appearance on imaging. It forms where the labrum meets the adjacent articular cartilage. Several sulci can be identified which include several sublabral sulci posterosuperior ~ 48% anterosuperior ~ 44% anteroinferior ~ ...

Acetabular labral tears

Acetabular labral tears, as the name implies, are tears involving the acetabular labrum of the hip. With the increasing use of hip arthroscopy in orthopedic surgery since the 1970s pathologies of the acetabular labrum as a possible cause of chronic hip and groin pain have become more familiar...

Acetabular labrum

Acetabular labrum acts to deepen the acetabulum and increase contact between the pelvis and the femoral head. Its exact biomechanical role remains to be fully elucidated. Gross anatomy The acetabular labrum is a C-shaped fibrocartilaginous structure with an opening anteroinferiorly at the site...

Acetabular notch

The acetabular notch is a depression in the margin of the acetabulum located anteroinferiorly. It is bridged by the transverse ligament, and thus forms the acetabular foramen. The ligamentum teres has part of its origin from the acetabular notch.

Acetabulum

The acetabulum (plural: acetabula) is the large cup-shaped cavity on the anterolateral aspect of the pelvis that articulates with the femoral head to form the hip joint. Gross anatomy All three bones of the pelvis (the ilium, ischium, and pubis) together form the acetabulum. The three bones ar...

Achalasia

Achalasia (primary achalasia) is a failure of organized esophageal peristalsis causing impaired relaxation of the lower esophageal sphincter, and resulting in food stasis and often marked dilatation of the esophagus. Obstruction of the distal esophagus from other non-functional etiologies, not...

Acheiria

Acheiria refers to absence of one or both hands and can occur in a number of situations, including: amniotic band syndrome: particularly if unilateral Cornelia de Lange syndrome 1 fetal hydantoin syndrome 2 Epidemiology Rare defect occurring in 1:65 000 live births. See also absent thumb...

Achievements

Achievements are a recent feature of your public profile. They are a way of showcasing to the world some of the ways you have contributed to Radiopaedia. Achievements There are 6 achievements currently, two of them representing 'current status' and representing the current level of a specific...

Achilles tendinopathy

Achilles tendinopathy refers to a combination of pathological changes affecting the Achilles tendon usually due to overuse and excessive chronic stress upon the tendon. It can be seen both in athletes and non-athletes. It is hard to differentiate clinically from a paratendinopathy (which is most...

Achilles tendon ossification

Achilles tendon ossification is an uncommon condition that consists of characteristic bone formation within the substance of the Achilles tendon. Epidemiology Achilles tendon ossification is more common in males 4. Pathology The etiology is unknown but Achilles tendon ossification is seen ...

Achilles tendon tear

Achilles tendon tears are the most common ankle tendon injuries, and are most commonly seen secondary to sports-related injury, especially squash and basketball. Epidemiology There is strong male over-representation presumably as a result of the predominantly sport related etiology. Patients a...

Achilles tendon thickening

Achilles tendon thickening can occur for a number of reasons. The Achilles tendon has an average AP diameter of 6 mm 1. Thickening of the tendon is when it exceeds 8 mm in AP diameter and can result from: Achilles tendinosis/tear post-surgical thickening retrocalcaneal bursitis degeneration...

Achilles tendon xanthoma

Achilles tendon xanthomas are painless soft tissue masses occurring most commonly at the distal one-third of the tendon and are usually bilateral and symmetrical. Pathology It is characterized by localized accumulation of lipid-laden macrophages, inflammatory cells and giant cells secondary to...

Achondrogenesis

Achondrogenesis refers to a group of rare and extreme skeletal dysplasias. Epidemiology The estimated incidence is 1:40,000 with no recognized gender predilection. Pathology It is classified as an osteochondrodysplasia, meaning deficiency of both bone and cartilage development. Subtypes T...

Achondrogenesis 1A

Achondrogenesis type Ia, also known as the Houston-Harris subtype, is a subtype of achondrogenesis. It is an extremely rare lethal skeletal dysplasia (chondrodysplasia) with a characteristic severe disarrangement of endochondral ossification. Clinical presentation Some of the common clinical f...

Achondrogenesis 1B

Achondrogenesis type Ib, also known as Parenti-Fraccaro subtype, is a subtype of achondrogenesis and is an extremely rare skeletal dysplasia (chondrodysplasia). Pathology Undersulphation has a pronounced effect on the composition of the extracellular matrix of cartilage, and this has been sh...

Achondroplasia

Achondroplasia is a congenital genetic disorder resulting in rhizomelic dwarfism and is the most common skeletal dysplasia. It has numerous distinctive radiographic features. Epidemiology It occurs due to sporadic mutations in the majority of cases but can be inherited as an autosomal dominan...

Achondroplastic base of skull abnormalities

Achondroplasia is the most common cause of short-limb dwarfism. (For a general discussion, see the generic article on achondroplasia.) As the skull base forms by endochondral ossification whereas the skull vault by membranous ossification, there is a marked discrepancy in relative size as the s...

Acinar-predominant adenocarcinoma of the lung

Acinar-predominant adenocarcinoma of the lung is a histological subtype of non-mucinous invasive adenocarcinoma of the lung. Terminology In 2011, the International Association for the Study of Lung Cancer (IASLC), American Thoracic Society (ATS), and European Respiratory Society (ERS) 4 introd...

Acinic cell carcinoma (lung)

Acinic cell carcinoma of the lung (also known as a Fechner tumor) is a type of lung carcinoma of the salivary gland type. It is extremely rare, especially when it presents in the form of primary acinic cell carcinoma. Pathology Histologically, they are comprised of clear cells with abundant gr...

Acinic cell carcinoma (salivary glands)

Acinic cell carcinomas of the salivary glands are rare malignant neoplasms that account for 1-3% of all salivary gland tumors. Pathology Pathology may superficially resemble normal serous (acinar) cells of the salivary glands. It is considered a low-grade, indolent malignancy, but with a tend...

Acoustic enhancement

Acoustic enhancement, also called posterior enhancement or enhanced through transmission, refers to the increased echoes deep to structures that transmit sound exceptionally well. This is characteristic of fluid filled structures such as cysts, the urinary bladder and the gallbladder. The fluid...

Acoustic impedance

Acoustic impedance (Z) is a physical property of tissue. It describes how much resistance an ultrasound beam encounters as it passes through a tissue. Acoustic impedance depends on: the density of the tissue (d, in kg/m3) the speed of the sound wave (c, in m/s) and they are related by: Z = ...

Acoustic shadowing

Acoustic shadowing on an ultrasound image is characterized by a signal void behind structures that strongly absorb or reflect ultrasonic waves. This happens most frequently with solid structures, as sound conducts most rapidly in areas where molecules are closely packed, such as in bone or stone...

Acquired aortic conditions

There are many acquired aortic conditions. These include aortic dissection aortic rupture / transection ascending aortic aneurysm aortitis thoracic aortic injury abdominal aortic aneurysm inflammatory abdominal aortic aneurysm

Acquired cholesteatoma

Acquired cholesteatomas makeup 98% of all middle ear cholesteatomas and are almost always closely related to the tympanic membrane, from which most are thought to arise. Clinical presentation The vast majority of acquired cholesteatomas develop as a result of chronic middle ear infection and ...

Acquired cystic kidney disease

Acquired cystic kidney disease (ACKD) is a condition that occurs in patients with end-stage renal disease (ESRD), especially when on dialysis treatment. They do not have a history of other cystic renal disease. Epidemiology Its incidence increases with the amount of time a patient is azotemic...

Acquired hepatocerebral degeneration

Acquired hepatocerebral degeneration is an uncommon irreversible extrapyramidal neurodegenerative condition encountered in patients with cirrhotic chronic liver disease, resulting in widespread cerebral, basal ganglia and cerebellar damage. Terminology Acquired hepatocerebral degeneration is ...

Acquired tonsillar ectopia

Acquired tonsillar ectopia is usually thought of as a subgroup of cerebellar tonsillar ectopia in which downward displacement of the cerebellar tonsils is secondary to another well defined and distinct pathological process. This is to distinguish it from Chiari I malformations and low-lying tons...

Acquired tracheo-esophageal fistula

An acquired tracheo-esophageal fistula refers to a pathological communication between the trachea and esophagus due to a secondary cause Pathology Acquired causes of tracheo-esophageal fistulae can be divided into those that are related to malignancy (common) and those from other causes (unco...

Acquisition time

The time of acquisition for a conventional spin echo or gradient echo sequence is the product of the repetition time, phase encoding steps, and number of averages (TR x phase steps x NEX). For example, with a one second TR, 128 phase steps, and two averages we would get an acquisition time of ab...

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