Chronic obstructive pulmonary disease (COPD) increases the risk of neuronal damage and cognitive dysfunction that increases the mortality and morbidity of the disease . Neuronal damage could be due to chronic hypoperfusion, chronic systemic inflammation, which is observed with repeated acute exacerbations, or associated comorbidities as vascular disease and smoking [1, 12, 20].
Conventional MRI images helped in the illustration of the diffuse white matter changes associated with COPD. Diffusion-weighted magnetic resonance imaging (DWI or DW-MRI) can give details about tissue architecture, either diseased or normal through detection of water molecule diffusion patterns. Recently, a more advanced type of DWI has been available, which is DTI that played a crucial role in the assessment of the integrity of white matter tracts .
Fractional anisotropy and MD are DTI measures. FA is a measure of tissue integrity while MD is an index for tissue ultrastructure. Thus, FA will decrease, and MD will increase with tissue microstructural damage
The FA is a mathematical value ranging from “0” to “1” and corresponds to the directionality and anisotropy per voxel; it is higher in more organized, cylindrical anisotropic media like corpus callosum approaching “1,” while decreases in less organized and isotropic media as grey matter and approaches “0” in the CSF [23,24,25,26].
The MD value is an average combination of the three eigenvectors, and it reflects the general diffusivity and the overall water motion without any directionality. It is an index to the amount of water in the extracellular space, so it is increased in vasogenic edema, axonal and myelin loss [9, 22, 24].
Many studies performed on structural brain damage in COPD patients found a decrease in FA and increase in MD metrics throughout brain regions, mainly in cingulate gyrus, uncinate fasciculus and superior longitudinal fasciculus, which play a major role in cognitive performance [11, 21, 22]. Patients have been found to have abnormalities in cognition and daily activities [27,28,29]; this signifies that impaired brain microstructure and function related to cognition are recognized complications of COPD.
This study investigated the white matter tracts integrity in COPD patients using DTI measures and revealed a highly significant difference between the FA and MD of white matter tracts of patients and the control group (p < 0.001). DTI matrices increased diagnostic performance of MR imaging in assessing the microstructural changes occurring in the cerebral white matter of severely affected COPD patients. The decrease in FA values was not significantly correlated with increased smoking index and increased exacerbation frequency, while MD values increased with increased smoking index and increased number of exacerbations significantly.
In our study, FA values were typically decreased, and the MD values increased in a broad range of white matter regions with a high significant difference in comparison with healthy controls mainly in uncinate fasciculus and cingulate gyrus. Decreased FA is attributed to compromised myelin structure and white matter microstructure damage, while the increase in MD owed to myelin loss, axonal degeneration and increase in the extracellular amount of water leading to increased diffusivity.
In this work, the FA values were decreased with increasing the SI, but without significant difference, and this can suggest that impaired white matter integrity is not depending on smoking only; other factors may have a role. A previous study reported the lack of significance of reduced white matter integrity in relation to smoking .
The correlation between the number of exacerbations and FA values of white matter tracts was not significant; this may suggest the improper self-reported exacerbation frequency .
In this study, MD had a significant positive correlation with SI and the number of exacerbations in the majority of selected white matter tracts. This may be attributed to increased sensitivity to excess extracellular fluid edema and myelin loss. FA is sensitive to the number of the dominant fibre directions, fibre’s size and organization of the tissue membranes within each voxel and can be influenced by partial volume effects from neighbouring grey matter, but less specific for the type of microstructure change [10, 31].