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

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.

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.

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.

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.