atlantoaxial rotatory fixation – This refers to a rotational dislocation of the normal alignment of the first vertebra of the spine (C! or Atlas) from the second vertebra (C2 or Axis) with the dislocation being of such a severity that it will not spontaneously relocate back into a normal alignment. In such cases when the head is maximally rotated to the opposite side the rotational malalignment of C1 on C2 is unchanged or “fixed”.
Arterial Blood Gas – This is a test that measures the amount of oxygen (PaO2), carbon dioxide (PaCO2) and the acidity of the arteries blood. It requires obtaining blood from the artery. The test is used to access the effectiveness of respiration, blood and the body’s metabolism.
Apparent diffusion coefficient – A number used to quantify the degree of signal abnormality on Diffusion MRI. Variations in its value can reflect several mechanisms of change in the tissue being imaged and have been correlated to disease states in brain tissue.
Antidiuretic hormone (AKA vasopressin) – A hormone secreted by the pituitary gland that is responsible for increasing water absorption by the kidney. It also increases the resistance to blood flow in the peripheral vascualr system with a resulting increase in blood pressure.
Atlanto-dens interval. The distance from the posterior cortical table of the atlas or C1 body to the anterior cortical edge of the dens or odontoid.
The Atlanta-De3ntal Interval. The horizontal distance from the posterior or ventral, midline surface of the anterior (or ventral) portion or arch of C1 to the anterior (or ventral) surface of the dens of C2 of the cervical spine. The normal interval is <5 mm for children, <3 mm for adult males and <2.5 for adult females..
The inlet of the Aqueduct of Sylvius. The inlet may also be termed the adytum.
Adenosine diphosphate – a critical molecule in the energy metabolism cycle in living beings. It is the resulting molecule when the molecule ATP or adenosine triphosphate looses a phosphate group to supply energy to a metabolic process.