Magnetic resonance diffusion tensor imaging for occult lesion detection in multiple sclerosis.
Chen J, Zhou C, Zhu L, Yan X, Wang Y, Chen X, Fang S. Abstract It remains challenging to locate occult lesions in patients with multiple sclerosis (MS). Diffusion tensor imaging (DTI) has been demonstrated to have the potential to identify occult changes in MS lesions. The present study used 3.0T magnetic resonance DTI to investigate the characteristics of different stages of MS lesions. DTI parameters, fractional anisotropy (FA), mean diffusivity (MD), λ// and λ┴ values of lesions were compared at the different stages of 10 patients with MS with 10 normal controls. The results demonstrated that FA and λ// values of MS silent and subacute lesions are decreased and MD and λ┴ values are increased, as compared with those of normal appearing white matter (NAWM) and normal controls. NAWM FA values were lower, and MD, λ//, and λ┴ values were higher than those of normal controls. It was also indicated that MS lesions had reduced color signals compared with the controls, and the lesion area appeared larger using DTI as compared with diffusion-weighted imaging. Furthermore, fiber abnormalities were detected in MS lesions using DTT, with fewer fibers connected to the lesion side, as compared with the contralateral side. FA, MD, λ// and λ┴ values in the thalamus were increased, as compared with those of normal controls (P<0.05); whereas MD, λ// and λ┴ values were significantly increased and FA values significantly decreased in the caudate nucleus and deep brain gray matter (DBGM) of patients with MS, as compared with the controls (P<0.05). λ// and λ┴ values were also significantly increased in the DBGM of patients with MS as compared with normal controls (P<0.05). The present findings demonstrate that DTI may be useful in the characterization of MS lesions.
These are the known knowns - MS lesions visible to the naked eye in eloquent areas lead to a neurological deficit; there are known unknowns - not all MS relapses correspond to a visible lesion on MRI. This is the quandary which is the silent MS. Chen and colleagues suggest diffusion tensor imaging (DTI), a particular MRI sequence can help identify these silent MS lesions. "MS lesions
can be identified by conventional magnetic resonance imaging (MRI);
however, it remains challenging to detect lesions in early stage or
occult MS lesions. Because of this, clinicians are unable to
connect all clinical symptoms with the findings on MRI". DTI uses the flow of water in neurones to outline their structure and thereby their loss using parameters such as fractional anisotropy (FA) and mean diffusivity λ┴ and λ//. They demonstrate a case example of this in a 27 year old who developed slurred speech for 1 week (see figure below).
Figure A: Abnormal signal in the right corona radiata (the whitish hyperintensity next to the ventricles which is the back H-shape in the center), B: FA showing loss of signal at the lesion, C: Colour-map showing reduced signal at the lesion, D and E: The neuronal fiber bundles showing interruption and fewer fibers on the affected side. Therefore DTI maybe able to overcome the limitations of conventional MRI in identifying silent MS lesions. In particular, FA and λ// were lower in these lesions i.e. the directionality of the diffusion (secondary to axonal damage and loss of myelin) and diffusion parallel to the nerve fibers are affected (secondary to axonal degeneration). Clearly, what DTI provides is an image of broken nerve fibers and disintegration of fibers distal to lesions. It reveals a story greater than the sum of its part. If were are to repair this, we would have to become sophisticated structural designers. Or we let the brain do it; the unknown unknowns - I wouldn't mind being a fly on the wall to watch that process.