Cropped thumbnails with ImageScience

I have a Photo model (below) and as you can see I create some
thumbnails. However I want to create a proportionally thymbnail, which
basically will be a cropped version, so that the thumbnail will be a
perfect square.
I use attachment_fu and ImageScience to handle photo uploads. Can
anyone help me or give me a hint on how to create my cropped
thumbnails ?

class Photo < ActiveRecord::Base

  belongs_to :user

  has_attachment :content_type => :image,
                   :storage => :file_system,
                   :size => 1.byte..12.megabytes,
                   :thumbnails => { :thumb => '125x83>' , :MS =>
'500x333>' },
                   :path_prefix => "/public/photos/"