Magnetic resonance imaging of the subthalamic nucleus for deep brain stimulation

Abstract

F uture progress to refine targets for deep brain stimulation (DBS) is critically dependent on precise placement and verification of electrodes in the brain. No stereotactic target is of more clinical importance or more nuanced regarding its imaging than the subthalamic nucleus (STN). The STN is difficult to image because of its biconvex shape, small size, oblique spatial orientation in 3 planes, and its close proximity to the substantia nigra (SN).3 A high level of accuracy is required as the motor region of the STN is posterolateral, and unwanted stimulation side effects may occur as a result of direct stimulation or spread to the anteromedial nonmotor area and the adjacent corticospinal tract. 9,18,31,56 Current imaging of STN borders is difficult.Targeting of the STN has historically been performed indirectly; the neurosurgeon predicts the location of the STN based on coordinates derived from atlases. These coordinates have often been related to the midcommissural point, which was derived from x-ray ventriculography but also from CT and MRI. Retaining this indirect, coordinate-based approach has been attractive to neurosurgeons. Experience in using this approach has been handed down from the past and can be used even with modern noninvasive imaging, and targeting preferences can be easily communicated between surgeons. The problem with indirect targeting is that different patients have different STN sizes, shapes, and positions.38 Direct targeting is an attempt to locate the STN in each individual patient.Poor image quality on traditional 1.5 tesla (T) scanners and primitive pulse sequences that do not provide adequate contrast at the level of the STN have limited the progression of direct targeting in some centers. Also hindering the development of stereotactic MRI are concerns about image distortion with higher field strength; traditional stereotactic head frames and localizers are too bulky to permit the use of high-quality, multichannel MRI head coils.Despite these challenges, higher field strength scanners and new pulse sequences have improved image quality to a point that some groups have adopted direct targeting of the STN, based on MRI.3,9,37 Difficulty delineating the STN and spatial inaccuracy has resulted in reluctance from others to abandon adjuncts such as CT/MRI merge and intraoperative electrophysiology.Traditionally, T2-weighted imaging (T2WI) has been used to plan for direct targeting of the STN using a variety of imaging parameters (Fig. 1).9 Although T2WI is useful, it is difficult to clearly delineate the STN from the SN and, to a lesser extent, the zona incerta (ZI). 22,32,37 abbreviatioNs CNR = contrast-to-noise ratio; DBS = deep brain stimulation; DTI = diffusion tensor imaging; FA = fractional anisotropy; FGATIR = fast gray matter acquisition T1 IR; FLAIR = fluid attenuated IR; FLASH = fast low-angle shot; FSE = fast spin echo; GRE = gradient echo; IR = inversion recovery; NSA = number of signal averages; PSIR = phase-sensitive IR; QSM = quantitative susceptibility mapping; SE = spin ...

Clinical quantitative susceptibility mapping (QSM): Biometal imaging and its emerging roles in patient care

“…8)(140). Therefore, QSM is an ideal technique to accurately and precisely guide electrode placement in DBS treatment for advanced PD patients and other patients with tremor and dystonia (140,141). Current promising results strongly argue in favor of additional clinical evaluations across multiple DBS centers to evaluate whether the inclusion of QSM in presurgical MRI (rather than current T2w protocols) can overall improve health outcomes in PD patients undergoing DBS.…”

Estimating Accurate Target Coordinates with Magnetic Resonance Images by Using Multiple Phase-Encoding Directions during Acquisition

“…Atlas-based indirect targeting or direct targeting of images have also been used at some centers [4, 8, 11, 12]. Each of these methods has its own advantages and disadvantages [13].…”

“…Many investigators have reported various methods for image acquisition in preparation for DBS surgery. For some structures, such as the pedunculopontine nucleus, there is no consensus on the optimal target site, and therefore it is even more important to devise and introduce targeting methods [3, 4]. …”

Using “Functional” Target Coordinates of the Subthalamic Nucleus to Assess the Indirect and Direct Methods of the Preoperative Planning: Do the Anatomical and Functional Targets Coincide?

“…The second is the direct method, which was developed by the work of Bejjani et al in 2000 [7] in which the STN is directly visualized as a hypointense structure on T2 weighted images. Direct localization became easier with the improvements of magnetic resonance imaging (MRI) sequences and the use of special stereotactic software to perform 3D image reconstruction and help in the calculations [8,9,10,11,12]. Many studies assessed the direct coordinates and compared them to the coordinates obtained by the indirect method [13,14,15,16,17].…”