Caching rails generated images


I use Ruby on Rails to generate images, which are delivered via
mod_fastcgi to apache2.
These images should be cached using mod_cache and mod_disk_cache.
But it does not work.
In Rails I set my http headers like this:

  expires_in 30.hours, :private => false
  @response.headers['Last-Modified'] = im.serie.received
  @response.headers['Content-Length'] = data.size
  send_data data, :type =>  'image/png', :disposition => 'inline'

The important settings from apache2 conf look like this:

FastCgiServer /home/hmeyer/devel/dicomweb/public/dispatch.fcgi \ -initial-env RAILS_ENV=development \ -processes 5 \ -idle-timeout 60

<VirtualHost *>
DocumentRoot /home/hmeyer/devel/dicomweb/public
ErrorLog /home/hmeyer/devel/dicomweb/log/apache.log
CustomLog /home/hmeyer/devel/dicomweb/log/access.log combined
SetEnv RAILS_ENV development

    <IfModule mod_cache.c>
            <IfModule mod_disk_cache.c>
                    CacheRoot /var/www/cache/
                    CacheSize 256000
                    CacheEnable disk /image
            CacheDirLevels 5
            CacheDirLength 3

<Directory /home/hmeyer/dicomweb/public/>
Options ExecCGI FollowSymLinks
AddHandler cgi-script .cgi
AllowOverride all
Order allow,deny
Allow from all
ExpiresByType image/png “now plus 1 month”
ExpiresActive On

What am I doing wrong?

Thanx for your help!