Watermarking Techniques in Medical images - A Survey Dr. S. Karthikeyan Dr. Alex Mathew College of Applied Sciences, Sohar Sultanate of Oman [email protected] College of Applied Sciences, Sohar Sultanate of Oman [email protected] Mrs. Sathya Degala College of Applied Sciences, Sohar Sultanate of Oman [email protected] Abstract—Medical imaging modalities generate colossal dimensions of medical images every day. Medical images are often transmitted over insecure channel. Hence protection of medical image is very crucial for accurate diagnosis and patient confidentiality. Furthermore there is a rise of various diseases, for which few diagnoses are insufficient; therefore to achieve a correct diagnostic, there is need to exchange the data over internet. The key problem of exchanging the data over internet is to maintain their authenticity, integrity and confidentiality. Therefore, we need a system for effective transmission and access of medical images keeping its authenticity, integrity and confidentiality. In this paper, we discuss various types of water marking, features, requirements, techniques and its pros and cons were analysed. Keywords—Medical Images, Watermarking, DCT, Least Significant Bit Modification (LSB), Discrete Fourier Transform (DFT), Discrete Cosine Transform (DCT), Discrete Wavelet transforms (DWT), Curvelet Transform. I. INTRODUCTION Medical Imaging is an essential part of medical care in the world. It means examining patients with DICOM images to help doctors and other health-care professionals make the correct clinical diagnosis and use this critical information to decide on the appropriate treatment. Medical Images are produced by varying imaging modalities such as Computer Tomography (CT), Magnetic Resonance Imaging (MRI), Ultra Sound (US). Picture archiving and communication system (PACS) is a medical imaging technology which provides economical storage of and convenient access to images from multiple modalities. PACS are installed in medical center to manage the medical image information. Medical images must need well-built security in view of its importance in clinical diagnosis and treatment. Now-adays exchange of medical images between medical centers located in different geographical location is very common. In this case, to achieve a correct diagnostic, there is need to exchange the data over Internet. The key problem of exchanging the data over internet is to maintain their authenticity, integrity and confidentiality. Therefore, we need a system for effective transmission and access of medical images keeping its authenticity, integrity and confidentiality [11]. Digital watermarking is a security technique which is capable to handle all security issues. It is defined as a pattern in which bits are inserted into audio, digital image, text file or video which detects the copyright information of the file. In the digital watermarking the secret information are implanted into the original data for protecting the ownership rights of the multimedia data. This paper discusses various features of the watermarking, water marking requirements, suitable Watermarking techniques in medical images and its pros and cons were analysed and presented. The rest of this paper is organized as follows. Section 2 describes Digital Watermarking Feature, Classifications, Types, Techniques and its pros and cons were presented. Finally, some conclusions are drawn in Section 3. II. DIGITAL WATER MARKING In the year 1988, the digital watermarking is used by Komatsu and Tominaga. Watermark is defined as a pattern in which bits or digital signals are inserted into audio, digital image, text file or video which detects the copyright information of the file. These watermarks are the faintly visible imprints in the stationary and the digital watermarks protect fully the intellectual property available in the digital format. The main requirements of digital watermarking are invisibility, robustness and data hiding capacity [5]. A. Need for Watermarking in Medical Images Digital Watermarking is very important in Medical image due to (i) Medical image transmission cannot be accessed by unauthorized parties (confidentiality) (ii) Received images are not modified during transmission (integrity), (iii) Images are from correct sources to the claimed receivers (authentication). Continuously updated Digital Imaging and Communication in medicine (DICOM) standards provide guidelines to ensure authentication, integrity and confidentiality of medical images [5]. B. Digital Watermarking Classifications Some of the important types of watermarking based on different watermarks [12] [4] [6] are given below: Visible watermarks: Visible watermarks are an extension of the concept of logos. Such watermarks are applicable to images only. These logos are inlaid into the image but they are transparent. Such watermarks cannot be removed by cropping the center part of the image. Further, such watermarks are protected against such as statistical analysis. Watermark Embedding Key Watermark Recovery Watermark Watermarked image Watermark Key Image Watermarked Image. The drawbacks of visible watermarks are degrading the quality of image and detection by visual means only. Thus, it is not possible to detect them by dedicated programs or devices. Such watermarks have applications in maps, graphics and software user interface. Invisible watermark: invisible watermark is hidden in the content. It can be detected by an authorized agency only. Such watermarks are used for content and /or author authentication and for detecting unauthorized copier. Public watermark: Such a watermark can be read or retrieved by anyone using the specialized algorithm. In this sense, public watermarks are not secure. However, public watermarks are useful for carrying IPR information. They are good alternatives to labels. Fragile watermark: Fragile watermark are also known as tamper-proof watermarks. Such watermark are destroyed by data manipulation or in other words it is a watermarks designed to be destroyed by any form of copying or encoding other than a bit-for-bit digital copy. Absence of the watermark indicates that a copy has been made. Private watermark: Private watermarks are also known as secure watermarks. To read or retrieve such a watermark, it is necessary to have the secret key. C. Features of Digital Watermarking Various features of watermarking are as follows. Imperceptibility: Watermark cannot be seen by human eye or not be heard by human ear, only be detected through special processing. To preserve the quality of the marked document, the watermark should not noticeably distort the original document. Ideally, the original and marked documents should be perceptually identical. Robustness: Robustness refers to that the watermark embedded in data has the ability of surviving after a variety of processing operations and attacks. Robustness means that it must be difficult to defeat a watermark without degrading the marked document severely-so severely that the document is no longer useful or has no value. Security: A watermark system is said to be secure, if the hacker cannot remove the watermark without having full knowledge of embedding algorithm, detector and composition of watermark. A watermark should only be accessible by authorized parties. This requirement is regarded as a security and the watermark is usually achieved by the use of cryptographic keys. Watermark information owns the unique correct sign to identify, only the authorized users can legally detect, extract and even modify the watermark, and thus be able to achieve the purpose of copyright protection [13]. No reference to original document: For some applications, it is necessary to recover the watermark without requiring the original, unmarked document (which would otherwise be stored in a secure archive) [8]. Multiple watermarks: It may also be desirable to embed multiple watermarks in a document. For example, an image might be marked with a unique watermark each time it is downloaded [8]. Verifiability: Watermark should be able to provide full and reliable evidence for the ownership of copyright-protected information products. It can be used to determine whether the object is to be protected and monitor the spread of the data being protected, identify the authenticity, and control illegal copying [13]. Computational cost: In order to reduce computational cost, a watermarking method should be less complex. Watermarking methods with high complex algorithms will require more software as well as hardware resources and thus incur more computational cost. Computational simplicity usually preferred in resource-limited environments like mobile devices [13]. D. Types of Digital Watermarking Watermarking algorithms are divided into two categories. Spatial-domain techniques work with the pixel values directly. Spatial domain techniques are simple and fast. Frequencydomain techniques employ various transforms, either local or global. Transform domain techniques are more robust to attacks several widely recognized techniques are described subsequently. Spatial Domain Watermarking Methods In this technique, the watermark is inserted in the cover image changing pixels or image characteristics [2]. Least Significant Bit Modification (LSB): LSB is the most familiar technique in hiding a watermark in an image it depends on modifications done to the least significant bits of certain pixels in the image. LSB is classified as a spatial domain technique in which the watermark size is very smaller than the cover object, the watermark can be embedded repeatedly in certain places in the cover object, these marks maybe lost during the opponent attacks but part of the marks will resist the attacks. LSB use some kind of cryptography on the watermark message before the embedding, process [14]. Merits: Least Significant Bit algorithm is strong and less perceptible. Watermark is embedded into an image by replacing least significant bit to hide the information. Demerits: It suffers from many drawbacks such as highly sensitive to noise and easily destroyed. It can survive simple operation such as cropping, any addition of noise. However lossy compression is going to defeat the watermark. An even better attack is to set all the LSB bits to ‘1’ fully defeating the watermark at the cost of negligible perceptual impact on the cover object. Furthermore, once the algorithm was discovered, it would be very easy for an intermediate party to alter the watermark [1]. Frequency Domain Watermarking Method Compared to spatial-domain methods, frequency-domain methods are more widely applied. The produce of high quality watermarked image is by first transforming the original image into the frequency domain by the use of Discrete Fourier Transform (DFT), Discrete Cosine Transform (DCT), Discrete Wavelet transforms (DWT), Curvelet Transform method. With this technique, the marks are not added to the intensities of the image but to the values of its transform coefficients. Then inverse transforming the marked coefficients forms the watermarked image. The use of frequency based transforms allows the direct understanding of the content of the image; therefore, characteristics of the human visual system (HVS) can be taken into account more easily when it is time to decide the intensity and position of the watermarks to be applied to a given image. Some of its main algorithms are discussed below: Discrete Cosine Transform (DCT): DCT like a Fourier Transform, it represents data in terms of frequency space rather than an amplitude space. This is useful because that corresponds more to the way humans perceive light, so that the part that are not perceived can be identified and thrown away. DCT based watermarking techniques are robust compared to spatial domain techniques. Such algorithms are robust against simple image processing operations like low pass filtering, brightness and contrast adjustment, blurring etc. However, they are difficult to implement and are computationally more expensive. At the same time they are weak against geometric attacks like rotation, scaling, cropping etc. DCT domain watermarking can be classified into Global DCT watermarking and Block based DCT watermarking [3]. Embedding in the perceptually significant portion of the image has its own advantages because most compression schemes remove the perceptually insignificant portion of the image. Discrete Wavelet Transforms (DWT): Wavelet Transform is a modern technique frequently used in digital image processing, compression, watermarking etc. The transforms are based on small waves, called wavelet, of varying frequency and limited duration. The wavelet transform decomposes the image into three spatial directions, i.e. horizontal, vertical and diagonal. Hence wavelets reflect the anisotropic properties of HVS more precisely. Magnitude of DWT coefficients is larger in the lowest bands (LL) at each level of decomposition and is smaller for other bands (HH, LH, and HL). The Discrete Wavelet Transform (DWT) is currently used in a wide variety of signal processing applications, such as in audio and video compression, removal of noise in audio, and the simulation of wireless antenna distribution. Wavelets have their energy concentrated in time and are well suited for the analysis of transient, time-varying signals. Since most of the real life signals encountered are time varying in nature, the Wavelet Transform suits many applications very well [16]. One of the main challenges of the watermarking problem is to achieve a better tradeoff between robustness and perceptivity. Robustness can be achieved by increasing the strength of the embedded watermark, but the visible distortion would be increased as well [16]. However, DWT is much preferred because it provides both a simultaneous spatial localization and a frequency spread of the watermark within the host image [5]. The basic idea of discrete wavelet transform in image process is to multidifferentiated decompose the image into sub-image of different spatial domain and independent frequencies [15]. Advantages of DWT over DCT 1) Wavelet transform understands the HVS more closely than the DCT. ]2) Wavelet coded image is a multi-resolution description of image. Hence an image can be shown at different levels of resolution and can be sequentially processed from low resolution to high resolution. 3) Visual artifacts introduced by wavelet coded images are less evident compared to DCT because wavelet transform doesn't decompose the image into blocks for processing. At high compression ratios blocking artifacts are noticeable in DCI; however, in wavelet coded images it is much clearer. 4) DFT and DCI are full frame transform, and hence any change in the transform coefficients affects the entire image except if DCT is implemented using a block based approach. However DWT has spatial frequency locality, which means if signal is embedded it will affect the image locally [9]. Hence a wavelet transform provides both frequency and spatial description for an image. Disadvantages of DWT over DCT 1) Computational complexity of DWT is more compared to DCT' [12]. As Feig (1990) pointed out it only takes 54 multiplications to compute DCT for a block of 8x8, unlike wavelet calculation depends upon the length of the filter used, which is at least 1 multiplication per coefficient [3]. Discrete Fourier Transform (DFT): Transforms a continuous function into its frequency components. It has robustness against geometric attacks like rotation, scaling, cropping, translation etc. DFT shows translation invariance. Spatial shifts in the image affects the phase representation of the image but not the magnitude representation, or circular shifts in the spatial domain don't affect the magnitude of the Fourier transform. Advantages of DFT over DWT and DCT: DFT is rotation, scaling and translation (RST) invariant. Hence it can be used to recover from geometric distortions, whereas the spatial domain, DCT and the DWT are not RST invariant and hence it is difficult to overcome from geometric distortions [16]. Disadvantage of DFT over DWT and DCT: The main disadvantage of the DFT is that the output of the DFT is always in complex value and it requires more frequency rate. Its computational efficiency is very poor. So, the DFT not used because of these disadvantages. Curvelet Transform The Curvelet transform has provided stable, highly effective expression for image with the smooth irregularity curve. The Curvelet transform has anisotropy and the very strong directivity. It is more suitable for image processing than the wavelet and can represent smooth and edge of image with sparsity [7]. TABLE 1 : COMPARISON OF WATERMARKING TECHNIQUES Methods LSB DCT DWT DFT Merits 1. Easy to implement and understand 2. Low degradation of image quality 3. High perceptual transparency. 1. The watermark is embedded into the coefficients of the middle frequency, so the visibility of image will not get affected and the watermark will not be removed by any kind of attack. 1. Allows good localization both in time and spatial frequency domain. 2.Higher compression ratio which is relevant to human perception. DFT is rotation, scaling and translation (RST) invariant. Hence it can be used to recover from geometric distortions Demerits 1. It lacks basic robustness 2. Vulnerable to noise 3. Vulnerable to cropping, scaling. 1. Block wise DCT destroys the invariance properties of the system. 2. Certain higher frequency components tend to be suppressed during the quantization step. Curvelet transform Good invisibility and robustness Complexity is more and large amount of redundancy E. Issues in medical image watermarking Medical image watermarking is a special type of watermarking technique where the watermarked medical images should not differ perceptually from the original images, because the diagnosis must not be affected due to the presence of watermark. The biggest challenge in medical image watermarking is that, the image may not undergo any major degradation that will affect the quality of images with visible alteration to their original form [15]. The second issue is either the watermarks are reversible or permanent [7]. The third issue is if we decide to have watermarks for content authentication, compression should be distinguished from other manipulations. Conclusion Watermarking has its place in security. It is not intended to replace cryptography or steganography but supplement them. Watermarking technologies are aimed at Robust proof of copyright ownership. The most important use of watermarking techniques will probably be lying in hiding the data without visible to the viewer. In this survey most important aspect of medical image watermarking such as classification, features, techniques are studied. Medical image watermarking technique still has not matured. Though several techniques have been proposed still the research on Medical Image Watermarking has to continue to solve many watermarking issues. Digital watermarking seems to be the only potential encryption technology to provide protection even after data is decrypted. 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