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.
Highlights
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
Our results show that for DTI of the kidney at 1.5T, the best compromise between acquisition time and image quality seems to be the application of b‐values between 300 and 500 s/mm2 combined with 15 encoding directions
In this study we were able to confirm that optimal b‐values for DTI of the kidney range between 300 and 500 s/mm2 and that increasing the number of encoding directions lead to better image quality as a result of improved corticomedullar discrimination, taking longer acquisition times into account
Summary
There is an increasing number of methods for functional imaging of the kidneys,[1] e.g. dynamic MRI with gadolinium‐based contrast media, diffusion weighted imaging (DWI),[2,3,4,5] blood‐oxygenation level dependent imaging (BOLD),[6,7] arterial spin labeling (ASL),[8,9] and MR‐elastography.[10,11,12]DWI can help in the detection of renal abnormalities[3,13,14,15] and characterization of focal renal lesions[16] using the apparent diffusion coefficient (ADC). In diffusion tensor imaging (DTI), the diffusion‐induced signal attenuation is measured for water displacement along at least six non‐collinear directions. This allows a full description of the second‐rank diffusion tensor and a quantitative assessment of diffusion anisotropy. Biological structures, such as the tubuli in the kidneys may restrict the Brownian motion of the water molecules in a direction dependent manner and cause anisotropy of the diffusion.[18] Renal structures in the medulla are oriented in a radial fashion resulting in stronger anisotropy effects in the medulla than in the cortex.[19]
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