Intrascan intensity inhomogeneities are a common source of difficulty for MRI segmentation. We estimate the bias field by Legendre polynomials.The bias field could be the best when we get minimum entropy. It needs to work out parameters of the base function in the process of finding bias field

but conventional methods such as gradient descent method often find local best. To find global best

we present genetics algorithm to find best parameters to estimate the bias field

however the result was not satisfying. Then we make some modification of genetics algorithm to make it easier to find global best. Experiments on the segmentation of brain magnetic resonance images show our modification can achieve optimal bias field and accurate segmentation results.