Leveraging field_get_items variable in a custom field template
You need TEMPLATE_preprocess_field() function at template.php

You can use this preprocess function in same way as you are doing for NODE. Like -

function MYTHEME_preprocess_field(&amp;$vars) {
  if ($node = menu_get_object()){
    $node = $vars['node'];
    $field_image_items = field_get_items('node', $node, 'field_image');
    $vars['alt_caption'] = $field_image_items[0]['alt'];
  }
}


Then go to your TPL made for your particular field and print it where ever you like just like you have mentioned --

&lt;?php print $alt_caption; ?&gt;


And it will work like charm!! I have implemented like this several times. 

But yeah...don't forget to clear your cache after this implementation.