Cerebral edema and venous infarction may be apparent on any modality, but for the detection of the thrombus itself, the most commonly used tests are computed tomography (CT) and magnetic resonance imaging (MRI), both using various types of radiocontrast to perform a venogram and visualise the veins around the brain.Computed tomography, with radiocontrast in the venous phase (CT venography or CTV), has a detection rate that in some regards exceeds that of MRI. The test involves injection into a vein (usually in the arm) of a radioopaque substance, and time is allowed for the bloodstream to carry it to the cerebral veins – at which point the scan is performed. It has a sensitivity of 75–100% (it detects 75–100% of all clots present), and a specificity of 81–100% (it would be incorrectly positive in 0–19%). In the first two weeks, the "empty delta sign" may be observed (in later stages, this sign may disappear). The empty delta sign is characterized by enhancement of the dural wall without intra-sinus enhancement.Magnetic resonance venography employs the same principles, but uses MRI as a scanning modality. MRI has the advantage of being better at detecting damage to the brain itself as a result of the increased pressure on the obstructed veins, but it is not readily available in many hospitals and the interpretation may be difficult.Cerebral angiography may demonstrate smaller clots than CT or MRI, and obstructed veins may give the "corkscrew appearance".
A callosity is another name for callus, a piece of skin that has become thickened as a result of repeated contact and friction. Primates
All Old World monkeys, gibbons, and some chimpanzees have pads on their rears known as ischial callosities. Ischial relates to the ischium: it forms the lower and back part of the hip bone. The pads enable the monkeys to sleep sitting upright on thin branches, beyond reach of predators, without falling. The ischial callosities are one of the most distinctive pelvic features which separates Old World monkeys from New World monkeys. Right whales
In whales, the term callosity refers to the rough, calcified skin patches found on the heads of the three species of right whales. These callosities are a characteristic feature of the whale genus Eubalaena; because they are found on the head of the whale and appear white against the dark background of the whales skin, they make it very easy to identify these species. The callosities themselves are grey; the white appearance is due to large colonies of whale lice, whale barnacles and parasitic worms which reside on them. Young whales and diseased individuals are often infested with a different species of cyamid, which gives an orange hue rather than white on these whales.