The purpose of this study was to evaluate the feasibility of diffusion tensor imaging of the kidney at a field strength of 3T. We assessed fractional anisotropy (FA) and apparent diffusion coefficients (ADC) of various acquisition protocols and determined the reproducibility of these measurements. FA, ADC, signal-to-noise ratios (SNR), and contrast-to-noise ratios (CNR) were compared with those acquired at 1.5T.
Material and Methods:
Ten healthy volunteers were examined with a respiratory-triggered echo-planar imaging sequence (TR: 1800 ms, TE: 58 ms, b = 0, 300 s/mm2) on a 3-Tesla whole-body MR scanner. Protocol variations included diffusion measurements during free-breathing, in 6 or 12 directions, and an additional b-value of 50 s/mm2. A breath-hold protocol was also integrated (TR: 820 ms, TE: 58 ms, b = 0, 300 s/mm2). Measurements with 2 b-values and 6 diffusion directions were also acquired at 1.5 T. SNR was calculated with the difference-image method. Statistical analysis was performed with Wilcoxon signed-rank tests. Intrareader correlation was assessed with weighted kappa coefficients and reproducibility with the root-mean-square-average and the Bland-Altman-method.
At 3T, SNR of cortex and medulla and CNR of cortex/medulla were significantly higher than at 1.5T, leading to improved corticomedullary discrimination. There were no significant FA- and ADC differences with 2 b-values and 6 diffusion directions between measurements at 1.5T and 3T. FA of the medulla was significantly higher than that of the cortex in all measurements. Tractography visualized a typical radial diffusion direction in the medulla. Best image quality was achieved with a respiratory triggered protocol with 12 acquisition directions. Measurements with 3 b-values led to decreased ADCs. Acquisition in 12 directions resulted in decreased cortical FA. FA and ADC of breath-hold and free-breathing acquisitions were significantly higher than that of the respiratory-triggered protocol. Intrareader correlation ranged from kappa 0.60 to 0.96. Variance of the respiratory-triggered protocol was smaller than that of breath-hold and free-breathing protocols. Variance was highest for medullary FA in all protocols with reproducibility coefficients ranging from 0.36 to 0.46.
Diffusion tensor imaging of the kidney at 3T is feasible and yields significantly higher SNR and CNR. FA and ADCs do not significantly differ from 1.5T. Number of b-values influences ADC-values. Acquisitions in 12 directions provide lower cortical FA-values. We recommend a respiratory-triggered protocol because of improved image quality and reproducibility.