Display sub-category list -Magento

Sub-Category Wise Get The Products In Magento show product sub-Category wais

<?php
    $_category  = $this->getCurrentCategory(); 
    $collection = Mage::getModel('catalog/category')->getCategories($_category->entity_id);
    $helper     = Mage::helper('catalog/category');
    ?>

    <ul>
        <?foreach ($collection as $cat):?>
            <?php if($_category->getIsActive()):?>
            <?php 
                 $cur_category = Mage::getModel('catalog/category')->load($cat->getId());
                 $_img = $cur_category->getImageUrl();  
            ?>
            <li>
                    <a href="<?php echo $helper->getCategoryUrl($cat);?>">
                         <img src="<?php echo $_img?>" title="$cat->getName()"/>
                         <span><?php echo $cat->getName();?></span>
                    </a>
                </li>
            <?php endif?>
        <?php endforeach;?>
    </ul>