Accessing a enum from XAML to access a Dictionary-object

See more:

C#

XAML

dictionary

enum

So the problem is the following:

I have a Dictionary<FormatEnum,string> and I want to access this Dicitionary from XAML.
FormatEnum has the types Small, Medium and Big, and the string contains a URL to a picture.

I tried the following code:

<ItemsControl Name="MixControl">
 <ItemsControl.ItemTemplate>
  <DataTemplate>
   <Border CornerRadius="5" BorderThickness="2" BorderBrush="Black" Width="56" Height="56">
    <Border.Background>
     <ImageBrush ImageSource="{Binding CoverUrls[{x:Static tracks:CoverFormat.Small}]}"/>
    </Border.Background>
   </Border>
  </DataTemplate>
 </ItemsControl.ItemTemplate>
</ItemsControl>

tracks is the namespacereference, and I read somewhere to use x:Static... But it won't work ...
Thanks for your help!