Help with a get_categories () drop down menu - I want it to show in heirachial form

Question

I have a drop down menu that displays all of the terms for one of my custom taxonomies using get_categories (). It displays fine in alphabetical form. However, what i want to know is how can i have the terms listed in a hierarchial form in a drop down menu, so they appear like this -

Parent Category 1

• Child One
• Child Two

Parent Category 2

• Child One

Here is my code for the drop down menu:

</select> 

<span>By Category: </span>
<select name="category"> 
<option name="all" value="">All Categories</option>
<?php $categories_filter = get_categories( 'taxonomy=tshirt_categories' ); 
    foreach( $categories_filter as $category_option ) { 

    echo '<option name=' . $category_option->slug . ' value="' .  $category_option->slug . '" /> ' . $category_option->name . '</option>' . "\n";
    }
?> 
</select>  

Once again thanks for any advice you guys may have!

Mike