Einsatz von Fiber tracking in der Darstellung des kaninen Rückenmarks

Intervertebral disk disease (IVDD) is one of the most common neurologic problems in dogs with an incidence of up to 2% of the dogs’ population. To date, magnetic resonance imaging (MRI) is considered to be the most sensitive diagnostic tool to assess the diagnosis of IVDD. Moreover, findings of MRI such as the occurrence of a hyperintensity in T2-weighted images and its extent might give indications to establish a reliable prognosis. In human medicine, diffusion tensor imaging (DTI) is a widely used technique for the evaluation of the fiber integrity in the brain and spinal cord. In veterinary medicine reports about the use of DTI are rare. The first part of this study therefore aimed at establishing the technique of fiber tracking for the dog. Thirteen dogs with healthy spinal cords were included in the study and DTI was performed subsequently to the standard MRI protocol using a 3.0T magnetic resonance scanner. In all dogs the spinal cord could be visualized in a 3D reconstruction. Spinal cord fibers were depicted in blue color due to the mainly craniocaudal diffusion direction of the water molecules. Morevoer, the dorsal and ventral nerve roots could be visualized by a slight modification in the maximum angle variation from 27° to 45° and were colored in green and red due to their main water diffusion (dorsoventral and right-left direction, respectively). DTI measurements of three defined regions of interest (ROIs) on the spinal cord enabled the calculation of data for the fractional anisotropy (FA) that provides information about diffusion direction and the apparent diffusion coefficient (ADC) as a means for the diffusion strength. Data were evaluated for the 13 dogs of different breeds and ages for cervical and thoracolumbar spinal cord. No statistically significant difference was detected for the different breeds, ages and spinal cord segments and therefore the data for FA and ADC were pooled each. For FA a mean of 0.447 (x) and for the ADC a value of 0.560 x10-3mm2/s (x) was assessed. In this part of the study quantifiable values were established that can serve as reference values for comparative studies in dogs with spinal cord diseases. In the second part of the study fiber tracking was applied to dogs with spinal cord injury after IVDD. Twenty dogs with IVDD and different severity of neurological signs were included in the study. Evaluation of the data assessed showed a decrease in FA in the dogs with IVDD compared to the healthy control dogs whereas the ADC was increased. These effects however, did not reach the level of significance. With further differentiation of the results for the ROIs a significant decrease of the FA in the spinal cord segment caudal to the IVDD was detected compared to the segment cranial to the IVDD and at the IVDD epicenter. A decreased FA can account for various pathological features in the spinal cord such as mechanical disruption of spinal cord fibers or myelin sheaths, demyelination, liquefaction, hemorrhage and and local extracellular edema preventing the free movement of water diffusion. Interestingly, the FA decrease in the segment caudal to the IVDD was more distinct in dogs with severe neurological deficits potentially reflecting the ongoing damage in the spinal cord. The ADC only showed a slight tendency for decreased values in the dogs with IVDD compared to the healthy controls that denotes for cytotoxic edema characterized by intracellular water inflow consequently leading to cell lysis. Evaluation of the fiber architecture in the spinal cord visualized alterations in the color accounting for disturbances in water molecule diffusion along axonal bundles and therefore indicating disruption of the fiber integrity. This finding correlated with the functional outcome as dogs with fiber ruptures did not show neurological improvement. Fiber tracking enables the reconstruction of spinal cord axonal bundles and offers the acquisition of quantitative data to objectively assess fiber integrity. Prospective studies with a larger cohort of dogs with IVDD and follow-up studies using DTI have to be performed to elucidate whether FT can be used as a prognostic indicator for functional recovery in dogs with IVDD.