pixel_url = TOPSPIN_URL_WRITER.url_for(:beacon, '',
        :k           => key.key,
        :ua          => key.fan_id,
        :aa          => campaign.artist_id,
        :campaign_id => campaign.id,
        :ec          => TrackEventType::EMAIL_OPEN,
        :es          => EventSource::PIXEL_BEACON
      )