Source of the midline echo and its implications in echoencephalography.

Ultrasound as a technique in neurological diagnosis was introduced by Leksell in 1955, and is now gaining acceptance as a useful screening test for intracranial tumours and haematomas. The technique uses a pulsed ultrasonic beam transmitted from a probe (transducer) applied to the temporal region of the skull. The ultrasound is then reflected from a 'midline structure' within the cranium. Spaceoccupying lesions are suspected when the midline echo is displaced laterally. Although the method is simple to use, errors in interpreting the reflected midline echo occur. Clinical studies by Lithander (1960), Ford and Ambrose (1963), and Brinker, King, and Taveras (1965) record false midline displacements in 6 to 12% of cases and normal midline echos associated with spaceoccupying lesions in 0 5 to 5-7 %. Failure in technique is partly responsible, as these authors have shown. However, in some situations where bilateral chronic subdural haematomas or bitemporal lobe contusions have balanced each other, or, as Lithander (1961) has shown, when moderate unilateral extracerebral haematomas have collected high over the convexity of the hemisphere, there may be no displacement ofthe midline echo, whereas a unilateral frontal mass, although displacing the anterior structures locally, need have no effect on those more posteriorly situated. In view of these interpretive difficulties the generalized term 'midline echo' appeared unsatisfactory. Indeed as the midline echo could be constantly produced with similar characteristics in different individuals it was probable that the echo signal did occur from a specific anatomical structure. Its identity has been previously investigated and variously held to be the longitudinal fissure (de Vlieger and Ridder, 1959), septum lucidum (Gordon, 1959), third ventricle (Ford and Ambrose, 1963), pineal body (Leksell, 1958; Jeppsson, 1961; Brylski and Izenstark, 1965), and structures in the region of the posterior part of the third ventricle (Brinker et al., 1965). Evidence from the present study into the reflecting source of the midline echo favours the pineal body. ECHOENCEPHALOGRAPHY