The preoperative assessment of the surgical cleavage plane at the tumor-brain interface in meningiomas is important for surgical planning and accurate assessment of post-surgical prognosis. Meningiomas that are adherent are much more difficult to resect resulting in extended surgical procedures, increased risk of complications and incomplete tumor removal, which could lead to tumor recurrence¹. Previous studies have described the usefulness of MRI, CT, and angiography in predicting the cleavage plane of meningiomas^2,3. However, these studies do not provide a direct measure of the degree of tumor cleavage, i.e., the degree of tumor adherence at the tumor-brain interface. A recently developed slip interface imaging (SII) technique is uniquely capable of directly assessing the tumor-brain interface. This technique has been successfully applied to assess the tumor-brainstem adhesion in vestibular schwannomas⁴. The goal of this study was to determine the extent to which SII can be used to assess the surgical cleavage plane between the tumor and the underlying brain in meningiomas.With institutional review board approval and written informed consent, 20 patients (5M and 15F, 61±11 years) with pathologically proven meningiomas underwent the preoperative MRI and SII assessments on 3T MR scanners. The SII technique is based on MR elastography (MRE)⁴. Low amplitude mechanical vibrations at 60 Hz were introduced into the head with a pillow-like MRE driver. The resulting shear waves were imaged by a single-shot, flow-compensated, SE-EPI MRE pulse sequence with the same scanning parameters as reference (4). The MR phase data were processed by shear line analysis to generate shear line images and quantitative octahedral shear strain (OSS) maps as described in reference (4). According to the criteria in Table 1, SII results were classified into three categories: complete slip interface (SI), partial SI, and no SI, and then correlated with three types of surgical cleavage planes (extrapial, mixed, and subpial) encountered at surgery. The correlation was assessed by using the Chi-square test (significance level <0.05). The Cohen κ coefficient was used to study the agreement between the SII prediction and surgical findings.The slip interface between the tumor and the brain was represented as a hypo-intensity contour on shear line images and high OSS values along the interface. Figure 1 shows a representative case illustrating a tumor with complete SI, where the tumor was completely removed via an extrapial surgical plane. A tumor with the absence of a well-delineated slip interface in Figure 2 was classified as no SI, and a subpial plane was found at surgery. As shown in Table 2, SII agreed with the intraoperative assessment in 15 (75%) of 20 cases (moderate agreement: κ=0.52). Twelve (87%) of the 14 tumors with complete SI had an extrapial plane, but the other two had mixed plane findings. When a tumor was classified as partial or no SI, the surgical plane was non-extrapial. Overall, the correlation between the SII results and the surgical plane of cleavage was statistically significant (p=0.0014). In all of the 12 cases with a surgical extrapial plane of cleavage, SII demonstrated a well-delineated complete SI on both shear line image and OSS maps. The results clearly show that a complete SI may be a predictor of an extrapial plane. If a safe (i.e. extrapial) surgical plane of cleavage exists, the low-amplitude (~microns) mechanical vibration of SII can lead to a large differential motion between the tumor and the adjacent brain surface on both sides of the interface due to the loose connection. This relative motion can create a large phase shift across the interface, leading to a signal loss at the interface due to intravoxel phase dispersion⁵. The large shear deformation caused by a slip boundary can also be captured at the same time by measuring OSS, which reflects the maximum shear strain across all possible planes. However, in patients with a non-extrapial (mixed or subpial) surgical plane, 5 of the 8 cases were discordant. Our findings showed that the discrepancies mostly happened in the tumors with prominent adjacent edema, calcified rim or portions, or internal cysts. The existence of these multiple tissue planes inside or outside the tumor likely complicates shear motions at tumor boundaries, which could interfere with the SII predictions. Future studies will focus on understanding and improving the accuracy of SII prediction of surgical planes with a larger patient cohort.SII can provide a method for preoperative prediction of the surgical plane of tumor cleavage in meningiomas. The presence of a complete SI suggests that the tumor can be removed using a dissection plane outside the pia mater.