ZBrushCentral

Displacement problem

hi im new to digital sculpting and Zbrush and looking for some help, when rendering my displacement map in mental ray the image appears to have a grainy almost wood like effect to it as shown in images
Im unsure if the problem is with workflow in zbrush or maya but i was hoping one of the many talented and knowledgeable artists on zbrush central could help me with this problem.

Thanx in adavance

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