b values

Last revised by Ayush Goel on 4 Jul 2020

b value measures the degree of diffusion weighting applied, thereby indicating the amplitude (G), time of applied gradients (δ) and duration between the paired gradients (Δ) and is calculated as:

b = γ² G² δ² (Δ−δ/3)

Therefore, a larger b value is achieved by increasing the gradient amplitude and duration and by widening the interval between paired gradient pulses.

To sense slow moving water molecules and smaller diffusion distances, b values should be higher (e.g. b = 500 s/mm2).

Apparent diffusion coefficient is calculated using different b-values (e.g 0-1000 s/mm2).

A useful rule of thumb is to choose the b value such that (b X ADC) is nearly equal to 1.

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Cases and figures

  • Effect of B values on DWI
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