Deep brain stimulation (DBS) had become a widely used method for treatment of Parkinson disease (PD), and the pedunculopontine nucleus (PPN), as a potential DBS target for the patients to improve gait and posture [1]. Until now, only a few results of the location of PPN has been published, due to its deep location in the pons, small structure and no clear boundary. In this study, 7T ultra-high field MR system and high resolution MP2RAGE [2] sequence were used to locate the PPN by a direct view, and a new coordinate designed for PPN location was introduced and test.In vivo data was collected at Siemens Magnetom 7T scanner, using MP2RAGE sequence. The scanning parameters were: 0.7 isotropic voxels with 384*384*224 data matrix, TR = 4000ms, TE = 3.08ms, TI1/TI2 = 900/2750ms, FA1/FA2 = 4/5deg, Bandwidth = 240Hz/Px. Four volunteers were scanned using this protocol.
A new coordinate for localizing the PPN was presented. An oblique axial slice start from the middle of corpora quadrigemina point Q and tagented to the superior border of the pons at point P; the coronal slice crossed the cerebral aqueduct (CA) and perpendicular to the oblique axial slices, and the original point was set as point C, the axial slice was defined as C-P plane.
The rostral and caudal boundary of PPN was measured manually as follows: 1) moving the axial slices to cross the inferior colliculus; 2) moving the coronal slice to the small mass of low signal voxels anterolateral the periaqueductal gray (PAG); 3) changing the slice number of coronal plane until the fuzzy boarders of PPN would be seen; 4) moving axial slice superior or inferior to find the clear boundary with the reference of coronal slice; 5) localizing the rostral and caudal point of PPN with the fine tuning at the sagittal plane.
Orthographic views with C-P coordinate across the PPN and other structures was present as figure 2. The rostral and caudal point of PPN was confirmed with the coordinate as reference [3], all the final results were compared between the two methods, as table 1 and table 2.The B-F [3] and C-P coordinate for brain stem were displayed in figure 1. The two coordinates had different original and different axis across the brain.
The narrow boomerang shape PPN could be seen from figure 2, it is located at the anterolateral of the pons, confined by superior cerebellar peduncle and its decussation (Dec SCP), the central tegmental tract (CTT) and curved band of the lemniscal system. Other fine structures also could been seen.
The manually measured coordinates of PPN boundary points listed in table 1 and 2. B-F coordinate had a similar result as reference [3]. Compared with the B-F, the C-P coordinate had a basically consistent result of the localization of PPN. Except the anterior-posterior of caudal part, the other result of PPN boundary from C-P system presented a smaller variation than that of B-F system, as could been seen from the standard deviation and range of the result.A clear location of PPN could be seen directly. The new C-P coordinate, provided a better view for PPN and showed a results had less standard deviation and range than the common used B-F coordinate system.High resolution imaging can provide better results. That’s why PPN is visible directly by MR imaging using MP2RAGE at 7T.
The reasons for less variation to localize PPN under C-P coordinate: first, the two coordinate system had different origin point and initial slice, the axial slice of C-P coordinate across the origin point past through the PPN, and B-F coordinate was far from the boundary of PPN. Less variation from brain structure between subjects would be introduced by C-P system. Second, the axial slice of two coordinate system had different angle, under C-P coordinate, a larger area would be got under a bigger forward lean degree, and this was more suitable for displaying and measuring the small structures in the pons such as PPN.
The new C-P coordinate introduced in this study, with the help of middle of corpora quadrigemina and superior border of the pons, was very easy to define in the brain, and had a good consistency when measuring the boundary of PPN between each subject. This new coordinate could be used in localizing the PPN before DBS surgery even when the boundary of PPN could not be seen in the low resolution images from low field scanner or an inevitable fast scan.