Importance of Fractional b Value for Calculating Apparent Diffusion Coefficient in DWI.

OBJECTIVE: The purpose of this study is to examine how the fractional b value affects the calculation of apparent diffusion coefficient (ADC) using DWI. The fractional b value is the point of intersection between the fast and slow components of biexponential decay in DWI. SUBJECTS AND METHODS: Human brains were imaged using multiple b values on echo-planar DWI. The ADCs of white matter, gray matter, and thalamus were calculated using the combination of b values by two-point and multipoint methods, and the characteristics of each ADC value were compared. RESULTS: When the two selected points for calculation were smaller than the fractional b value (b = 1700 s/mm2), the ADC value was 0.0007-0.0008 mm2/s, but when the two points used for calculation were greater than the fractional b value, the ADC value was 0.0003-0.0004 mm2/s. When a range of b values was included in the fast and slow components by use of the multipoint method, the ADC value showed a statistically significant increase as the number of multiple b values increased. CONCLUSION: The ADC value fluctuated when the b values used for calculation were higher than the fractional b value. Therefore, it is important to determine the fractional b value of the target tissue.

Optimal b Values for Generation of Computed High-b-Value DW Images.

OBJECTIVE: The purpose of this study was to investigate the optimal b values required for the generation of computed high-b-value DW images. SUBJECTS AND METHODS: Brain DWI was performed for eight subjects using 16 b values, ranging from 0 s/mm(2) to 5000 s/mm(2). The fractional value as the turning point between the fast and slow components was determined using a diffusion decay curve. Image contrast in the white matter, gray matter, thalamus, putamen, and lateral ventricle was compared between the acquired high-b-value images and the computed high-b-value images derived from various b value images. In addition, the signal-to-noise ratio of each computed image and acquired image was measured and compared. RESULTS: The fractional values obtained from the diffusion decay curve were between 1200 and 1800 s/mm(2). Image contrast in all regions was nearly equivalent between the acquired and computed high-b-value images when b values higher than the fractional value were used, whereas image contrast was significantly different between the two sets of images when b values lower than the fractional value were used (p < 0.05). The computed images derived from low-b-value images showed a higher signal-to-noise ratio, compared with the acquired images. CONCLUSION: Optimal b values should be considered when acquiring images for the derivation of computed DW images.