In this study, we employed DSI sequences to compute the diffusion rates of periventricular projection and association fibers in T2DM patients and determined their ALPS indices. We conducted a quantitative evaluation of lymphatic system function in T2DM patients using the DTI-ALPS index. The findings revealed abnormalities in projection fibers and ALPS in T2DM patients. Furthermore, these abnormalities (Dxassoc and ALPS) exhibited a correlation with the TyG index. This discovery suggests a potential involvement of insulin resistance in lymphatic system damage in T2DM patients.
The non-invasive, in vivo assessment of the brain's lymphatic system function, as exemplified by the DTI-ALPS index, shows promise in addressing the challenges and safety concerns associated with lymphatic imaging techniques. Notably, the brain's projection fibers follow the vertical axis, while association fibers follow the anterior-posterior axis. However, both types of fibers do not run parallel to the perivascular spaces around venous vessels. Consequently, analyzing changes in the diffusion rates of these two fiber types in the X, Y, and Z axes can provide insights into the pathophysiological alterations in the perivascular spaces(Taoka et al., 2017). Previously, DTI-ALPS has found widespread applications in clinical research for conditions such as obstructive sleep apnea syndrome(Lee et al., 2022; Roy et al., 2022), epilepsy(Lee et al., 2022; Lee et al., 2022), and Parkinson's disease(Si et al., 2022). Nevertheless, the DTI model is relatively simplistic and may not effectively capture the specific characteristics of white matter fibers. In contrast, DSI represents a generalized q-space sampling imaging technique and is currently the sole method capable of non-invasively providing high-resolution whole-brain fiber tract imaging in living subjects. It offers exceptional cross-fiber resolution and high spatial resolution of diffusion data, facilitating the simultaneous fitting of various diffusion quantitative parameters(Wang et al., 2022). Research by Taoka et al.(Taoka et al., 2022) has demonstrated a strong correlation between ALPS indices acquired with different scanning parameters, affirming the feasibility of assessing lymphatic system function using multi-b-value diffusion MRI data obtained with DSI sequences. In a prior study, Qin et al.(Qin et al., 2023) obtained DSI images from subacute ischemic stroke patients, from which they derived ALPS indices. They observed a significant reduction in the ALPS index on the affected side in comparison to the unaffected side and the same-side ALPS index in healthy subjects. In this study, a total of 129 DWI images were acquired for each subject, encompassing 18 different b-values ranging from 0 to 3000 s/mm². Consequently, the diffusion rates of projection fibers and association fibers in the X, Y, and Z axes were lower than those obtained with traditional DTI sequence acquisition(Lin et al., 2023), while the ALPS index remained consistent with previous research findings.
This study uncovered that the ALPS index, determined by DSI, was lower in T2DM patients compared to healthy individuals, implying impaired lymphatic system function in the examined T2DM patient population. In a study by Yang et al.(Yang et al., 2020), DTI data were collected from 10 individuals with T2DM of over 10 years' duration, 10 with T2DM of less than 10 years duration, and 10 healthy volunteers. The research revealed that both groups of T2DM patients exhibited reduced DTI-ALPS indices. Additionally, it was observed that in T2DM patients with a longer disease duration, the diffusion rates of both projection fibers and association fibers were lower in comparison to the other two groups. In this study, the diffusion rate of T2DM patients' projection fibers along the X-axis was lower than that of normal subjects. This could potentially result from the accumulation of soluble proteins in the perivascular spaces around venous vessels, hindering the flow of water molecules in that direction. Simultaneously, the diffusion rate of T2DM patients' projection fibers along the Y-axis was higher than that of normal subjects, indicating potential compromised white matter integrity of the projection fibers in T2DM patients, leading to an increase in radial diffusion of water molecules. Utilizing DSI technology, this study further corroborated the presence of lymphatic system dysfunction in T2DM, along with structural abnormalities in projection fibers.
Moreover, of significant importance, a negative correlation was identified between the ALPS index in T2DM patients and the TyG index. This suggests that insulin resistance in T2DM patients may indirectly signify alterations in lymphatic system function. Previous research has indicated an association between elevated fasting blood glucose and reduced white matter integrity in individuals with metabolic syndrome(Huang et al., 2023; Yau et al., 2014). Jing et al.(Jing et al., 2022) emphasized that T2DM patients exhibited decreased FA values and increased mean diffusivity (MD) values in areas encompassing projection fibers, such as the bilateral anterior corona radiata, further highlighting that abnormal metabolic levels might lead to white matter damage in projection fiber regions.
In this study, a negative correlation was observed between the diffusion rate of T2DM patients' association fibers along the X-axis and the TyG index. These findings imply that insulin resistance in T2DM might be linked to restricted diffusion of water molecules within the perivascular spaces around association fibers. Insulin resistance has the potential to elevate the concentration and toxicity of β-amyloid proteins, thereby promoting their further deposition in the perivascular spaces(Michailidis et al., 2022). This suggests that insulin resistance exacerbates the deposition of β-amyloid proteins in the perivascular spaces of association fiber regions. It's essential to note that the differences in the Dxassoc group between the recruited T2DM patients and the normal subjects were not statistically significant, which does not entirely align with previous research(Yang et al., 2020). This inconsistency may be attributed to sample heterogeneity and the utilization of DSI technology. Future studies with larger sample sizes and the elimination of potential confounding factors will be required to validate the robustness of these results.
This study has several limitations: 1. Some of the recruited T2DM patients exhibited well-controlled blood sugar levels and good dietary habits, resulting in less pronounced increases in the TyG index. This could potentially influence the final experimental outcomes. 2. Due to the nature of perivascular space tensor imaging, there might be some errors in the manual delineation of ROI. 3. The relatively low b-values in the collected diffusion MRI data may have a greater influence from highly mobile water molecules. Future research should place greater emphasis on establishing appropriate b-value parameters.