Radiation Effects in the Rat Spinal Cord: Evaluation with Apparent Diffusion Coefficient versus T2 at Serial MR Imaging

To prospectively determine whether apparent diffusion coefficients (ADCs) are more sensitive to radiation-induced changes in the rat spinal cord than T2 relaxation times.The study was approved by the institutional ethical committee on animal welfare. One centimeter of the thoracolumbar spinal cord of six rats was irradiated with 36 Gy. For 3-6 months after irradiation, five 7.0-T magnetic resonance (MR) imaging measurements were performed in each rat until motor impairment developed. Six age-matched rats were examined as controls. Measurements were performed by using diffusion-weighted imaging with five b values and a spin-echo sequence with 20 echoes. ADC and T2 values were calculated, and the spatiotemporal evolution of the radiation-induced lesions was determined semiautomatically. The final MR measurements were compared with the histologic findings.Shortly before the neurologic signs appeared, the first radiation effects manifested as well-circumscribed white matter (WM) lesions with a low longitudinal ADC and normal or high T2. WM lesions with high T2 correlated with confluent necrosis at histologic analysis, whereas WM lesions with normal T2 correlated with focal necrosis and demyelination. In the gray matter (GM), lesions with diffusely high T2 were present and were attributed to edema. T2 changes in the GM preceded T2 and ADC changes in the WM.In the WM, longitudinal ADC was more sensitive for the detection of radiation damage than T2, but in the GM, T2 was more sensitive.