Our study showed dMRI is potentially valuable for prediction of IDH1 genotype in LrGGs, where α is the most effective predictor of IDH1 mutation status. Also, dMRI parameters are promising in the assessment of cell proliferation, especially maximal correlation coefficient was found between MK and proliferation index.
Radiological differences were found between IDH1-mutant and wild-type groups in LrGGs. We suspected that IDH1 wild type existed in the LrGGs when more complex organizational structure, more abundant microvasculature, higher cell density and more diffusion barriers appeared. However, none of the parameters showed significant differences in the identification of IDH1-mutant and wild-type groups in GBMs due to a single parameter with insufficient ability to discriminating diffusion and perfusion patterns of highly malignant and structurally complex. This hints us to further explore differences in the GBM groups using a combination of multiple parameters in the future.
Previous studies have demonstrated the ability of conventional DWI and DTI to distinguish IDH-mutant gliomas from IDH wild-type gliomas[15, 16]. In our study, ADC and MD values of IDH1 wild-type group were significantly lower than that of IDH1-mutant group in LrGGs. FA showed ineffective prediction on IDH1 genotype, possibly due to the high heterogeneity in FA values for the solid component of the tumor, which is consistent with the results of Tan et al [17].
Many studies have found that DKI has better efficacy in identifying molecular subtypes of gliomas [18, 19]. A DKI parameter MK reflects the complexity of the tissue microenvironment under the assumption of non-Gaussian distribution in the organism. Similar to the results reported by Zhao et al [20], our finding showed MK effectively discriminated IDH1-mutant group from wild-type group in LrGGs.
ODI and ICVF computed based on the NODDI model represent neurite dispersion characteristics and neurite density, respectively. Since both neuronal density and directional dispersion affect FA value [21], our results showed both ODI and ICVF significantly distinguished IDH1-mutant group from IDH1 wild-type group in LrGGs. IDH1 wild-type gliomas may be more proliferative and aggressive due to more complex microstructure and higher dispersion of neurites and thus possess higher ODI and ICVF values. Zhao et al. [22] reported that the mean ICVF was significantly higher in GBMs with IDH1 mutation than that without IDH1 mutation. However, only 4 cases of IDH1-mutant GBMs were collected in their study, the findings still need to be confirmed even different to ours.
The IVIM model was proposed by Le Bihan et al [23], where D represents the diffusion movement of water molecules inside and outside the cell, D* reflects the blood perfusion of the microcirculation, and f represents the abundance of capillaries in the tissue. In the current study, the performance of D was slightly better than that of ADC in identifying the mutation status of IDH1 genotype in LrGGs perhaps due to D eliminates the influence of perfusion and more accurately reflects the diffusion and movement of water molecules. The D* value of IDH1 wild-type group is higher than that of IDH1-mutant group in LrGGs, indicating that IDH1 wild-type glioma has more abundant blood perfusion. However, interobserver agreement and the AUC value of D* was low even it had significant difference in the identification of IDH1 mutation status in LrGGs. The instability of the parameters may lead to a limited application of D* in glioma IDH1 genotype prediction. Furthermore, f value is higher in IDH1mutant gliomas, inconsistent with Wang et al [24]. The same contradictory results exist in studies of glioma grading, where f values are higher in low-grade gliomas than in high-grade gliomas [25]. Le Bihan et al. [26] suggested that the IVIM model is sensitive to fluid flow distributed within any voxel, not just blood flow. More relatively unrestricted water molecules outside the IDH1-mutant glioma cells may have contributed to the increase of f values. Alternatively, these differences may be due to different IVIM model parameters, fitting methods, and ROI plotting methods [27].
The stretched-exponential DWI model showed excellent efficacy in IDH1 genotype discrimination in our study, and α was able to distinguish IDH1 mutation status in LrGGs with the largest AUC value and high sensitivity and specificity. Lower α values indicate that the diffusion of water molecules in the tissue was inhomogeneous, and the heterogeneity of the tissue was higher[28]. We speculate that the microenvironment of IDH1 wild-type glioma is more complicated, such as cell swelling and vascular proliferation, so it exhibits greater heterogeneity of intra-voxel diffusion.
In this study, the six diffusion models all provide at least one parameter with effective prediction performance on the IDH1 genotype of LrGGs. This is of great clinical importance for the prediction of IDH1 wild-type LrGGs, which have a malignant clinical course despite being pathologically relatively inert alterations. Therefore, accurate and non-invasive prediction of the IDH1 genotype in LrGGs allows for timely treatment planning to impede malignant transformation of the disease.
We also investigated the prediction of glioma grading by dMRI under the same IDH1 genotype. Generally speaking, high-grade gliomas tend to be more heterogeneous, as our findings. However, IDH1 wild-type LrGGs and GBMs showed only statistically different ADC and DDC. Some studies have found that even in patients with IDH wild-type LrGG, tumors exhibit high levels of aggressiveness, with overall survival times similar to those of IDH wild-type GBM[29, 30]. This may explain our results, probably because the similar high heterogeneity and aggressiveness of IDH1 wild-type gliomas, resulting in most parameters that do not differ significantly between LrGGs and GBMs both with wild-type IDH1. In a word, with the increase of pathological grade, the tumor microstructure is more complex, with higher cell density and more disturbed water molecule movement, but IDH1 gene phenotype will affect the development of gliomas at a microscopic point of view. Compared with previous pathological grading studies[28], our study combined the pathological grading of gliomas with molecular phenotypes, which contributes to a more comprehensive understanding of the characteristics and microstructure of gliomas.
Nuclear protein Ki-67 is associated with cell proliferation specifically expressed in tumor cells [31, 32]. As the malignancy of the tumor increases, the blood supply becomes more abundant, the number of cells increases, malignant biological behavior ensues, such as hemorrhage and necrosis, and neovascularization forms further [33, 34]. And these aforementioned alterations can affect the complexity and heterogeneity of tumor microstructure, when cellular gaps are smaller, water molecules diffusion is more restricted and movement is more disturbed. Zhang et al. [35] found MK and D have considerable potential to predict the degree of proliferation in diffuse astrocytomas. This is similar to our findings, where MK has maximal correlation coefficient with cell proliferation index. However, no significant correlation was found between FA and Ki-67 LI, which may be because the level of cell proliferation in response to Ki-67 only affects the size of the diffusion and not the pattern of diffusion routes. No significant correlation was also found between D* and Ki-67 LI. We speculate that D* reflects more perfusion-related information and is not sensitive enough to changes in cell proliferation.
Our study had some limitations. First, the sample size of this study was small and it was a single-center study. Future multicenter studies with large sample sizes are needed to validate the results of this study. Secondly, only one molecule, IDH1, was considered in this study, but many other molecules status such as 1p/19q codeletion and O6-methylguanine-DNA methyltransferase promoter methylation also play an important role in the development of gliomas, which needs to be further investigated. Finally, we performed 2D ROI placement at the tumor parenchyma site with the lowest ADC value, which may ignore the overall tumor condition.