treeview expanded level

when redirect  to the content form, the treeview level  expanded   view is lost, its gone to its 0 level . Please can you help me with the code

to keep the same expanded level of master form even in content form.

protected void TreeView1_SelectedNodeChanged(object sender, EventArgs e)
        {
            string _Subjects = "";
            string _GroupName = "";
            Session["GroupName"] = TreeView1.SelectedNode.Text;
            if (TreeView1.SelectedNode.ChildNodes.Count == 3)
            {
                Session["Subjects"] = TreeView1.SelectedNode.Text;
            }
            if (TreeView1.SelectedNode.ChildNodes.Count <= 1)
            {
                Session["Subjects"] = TreeView1.SelectedNode.Parent.Text;
            }
            _GroupName = Session["GroupName"].ToString();
            Response.Redirect("~/Subjects.aspx?Subjectid=" + _GroupName);
}

<asp:TreeView ID="TreeView1" runat="server" CssClass="style5" 
                DataSourceID="XmlDataSource1" Width="169px" ImageSet="Arrows" 
                onselectednodechanged="TreeView1_SelectedNodeChanged" ExpandDepth="1" ><DataBindings><asp:TreeNodeBinding DataMember="COMPUTER" TextField="#Name" /><asp:TreeNodeBinding DataMember="field" ValueField="#InnerText" /><asp:TreeNodeBinding DataMember="STUDY" TextField="#Name" /><asp:TreeNodeBinding DataMember="field" ValueField="#InnerText" /></DataBindings><HoverNodeStyle Font-Underline="True" ForeColor="#5555DD" /><NodeStyle Font-Names="Tahoma" Font-Size="10pt" ForeColor="Black" 
                    HorizontalPadding="5px" NodeSpacing="0px" VerticalPadding="0px" /><ParentNodeStyle Font-Bold="False" /><SelectedNodeStyle Font-Underline="True" ForeColor="#5555DD" 
                    HorizontalPadding="0px" VerticalPadding="0px" /></asp:TreeView>

Please help