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Second gradient: Rephasing
Moving protons
Stejskal-Tanner sequence Three principal axes
The B-value
• Expresses strength and duration of
gradient field
• Low B-values:
– B = 0: T2-weighted image
– B = 50 - 400
» Signal loss in fast moving protons
(perfusion)
• High B-values
– B > 400
» Signal loss in slowly moving protons
(diffusion)
The ADC
• ADC = apparent diffusion coefficient
• Slope of straight line connecting two
values obtained with different B-values
Signal intensity
0
22,5
45
67,5
90
B-value
50 800
Low b-value High b-value
Calculation of
signal loss/pixel
ADC image
Calculation of the ADC
( )
3
3 2 1 λ λ λ + +
= ADC