Rationale and Objectives
To measure the anisotropic diffusion in kidney and to demonstrate the feasibility of renal tractography.
Materials and Methods
Diffusion tensor imaging was acquired in kidney from 10 healthy volunteers and 5 patients with chronic kidney disease. Diffusion indices were calculated from the tensor, including fractional anisotropy, intervoxel diffusion coherence, and mean/axial/radial diffusivity.
Results
Acquisitions with respiratory triggering could provide improved image quality in all diffusion indices, as compared to that by breathhold. It is sufficient to use five to seven scan averages when the measured diffusion indices converge to a steady state in medulla, which reduced the acquisition time in a triggered measurement down to a clinically tolerable limit. Second, the measured diffusion indices can be affected by the diffusion weighting. An increased diffusion weighting will lead to an underestimation in all diffusion indices. Finally the direction of water diffusion is consistent in the kidney cortex, which was properly reflected in intervoxel diffusion coherence. In a feasibility study in healthy volunteers and patients, renal tractography was performed that visualized the organized renal structure and as it declined with the progress of chronic kidney disease.
Conclusion
When compared to conventional breath hold technique, the significant improvement in image quality compensated for the prolonged acquisition time. Therefore, triggered acquisition is preferred in a clinical setting because it required less from patient cooperation.
Conventional magnetic resonance imaging (MRI) is capable to provide morphological assessment in kidney. Recent development in diffusion imaging could allow the detection of the functional anatomical alteration. Apparent diffusion coefficient (ADC) could provide information potentially related to the tissue microstructure. Interest in measuring water diffusion in kidney arises because of its special role in water transportation function. The measured ADC could be related to the pathological changes in several renal diseases. For example, the ADC values in both the cortex and medulla in renal failure kidneys were significantly lower than those of the normal . In rats with diabetes mellitus, ADC was reported significantly lower as well . The ADC measurements were thought to be related to the histopathological changes or cellular edema within the tissue from ischemia in animal with diabetic nephropathy . Yet, these studies did not address the changes in diffusion anisotropy.
Diffusion anisotropy is related to the structural organization and therefore could be compromised during a pathological process. Diffusion tensor imaging (DTI) is a further development from diffusion-weighted MRI, which allowed the quantification of the differential diffusion along the principal directions. Indices derived from the diffusion tensor have been in clinical use, with their properties investigated . Three indices related to the magnitude of diffusivity are derived from the tensor, mean, axial and radial diffusivity (MD, AD, and RD, respectively) . MD is the averages of the ADC along all directions, a reflection of the magnitude of the tensor. In a cylindrical model, AD is the first eigenvalue, which is along the longitudinal direction of the tensor. RD is the average of the second and third eigenvalues, which is related to the diffusion along the radial direction in the cylindrical model. Indices related to diffusional anisotropy are fractional anisotropy (FA) and intervoxel diffusion coherence (IVDC) . FA quantifies the difference of directionally dependent diffusion within a voxel of interest. In a cylindrical model, it is 1. FA is 0 if the tensor is spherical. IVDC was proposed to quantify the orientational coherence among the principal diffusion directions between a voxel of interest and its closest neighborhood. The interest of measuring the diffusion anisotropy in kidney is because renal structures, such as vessels, tubules, and collecting ducts, are radially orientated, which leads to potential diffusion anisotropy .
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Methods and materials
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Subjects
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Image Acquisition
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Image Analysis
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Region of Interest Selection and Statistical Analysis
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Result
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Table 1
Effect from Number of Averages on FA
No. of Averages 1 2 3 4 5 6 7 8 9 10 11 Medulla TH 8.1E-51 ∗ 2E-24 ∗ 1E-13 ∗ 7E-08 ∗ 3E-05 ∗ 0.001 ∗ 0.015 ∗ 0.072 0.108 0.301 0.591 TL 2.1E-33 ∗ 1E-15 ∗ 5E-09 ∗ 5E-05 ∗ 0.007 0.075 0.208 0.435 0.537 0.695 0.759 BH 1.4E-50 ∗ 2E-37 ∗ 2E-25 ∗ 2E-17 ∗ 2E-10 ∗ 2E-07 ∗ 8E-06 ∗ 0.001 ∗ 0.009 ∗ 0.129 0.446 Cortex TH 4.5E-50 ∗ 1E-36 ∗ 2E-27 ∗ 1E-25 ∗ 4E-19 ∗ 1E-14 ∗ 9E-11 ∗ 6E-07 ∗ 6E-05 ∗ 0.031 ∗ 0.237 TL 5E-56 ∗ 2E-43 ∗ 2E-29 ∗ 6E-21 ∗ 5E-13 ∗ 7E-08 ∗ 2E-05 ∗ 0.004 ∗ 0.079 0.260 0.525 BH 1E-53 ∗ 8E-37 ∗ 1E-26 ∗ 3E-23 ∗ 1E-15 ∗ 4E-10 ∗ 6E-07 ∗ 0.0002 ∗ 0.012 ∗ 0.065 0.326
FA from different number of scan averages was compared to that by 12 averages using a Student’s t -test. The table contained the p values. The region of interest was located in either cortex or medulla, respectively.
BH, breath hold; FA, fractional anisotropy; TH, triggered-high; TL, triggered-low.
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Table 2
Diffusion Index in Kidney
FA MD AD RD Medulla TH 0.34 ± 0.10 1.23 ± 0.22 1.56 ± 0.29 1.07 ± 0.21 TL 0.44 ± 0.14 1.69 ± 0.30 2.30 ± 0.41 1.38 ± 0.31 BH 0.46 ± 0.12 0.99 ± 0.43 1.37 ± 0.55 0.82 ± 0.38 Cortex TH 0.18 ± 0.03 1.54 ± 0.16 1.74 ± 0.18 1.43 ± 0.16 TL 0.22 ± 0.05 2.01 ± 0.28 2.41 ± 0.22 1.89 ± 0.19 BH 0.26 ± 0.06 1.52 ± 0.58 1.81 ± 0.67 1.37 ± 0.53
The table showed the mean value of diffusion index at 12 averages. FA is dimensionless. MD/AD/RD are given in 10 −3 mm 2 /s.
BH, breath hold; FA, fractional anisotropy; TH, triggered-high; TL, triggered-low.
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Discussion
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Breathhold and Respiratory Triggering
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Diffusion Weighting
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Directional Coherence and Diffusion Anisotropy
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Renal Tractography and its Clinical Implications
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