L7 Li AP 0 CL Pf ml CM sPf VPMpc fct RN 456 Brain Struct Funct (2008) 212:443–463 MDpc CL L8 CL MDpc VLpv VM ft PTT STh mcl V2 GPe VAmc fl ZI GPi VApc R al ic STh Li bic AP 0 fl PTTft fct CTT CM Pf ml VPM CTT fct RN fct fx mtt RN Hb1 Hb2 Hb3 Hb5 VLpv VM ft STh mcl mcl VLa ZI fl VApc R al DV 0 Fig. 12 Variability of the ﬁbre tracts in the subthalamic area in sagittal (upper panels) and horizontal (lower panel) atlas maps of different brains. In sagittal sections (upper panels, L7 and L8), red contours and ﬁlling correspond to brain Hb1 and black contours and gray ﬁlling correspond to brain Hb4. In the horizontal section represented in lower panel, red and black thalamic and basal ganglia contours correspond to cases Hb1 and Hb3, respectively (see also Table 1). For reference, the STh is represented in green. In the horizontal section (lower panel), the delineation of fct, ft, and ﬂ in than VPMpc) by remaining relatively medial. The locations of the targeted PTT and CTT in relation to the variability of the positions of the pallido- and cerebellothalamic tracts are represented in Fig. 12 on sagittal (upper panels) and horizontal (lower panel) atlas maps. The use of antero- posterior mcl level, instead of the pc level, and of physiological control for depth assessment, allows adjust- ing the site for RFL in different patients. For mediolateral coordinates, some correction can be made according to the width of the thalamus and subthalamic area assessed by the border of the internal capsule. However, corrections are minimized by the fact that targeting is directed at more medial portions of the two tracts where interindividual variations are less marked (Morel 2007; see also Fig. 12, lower panel). Figure 14 illustrates PTT of ﬂ and ft, and CTT of fct in two patients with PD and ET, respectively, on 2-day postoperative axial T1-weighted MRI (panels a and b) and projections onto a horizontal section of the atlas (panel c). The PD patient suffered from a tremor dominant unilateral form of the disease and at 1-year follow-up, beneﬁted from a complete tremor relief, without arising of new symptoms. He experienced a strong improvement of 123 four different cases is represented by different dotted lines and the area of maximal overlap between the tracts in Hb1, Hb2, and Hb5 is depicted in yellow. In all panels, the locations and sizes of the targeted PTT and CTT are represented in dark gray areas. Positions of the horizontal intercommissural (DV0), posterior commissural (AP0) and midcommissural (mcl) planes are also indicated. Midcommissural level in the sagittal maps corresponds to that of case Hb4 (13 mm). Scale bars: 2 mm his quality of life and daily activities. The ET patient manifested at the 1.5-year follow-up, a complete control of both the postural and kinetic components of his prominent action tremor. Discussion This study was designed to provide an anatomical basis for stereotactic targeting of the pallido- and cerebellothalamic ﬁbre tracts in motor disorders, particularly in PD and ET. Indeed, there is an increasing evidence that RFL or DBS partially or completely involving subthalamic ﬁbre tracts from the pallidum (in case of PD) or from the cerebellar nuclei (in case of ET or other types of kinetic tremors) to the thalamus are more efﬁcient for relieving PD or ET symptoms than targets placed in the thalamus or STh (Voges et al. 2002; Zonenshayn et al. 2004; Breit et al. 2006; Godinho et al. 2006; Plaha et al. 2006; Hamel et al. 2007; Herzog et al. 2007a, b). Our surgical approach, which aims at directly interrupting the ﬁbre tracts in sub- thalamic area with a RFL (Magnin et al. 2001a, b;