PURPOSE

Diffusion MRI allows for the visualization and quantification of renal tissue and function without administration of exogenous contrast materials^1,2. However, there is no consensus about the origin of anisotropy in the measured diffusion signal, as it can reflect multiple structures and active flow processes, such as the radial orientation of nephron tubules in the kidney medulla, perfusion and glomerular filtrate flow in the tubules. Hence, the purpose of this study was to study the anisotropy of different flow processes combining diffusion tensor imaging and intravoxel incoherent motion (IVIM) bi-exponential fitting.

METHODS

Approval of our institution’s ethical committee was obtained for this research. Five healthy volunteers were scanned on a 3T Philips Ingenia clinical scanner. Subjects underwent T2 TSE imaging (TE: 100ms, TR: 2418ms, Matrix size: 400x320, FOV: 450x450mm², voxel size: 1.13x1.41x3.0mm) as well as DTI (TE: 39ms, TR: 1267ms, Matrix size: 112x68, FOV: 336x204mm2, voxel size: 3.0x3.0x3.0, b-values: 0, 100, 300 s/mm², gradient directions: 12) and IVIM imaging (TE: 45ms, TR: 1344ms, Matrix size: 112x68, FOV: 336x204mm², voxel size: 3.0x3.0x3.0mm, b-values: 0, 10, 25, 40, 75, 100, 200, 300, 500, 700 s/mm² gradient directions: 6). The DTI and IVIM acquisitions were respiratory triggered using a navigator. The diffusion tensor was calculated from the DTI data (“conventional DTI”) using ExploreDTI³ and D and D* were obtained from a two tensor fit using the IVIM scan with a custom-made MATLAB script. Whole kidney tractography was performed with D_DTI, D_IVIM and D*_IVIM obtained tensors using ExploreDTI.

RESULTS

Tracking was possible using D_IVIM and the D*_IVIM derived tensors. Figure 1 shows the tractography results based on the tensor DTI (A) and two-tensor IVIM fit (B and C). The FA and MD for conventional DTI and IVIM based DTI calculation are displayed in table 1. The MD from D_IVIM is lower than from DTI whereas the MD from D*_IVIM is (about 50 times) higher. FA for D_IVIM and D*_IVIM are both higher than DTI derived FA

DISCUSSION

Previous studies have demonstrated the feasibility of DTI tractography in the kidney, resulting in radially oriented tracts that congregate into multiple papillae at the centre of the kidney^1,2. Because these structures accord with the known microstructures of the kidney, i.e. tubules organised in multiple renal pyramids that terminate in papillae at the edge of the pyelum, it was concluded that diffusion tractography reflects tissue integrity of the renal medulla. An earlier combined DTI and IVIM study of the kidney showed that the pseudodiffusion component is also anisotropic.⁴ Here, we show that tractography is feasible based on the IVIM derived diffusion coefficient, associated with random diffusion, as well as the pseudodiffusion coefficient, associated with tissue perfusion. This suggests that the coherent diffusion anisotropy in the kidney that originates both in the radial organization of tubules in the renal pyramids and in perfusion by radially organized vasculature in the kidney medulla. An interesting point for further research is how these components each contribute to FA in conventional DTI: if the tracts from diffusion and pseudodiffusion DTI are aligned, the anisotropy from these components would strengthen each other, resulting in a higher DTI derived FA, whereas if they were dispersed, the IVIM derived anisotropies would counteract, resulting in a lower DTI derived FA. FA’s of the diffusion and pseudodiffusion components are each higher than the DTI derived FA, which suggests that the anisotropies are not aligned. However, differences in FA can also be the result of a lower SNR in diffusion and pseudodiffusion DTI compared to the total signal SNR. In addition, in the current tractography images, the exact orientations of the tracts cannot be verified.

CONCLUSION

Diffusion and pseudodiffusion are both anisotropic which can be visualized using tractography. This indicates that they likely both contribute to the coherent diffusion anisotropy that is detected in conventional monoexponential DTI.

Acknowledgements

No acknowledgement found.