Trigonometric method of computing the coordinates of invisible targets in functional neurosurgery.

Accessible online at: www.karger.com/journals/sfn We read, with great interest, the paper by K. Ott ‘An Algorithm for the Empirical Determination of Intracranial Stereotactic Targets’, Stereotact Funct Neurosurg 1998;71: 29–35. In the Neurosurgical Department of Florence City Hospital we routinely employ basically the same method of calculation since 1996 for the surgical treatment of Parkinson’s disease by deep brain stimulation. Since December 1996, we have implanted 54 deep electrodes (1 VIM, 10 GPi, 42 STN, 1 in the pontomesencephalic reticular formation for persistent vegetative state) with anatomical postoperative CT and MRI confirmation of the correct position of the electrode. We autonomously developed this trigonometric method of calculation in close collaboration with mathematicians of the National Research Council of Italy. Using a CT scan, the x, y, z coordinates of a point which is at a prefixed distance on the three cartesian planes from a reference system are computed. The latter contains the ‘origo’ from which the distances are measured, and is identified by points of known coordinates. In clinical terms, we define the coordinates of an ‘invisible target’ (e.g. VIM, GPi, STN) referred to a reference system, namely the intercommissural line (AC–PC) and the foramen of Monro – posterior commissure line (FM–PC), or the midsagittal plane of the third ventricle which contains both of them. This plane is univocally identified by three points: A, AC, the anterior commissure, which is localized 3–4 mm below the lower border of the foramen of Monro; P, PC, the posterior commissure, which can be seen on the slice immediately superior to the quadrigeminal plate or the upper end of the aqueduct, and inferior to the pineal calcification: I, the