Research On Lossless Watermarking System For Medical Data

In a modern integrated health care environment, digital information systems are playing an ever more important role. The digital representation of medical data has lots of advantages over its analogy counterpart in data storage and transmission. On the other hand, with current techniques, it is fairly easy for malicious adversary to intercept or tamper sensitive medical data when the public network is being used for telemedicine. It is common view that there is an urgent need of security measures in medical information system.Digital watermarking, which imperceptibly embeds additional information within a host signal, is an emerging technique for protecting multimedia data. In this dissertation, lossless watermarking systems for medical images and biomedical signals are studied.1. A lossless watermarking scheme, in the sense that the original image can be exactly recovered from the watermarked one, is proposed with the purpose of verifying the integrity and authenticity of medical images. The embedding method includes imposing invertible modulations that induce low distortion level, as well as losslessly compressing the status of the original image and carrying such information in the watermarked image. Digital signature of the whole image is embedded for verifying the integrity of the image. An identifier also presented in the electronic patient record (EPR) is embedded for verifying the authenticity by simultaneously processing of the watermarked image and the EPR. Combining with fingerprint system, patient's fingerprint information is further embedded into several image slices and then extracted for verifying the authenticity.2. A novel region-based lossless watermarking scheme is proposed for medical images. The scheme has the capability of not introducing any embedding-induced distortion in the region of interest (ROI) of a medical image. Difference expansion of adjacent pixel values is employed to embed several bits. A region of embedding, which is represented by a polygon, is chosen intentionally to prevent introducing embedding distortion in the ROI. Only the vertex information of a polygon is transmitted to the decoder for reconstructing the embedding region, which improves the embedding capacity considerably.3. A lossless watermarking scheme for verifying the integrity of medical images with tamper localization is proposed by incorporating such functionality into the region-based lossless watermarking scheme, instead of using the conventional irreversible block-based watermarking approach. The tamper localization is achieved by partitioning an image into certain non-overlapping regions and appending the associated local authentication information directly into the watermark payload. A region of authentication, which can be flexibly specified by the user, is partitioned into small regions in a multilevel hierarchical manner. Such hierarchical structure allows the user to easily adjust the localization accuracy, and makes the tamper detection efficient.4. A region-based reversible data-embedding algorithm is proposed for ECG signals. The algorithm enables selecting some region to embed watermark information. When the embedding-induced distortion is outside the region of interest, the watermark extraction and host signal restoration can be safely neglected. It also enables authenticating the integrity of the signal with tamper localization capability.