Eur Radiol (2012) 22:1797–1802 1799 Fig. 1 Diffusion metric maps of the cervical spinal cord in the axial plane. Apparent diffusion coefficient (a), fractional anisotropy (b), full width at half maximum of probability density function (c) and mean diffusional kurtosis (d) at the C3–C4 vertebral level and ADC could be calculated by using part of the q-space data. Next, the full width at half maximum (FWHM) of the probability density function (PDF) and mean DK maps were obtained. Details of the new diffusion metrics and their calculation procedure were as previously described [8–19]. Briefly, the key principle in q-space analysis is that a Fourier transform of the signal attenuation with respect to q (or the b value) provides the PDF for diffusion by using multiple q values [10]. The shape of the computed PDF can be char- acterized by the FWHM and the maximum height of the curve. In the specific case of unrestricted Gaussian diffu- sion, the diffusion constant D and the RMSD for one- dimensional diffusion can be computed from the FWHM. Mean RMSD was calculated from the FWHM values (RMSD 0 0.425 × FWHM) [10, 11]. Moreover, as described in previous papers [14, 22], the DK for a single direction can be determined by acquiring data at three or more b values (including b 0 0) and fitting them to Eq. (1): ln½SðbÞ ¼ ln½Sð0Þ b Dapp þ 1=6 b2 2 Dapp Kapp 1 ð Þ where Dapp is the apparent diffusion coefficient for the given direction and Kapp is the apparent kurtosis coefficient, which is dimensionless. By referring to T2-weighted images, two experienced neuroradiologists (M.H. and S.A.) classified the patients Fig. 2 T2-weighted fast spin- echo images in the sagittal plane of one patient from group 1 (a) and one patient from group 2 (b). High intensity indicates cerebrospinal fluid between the cervical spinal cord and dura in a (arrow); this is absent in the compressed areas in b into two groups in consensus: (1) uncompressed spinal cords (residual cerebrospinal fluid (CSF) signal between the spinal column and spinal cord on the T2-weighted images); and (2) compressed spinal cords (cervical canal narrowing and no residual CSF signal between the spinal column and spinal cord at the level of narrowing on T2- weighted images) (Fig. 2). On the axial images of the calculated maps, a region of interest (ROI) was drawn manually on the spinal cord by an experienced neuroradiol- ogist M.H.; the ROI included both white matter and grey matter, excluding any CSF contribution, at spinal canal levels C3–C4, C4–C5 and C5–C6. Therefore, the shape of each ROI varied according to the shape of the spinal cord. The dTV II FZR software allowed for copying of the ROIs and guaranteed the evaluation of the same region with diffusion metric maps. FA, ADC, RMSD and mean DK values were measured in each area. Areas with severe signal loss or calculation errors were excluded from the analysis. Statistical analysis A preliminary analysis was performed by using the Anderson– Darling test to evaluate whether the data were normally distributed. Then, statistical comparisons between the two groups were performed by using IBM SPSS Statistics software (version 19.0; SPSS, Chicago, IL). A P value less than 0.05 was considered to indicate a statistically significant difference.