# Articles

Articles are a collaborative effort to provide a single canonical page on all topics relevant to the practice of radiology. As such, articles are written and continuously improved upon by countless contributing members. Our dedicated editors oversee each edit for accuracy and style. Find out more about articles.

Article

#### Cyst of the medullary conus

Cyst of the medullary conus is a rare benign ependymal cyst of the conus medullaris which probably relates to abnormal persistence and cystic dilatation of the ventriculus terminalis or "5th ventricle". This entity can be symptomatic and present in adulthood with bladder or bowel sphincter distu...

Article

#### Ischemic stroke

Ischemic stroke is an episode of neurological dysfunction due to focal infarction in the central nervous system attributed to arterial thrombosis, embolization, or critical hypoperfusion. While ischemic stroke is formally defined to include brain, spinal cord, and retinal infarcts 1, in common u...

Article

#### Baumann angle

Baumann angle, also known as the humeral-capitellar angle, is used for the evaluation of the displacement of pediatric supracondylar humeral fractures. It is measured on a frontal radiograph, with elbow in extension.
This angle is formed by the humeral axis and a straight line through the epiph...

Article

#### Alpha vs beta error

Clinical trials may have incorrect results due to random error or bias. The 2 types of random error are called alpha (α) and beta (β), also known as type I and type II errors respectively. For Radiologists, it may be helpful to think of α or type I error as a false positive (FP) and β or type II...

Article

#### Power

The power of a clinical trial is the probability that the trial will find a difference between groups if there is one. Power can be defined as the probability of a true positive trial result and is often written as:
power = (1 - β)
where β is the probability of missing a difference between gro...

Article

#### Receiver operating characteristic curve

The receiver operating characteristic (ROC) curve is a statistical relationship used frequently in radiology, particularly with regards to limits of detection and screening.
The curves on the graph demonstrate the inherent trade-off between sensitivity and specificity:
y-axis: sensitivity
x-a...

Article

#### Hypertrophic olivary degeneration

Hypertrophic olivary degeneration (HOD) is a rare condition characterized by a unique pattern of trans-synaptic degeneration. Typically it is caused by a lesion in the brainstem/cerebellum interrupting the triangle of Guillain and Mollaret, resulting in hypertrophy of the inferior olivary nucleu...

Article

#### Epilepsy

Epilepsy is a common neurological disorder that is characterized by a predisposition to having epileptic seizures and can take many clinical forms and have a veritable Augean stable of etiologies.
Epilepsy is defined by the International League Against Epilepsy (ILAE) as 1:
at least two or mor...

Article

#### Likelihood ratios

Likelihood ratios (LR) are an alternative to positive and negative predictive values for estimating the likelihood of disease after diagnostic testing. The general formula for a likelihood ratio is the probability (P) that someone with a disease will have a particular test result divided by the ...

Article

#### Negative predictive value

Negative predictive value (NPV) is one of the 4 basic diagnostic test metrics in addition to sensitivity, specificity and positive predictive value. Negative predictive value is a measure of how often someone who tests negative for disease does not have disease and is calculated by dividing the ...

Article

#### Positive predictive value

Positive predictive value (PPV) is one of the 4 basic diagnostic test metrics in addition to sensitivity, specificity and negative predictive value. Positive predictive value is a measure of how often someone who tests positive for disease actually has disease and is calculated by dividing the n...

Article

#### Specificity

Specificity is one of the 4 basic diagnostic test metrics in addition to sensitivity, positive predictive value and negative predictive value. Specificity is a measure of how good a diagnostic test is at identifying people who are healthy and is calculated by dividing the number of true negative...

Article

#### Sensitivity

Sensitivity is one of the 4 basic diagnostic test metrics in addition to specificity, positive predictive value and negative predictive value. Sensitivity is a measure of how good a diagnostic test is at detecting disease when it is present and is calculated by dividing the number of true positi...

Article

#### Probability vs odds

The likelihood of an occurrence (called an “event”) can be expressed as a probability or as odds.
Probability (P) tells us how often a particular event occurs on average over the course of many trials. For example, the probability of rolling a 4 with a fair 6-sided die is 1/6. Think of probabil...

Article

#### Chiari II malformation

Chiari II malformations are relatively common congenital malformations of the spine and posterior fossa characterized by myelomeningocele (lumbosacral spina bifida aperta) and a small posterior fossa with descent of the brainstem, cerebellar tonsils, and vermis through the foramen magnum. Numero...

Article

#### Diagnostic test accuracy

The accuracy of a diagnostic test is defined as how often the test correctly classifies someone as having or not having the disease. The formula for accuracy is:
(true positive + true negative) / (true positive + true negative + false positive + false negative)
or correct results / all results...

Article

#### Focal nodular hyperplasia

Focal nodular hyperplasia (FNH) is a regenerative mass lesion of the liver and the second most common benign liver lesion (the most common is a hemangioma). Many focal nodular hyperplasias have characteristic radiographic features on multimodality imaging, but some lesions may be atypical in app...

Article

#### Rickets

Rickets, less commonly known as rachitis, refers to deficient mineralization of the growth plate in the pediatric population. In contrast, osteomalacia refers to deficient mineralization of the bone matrix, which co-occurs with rickets but can also occur even after growth plate closure, in adult...

Article

#### Coca-Cola bottle sign (thyroid eye disease)

The Coca-Cola bottle sign refers to the appearance of the muscles of the orbit in thyroid eye disease. The belly of the muscle enlarges with sparing of the tendinous insertion, giving the appearance of the traditional Coca-Cola bottle.
The enlargement of the muscles follows the I'M SLOW format....

Article

#### Riche-Cannieu anastomosis

Riche-Cannieu anastomosis describes a common interneural communication between the median nerve and the ulnar nerve in the hand.
Gross anatomy
Generally, this anastomosis is specifically between the deep palmar branch of the ulnar nerve and the recurrent branch of the median nerve, although ma...