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/"

  validates_as_attachment

end

Thanks, Demetris.