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/mm(2)) 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/mm(2) 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/mm(2). 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) mm(2)/s were observed. In the renal cortex, the corresponding ranges were 0.24-0.25 (FA) and 2.2-2.8 × 10(-3) mm(2)/s (MD). Including b-values below 300 s/mm(2), 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/mm(2). Including lower b-values allows for assessment of fast diffusing spin components.

Keywords: Diffusion tensor imaging; diffusion weighted imaging; kidneys.

Figure 1

(a) Exemplary image to illustrate the orientation of the oblique-coronal plane. (b) Typical ROI placement in a fractional anisotropy map (b = 300 s/mm², 15 encoding directions).

Figure 2

Comparison of image quality of FA and MD maps: Upper two rows were calculated with two b-values: b = 0 and b-values of 300 and 700 s/mm² using 15 encoding directions (2b 15d b300 and 2b 15d b700). Third row with all 6 b-values with 15 encoding directions. In the parametrical MD map computed from 6 b-values, markedly increased diffusion is displayed without significant alterations in the FA maps compared to the parametrical images of the upper row.

Figure 3

Comparison of image quality of FA and MD maps with the different encoding directions: The upper row shows the b0 image, the MD and FA map with 6 encoding directs with b-value 0 and 500 s/mm², the second row shows the same order with 15 encoding directions with b-value 0 and 500 s/mm² and the third row shows the b0 image, MD and FA map with 32 encoding directions, again with b-values 0 and 5 s/mm². Best corticomedullary discrimination is seen with 15 directions.

Figure 4

Left side: b = 0 s/mm² image of the left kidney. Corresponding parametrical MD map (center) and FA map (right side) acquired with a b-value of 500 and 15 encoding directions (2b 15d b500). B =0 s/mm² image and MD map show no significant artifacts, whereas in the FA map severe artifacts are visible in the upper pole of the kidney.

Figure 5

Minimal signal-to-noise ratio (SNR) decreases with increasing b-value. A b-value of 700 s/mm² results in low SNR with an underestimation of mean diffusivity and degradation of image quality.

Figure 6

Corticomedullary contrast-to-noise-ratio (CNR) remains stable for b-values between 300 and 500 s/mm², whereas a b-value of 700 s/mm² leads to a reduction of CNR.