DTI metrics have been used to monitor structural changes in aphasia after stroke due to marked sensitivity in detecting structural tissue abnormalities. DTI parameters can also be used as prognostic markers of disease evolution [1, 2, 17,18,19]. In this study, mean FA and MD of the AF and SLF and number of TF were found to be accurate parameters for detecting differences and changes between patients (with affected language areas and tracts) and controls.
FA is a common measurement used in DTI studies that indicates diffusion orientation and is high along well-defined pathways such as the corpus callosum, pyramidal tracts, and optic radiations. FA value is a potential marker of axonal structural integrity; higher FA values indicate increased parallel movement of water molecules to fiber tracts, denoting increased structural connectivity [1,2,3, 18, 19]. Decreased FA value is related to the disintegration of the neural tract. In our study, we found that the mean FA of AF and SLF in patients with aphasia was significantly lower than controls with a positive correlation between FA of SLF and MASAT. There was also a negative correlation between the FA of the SLF and NIHSS, indicating that decrease in FA value was associated with a higher NIHSS score. This was in agreement with previous DTI studies [4, 5, 20, 21] that showed negative correlations between comprehension abilities and FA decline.
MD value indicates the magnitude of water diffusion, which can increase with vasogenic edema or accumulation of cellular debris due to axonal damage. The mean MD value of AF and SLF in patients with aphasia in our results was significantly higher than the mean MD value in controls. This study's findings demonstrated that injuries of the AF and SLF could cause conduction aphasia in stroke patients. This was coping with other studies [5] which showed that lower FA and higher MD values of the AF and SLF of the affected hemisphere were found to show a correlation with a decreased ability of repetition and comprehension deficits after stroke, and also showed an association with lower FA value of the affected AF [1, 2, 17, 18].
In our study changes of the AF and SLF in patients were in the form of decreased FA value, increased MD value, and decreased number of TF on DTI which can be used as follow-up DTI parameter in stroke patients. This was in agreement with other studies which revealed that AF has a critical role in determining aphasia severity with decreased FA value and a smaller number of TF of the affected AF, compared with the control AF [5, 20, 21].
In our study during processing of cases, we have noticed that atrophic brain changes led to marking thinning of the affected tract fibers with evident degeneration and atrophy, and patients with mild NIHSS showed more preserved shape and number of TF of the AF and SLF, while aged patients with brain atrophic changes showed a decreased number of TF of the AF and SLF as a part of the aging process. A previous study reported that higher FA values and a higher number of tract fibers in corpus callosum suggest intact fibers, and also reported that density and volume of corpus callosum can be affected by brain size [24] which was in agreement with our results (Fig. 5).
In our study, the FA values of the affected posterior segments of the AF, and the anterior segments of the AF in patients with Wernick's-like conduction aphasia and in patients with Broca's-like conduction aphasia, respectively, were smaller than those of controls. However, there was no significant difference detected in the mean FA value in BA and WA between patients and control which may be attributed to the presence of Lt-sided infarction in all patients interfered with accurate ROI placement at the anatomical site of the BA and WA.
The integration of both DTI and conventional MRI measures together with connectivity-based regional assessment and the development of novel image analysis and visualization techniques could provide better means to understand the nature and the location of WM abnormalities. The relationship between WM disruption, WM connectivity, and clinical measures will potentially allow clinicians to correlate fiber tract disruption and cognitive impairment better. Furthermore, it would ultimately lead to improved monitoring of patients, better prediction of the course of the disease, and more rapid assessment of new treatments or therapies [1,2,3,4, 19].
In this study, there is a correlation between NIHSS and DTI parameters. NIHSS quantifies the impairment caused by a stroke [7]. MASAT is a valid and reliable brief assessment tool that can be completed on the first clinic appointment that detects the type and severity of Arabic-speaking aphasic patients; the tool is designed to be used by either phoniatricians who are physicians in Egypt or speech-language pathologists [8]. Provided clinical data by these tests could be correlated with imaging findings and parameters detected by DTI; minor or moderate stroke symptoms with subsequent mild to moderate aphasia are expected to have less damage to and more preserved functions of the affected language areas as well as maintained number of TF, in contrary to cases with moderate to severe and cases with severe stroke symptoms and aphasia.
There are a few limitations of this study. First, this study includes a small number of patients. Further studies done upon a large number of patients are recommended and also patients recruited at different stages of post-stroke recovery and were grouped together over a time period ranging from two to six weeks after stroke, which might have affected the results. Second, this study used DTI at a 1.5-Tesla machine. Further studies that used multi-parametric MR imaging include arterial spin labeling, perfusion MR imaging, and MR spectroscopy will improve the results [25,26,27,28,29,30,31,32,33,34]. Third, there is no follow-up of these patients. Further studies with longitudinal assessment of these patients after therapy are recommended.