How to get top level category 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; ?>