We image an ex vivo human basal ganglia to demonstrate the potential of MRI/OCT for neurosurgery guidance. iMRI has been applied to many stereotactic neurosurgeries, including brain tumor biopsy,10 catheterization,11 and deep brain stimulation.3,12 However, the resolution and the contrast of MRI are not high enough to detect certain target nuclei in basal ganglia.13 On the other hand, OCT can locate these nuclei in human brain ex vivo14 and rat brain in vivo.15 We place a slab of frozen human brain tissue containing basal ganglia in a gelatin holder filled with saline at room temperature and wait until it is completely thawed. Then, we acquire preoperative T2 MRI images to plan the trajectory. We monitor the advancement of the needle by dynamic MRI and OCT in real time [Fig. 4(e), Video 2]. In OCT video, we can easily identify the boundary between the gray matter (GM) and the white matter (WM), whereas most of structures are occult in the dynamic MRI video. Figure 4(a) shows a photo of the brain tissue. In the high-resolution MRI image [Fig. 4(b)], the transitions between the GM nucleus, putamen (PUT, white on T2 image) and its surrounding WM bundles, white matter lamina (WL), and internal capsule (IC) (dark on T2 image) are clearly visible. We reconstruct a full-track OCT image [Fig. 4(c)] from the OCT video and correlate it with the large-scale MRI morphology. From each frame of the OCT video, we select a band of A-lines with 66.67-μm width ( sampling rate, same as the needle insertion speed) from the same location. Then, we stitch these bands together to form Fig. 4(c). We align the pre-operative MRI to the intra-operative OCT by matching them to the dynamic MRI images. The accuracy of the alignment is limited by the resolution of the dynamic MRI image, which is 2 mm. In Fig. 4(d), we plot the normalized MRI intensity (green line), normalized OCT backscattering intensity (blue line), and normalized attenuation coefficient (red line). The normalized OCT attenuation coefficient and OCT backscattering in Fig. 4(e) are smoothed for better visualization by a moving average filter with 2.35 mm average width. In the full-track OCT image, the transitions between PUT and its surrounding WM bundles (WL and IC) are obvious and well correlated with the MRI image. The full-track OCT shows that the GM nucleus (PUT) has lower backscattering intensity, deeper penetration depth, and thus smaller attenuation coefficient [Fig. 4(d)], whereas myelinated WM tracks have higher backscattering intensity, shallower penetration depth, and larger attenuation coefficient. This result agrees well with previous study.16 In addition, the transition from IC to thalamus is not visible in MRI. The thalamus is a GM nucleus containing uniformly distributed WM fibers. The mixture of WM and GM gives it very low contrast to the surrounding WM tracks in MRI. However, we can easily identify the boundary between IC and thalamus in OCT. Compared to the PUT, high level of WM in thalamus reduces the optical penetration depth and increases the attenuation coefficient [Fig. 4(d)]. Also, the backscattering intensity from the thalamus is not as high as that from the WM bundles. This result also agrees with a previous study.14Figure 4(d) shows that the transitions between PUT and its surrounding tissues are obvious in both imaging modalities, but only OCT provides good contrast between IC and thalamus. Figure 4(f) shows that the attenuation coefficient of thalamus (GM/WM mixture) is between IC (WM) and PUT (GM), and the difference is statistically significant (). The attenuation coefficients in Fig. 4(f) are calculated from 1-cm-wide unsmoothed data at the location indicated by the black arrows in Fig. 4(d). This result suggests that OCT may provide a different contrast to MRI, which could be vital for targeting nuclei in basal ganglia. Contrast in MRI images depends on the magnetic property of the tissue, whereas contrast in OCT lies in light-scattering property. Although the source of the contrast of both imaging modalities comes from two different physical phenomena, OCT is able to produce images at a very high resolution over a small field of view. Such high resolution is currently not possible with MRI. However, these two modalities can complement each other, where MR images may provide guidance to an OCT catheter to a specific location from which high-resolution images may be obtained.