private function getImageRtfHeight()
    {
        if ($this->_height > 0) {
            return PHPRtfLite_Unit::getUnitInTwips($this->_height);
        }

        $imageHeight = $this->_imageHeight ? $this->_imageHeight : 100;
        if ($this->_width > 0) {
            $imageWidth = $this->_imageWidth ? $this->_imageWidth : 100;
            $height = ($imageHeight /$imageWidth) * $this->_width;
            return PHPRtfLite_Unit::getUnitInTwips($height);
        }

        return PHPRtfLite_Unit::getPointsInTwips($imageHeight);
    }