Diffusion Tensor Imaging of the Kidneys: Influence of b-Value and Number of Encoding Directions on Image Quality and Diffusion Tensor Parameters

Abstract

Objectives:The purpose of this study was to evaluate to which degree investment of acquisition time in more encoding directions leads to better image quality (IQ) and what influence the number of encoding directions and the choice of b-values have on renal diffusion tensor imaging (DTI) parameters.Material and Methods:Eight healthy volunteers (32.3 y ± 5.1 y) consented to an examination in a 1.5T whole-body MR scanner. Coronal DTI data sets of the kidneys were acquired with systematic variation of b-values (50, 150, 300, 500, and 700 s/mm2) and number of diffusion-encoding directions (6, 15, and 32) using a respiratory-triggered echo-planar sequence (TR/TE 1500 ms/67 ms, matrix size 128 × 128). Additionally, two data sets with more than two b-values were acquired (0, 150, and 300 s/mm2 and all six b-values). Parametrical maps were calculated on a pixel-by-pixel basis. Image quality was determined with a reader score.Results:Best IQ was visually assessed for images acquired with 15 and 32 encoding directions, whereas images acquired with six directions had significantly lower IQ ratings. Image quality, fractional anisotropy, and mean diffusivity only varied insignificantly for b-values between 300 and 500 s/mm2. In the renal medulla fractional anisotropy (FA) values between 0.43 and 0.46 and mean diffusivity (MD) values between 1.8-2.1 × 10-3 mm2/s were observed. In the renal cortex, the corresponding ranges were 0.24-0.25 (FA) and 2.2-2.8 × 10-3 mm2/s (MD). Including b-values below 300 s/mm2, notably higher MD values were observed, while FA remained constant. Susceptibility artifacts were more prominent in FA maps than in MD maps.Conclusion:In DTI of the kidneys at 1.5T, the best compromise between acquisition time and resulting image quality seems the application of 15 encoding directions with b-values between 300 and 500 s/mm2. Including lower b-values allows for assessment of fast diffusing spin components.

Assessment of renal allograft function early after transplantation with isotropic resolution diffusion tensor imaging

“…Longer TE also increases the sensitivity of anisotropy measurement and improves fibre tracking; thus we used a longer TE in the present study which also increased the FA values in anisotropy tissues [25,26]. As the results showed when using b values from 300 to 500 s/mm 2 in our primary test, the flow effects in smaller b values contributed to the higher ADC, but did not have a significant effect on FA which is consistent with a previous report [27].…”

In vivo demonstration of microscopic anisotropy in the human kidney using multidimensional diffusion MRI

“…Therefore, assuming a maximum FA of 0.5 (see Results for details of our FA estimates in this data set), ~11 directions yield a rotation invariant powder signal for b × MD < 3, 39 where MD = mean diffusivity. As such, considering the highest b-value used in this study (1000 s/mm 2 ), ~11 directions yield a rotation invariant powder signal for a MD of 3 × 10 −3 mm 2 /s (diffusion coefficient of water at body temperature), 40 which is above the typically observed MD in the renal cortex and medulla [41][42][43] (also verified in our data, see Results). The number of encoding directions was therefore set to 12 for all LTE and STE acquisitions.…”

Fast diffusion kurtosis imaging of fibrotic mouse kidneys

“…For the present purpose we assume isotropic tissue so that D̄ = D ( n̂ ) and W̄ = W ( n̂ ) meaning that data acquired along one direction is sufficient for the optimization. This assumption is valid in kidney cortex where FA is low but may apply to more regions of the kidney as FA is generally reported to be <0.4 in most of the organ (15,43,44). Prior to analysis, data was smoothed using a Gaussian kernel with FWHM = 1.75 x voxel size.…”

“…in most of the organ. 15,43,44 Prior to analysis, data was smoothed using a Gaussian kernel with a full width at half-maximum of 3 | RESULTS…”

“…12 However, the results in past studies show great discrepancy because of the lack of consensus in the measurement schemes (different b-values) as pointed out by different research groups. [13][14][15][16] Inappropriate choice of b-values may cause measurements to be contaminated by intravoxel incoherent motion (IVIM) effects 17,18 from perfusion and filtration in the kidney, which will skew ADC estimates. Moreover, basic dMRI metrics, such as ADC, might not be the most sensitive diffusion marker to detect fibrosis, and more advanced diffusion techniques now offer parameters sensitive to more aspects of the microstructure.…”