This paper presents a new local volume estimator, the spatial rotator, which is based on measurements on a virtual 3D probe. The basic design of the probe builds upon the rotator principle which requires only a few manual intersection markings and thus the spatial rotator is fast to use. Since a 3D probe is involved it is expected that the spatial rotator is more efficient than the traditional local volume estimators, the nucleator and the planar rotator, which are based on measurements in a single plane. An extensive simulation study shows that the spatial rotator may be a factor 10 more efficient than the traditional local volume estimators. Furthermore, the spatial rotator can be seen as a derivative of the Cavalieri estimator which does not require randomization of sectioning or viewing direction. The tissue may thus be sectioned in any arbtrary direction where it is easy to identify the specific tissue region under study. In order to use the spatial rotator in practice, however, it is necessary to be able to identify intersection points between cell boundaries and test rays in a series of parallel focal planes, also at the peripheral parts of the cell boundaries. Under- and over-projection phenomena should therefore be negligible or corrected for if the spatial rotator is to be applied.

Keywords: Cavalieri, local stereology, particle volume, rotator, virtual probe