I am attempting to measure the FA, ADC, RD, and AD along a tract that I have generated and the ADC, RD,and AD values are all very small when I use tcksample. They range from around -.007 to .002. The FA values appear to be within normal range though so I am confused as to what is going wrong.
Any input is appreciated.
What do you consider a normal range for diffusivity? At body temp, ADC in free water is around 0.003 mm^2/s. So any diffusivities you measure in the white matter are expected to be lower than that number… As for the negative values: these are probably due to noise or outliers, resulting in negative eigenvalues of the diffusion tensor (there is no positivity constraint in place).