In Gridview Set Image Column programatically

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In Gridview I have column representing Thumbnail image which works fine.:
<asp:ImageFie ld DataImageUrlFie ld="PicFile1"
DataImageUrlFor matString="Imag es/A009/Thumbs/{0}">
</asp:ImageField>

I want the fixed 'A009' to be replaced by Session variable - "ClCode"

VB in code-behind I have:
iPicFile = ImageString & Session("ClCode ") & "/Thumbs/{0}"
How can "iPicFile" variable be used in DataImageUrlFor matString?
With thanks