Analysis of Photoplethysmographic Signals Analysis of Photoplethysmographic Signals Modified by Reversible Watermarking Modified by Reversible Watermarking Technique using Prediction-Error in Technique using Prediction-Error in

Wireless TelecardiologyWireless TelecardiologyPresented byDr. Sheli Sinha Chaudhuri

CO-AUTHORS – ANAMITRA BARDHAN RAY, NILANJAN DEY, SHOUVIK BISWAS, DR. ACHINTYA DAS

Introduction

Doctors and medical practitioners often require to exchange medical signal between various hospitals and diagnostic centers for mutual availability of diagnostic and therapeutic case studies.

To increase the level of reliability and security while exchanging data between hospitals or diagnostic centres, watermarking technique can be used to hide electronic patient record (EPR) and logo of hospitals within the bio-medical signals.

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This paper emphasizes on evaluating the minutiae’s of PPG signal along with ECG signal and prediction error based reversible watermarking for higher security while achieving least distortion of the PPG signal to reveal diagnostic medical details.

Second part of our work involves a comparative study of Pulse Transit Time (PTT), Distance and PWV between the recovered PPG signal and different levels of wavelet based de-noised recovered PPG signal is done.

Photoplethesmography

• The word ‘Photoplethysmography’ basically came from the Greek word ‘plēthusmos’ which stands for enlargement and ‘graphein’ which means write.

• Photoplethysmography (PPG) is a non-invasive method to detect cardio-vascular pulse wave that propagates through the body using a light source and a detector.

• It is capable of measuring relative blood volume changes in the blood vessels close to the skin.

• Blood Pressure (BP) is directly proportional to the rate at which the blood travels in arteries and flow rate is calculated from pulse wave velocity.

As it is hard to measure Pulse Wave Velocity, it can be replaced by Pulse Transit Time (PTT) and can be related to BP.

• PTT can be measured easily by PPG test instead of traditionally used Echocardiography(ECG).

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Fig 1: Extracted PPG pulse amplitude (AMP) and timing (PTTf) characteristics, with cardiac timing reference ECG. 

The bit of the image is to be embedded by a predictor in a selected cell of 2D matrix with intensity .The predictor operates on a three-neighbour context and predicts its intensity x’.

The contexts are chosen as follows:

Prediction-Error Calculation

The prediction function works as,

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The prediction-error, will be

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Watermark Embedding:

The embedded cell intensity is calculated from the original intensity, prediction-error and fractional value of bit .

Watermark Extraction:At the decoder end the predictor predicts

the intensity of the embedded cell, which will be (from the same neighbourhood context). The prediction-error at the decoder end is,

Contd. …

The embedded bit, i is extracted as following,

The original cell intensity is restored after calculating true prediction error,

Proposed Method

Watermark Embedding :Watermark Embedding :

Step 1 : PPG signal is taken and converted into 2D matrix

Step 2 : 2D matrix is divided into 2x2 sub matrix.

Step 3: Apply prediction-error algorithm on individual sub matrices.

Step 4: Watermark binary image is embedded in the 2D matrix.

Step 5: Embedded watermarked 2D matrix is reshaped into 1D matrix as Watermarked PPG signal.

Proposed Method

Watermark Watermark ExtractionExtraction : :

Step 1: Watermarked PPG signal is reshaped into 2D matrix.

Step 2: 2D matrix is divided into 2x2 sub matrix.

Step 3: The prediction error algorithm is reapplied.

Step 4: The watermark from the 2D matrix is extracted.

Step 5: After extraction of the watermark, the resultant 2D matrix is reshaped into 1D matrix as recovered PPG signal.

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PPG signal Corresponding with ECG Signal :

Step 1:   The PTT, Distance, PWV, amplitude between the original ECG signal and the Original PPG Signal

is measured.

Step 2: The PTT, Distance, PWV, amplitude between the original ECG signal and the Recovered PPG

Signal is measured.

Step 3: Decision of diagnostic value changes is taken based on the measured values.

Result and Discussions

Fig 2 (a) : Original ECG signal and PPG Signal

Fig 2 (b) : Original Watermark

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(c) Watermarked PPG Signal

(d) Recovered PPG Signal

(e) Recovered Watermark

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Figure 3. (a) R Peak detected ECG & Primary peak detected Original PPG Signal, (b) R Peak detected ECG & Primary peak detected Watermarked PPG Signal, (c) R Peak detected ECG & Primary peak detected Recovered PPG Signal.

(a) (b)

(c)

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Advantages of the Proposed Method

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In this proposed technique of watermarking in PPG signal, PSNR between the original PPG signal and watermarked PPG signal is 155.1545 and the PSNR between the original PPG Signal and the extracted PPG Signal is 105.7002 .

Correlation between original Watermark image and recovered Watermark Image is 1, which implies no change in the recovered watermark.

Conclusion

As pulse rate is approximately same as that of heart rate, it can be measured from PPG signal which is a simpler process.

Proposed technique of reversible blind-Watermarking Technique is useful in telecardiology applications for authentication of the source of the information.

The values of correlation and PSNR are very much encouraging regarding the accuracy of the recovered image and quality of reconstruction of the PPG signal, respectively, resulting to delivery of the aforesaid information with a much higher relevance sustaining with high security.

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