Wordpress Wp_Get_Attachment_Metadata

I have a problem with some of my custom gallery functions, that I have used for years in WordPress. Now with WordPress Version 4.0 the wp_get_attachment_metadata function isn't working correctly, so that I have no alt-attribute. Everything else is working perfectly. Images are visible and in correct order.

<!-- language: lang-php -->

/* CUSTOM GALLERY ############################################### */

add_filter('post_gallery', 'my_post_gallery', 10, 2);
function my_post_gallery($output, $attr) {
    global $post;

    if (isset($attr['orderby'])) {
        $attr['orderby'] = sanitize_sql_orderby($attr['orderby']);
        if (!$attr['orderby'])
            unset($attr['orderby']);
    }

    extract(shortcode_atts(array(
        'order' => 'ASC',
        'orderby' => 'menu_order ID',
        'id' => $post->ID,
        'itemtag' => 'dl',
        'icontag' => 'dt',
        'captiontag' => 'dd',
        'columns' => 3,
        'size' => 'thumbnail',
        'include' => '',
        'exclude' => ''
    ), $attr));

    $id = intval($id);
    if ('RAND' == $order) $orderby = 'none';

    if (!empty($include)) {
        $include = preg_replace('/[^0-9,]+/', '', $include);
        $_attachments = get_posts(array('include' => $include, 'post_status' => 'inherit',
        'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order,
        'orderby' => $orderby));

        $attachments = array();
        foreach ($_attachments as $key => $val) {
            $attachments[$val->ID] = $_attachments[$key];
        }
    }

    if (empty($attachments)) return '';

    $output = "<div class=\"slideshow-wrapper\">\n";
    $output .= "<div class=\"preloader\"></div>\n";
    $output .= "<ul data-orbit>\n";

    foreach ($attachments as $id => $attachment) {
        $img = wp_get_attachment_image_src($id, 'full');
        $image_data = wp_get_attachment_metadata($id, FALSE );
        $output .= "<li>\n";
        $output .= "<img src=\"\" data-src=\"{$img[0]}\" width=\"{$img[1]}\" 
        height=\"{$img[2]}\" alt=\"{$image_data['image_meta']['title']}\" 
        class=\"gallery-img\"/><noscript><img src=\"{$img[0]}\" 
        data-src=\"{$img[0]}\" width=\"{$img[1]}\" height=\"{$img[2]}\"
        alt=\"{$image_data['image_meta']['description']}\" 
        class=\"gallery-img\"\></noscript>";
        $output .= "</li>\n";
    }

    $output .= "</ul>\n";
    $output .= "</div>\n";

    return $output;
}