Imagick出错

I have a problem with Imagick. Imagick is correctly install (I can see it in my php.ini) but it does not work.

When I try to use the library in my PHP code, I have error.

Error is : ImagickException: UnableToOpenBlob `mypdf.pdf': No such file or directory @ error/blob.c/OpenBlob/2642 in C:\wamp\www\testimagick\test.php on line 3

Here's my code :

<?php 
$myurl = 'mypdf.pdf[1]';
$image = new Imagick($myurl);
$image->setResolution( 300, 300 );
$image->setImageFormat( "png" );
$image->writeImage('newfilename.png');
?>

Thanks you !