Imagick : generate image from firs page pdf document - imagick

Question

I use this code (CMS used CMSMS)
$obj = $params['item_object'];
$fd = $obj->fielddefs['envoipdf'];
$fd = $fd->GetImagePath(true) . '/' . $fd->value;
$ali = $obj->alias;
$im = new Imagick();
$im->setResolution(72,72);
$im->readImage($fd."[0]");
If I replace
$im->readImage($fd."[0]");
by
$im->readImage($fd);
and put a single page pdf document, no matter the image is well generated.
The returned path is like
https://www.maropclac.com/uploads/gabkio/placa/visuel-exercice-003.pdf
But with a multiple page pdf document and
$im->readImage($fd."[0]");
this returns
Fatal error: Uncaught ImagickException: Imagick::readImage(): HTTP request failed! HTTP/1.1 404 Not Found in /home/tukbix92dqp/testserv/lib/classes/class.usertagoperations.inc.php(305) : eval()'d code:7 Stack trace: #0 /home/tukbix92dqp/testserv/lib/classes/class.usertagoperations.inc.php(305) : eval()'d code(7): Imagick->readImage('https://www.maro...') #1 /home/tukbix92dqp/testserv/lib/classes/class.usertagoperations.inc.php(285): cms_user_tag_slthumb(Array, Object(Smarty_CMS)) #2 /home/tukbix92dqp/testserv/lib/classes/class.Events.php(126): UserTagOperations->CallUserTag('slthumb', Array) #3 /home/tukbix92dqp/testserv/modules/LISE/lib/class.LISEItemOperations.php(232): Events::SendEvent('LISEjourmaro', 'PostItemSave', Array) #4 /home/tukbix92dqp/testserv/modules/LISE/LISE.module.php(631): LISEItemOperations::Save(Object(LISEjourmaro), Object(LISEItem)) #5 /home/tukbix92dqp/testserv/modules/LISE/framework/action.admin_edititem.php(218): LISE->SaveItem(Object(LISEItem)) #6 /home/tukbix92dqp/testserv/modules/LISE/l in /home/tukbix92dqp/testserv/lib/classes/class.usertagoperations.inc.php(305) : eval()'d code on line 7
Perhaps someone can help me....