# A Hybrid Watermarking Approach for DICOM Images Security
## 1 Introduction  
The work presented in this paper extends and enhances the work originally presented in [1]. Telemedicine uses telecommunications and information technology to offer health services in distant places. It adds access to medical services that are not available in distant rural areas. When using telecommunications technology, patients who live in distant communities can benefit from the medical analysis of remote professionals, so the patient does not have to travel to visit a doctor for consultation. Telemedicine facilitates the transmission of medical images and information to doctors and radiologists for diagnosis and treatment. It offers doctors the opportunity to send medical data and images from one hospital to another. So, doctors can help other doctors to find out adequate analytical methods to treat patients [2].  
During transmission or storage, medical images may be intentionally or accidentally modified. Therefore, the authenticity of medical images needs to be controlled by the specialist to avoid a wrong diagnosis that may be caused by their modification [3].  
Digital watermarking is a promising method to secure medical images and patient data. Indeed, it provides a solution making it possible to authenticate images at different steps of their use: during their storage, during their transmission over the network, and after their reception. There are two main watermarking principles. The first one consists in embedding a watermark in the image and extracting it later for image authentication. We will refer to watermarking approaches based on this principle as non-zero-watermarking approaches. The second principle is to generate a watermark from the image and to use the watermark as a key that will be used later to authenticate the original image by regenerating the key and comparing it to the original one. Watermarking approaches based on this second principle are called zero-watermarking approaches.  
Medical images contain two disjoint areas respectively called Region Of Interest (ROI) and Region Of Non-Interest (RONI). The ROI is part of the image that is useful for the diagnosis and the RONI is part of the image that has no impact on the diagnosis.  
There are four main categories of medical image watermarking methods [4]:  
* Watermarking in RONI [5; 6; 7]
* Reversible watermarking [8; 9; 10]
* Zero-watermarking [11; 12; 13]
* Watermarking with image change not affecting diagnosis [14].  
In this paper, we propose a watermarking approach that provides strong security for DICOM medical images by combining a zero-watermarking in ROI and a non-zero-watermarking in RONI, by using and ensuring the confidentiality of patient data available in the header of the DICOM image. DICOM is the format commonly used for the transfer of medical images. By embedding the watermark only in RONI, the diagnosis is not affected and the sensitive information of the patient is secured and not modified. Furthermore, by securing the ROI part with a zero-watermarking method, the ROI part is not modified because in zero-watermarking the watermark is not inserted directly into the original image. The proposed watermarking approach also takes into account the real-time constraints of medical applications.  
The remainder of the paper is organized as follows. The second section is dedicated to related works. The proposed watermarking approach is described in section three. Experimentations performed for the evaluation of the proposed solution as well as the results obtained and their analysis are presented in section four. The paper ends with a conclusion and future works in section five.