Creating a list of Folders in an ItemGroup using MSBuild

I'm trying to build an ItemGroup in an MSBuild script which contains a list of folders directly below a given 'Root' folder. So - in this example...

+ Root folder
---- Sub Folder 1
-------- Sub-Sub Folder 1
-------- Sub-Sub Folder 2
---- Sub Folder 2
---- Sub Folder 3

... I would want my ItemGroup to contain "Sub Folder 1", "Sub Folder 2" and "Sub Folder 3".

There may be a number of files at any point in the hierarchy, but I'm only interested in the folders.

Can anyone help!?