2.1.1. Intraoperative CT

In some centers, intraoperative computed tomography (iCT) during surgery is used to verify the accuracy of lead placement (Table 1). This is achieved through fusion of iCT scans and preoperative MRI scans after intracranial electrode implantation [31,37,41,44,47]. In awake DBS surgery with MER guidance, iCT provides useful information, such as hemorrhage and a general idea of electrode location when fused with preoperative MRI [21]. According to a study about the accuracy of microelectrode trajectory in patients receiving MER-guided awake DBS using iCT, median (IQR) radial error 0.59 (0.64) mm, and median (IQR) absolute x and y coordinate errors were 0.29 (0.52) and 0.38 (0.44) mm, respectively [21]. Burchiel et al. fused and compared iCT and trajectory planning images after electrode implantation for various targets [37]. The mean vector error and mean deviation of trajectory was 1.59 ± 1.11 mm and 1.24 ± 0.87 mm, respectively, and the intraoperative replacement was performed on one electrode with a vector error of more than 3 mm. There was a significant correlation between the distance from the ventricle and the error. Kremer et al. stated that the mean di fference between lead tips was 0.98 ± 0.49 mm, and the upper confidence interval did not exceed the non-inferiority margin described when comparing postoperative MRI with iCT [48].

Some centers use MER without test stimulation but with intraoperative imaging to verify the intended target acquisition in STN DBS surgery under GA [32,49–58]. A recent study compared the mean errors of MER-guided electrode implantation in DBS surgery under LA and those of iCT scan-guided intracranial electrode implantation in STN DBS surgery under GA [42]. When targeting STN, mean radial errors of the LA and GA group was about 0.9 ± 0.3 mm without significant di fference (P = 0.70). The average number of brain penetration for electrode implantation in DBS surgery under LA and GA was similar (1.1 ± 0.2 and 1.1 ± 0.3 penetrations, *p* = 0.97). Brodsky et al. compared 6-months of the clinical outcomes between the group of LA and GA with iCT [59]. There was no significant di fference in the improvement in UPDRS III and II, but the improvement in summary index (*p* = 0.004), subscores for cognition (*p* = 0.011), communication (*p* < 0.001), and speech outcome (category, *p* = 0.0012; phonemic fluency, *p* = 0.038) was found better in the GA group.

A few authors have published the results of a study using the intraoperative O-arm. Sharma et al. performed STN DBS surgery under GA using intraoperative O-arm without MER for various targets, and no significant targeting error due to incorporation of iCT images into preoperative CT or MRI was observed [60]. Carlson et al. also reported that intraoperative O-arm images provided a higher accuracy in determining the location of STN DBS electrodes than postoperative CT and MRI images [61].
