public static function checkEmptyMedia($qti){
        $doc = new DOMDocument();
        $doc->loadHTML(self::loadQtiXml($qti)->saveXML());

        $imgs = $doc->getElementsByTagName('img');
        foreach ($imgs as $img) {
            if(empty($img->getAttribute('src'))){
                throw new QtiModelException('image has no source');
            }
        }

        $objects = $doc->getElementsByTagName('object');
        foreach ($objects as $object) {
            if(empty($object->getAttribute('data'))){
                throw new QtiModelException('object has no data source');
            }
        }

        $objects = $doc->getElementsByTagName('include');
        foreach ($objects as $object) {
            if(empty($object->getAttribute('href'))){
                throw new QtiModelException('object has no data source');
            }
        }
    }