how to get javascript to enable / disable radio buttons

So below are the HTML and the javascript where all I'm trying to do is enable / disable (and clear) a second set of radio buttons based on yes / no for the first ones.  I know the code runs, as I've put alerts in to make sure the test of the first ' yes / no' is being found.   (the javascript is tied to the radio buttons via the code behind:

        ClientScript.RegisterClientScriptInclude("activateImagingStage", "activateImagingStage.js")
        Imaging_Yes.Attributes.Add("onclick", "activateImagingStage();")
        Imaging_No.Attributes.Add("onclick", "activateImagingStage();")

In IE8, the second set of radio buttons gets activated / deactivated based on the first, but the second set don't get cleared when they are disabled.

In FF, nothing gets enabled / disabled.

Seems like it should be so simple...

<asp:TableRow ID="TableRow28" runat="server">
                                    <asp:TableCell  runat="server" ColumnSpan="4">22.  Did you perform imaging studies?
                                       
                                    </asp:TableCell>
                                    <asp:TableCell  CssClass="answer" runat="server">
                                        <asp:RadioButton ID="Imaging_Yes" GroupName="imaging" runat="server" />
                                    </asp:TableCell>
                                    <asp:TableCell  CssClass="answer" runat="server">
                                        <asp:RadioButton ID="Imaging_No" GroupName="imaging" runat="server" />
                                    </asp:TableCell>
                                </asp:TableRow>
                                <asp:TableRow ID="Question23a" runat="server" Enabled="false" >
                                    <asp:TableCell  runat="server" ColumnSpan="4">23.  What stage was the diagnostic imaging ordered?
                                   
                                    </asp:TableCell>
                                </asp:TableRow>
                                <asp:TableRow ID="Question23b" runat="server"  >
                                    <asp:TableCell></asp:TableCell>
                                    <asp:TableCell ID="tc8b" runat="server" ColumnSpan="4">
                                        <asp:RadioButtonList ID="Imaging_Stage" runat="server">
                                            <asp:ListItem  Value="1" runat="server" Text="Stage 0 or 1A" />
                                            <asp:ListItem  Value="2" runat="server" Text="Other" />
                                        </asp:RadioButtonList>
                                    </asp:TableCell>
                                </asp:TableRow>

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function activateImagingStage(r) {

    var Imaging_Stage = document.getElementsByName("<%= Imaging_Stage.ClientID %>".replace(/_/g, '$'));

		if ((document.getElementById('Imaging_Yes').checked) ) {
		    document.getElementById('Question23a').disabled = false;
		    document.getElementById('Question23b').disabled = false;
		    for (var i = 0; i < Imaging_Stage.length; i++) {
		        Imaging_Stage[i].disabled = false ;
		    }
 	        }
		
 	    else {
 	        for (var i = 0; i < Imaging_Stage.length; i++) {
 	            Imaging_Stage[i].disabled = true;
 	            Imaging_Stage[i].checked = false;
		    }
    		    document.getElementById('Question23a').disabled = true;
    		    document.getElementById('Question23b').disabled = true;
 	    }
}

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