https://doi.org/10.31449/inf.v47i10.5260 Informatica 47 (2023) 155–160 155 Variational Iteration Method for Solving Electrocardiography Inverse Problem Ebtihal Sabah Al-Bayati, Ahmed Farooq Qasem Department of Mathematics, College of Computer Science and Mathematics, University of Mosul, Mosul, Iraq E-mail: ebtehal.21csp19@student.uomosul.edu.iq , ahmednumerical@uomosul.edu.iq Keywords: bidomain model, variational iteration method, electrocardiography inverse problem Received: October 7, 2023 In this paper, the variational iteration method is proposed as a solution to an inverse problem in electrocardiography. The aim is to obtain approximate solutions to the model with the lowest error rate. This is crucial in determining the patient's heart activity and facilitating rapid medical intervention. The main challenges in accurately computing clinically relevant maps of cardiac depolarization stem from the ill-posed nature of the continuous problem and the presence of noise in the data. To tackle these difficulties, we have developed regularizing iterative algorithms based on domain decomposition techniques. These algorithms reformulate the inverse problem into several cases, depending on the solution area. This formulation has enabled us to establish a new stopping criterion that is more responsive and accurately reflects the behavior of the error on the non-accessible part of the boundary. The numerical results obtained through the variational iteration method demonstrate that the proposed approach effectively captures the error behavior on the non-accessible boundary. An additional advantage of these approaches is their ability to reduce execution time through their parallelized versions. Thus, we have successfully demonstrated the effectiveness of these methods in terms of quality (accuracy of approximation) and quantitative aspects (computation cost). Povzetek: Članek predstavlja variacijsko iteracijsko metodo za reševanje inverznih problemov v elektrokardiografiji z nizko stopnjo napake. 1 Introduction Due to the importance of mathematical simulation in science, several mathematical models were built for the electrical activity in the heart, including the bidomian model [1,2,3]. The heart muscle is anisotropic, the effects of these properties on heart tissue are discussed using the bidomain electrophoresis model, it is a microscopic model that describes the bioelectric behavior of the heart [4], The mathematical model of electrophysiology of the heart is a dynamic system that quantitatively describes the electrical processes occurring in the tissues of the heart [5]. The bidomain model is a system of nonlinear and convergent ordinary differential equations (ODEs), which used to model the gastric electrophysiology [6]. The use of the bidomain model is necessary for the correct modeling of the defibrillation response, and the error arising from the numerical solution of bidomain models is relatively small; various types of boundary condition can be imposed that can take into account the leakage of current to the surrounding tissues other than the myocardium [7]. In order for the model results to be useful, it is necessary to obtain accurate input of the models [8,9], Therefore, we will try to be precise in choosing the initial conditions to get better results and by using the Variational Iteration Method (VIM) which described and used to give approximate solutions for some non-linear problems [10,11,12], This method relying on The initial condition and based on Taylor series approximation, which applied as center difference, front difference, and back difference schemes[13], This method has the ability to reduce the volume of calculations and easily overcome the difficulty of the perturbation method[14]; Later, this method was used based on Laplace transformations to get the solution in the form of a series [15] , there is no need to linearize or treat the nonlinear terms to apply the method on nonlinear differential equations [16]. 2 Bidoman model It is a mathematical way to descript the electrical activity of heart which developed since the invention of the ECG measuring by Einthoren [17]. It is used to simulate cardiac electrical tissue computationally and to study the stimulation of cardiac tissue and defibrillation of the heart. The heart domain denoted by Ω and the conductivity tensor of intercellular and extracellular by 156 Informatica 47 (2023) 155–160 E.S. Al-Bayati et al. 𝜎 𝑖 ,𝜎 𝑒 , let 𝜈 𝑚 be atransmembrane potential and 𝑢 𝑖 , 𝑢 𝑒 be electric potential of intercellular and extracellular respectively , such that 𝜈 𝑚 =𝑢 𝑖 −𝑢 𝑒 . The bidomain model in two dimensional defined as: { 𝐴 𝑚 (𝐶 𝑚 𝜕 𝜈 𝑚 𝜕𝑡 +𝐼 𝑖𝑜𝑛 (𝜈 𝑚 ,𝑤 ))−𝑑𝑖𝑣 (𝜎 𝑖 ∇𝜈 𝑚 )=𝑑𝑖𝑣 ( 𝜎 𝑖 ∇ 𝑢 𝑒 ) 𝑖𝑛 Ω×]0,𝑇 [ 𝑑𝑖𝑣 (( 𝜎 𝑖 +𝜎 𝑒 )∇ 𝑢 𝑒 )=−𝑑𝑖𝑣 ( 𝜎 𝑖 ∇𝜈 𝑚 ) 𝑖𝑛 Ω×]0,𝑇 [ 𝜕 𝑡 𝑤 +𝑔 (𝜈 𝑚 ,𝑤 )=0 𝑖𝑛 Ω×]0,𝑇 [ 𝜎 𝑖 𝛻 𝜈 𝑚 .𝑛 =−𝜎 𝑖 𝛻 𝑢 𝑒 .𝑛 𝑜𝑛𝛴 ×]0,𝑇 [ ( 𝜎 𝑖 +𝜎 𝑒 )𝛻 𝑢 𝑒 .𝑛 =− 𝜎 𝑖 𝛻 𝜈 𝑚 .𝑛 𝑜𝑛𝛴 ×]0,𝑇 [ (1) Where 𝐴 𝑚 is defined to be the surface to volume ratio of the cell membrane, 𝐶 𝑚 the membrane capacitance, 𝐼 𝑖𝑜𝑛 is the current due to the ion exchange. 𝑔 (𝜈 𝑚 ,𝑤 ) is a function of fields having the same dimension of 𝑤 which is the concentrations of different chemical, 𝑛 is the outward unit normal on Ω, and 𝛴 = Г 𝑒𝑛𝑑𝑜 ∪ Г 𝑒𝑝𝑖 , Г 𝑒𝑛𝑑𝑜 internal boundaries in the rated endocardia and Г 𝑒𝑝𝑖 an outer boundaries, epicedial [18]. Figure 1: Cardiac field distribution 3 Variational iteration method The variational iteration method (VIM) proposed to find a numerical solution for linear and nonlinear equations by (He ,1998,1999) [10] , then it was developed to solve various types of integral equations and ordinary, partial and fractional differential equations. consider the general equation 𝐿 𝑢 +𝑁 𝑢 =𝑔 (𝑥 ) (2) where 𝐿 is linear operator , 𝑁 is the nonlinear operator and 𝑔 is inhomogeneous term. Assuming 𝑢 0 (𝑥 ) is the initial solution, then the variational iteration technique becomes: 𝑢 𝑛 +1 (𝑥 0 )=𝑢 𝑛 (𝑥 0 )+∫ 𝜆 (𝜉 ) ( 𝑥 0 0 𝐿 𝑢 𝑛 +𝑁 𝑢 𝑛 − 𝑔 )𝑑𝑥 (3) With 𝑢 0 as initial condition ,we can rewrite this equation as follow 𝑢 𝑛 +1 (𝑥 )=𝑢 𝑛 (𝑥 )+∫ 𝜆 (𝜉 ) ( 𝑥 0 𝐿 𝑢 𝑛 (𝜉 )+𝑁 𝑢 𝑛 (𝜉 )− 𝑔 (𝜉 ))𝑑𝜉 (4) Where 𝜆 is the general Lagrange multiplier [15] 𝜆 (𝜉 )= (−1) 𝑚 (𝑚 −1)! (𝜉 −𝑥 ) 𝑚 −1 (5) 𝑚 the highest order of the derivative, the final solution is u(𝑥 )= lim n→∞ 𝑢 𝑛 (𝑥 ). 4 Application The system (1) will be resolved using VIM in two cases: Case (1): The system (1) can be solved using VIM when 𝜆 =1 𝑖𝑛 Ω×]0,𝑇 [ { 𝑣 𝑚 𝑛 +1 =𝑣 𝑚 𝑛 +𝜆 ∫ (𝐴 𝑚 (𝐶 𝑚 𝜕 𝑣 𝑚 𝑛 𝜕𝑡 +𝐼 𝑖𝑜𝑛 (𝑣 𝑚 𝑛 ,𝑤 𝑚 𝑛 ))−𝑑𝑖𝑣 (𝜎 𝑖 𝛻 𝑣 𝑚 𝑛 )−𝑑𝑖𝑣 ( 𝜎 𝑖 𝛻 𝑢 𝑒 𝑛 ))𝑑𝑡 𝑡 0 𝑢 𝑒 𝑛 +1 =𝑢 𝑒 𝑛 +(𝑢 𝑒 𝑛 (t,0,y)+ 𝜕 (𝑢 𝑒 𝑛 (t,0,y) 𝜕𝑥 x )− 1 (𝜎 𝑖 +𝜎 𝑒 ) (𝜎 𝑖 𝑣 𝑚 𝑛 −𝜎 𝑖 𝑣 𝑚 𝑛 (t,0,y)−𝜎 𝑖 𝜕 𝑣 𝑚 𝑛 (t,0,y) 𝜕𝑥 x) + 𝜆 (𝜎 𝑖 +𝜎 𝑒 ) ∫ ∫ (−𝜎 𝑖 𝜕 2 𝑣 𝑚 𝑛 𝜕𝑦 2 −(𝜎 𝑖 +𝜎 𝑒 ) 𝜕 2 𝑢 𝑒 𝑛 𝜕𝑦 2 )𝑑𝑥 𝑑𝑥 𝑥 0 𝑥 0 𝑤 𝑚 𝑛 +1 =𝑤 𝑚 𝑛 +𝜆 ∫ 𝜕 𝑤 𝑚 𝑛 𝜕𝑡 +𝑔 (𝑣 𝑚 𝑛 ,𝑤 𝑚 𝑛 ))𝑑𝑡 𝑡 0 (6) with the initial conditions [18] 𝑣 𝑚 0 (0,x,y)=𝑣 0 (𝑋 ),𝑤 𝑚 0 (0,x,y)=𝑤 0 (𝑋 ) (7) 𝐼 𝑖𝑜𝑛 (𝜈 𝑚 ,𝑤 )= 𝑊 𝑇 𝑖𝑛 𝑉 2 (𝑉 −1)− 𝑉 𝑇 𝑜𝑢𝑡 , 𝑔 (𝜈 𝑚 ,𝑤 )={ 𝑤 −1 𝑇 𝑜𝑝𝑒𝑛 𝑖𝑓 𝑣 ≤𝑣 𝑔𝑎𝑡𝑒 𝑤 𝑇 𝑐𝑙𝑜𝑠𝑒𝑑 𝑖𝑓 𝑣 >𝑣 𝑔𝑎𝑡𝑒 (8) Where 𝑇 𝑖𝑛 ≤𝑇 𝑜𝑢𝑡 ≤𝑇 𝑜𝑝𝑒𝑛 ,𝑇 𝑐𝑙𝑜𝑠𝑒𝑑 and 0< 𝑣 𝑔𝑎𝑡𝑒 <1 , are given positive constants. Using iterative formula (6) with the initial conditions: Z { 𝑢 𝑒 0 =𝑥 6 +3𝑥 4 𝑦 2 +3𝑥 2 𝑦 4 +𝑦 6 −𝑥 2 𝑦 3 −3𝑥 4 −6𝑥 2 𝑦 2 −3𝑦 4 +3𝑥 2 +3𝑦 2 −1 𝑢 𝑖 0 =2𝑥 6 +6𝑥 4 𝑦 2 +6𝑥 2 𝑦 4 +2𝑦 6 −2𝑥 2 𝑦 3 −6𝑥 4 −12𝑥 2 𝑦 2 −6𝑦 4 +6𝑥 2 +6𝑦 2 −2 𝑣 𝑚 0 =𝑥 6 +3𝑥 4 𝑦 2 +3𝑥 2 𝑦 4 +𝑦 6 −𝑥 2 𝑦 3 −3𝑥 4 −6𝑥 2 𝑦 2 −3𝑦 4 +3𝑥 2 +3𝑦 2 −1 𝑤 𝑚 0 = 1 (𝑣 𝑚 0 ) 2 (𝑣 𝑚 0 −1) (9) Then the first iterative formula { 𝑢 𝑒 1 =ℎ+ 𝑟𝑡 𝑠 𝑢 𝑖 1 =2ℎ+2 𝑟𝑡 𝑠 𝑣 𝑚 1 =h+ 𝑟𝑡 𝑠 𝑤 𝑚 1 = 1 ℎ+( 𝑟𝑡 𝑠 ) 2 (ℎ−1+ 𝑟𝑡 𝑠 ) (10) where { 𝑟 =𝑥 6 𝑇 𝑖𝑛 +3𝑥 4 𝑦 2 𝑇 𝑖𝑛 +3𝑥 2 𝑦 4 𝑇 𝑖𝑛 +𝑦 6 𝑇 𝑖𝑛 −𝑥 2 𝑦 3 𝑇 𝑖𝑛 −3𝑥 4 𝑇 𝑖𝑛 −6𝑥 2 𝑦 2 𝑇 𝑖𝑛 −3𝑦 4 𝑇 𝑖𝑛 +3𝑥 2 𝑇 𝑖𝑛 +3𝑦 2 𝑇 𝑖𝑛 +𝑇 𝑜𝑢𝑡 𝑇 𝑖𝑛 −𝑇 𝑖𝑛 −𝑇 𝑜𝑢𝑡 𝑠 =𝑇 𝑖 𝑛 𝐶 𝑚 𝑇 𝑜𝑢𝑡 ℎ=𝑥 6 +3𝑥 4 𝑦 2 +3𝑥 2 𝑦 4 +𝑦 6 −𝑥 2 𝑦 3 −3𝑥 4 −6𝑥 2 𝑦 2 −3𝑦 4 +3𝑥 2 +3𝑦 2 −1 j=26.4t−13.2𝑥 2 𝑦𝑡 +79.2𝑥 4 𝑡 +79.2𝑦 4 𝑡 +158.4𝑥 2 𝑦 2 𝑡 −105.6𝑦 2 𝑡 −4.4𝑦 3 𝑡 −105.6𝑥 2 𝑡 and at the boundary conditions { [ 2ℎ 2ℎ−12(2𝑦 2 +6𝑥 2 + 1 3 𝑦 3 −𝑦 4 −6𝑥 2 𝑦 2 −5𝑥 4 −1)𝑡 … 2ℎ 2ℎ−12(6𝑦 2 +2𝑥 2 +𝑥 2 𝑦 −5𝑦 4 −6𝑥 2 𝑦 2 −𝑥 4 −1)𝑡 … ] [ ℎ 2 ⁄ ( ℎ 2 ⁄ )+(−66𝑥 4 −79.2𝑥 2 𝑦 2 −13.2𝑦 4 +4.4𝑦 3 +79.2𝑥 2 +26.4𝑦 2 −13.2)𝑡 … ℎ 2 ⁄ ( ℎ 2 ⁄ )+(−13.2𝑥 4 −79.2𝑥 2 𝑦 2 −66𝑦 4 +13.2𝑥 2 𝑦 +26.4𝑥 2 +79.2𝑦 2 −13.2)𝑡 … ] Variational Iteration Method for Solving Electrocardiograph… Informatica 47 (2023) 155–160 157 The general form { 𝑉 𝑉𝐼𝑀 =𝑥 6 +3𝑥 4 𝑦 2 +3𝑥 2 𝑦 4 +𝑦 6 −𝑥 2 𝑦 3 −3𝑥 4 −6𝑥 2 𝑦 2 −3𝑦 4 +3𝑥 2 +3𝑦 2 −1+ℎ+ 𝑟𝑡 𝑠 +⋯ 𝑢 𝑉𝐼𝑀 =𝑥 6 +3𝑥 4 𝑦 2 +3𝑥 2 𝑦 4 +𝑦 6 −𝑥 2 𝑦 3 −3𝑥 4 −6𝑥 2 𝑦 2 −3𝑦 4 +3𝑥 2 +3𝑦 2 −1+ℎ+𝑗 +⋯ 𝑤 𝑉𝐼𝑀 = 1 ℎ 2 +(ℎ−1) + 1 ℎ+( 𝑟𝑡 𝑠 ) 2 (ℎ−1+ 𝑟𝑡 𝑠 ) +⋯ (11) The exact solution for system (1) with initial conditions (9) and the values 𝑇 𝑜𝑝𝑒𝑛 =1 ; 𝑇 𝑐𝑙𝑜𝑠𝑒𝑑 =1 ; 𝑇 𝑖𝑛 =1 ; 𝑇 𝑜𝑢𝑡 =1 ; 𝐶 𝑚 =1; 𝜎 𝑖 =1 ;𝜎 𝑒 =0.2 is 𝑉𝑚 =((𝑥 2 +𝑦 2 −1) 3 −𝑥 2 𝑦 3 )𝑒 𝑡 (12) Table 1: Demonstrates comparison absolute error of numerical solution using VIM with the exact solution when 𝑛 =3,𝑥 ∈[−1.5,1.5];𝑦 ∈[−1.5,1.5];𝑡 =0.25. 𝑽 𝒆𝒙𝒂 𝒄 𝒕 𝑽 𝑽𝑰𝑴 𝑬𝒓𝒓𝒐𝒓𝒆 𝑽𝑰𝑴 55.77659477 55.77296875 3.626020×10 −3 19.00253201 19.00459321 2.061200×10 −3 4.944311443 4.948546790 4.235347×10 −3 0.9159855499 0.9208438900 4.858340×10 −3 0.1857372348 0.1907085100 4.971275×10 −3 0.1036097736 0.09859375000 5.016024×10 −3 0.6066703134 0.6015764900 5.093823×10 −3 1.040166975 1.035006110 5.160865×10 −3 1.040189078 1.035028210 5.160868×10 −3 0.6120414440 0.6069467900 5.094654×10 −3 Figure 2: Solution path for system (1) using VIM We notice from the table (1) and figure (2), that the numerical solution approaches the exact solution based on the parameters found using system (11), where the absolute error rate reaches 10 −3 with few iterations n=3, Whereas the interval that is approved is [−1.5,1.5]x[−1.5, 1.5]. Case 2: If we Segmentation the area of heart into an internal and an external area only, after substituting the second equation into the first and third equations of the system (1), we get the following system 𝑖𝑛 Ω×]0,𝑇 [ : { 𝐴 𝑚 (𝐶 𝑚 𝜕 𝜈 𝑚 𝜕𝑡 +𝐼 𝑖𝑜𝑛 (𝜈 𝑚 ,𝑤 ))+𝑑𝑖𝑣 ( 𝜎 𝑒 𝛻 𝑢 𝑒 )=0 𝑖𝑛 Ω×]0,𝑇 [ 𝜕𝑤 𝜕𝑡 +𝑔 (𝜈 𝑚 ,𝑤 )=0 𝑖𝑛 Ω×]0,𝑇 [ (13) and on the boundaries of the heart in figure (1), the system (1) will be as: { 𝐴 𝑚 (𝐶 𝑚 𝜕 𝜈 𝑚 𝜕𝑡 +𝐼 𝑖𝑜𝑛 (𝜈 𝑚 ,𝑤 ))=0 𝑜𝑛 𝛴 ×]0,𝑇 [ 𝜕𝑤 𝜕𝑡 +𝑔 (𝜈 𝑚 ,𝑤 )=0 𝑜𝑛 𝛴 ×]0,𝑇 [ (14) The systems (13) and (14) can be solved via VIM with the initial conditions (7) and: 𝐼 𝑖𝑜𝑛 (𝜈 𝑚 ,𝑤 )=𝑔 (𝜈 𝑚 ,𝑤 )=𝜈 𝑚 −𝑤 (15) Then { 𝑣 𝑚 𝑛 +1 =𝑣 𝑚 𝑛 +𝜆 ∫ (𝐴 𝑚 (𝐶 𝑚 𝜕 𝑣 𝑚 𝑛 𝜕𝑡 +𝐼 𝑖𝑜𝑛 (𝑣 𝑚 𝑛 ,𝑤 𝑚 𝑛 )) −𝑑𝑖𝑣 (𝜎 𝑒 𝛻 𝑢 𝑒 𝑛 ))𝑑𝑡 𝑡 0 𝑖𝑛 Ω×]0,𝑇 [ 𝑤 𝑚 𝑛 +1 =𝑤 𝑚 𝑛 +𝜆 ∫ 𝜕 𝑤 𝑚 𝑛 𝜕𝑡 +𝑔 (𝑣 𝑚 𝑛 ,𝑤 𝑚 𝑛 ))𝑑𝑡 𝑡 0 𝑖𝑛 Ω×]0,𝑇 [ (16) 158 Informatica 47 (2023) 155–160 E.S. Al-Bayati et al. At the boundary conditions, the system (14) becomes: { 𝑣 𝑚 𝑛 +1 =𝑣 𝑚 𝑛 +𝜆 ∫ (𝐴 𝑚 (𝐶 𝑚 𝜕 𝑣 𝑚 𝑛 𝜕𝑡 +𝐼 𝑖𝑜𝑛 (𝑣 𝑚 𝑛 ,𝑤 𝑚 𝑛 )))𝑑𝑡 𝑡 0 𝑖𝑛 𝛴 ×]0,𝑇 [ 𝑤 𝑚 𝑛 +1 =𝑤 𝑚 𝑛 +𝜆 ∫ 𝜕 𝑤 𝑚 𝑛 𝜕𝑡 +𝑔 (𝑣 𝑚 𝑛 ,𝑤 𝑚 𝑛 ))𝑑𝑡 𝑡 0 𝑖𝑛 𝛴 ×]0,𝑇 [ (17) The exact solution for system (1) with initial conditions (9) and the values 𝑇 𝑜𝑝𝑒𝑛 =1 ; 𝑇 𝑐𝑙𝑜𝑠𝑒𝑑 =1 ; 𝑇 𝑖𝑛 =1 ; 𝑇 𝑜𝑢𝑡 =1 ; 𝐶 𝑚 =1; 𝜎 𝑖 =1 ;𝜎 𝑒 =0 is 𝑉𝑚 =((𝑥 2 +𝑦 2 −1) 3 −𝑥 2 𝑦 3 )𝑒 𝑡 (18) At the boundaries of the heart, the boundary conditions towards 𝑥 are used when 𝑥 =𝑥 0 with 𝑦 =𝑦 0 , 𝑦 = 𝑦 𝐿 in system (17), while for the boundaries of the heart towards 𝑦 , then the boundary conditions towards 𝑦 are used when 𝑦 =𝑦 0 with 𝑥 =𝑥 0 , 𝑥 = 𝑥 𝐿 in system (17). Table 2: Demonstrates comparison absolute error of numerical solution using VIM with the exact solution when 𝑛 =3,𝑥 ∈[−1.5,1.5];𝑦 ∈[−1.5,1.5];𝑡 =0.25. 𝑽 𝒆𝒙𝒂𝒄𝒕 𝑽 𝑽𝑰𝑴 𝑬𝒓𝒓𝒐𝒓𝒆 𝑽𝑰𝑴 45.66601344 45.66601758 3.941900×10 −4 15.55795734 15.55795875 1.343000×10 −4 4.048059831 4.048060198 3.494200×10 −5 0.7499455391 0.7499456071 6.473700×10 −6 0.1520687861 0.1520688000 1.312600×10 −6 0.0848285079 0.0848285156 7.322400×10 −7 0.4966996426 0.4966996875 4.287600×10 −6 0.8516166907 0.8516167679 7.351300×10 −6 0.8516347875 0.8516348647 7.351300×10 −6 0.5010971524 0.5010971978 4.325500×10 −6 Figure 3: Solutions path for system (1) at the boundary conditions using VIM. We note from the table (2), the approximation solution is significantly improved when dividing the domain solution area of the heart, bringing the absolute error to 10 −6 . 5 Conclusion Using simulation to form a system of differential equations and solve them numerically offers the potential to reduce the error rate in disease diagnosis and avoid excessive treatment dosages. This approach is widely applicable across different fields, including electrocardiogram simulation. 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