6. Diffusion tensor MRI study of the spinal cord in patients with multiple sclerosis

Abstract

Introduction Anatomical MR contrasts (such as T1 and T2) fail to identify occult lesions and diffuse demyelination which are known to occur in multiple sclerosis (MS). Diffusion tensor imaging (DTI) can potentially discern axonal loss in white-matter tissue and serve as a marker for loss of tissue integrity (demyelination, axonal degeneration) [1,2]. Poor image quality and the limited resolution in spinal cord DWI has so far impeded focal evaluation of spinal cord tissue i.e. the differentiation between grey and white matter. Furthermore, adequate image quality was achievable only at the cervical level of the spinal cord. Recent improvements in MR pulse sequence design have overcome these problems, thereby providing a new instrument for the assessment of multiple sclerosis. We tested the newly available technique in a comprehensive clinical MS study and measured diffusivity values of the spinal cord at three levels. Methods Imaging was performed on a 3 T Philips Achieva (Philips Healthcare, Best, the Netherlands) using a dedicated spine coil. DTI data of 18 volunteers and 41 patients with relapsing remitting MS (RRMS), secondary progressive MS (SPMS) or primary progressive MS (PPMS) (Table 1) were acquired at cervical (~C5) and thoracic level (~T5) as well as at the lumbar enlargement of the spinal cord. In each region 6 transverse slices were acquired using an outer volume suppressed reduced field of view single-shot EPI sequence [3,4] (NEX = 6/12 for b = 0/b = 750 s/mm2, acquisition matrix = 176x44, FOV = 120x30 mm, z=5mm, TR = 4000 ms, TE = 49 ms, 60% partial-Fourier acquisition). After subsequent image co-registration fractional anisotropy (FA) and apparent diffusion coefficient (ADC) maps were calculated. Diffusivity values were evaluated in the posterior white matter (PWM) (Fig. A). Only normal appearing spinal cord tissue, i.e. spinal cord tissue without T2-hyperintense lesions, was evaluated. ROI sizes were kept constant for all evaluated subjects.