SECURE WATERMARKING TECHNIQUE FOR MEDICAL IMAGES WITH VISUAL EVALUATION

Signal & Image Processing : An International Journal (SIPIJ) Vol.9, No.1, February 2018
DOI : 10.5121/sipij.2018.9101 1
SECURE WATERMARKING TECHNIQUE FOR
MEDICAL IMAGES WITH VISUAL EVALUATION
Majdi Al-qdah
Department of Computer Engineering, University of Tabuk, Tabuk, KSA
ABSTRACT
This paper presents a hybrid watermarking technique for medical images. The method uses a combination
of three transforms: Discrete Wavelet Transform (DWT), Discrete Cosine Transform (DCT), and singular
value decomposition (SVD). Then, the paper discusses the results of applying the combined method on
different medical images from eight patients. The images were watermarked with a small watermark image
representing the patients' medical data. The visual quality of the watermarked images (before and after
attacks) was analyzed using five quality metrics: PSNR, WSNR, PSNR-HVS-M, PSNR-HVS, and MSSIM.
The first four metrics' average values of the watermarked medical images before attacks were
approximately 32 db, 35 db, 42 db, and 40 db respectively; while the MSSM index indicated a similarity
between the original and watermarked images of more than 97%. However, the metric values decreased
significantly after attacking the images with various operations even though the watermark image could be
retrieved after almost all attacks. In brief, the initial results indicate that watermarking medical images
with patients' data does not significantly affect their visual quality and they can still be used by medical
staff.
KEYWORDS
Transforms, Watermarking, medical images, visual metrics
1. INTRODUCTION
Data hiding has increasingly become an important tool in authentication of images and protection
of rightful owners copyright. Also, there is an increasing need to store and transfer patients'
medical images over the Internet and other computer networks for sharing among medical staff in
medical institutions all over the world. Image watermarking techniques that hides important
details inside cover images can be divided into two broad domains: spatial domain and frequency
domain [1, 2]. Various medical images based watermarking schemes have been proposed in
literature [3,4,5]. Three of the most important frequency watermarking methods are the discrete
cosine transform (DCT), discrete wavelet transform (DWT) and Singular Value Decomposition
(SVD). Many researchers have used a hybrid of two or more transforms in order to compensate
for the shortcomings of various transforms.
There are many examples of spatial domain techniques such as LSB substitution, spread
spectrum, and patchwork. Lin et al. [6] proposed a spatial watermarking methods where the
watermark logo is fused with noise bits first, and then XORed with the feature value of the image
by 1/T rate forward error correction (FEC), where T is the times of data redundancy. The
watermark bits are extracted by majority voting.
Rosiyadi et al.[7] proposed a robust hybrid watermarking method based on DCT and SVD. The
DCT is applied on the host image using the zigzag space-filling curve (SFC) for the DCT
coefficients and then the SVD is applied on the DCT coefficients. Horng et al. [8] proposed a

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robust adaptive watermarking method based on DCT, SVD and Genetic Algorithm (GA). The
host image luminance masking is used and the mask of each sub-band area is transformed into
frequency domain. Subsequently, the watermark image is embedded by modifying the singular
values of DCT-transformed host image with singular values of mask coefficients of host image
and the control parameter of DCT-transformed watermark image using GA. Singh et al. [9]
proposed a robust hybrid watermarking technique using DWT, DCT, and SVD. First, the host
image into first decomposed by DWT and the Low frequency band (LL) and watermark image
are transformed using DCT and SVD. Then the S vector of watermark image is embedded in the
S component of the host image and the watermarked image is generated by inverse SVD on
modified S vector and original U, V vectors followed by inverse DCT and inverse DWT.
2. METHODOLOGY
The following sections will give details of the used watermarking algorithm and evaluation
metrics.
2.1. Watermarking algorithms
The designed and implemented algorithm is a combination of three frequency domain techniques:
discrete wavelet transform (DWT), discrete cosine transform (DCT), and singular value
decomposition (SVD). DWT decomposes an image into frequency channels of constant
bandwidth on a logarithmic scale by separating an image into a set of four non-overlapping multi-
resolution sub bands denoted as lower resolution approximation image (LL), horizontal (HL),
vertical (LH) and diagonal (HH) with the availability of multiple scale wavelet decomposition.
The watermark is usually embedded into the high frequency detail sub-bands (HL, LH and HH
sub-band) because the human visual system (HVS) is sensitive to the low-frequency LL part of
the image. We can usually embed sensitive data such as medical information in higher level sub-
bands since the detail levels carry most of the energy of the image [10]. DWT achieves higher
robustness since it has the characteristics of space frequency localization, multi-resolution
representation, multi-scale analysis, adaptability and linear complexity [11].
Also, DCT has a very good energy compaction property. It separates the image into different
low, high, and middle frequency coefficients [12]. The watermark is embedded in the middle
frequency band that gives additional resistance to the lossy compression techniques with less
modification of the cover image. The DCT coefficients D(i, j) matrix of an image (N x M) with
pixel intensity I(x, y) are obtained as follows:
SVD of a rectangular matrix is a decomposition of the form
Where is a M x N matrix, U and V are orthonormal matrices, and S is a diagonal matrix
comprised of singular values of . The singular values are
unique values that appear in descending order along the main diagonal of S. They are obtained
by taking the square root of the Eigen values of and The U, V are not unique. In
the Singular Value Decomposition, the slight variations of singular values do not affect the visual

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perception of the cover image, which achieves better quality of the watermarked image and better
robustness against attacks. Also, singular values represent the intrinsic algebraic image properties
[12].
Figure 1 shows the approach taken in embedding the patients' data into a cover medical image;
First, DCT is applied on the LL component of the DWT transformed cover image; SVD is applied
to the DCT coefficients. Then, the watermark is DCT transformed and the singular values of the
SVD transformed coefficients are embedded in the singular values of the DWT transformed
coefficients of the cover image. Figure 2 shows the extraction approach of the patient's image
data from the watermarked image. The watermarked images is DWT and DCT transformed then
SVD is applied to the DCT coefficients; the watermark is extracted from the LL sub band of
DWT. For an added security, the watermark image can be encrypted before embedding it in the
cover image.
Figure1. Embedding process
Figure 2. Extraction process
2.2. Evaluation metrics
Four visual metrics (WSNR, MSSIM, PSNR-HVS-M, and PSNR-HVS) described by
Ponomarenko et. al. [13] are used for comparing the watermarked images with their originals.
Traditionally, the efficiency of an image processing operation ; i.e. lossy compression is usually
analyzed in terms of rate-distortion curves. These curves represent dependencies of PSNR (or
MSE) on bits per pixel (bpp) or compression ratio (CR) where PSNR and MSE are calculated for
some original image and the corresponding processed image.

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where denote the values of the original and processed pixels and N, M denote an image size
[14]. In order to obtain a high imperceptibility of the watermarked image, it is desirable to have a
high value of PSNR; meaning a lesser value of MSE.
Also, usually the similarity and differences between an original image and a processed image is
measured by the Normalized Correlation (NC). Its value is generally 0 to 1. Ideally it should be 1
but a value 0.7 or higher is usually acceptable [15].
where denote the values of the original and processed pixels and X, Y denote an image size.
Two different distorted images with the same PSNR value with respect to the same original image
may give significantly different visual impact. It is well known that conventional quality metrics,
such as MSE, SNR and PSNR do not always correlate with image visual quality [17,18].
Therefore, the choice of a proper visual quality metric for analysis and comparisons is always
problematic since the human visual system (HVS) is nonlinear and it is very sensitive to contrast
changes and to noise [19]. Many studies have confirmed that the HVS is more sensitive to low
frequency distortions rather than high frequency components. The best performance was achieved
by the metrics PSNR-HVS-M, PSNR-HVS, and WSNR [14] especially if there is noise or the
images are to be compressed. HVS-based models are the result of trade-off between
computational feasibility and accuracy of the model. HVS-based models can be classified into
two categories: neurobiological models and models based on the psychophysical properties of
human vision. Psychophysical HVS-based models are implemented in a sequential process that
includes luminance masking, colour perception analysis, frequency selection, and contrast
sensitivity [19].
Recently, processing of images is done using perceptual image quality assessment methods,
which attempt to simulate the functionality of the relevant early human visual system (HVS)
components. These methods usually involve a pre-processing process that may include image
alignment, point-wise nonlinear transform, low-pass filtering that simulates eye optics, and color
space transformation, a channel decomposition process that transforms the image signals into
different spatial frequency as well as orientation selective subbands, an error normalization
process that weights the error signal in each subband by incorporating the variation of visual
sensitivity in different subbands, and the variation of visual error sensitivity caused by intra- or
inter-channel neighbouring transform coefficients, and an error pooling process that combines the
error signals in different subbands into a single quality/distortion value [20].
PSNR-HVS takes into account the HVS properties such as sensitivity to contrast change and
sensitivity to low frequency distortions; while the PSNR-HVSM takes into account the contrast
sensitivity function (CSF). Similar to PSNR and MSE, the visual quality metrics PSNR-HVS and
PSNR-HVSM can be determined:
where I,J denote image size, K=1 [(I-7)(J-7)64] , are DCT coefficients of 8x8 image
block for which the coordinates of its left upper corner are equal to i and j, Xij e are the DCT
coefficients of the corresponding block in the original image, and is the matrix of
correcting factors [21].

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The Weighted Signal to Noise Ratio (WSNR) is a noise metric where the difference (residual)
between the original and the processed images must be noise. (WSNR) uses a Contrast Sensitivity
Function (CSF) given by the following:
where is a radial angular frequency
The WSNR between an original image (x) and a processed image (y) is:
The structural similarity index (SSIM) measures the similarity between two images [19]. SSIM
compares two images using information about luminous, contrast and structure. SSIM metric is
calculated on various windows of an image. The measure between two windows x and y of
common size N×N is given as follows:
MSSIM (Multi-Scale Structural Similarity) is a multi-scale extension of a SSIM metric. MSSIM
[22] is introduced to incorporate the variations of viewing conditions to the previous single-scale
SSIM measure. MSSIM is known as mean structural similarity index metric [22] and it is given
by:
where M is the correlation between two images x, y
Correlation is a similarity measure between two functions. The correlation measure between two
functions x(x,y) and s(x,y) in discrete form is defined as:
Where is the complex conjugate, x=0, 1,…….., M-1 and y=0, 1,……, N-1
3. RESULTS
Figure 3 shows the eight medical cover images of size [512×512] and the patients' data
watermark image of size [256×256] selected for the experiment. The medical images contain
medical information based on the characteristics of each image and the purpose of its capture.
The medical images reveal characteristics of the bones, tissues, vessels, nerves....etc. For example,
the finger print image shows the shape and size of the prints while the ultrasound image shows the size and
shape of the fetes. Thus, embedding a watermark image inside a medical cover image should
preserve the existing medical information in the cover medical image: the unique pattern of the
fingerprint, vessels and optical nerves inside the retina, bone fracture in the wrist, size and
development signs of the fetus, shape, the position of the torn ligament, and sliced layers and soft
tissue of the human skull. The patients' personal details can be embedded in the captured medical
image in textual or image format and saved in one file. The patients' personal details (watermark)
are embedded by the earlier discussed combined method of DWT, DCT, and SVD transforms;
while the imperceptivity of the watermarked images is evaluated using PSNR, P-HVS, P-HVS-M,
WSNR, and MSSIM. The metrics measure the imperceptivity of the watermarked images, which

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is an important factor in medical images watermarking. The experiment was run under
MATLAB simulation software.
Retina Broken wrist Fingerprint
Teeth Mammogram Torn ligament
Ultrasound Head
Watermark
Figure 3. Eight cover images and one watermark
The algorithm was evaluated using five quality metrics. Table 1 shows the PSNR, P-HVS, P-
HVS-M, WSNR, and MSSIM metrics among all the watermarked images before any attacks. It
can be observed that the PSNR average value is about 32 db, P-HVS average value is around 35
db, P-HVS-M average value is about 42 db, and the WSNR average value varies from 35 db to 47
db. The MSSIM metric shows that the watermarked images are highly visually similar to the
original images with similarity index values between the original and the watermarked images of
more than 0.97%. Also, it can be observed that there is no significant difference between the
average metric values among the various images; only the WSNR value of the of the Head image
varies from one image to another with approximately 15 db difference between the Fingerprints
image and the Head image; that is mainly due to the characteristics of the two images.

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Table 1. Metric values of the watermarked images with watermark "Copyright"- not attacked
Image PSNR P-HVS P-HVS-M WSNR MSSIM
Fingerprints 32.7049 34.8745 46.2079 47.0602 0.9920
Retina 32.9101 34.8738 40.4924 38.0317 0.9740
Torn Ligament 32.9784 34.8868 42.2467 39.5283 0.9846
Broken Wrist 32.7310 34.9020 40.7815 43.3029 0.9734
Teeth structure 32.7048 34.8898 41.4563 45.4571 0.9793
Ultrasound 33.2059 34.8428 41.3834 37.8052 0.9850
Head 33.3870 35.1103 40.0242 34.3916 0.9770
Mammogram 32.6940 34.8750 41.1925 46.0111 0.9738
To test the robustness, the watermarked image were attacked with various types of attacks.
Tables 2 shows the average values of the same metrics for each image after the watermarked
images are attacked with various operations (Gaussian noise, Salt & Pepper noise, 2D FIR filter,
Cropping, Rotation & Cropping, Weiner filter, Intensity adjustment, Gaussian filter, and
Sharpening). ). It is observed that the numerical values decrease after an attack operation is
performed on the images. Thus, there is a degradation in the quality of the attacked images. The
drop in the numerical values is not significant after the Gaussian Noise, Salt & Pepper Noise, and
2D FIR filter attacks. The PSNR and other HVS metric values are similar among all watermarked
images before and after attacks. The values of PSNR, P-HVS, P-HVS-M, and WSNR stay above
the value of 20 db and the MSSIM metric values remain above 0.82%. On the other hand, there
is a significant decrease in the values after the Cropping, Rotation & Cropping, Weiner Filter,
Intensity adjustment, Gaussian filter, and Sharpening image attack operations. The numerical
values of PSNR, P-HVS, P-HVS-M, and WSNR drop to less than 6 db while the MSSIM
similarity index drops to 10% approximately. The watermark images can be clearly recovered
after the Gaussian noise, Salt & Pepper noise, Intensity adjustment, Gaussian filter, and
Sharpening attacks; but the recovered watermarks are distorted after the 2D FIR filter, Rotation &
Cropping, and Weiner filter attacks. Even though the images are apparently distinguishable after
those attacks the metric values drop significantly. Finally, there is no correlation between the
drop in the metric values and the recovery of the watermark; for example, the P-HVS, P-HVS-M,
and the WSNR values drop greatly after the sharpening attack but the watermark is fully
recovered.
Table 2. Average metric values of all eight watermarked images after some attacks
Attack PSNR P-HVS P-HVS-M WSNR MSSIM
No attack 32.9878 34.9307 41.7779 40.1183 0.9803
Gaussian Noise 19.9203 19.9790 22.6201 27.0916 0.8212
Salt & Pepper Noise 24.6345 24.8935 27.9674 32.1470 0.9304
2D FIR filter 25.3646 26.6690 30.0951 35.1960 0.9618
Cropping 13.7111 9.5336 9.5670 8.1109 0.7391
Rotation & Cropping 5.9136 1.7664 1.7862 0.2728 0.0982
Weiner Filter 5.9212 1.7732 1.7950 0.2801 0.1029
Intensity adjustment 5.9431 1.7932 1.8150 0.3004 0.1113
Gaussian filter 5.9212 1.7743 1.7950 0.2802 0.1030
Sharpening 5.9214 1.7733 1.7951 0.2801 0.1031

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The limitation of this research is that the algorithms cannot determine how much of medical
information is lost after watermarking medical images or even after attacking the images. Only
medical doctors can decide the important segments of a medical image that are affected by
watermarking or by attacking. Also, the effects can vary from one image to another. Finally,
recovering the watermark after some attacks does not necessarily indicate that all medical
information is preserved.
4. CONCLUSIONS
The results of this limited research show that watermarking medical images with a watermark of
patients' personal details does not significantly affect the visual quality of the original medical
images; and they can be utilized for their medical purpose. It was experimentally quantitatively
demonstrated using Human Visual System (HVS) metrics that the watermarked medical images
were similar to their originals. Also, choosing the appropriate watermarking algorithm is essential
to obtain the robustness, imperceptivity and security needed to protect the patients' personal data
inside a medical image and there are many transform domain algorithms that are available and
can be utilized to preserve the characteristics of the original images. Artificial intelligence
methods will be used in the future to classify the effectiveness of new algorithms.
ACKNOWLEDGEMENTS
The authors would like to acknowledge financial support of this work from the Deanship of
Scientific Research (DSR), University of Tabuk, Tabuk, Saudi Arabia, under grant no.
S/0180/1438
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AUTHOR
Dr. Majdi is currently an assistant professor in department of computer engineering at the University of
Tabuk Saudi Arabia. His research interests include data hiding, cryptography, medical imaging, and other
various other current engineering topics.

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