Display Top level categories only in Magento

<?php $_helper = Mage::helper('catalog/category') ?>
<?php $_categories = $_helper->getStoreCategories() ?>
<?php if (count($_categories) > 0): ?>
<ul>
<?php foreach($_categories as $_category): ?>
<li>
<a href="<?php echo $_helper->getCategoryUrl($_category) ?>">
<?php echo $_category->getName() ?>
</a>
</li>
<?php endforeach; ?>
</ul>
<?php endif; ?>