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    Mathematical Biosciences 269 (2015) 1–9

    Contents lists available at ScienceDirect

    Mathematical Biosciences

     journal homepage: www.elsevier.com/locate/mbs

    Bioheat transfer problem for one-dimensional spherical

    biological tissues

    Emmanuel Kengne∗, Ahmed Lakhssassi

    Département d’informatique et d’ingénierie, Université du Qué bec en Outaouais, 101 St-Jean-Bosco, Succursale Hull, Gatineau (PQ) J8Y 3G5, Canada

    a r t i c l e i n f o

     Article history:

    Received 30 March 2015

    Revised 6 July 2015

    Accepted 20 August 2015

    Available online 1 September 2015

    Keywords:

    Pennes bioheat transfer model

    Point-heating source

    Bioheat transfer problems

    Tumors

    Spherical living biological bodies

    a b s t r a c t

    Based on the Pennes bioheat transfer equation with constant blood perfusion, we set up a simplified one-

    dimensionalbioheat transfer model of the spherical living biological tissues for application in bioheat transferproblems. Using the method of separation of variables, we present in a simple way the analytical solution of 

    the problem. The obtained exact solution is used to investigate the effects of tissue properties, the cooling

    medium temperature, and the point-heating on the temperature distribution in living bodies. The obtained

    analytical solution can be useful for investigating thermal behavior research of biological system, thermal

    parameter measurements, temperature field reconstruction and clinical treatment.

    © 2015 Elsevier Inc. All rights reserved.

    1. Introduction

    Spatiotemporal temperature distribution in living biological tis-

    sues plays a vital role in many physiological processes. Investigationof bioheat transfer problems requires the evaluation of temporal and

    spatial distributions of temperature. This class of problems has been

    traditionally addressed using the Pennes bioheat equation [1]. Scien-

    tific research in the bioheat transfer research field has paved a key

    foundation in hyperthermia cancer therapy, thermal diagnosis, cryo-

    genic surgery etc.   [2–8].  The quantitative, qualitative, and accurate

    analysis of bioheat transfer is to effectively understand and model

    the heat transfer mechanism of the biological system.

    Heat transfer analysis on thermal medical problems, such as the

    thermal diagnostics [9] and thermal comfort analysis [10,11], thermal

    parameter estimation [12–16], or burn injury evaluation [17], usually

    has to simultaneously face the transient or spatial heating both on

    skin surface and in interior of the biological bodies. The complexity

    underlying in this class of medical problems remains not only for its

    heterogeneity and anisotropy but also for conduction, convection,

    and radiation heat flow, cell’s metabolism, and blood perfusion etc.

    Therefore, to obtain a flexible solution, which is capable of solving

    any one of the above thermal medical problems, is very desirable.

    Indeed, analytical solutions reflect actual physical feature of the

    models and can be used as standards to verify the corresponding

    numerical results and as a proof to the reasonability of in-vitro mode

    ∗   Corresponding author. Tel.: +18196430402.

    E-mail address: [email protected][email protected] (E. Kengne).

    analysis. Although people relied too much on numerical approaches

    such as finite difference method (FDM), finite element method (FEM),

    and boundary element method (BEM) for solving thermal medical

    problems, the analytical solutions, if they can be obtained, areoften preferred [18,19]. Analytical solutions are very attractive since

    their efficiency depends weakly on the dimensions of the problem,

    in contrast to the numerical methods. Knowing analytical solution,

    temperature at a desired point at a given time canbe performed inde-

    pendentlyfrom that of the other points within thedomain, which can

    be an asset when temperatures are needed at only some isolated sites

    or times. It is also important to point out that analytical solutions

    of bioheat transfer problems will save computational time greatly,

    which is valuable in some hyperthermia practices. Motivated by the

    importance of exact solutions of bioheat transfer problems in hy-

    perthermia cancer therapy, thermal diagnosis, or cryogenic surgery,

    we aimed in this paper to present analytical solutions to the Pennes’

    bioheat model in one-dimensional spherical coordinate system with

    relatively complex boundary or volumetric heating conditions [20].

    Various techniques have been performed to obtain exactsolutions

    of bioheat transfer problems in one dimensional Cartesian coordinate

    [8,21–27]. In our knowledge, a combination of the method of separa-

    tion of variables with the Green’s function method has not yet been

    applied for explicitly solving bioheat transfer problems for spherical

    symmetry. In this paper, we combine the Fourier method (method

    of separation of variables) with the Green’s function method to de-

    rive the exact solution of one-dimensional model of the spherical liv-

    ing tissue. Such a combination of the two methods has been used

    by Durkee and Antich [28,29] when addressing the time-dependent

    Pennes’ equation in 1-D multi-region Cartesian and spherical

    http://dx.doi.org/10.1016/j.mbs.2015.08.012

    0025-5564/© 2015 Elsevier Inc. All rights reserved.

    http://dx.doi.org/10.1016/j.mbs.2015.08.012http://www.sciencedirect.com/http://www.elsevier.com/locate/mbsmailto:[email protected]:[email protected]:[email protected]://dx.doi.org/10.1016/j.mbs.2015.08.012http://dx.doi.org/10.1016/j.mbs.2015.08.012mailto:[email protected]:[email protected]://crossmark.crossref.org/dialog/?doi=10.1016/j.mbs.2015.08.012&domain=pdfhttp://www.elsevier.com/locate/mbshttp://www.sciencedirect.com/http://dx.doi.org/10.1016/j.mbs.2015.08.012

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    E. Kengne, A. Lakhssassi/ Mathematical Biosciences 269 (2015) 1–9   3

    Fig. 2.   Steady-state temperature fields prior to heating for tissue properties given in the text. (a) Effect of the blood perfusion on the steady-state temperature distribution. (b)

    Effect of the tissue thermal conductivity on the steady-state temperature distribution.

    third BC. Through using the following transformation:T (r , t ) = u(r , t )+   f (t ),   (5)

    Eq. (1) takes the following form:

    ρc 

    k

    ∂u

    ∂t  = 1

    r 2∂

    ∂r 

    r 2∂u

    ∂r 

    − αu + F ,   (6)

    where

    F (r , t ) = α(T a −   f (t ))+qm + Q 

    k  − ρc 

    k

    df 

    dt  .   (7)

    The corresponding boundary and initial conditions are then ex-

    pressed as

    k∂u

    ∂r 

    r =0 =0,   (8a)

    k∂u

    ∂r 

    r =R1

    + h f  u|r =R1 = 0   (8b)

    u(r , 0) = T 0(r )−   f (0).   (8c)Solving problems (6) and (8a)–(8c) with the method of separation of 

    variables combined with the Green’s function method  [8]  and using

    Eq. (5) yield

    T (r , t ) =   f (t )−  f (0)+ exp−kαρc 

    T 0(r )

    +k

    ρc    t 

    0 dτ    R1

    0 G(r , t ; ξ , τ)dξ ,   (9)where

    G(r , t ; ξ ,τ) = exp

    ρc (τ  − t )

    +∞n=1

    F (ξ , τ)

    U n2sin [λnξ ] sin [λnr ]

    ξ r   ,

    (10)

    λn are positive roots of equation

    tan [λR1] −λkR1

    k − R1h f = 0,   (11)

    and

    U n

    2

    =λnSi[2λnR1]

    sin2

    [λnR1]

    R1.

    3. Results and discussion

    In this section, we discuss the effect of major thermal parameters

    on thetemperature distribution of spherical living tissues. In oursim-

    ulations,the typical tissueproperties are applied as given in [9,32,33].

    The apparent heat convection coefficient due to natural convec-

    tion and radiation is taken as  ha = 10 W/(m2 ◦C),  while the forcedconvection coefficient is applied as   h f  = 100 W/(m2 ◦C)   and thesurrounding fluid temperature is chosen as   T e = 25 ◦C. Further, asdemonstrated in many works [34,35], the interior tissue temperature

    usually tends to a constant within a short distance such as 2–4 cm.

    Therefore 0.02 m ≤ L ≤  0.04 m will be used in our study. Curves of Fig. 2 depict the steady-state temperature distributions of biological

    bodies associated with the above typical tissue properties.  Fig. 2(a)

    and(b) shows theeffect of theblood perfusion andthe tissue thermal

    conductivity on the steady-state temperature distribution. Obviously,

    the steady-state temperature (initial temperature) of the tissue de-

    creases from the body core to the skin surface. It is seen from Fig. 2(a)

    that the steady-state temperature of the tissue increases as the blood

    perfusion decreases; moreover,  Fig. 2(a) shows that the blood per-

    fusion has significant effects on the temperature of the body core.

    Curves of  Fig. 2(b) show that the steady-state temperature near the

    body core (near the skin surface) decreases as the tissue thermal con-

    ductivity increases (increases with the tissue thermal conductivity).

    In our investigations, we study the temperature distribution when

    the biological body is subjected either to a point-heating source or to

    the most typical heat source that the heat flux decays exponentially

    with the distance from the skin surface.

     3.1. Temperature distribution under a point-heating source

    In our analysis, we first discuss the temperature response subject

    to a point-heating. Practical examples for such heating can be found

    in clinics where heat is depositedthough insertinga conducting heat-

    ing probein the deep tumor site. Here, the point-heating source to be

    used is [8]

    Q (r , t ) = P 1(t )δ(r − r 0),   (12)

    where P 1(t ) is thepoint-heating power, δ(r − r 0) is theDiracfunction,and r 0 is theposition of thepoint-heating source. FollowingAnderson

    and Burnside [36], a sinusoidal point-heating power

    P 1(t ) = q0 + qw cos [ω0t ]   (13)

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    4   E. Kengne, A. Lakhssassi / Mathematical Biosciences 269 (2015) 1–9

    will be used. Here,  q0  and  qw  are the constant term and the oscilla-

    tion amplitude of sinusoidal point-heating power, respectively, and

    ω0   is heating frequency. It is important to point out that the spatialsinusoidal heating was also proposed  [8]  to measure the blood per-

    fusion where the heat was deposited to the biological body using ul-

    trasound, and the temperature response was monitored at the skin

    surface. As the time-dependent temperature of the cooling medium,

    we will consider a sinusoidal temperature as follows:

     f (t ) = q0 f  + qw f  cosω0 f t 

    ,

    where q0 f  and qwf  are theconstantterm andthe oscillation amplitude,

    respectively, of sinusoidal temperature of the cooling medium, and

    ω0 f  is the temperature frequency.Under the above considerations, Eq. (10) becomes

    G(r , t ; ξ , τ) = exp

    ρc (τ  − t )

    +∞n=1

    1

    U n2

    P 1(τ)

    k

    sin [λnr 0]

    r 0

    + sin [λnξ ]ξ 

    αT a − α f (τ)+

    qmk  − ρc 

    k

    df 

    dt  (τ)

    sin [λnr ]

    r   .   (14)

    Therefore, the temperature response is obtained from  Eqs. (9)  and

    (14):

    T (r , t ) =   f (t )−  f (0)+ exp−kαρc 

    T 0(r )+

    +∞n=1

     g I n(t )

    U n2sin [λnr ]

    r   ,

    (15)

    where

     g I n(t ) = R1qw sin [λnr 0]r 0

    k2α2 +ω20ρ2c 2kα cos [ω0t ] +ω0ρc sin [ω0t ]

    − kα exp−kαρc 

    +

    R1q0 sin [λnr 0]

    αkr 0+ Si[λnR1]

    ×αkT a + qm − kαq0 f αk

    1 − exp

    −kαρc 

    +qw f Si[λnR1]

    exp

    −kαρc 

    − cos

    ω0 f t 

    .

    Fig. 3   depicts temporal temperature distributions in the tissues

    under a surface point-heating with the sinusoidal heating power

    P 1(t ) = 2500+ 2450 cos[0.02t ] at r 0 0. The sinusoidal temperature

    Fig. 3.   Temperature distribution at three positions when a surface point-heating with the sinusoidal point-heating power and a sinusoidal temperature of the cooling medium are

    adopted.

    Fig. 4.   Temperature distribution at different times when a surface point-heating with the sinusoidal point-heating power  P 1(t ) = 1500 + 1450 cos [0.02t ]  W/m3 is applied. A

    sinusoidal temperature of the cooling medium  f (t ) = 25+ cos [0.02t ] is used.

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    E. Kengne, A. Lakhssassi/ Mathematical Biosciences 269 (2015) 1–9   5

    Fig. 5.   Effect of some biological parameter on the temperature distribution for typical tissue parameters shown in Table 1. (a) Influence of the tissue density ρ . (b) Influence of the

    tissue thermal conductivity k.

     Table 1

    Typical material properties of the tissue.

    Paramete rs Val ue and uni t

    ρ, ρb   1000 [kg/m3]

    c, c b   4200 [J/(kg°

    C)]T a   37 [°C]

    k   0.5 [W/(m °C)]

    ωb   0.0005 [ml/(s ml)]

    qm   33,800 [W/m3]

    of the cooling medium   f (t ) = 25 + cos [0.02t ]   °C is used. Due to thesurface cooling by the flowing medium, the lowest tissue tempera-

    ture occurs at the skin surface ( x = L = 0.03) while the highest tissuetemperature reaches at the body core.

    Fig. 4 depicts the temperature distributions at time  t  = 100 s of biological bodies subject to a surface sinusoidal point-heating and

    sinusoidal temperature of the cooling medium. The surface point-

    heating is applied at r 0 = 0.01. Due to the surface cooling by the flow-ing medium, the tissue temperature decreases as both the time and

    the depth increase. At a given time, the position for the local high-

    est temperature is just stayed at the site of the point source. This is

    very beneficial for hyperthermia therapy since one can then selec-

    tively heat the deep regional tumor.

    Fig. 5   illustrates the temperature distribution at time

    t  = 100 s for the tissues subjected to point source   P 1(t ) =2500+ 2450 cos[0.02t ]   W/m3,   r 0 = 0.015 m under the sinusoidalcooling medium temperature   f (t ) = 25 + cos [0.02t ]   °C for the

    typical biological parameters shown in   Table 1.   The effect of the

    tissue density ρ and the effect of the tissue thermal conductivity k onthe temperature distribution are illustrated in Fig. 5(a) and Fig. 5(b),

    respectively. For each value of either the tissue density ρ or the tissuethermal conductivity k, the position for the local highest temperature

    is just stayed at the site of the point source. Plots of  Fig. 5(a) and (b)show that the position for the local highest temperature depends

    both on the tissue density and the tissue thermal conductivity. It

    is seen from Fig. 5(a) that the higher the tissue density, the higher

    the temperature near the skin surface. The effect of the tissue

    thermal conductivity on the temperature distribution is illustrated

    in Fig. 5(b); this figure indicates that the thermal conductivity has

    insignificant effect on the temperature distribution at the body core

    as it varied between 0.48 W/(m   °C) and 0.54W/(m   °C). The curves

    of  Fig. 5(b) indicate that the higher the tissue thermal conductivity,

    the lower the temperature near the body core and the higher the

    temperature near the skin surface. Such a phenomenon in thermal

    distribution of biological body produces a distinguishable elevation

    in either the skin surface or the body core temperature and is a tool

    for analyzing benign stage tumour.Fig. 6 depicts the influence of tissue specific heat  c  on the tem-

    perature distribution at time  t  = 100 s when tissue was subjectedto point-heating source Q (r , t ) = P 1(t )δ(r − 0.015), and the tempera-ture of cooling medium is f (t ) = 25 ◦C. It shows that, the larger tissuespecific heat, the higher temperature increases.

    Fig. 7 shows the temperature distributions at time  t  = 100 s forthe tissues subjected to constant point source  P 1(t ) = 2500 W/m3,

    Fig. 6.   Effect of the tissue specific heat  c  on the temperature distribution in biological tissues subject to a constant point-heating under a constant temperature of the cooling

    medium.

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    6   E. Kengne, A. Lakhssassi / Mathematical Biosciences 269 (2015) 1–9

    Fig. 7.   Influence of cooling medium temperature to tissue temperature distribution.

    Fig. 8.   Influence of heating power on tissue temperature distribution.

    r 0 = 0.018 m and under different cooling medium temperatures. Aswe can see from plots of this figure, the larger the temperature of the

    cooling medium, the lower the body core temperature decreases. It is

    also seen from this figure that themagnitude and position of thelocal

    highest temperature are changeless on the whole. This means that

    an optimum heating can be obtained through regulating the surface

    cooling.

    We show in Fig. 8 the influence of heating power on the temper-

    ature distribution when tissue was subjected to a constant point-

    heating source   Q (r , t )=

    P 1(t )δ(r −

    0.018),   and the temperature of 

    cooling medium is   f (t ) = 25 ◦C. It shows that although the effectof the heating power on the temperature distribution is negligible,

    the larger the heating power, the higher the temperature increases.

    Moreover, the position for the local highest temperature is fixed for

    all these heatings.

    Fig.9 gives outthe effects of thecooling medium temperature (top

    plot) and the effects of the heating power (bottom plot) on the tem-

    perature transients at skin surface. Curves of the top panel are ob-

    tained when the biological tissue was subjected to a constant point-

    heating source  Q (r , t ) = 2500δ(r − 0.018). To generate the plots of the bottom panel, we used the constant cooling medium temper-

    ature   f (t ) = 25 ◦C. As it is seen from the curves of the top panel,the skin surface temperature decreases when the cooling medium

    temperature increases. The bottom panel of  Fig. 9 indicates that, the

    larger the heating power, the higher the skin surface temperature

    increases.

     3.2. Temperature distribution under the most typical heating source

    In this section, we study the temperature distribution when the

    tissue is subjected to the typical heating source for which the heat

    flux decays exponentially with the distance from the skin surface. In

    other words, we work with the spatial heating [37]

    Q (r , t ) = ηP 0(t )exp [−ηr ],   (16)

    where   P 0(t ) is the time-dependent heating power on skin surface

    and η  is the scattering coefficient. We focus on the constant heat-ing power (P 0(t ) = q p = constant), reflecting the situation where thehuman skin was heated by a laser.

    In the present situation, the temperature response is obtained

    from Eqs. (9) and (14):

    T (r , t ) =   f (t )−  f (0)+ exp−kαρc 

    T 0(r )+

    +∞n=1

     g II n(t )

    U n2sin [λnr ]

    r   ,

    (17)

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    E. Kengne, A. Lakhssassi/ Mathematical Biosciences 269 (2015) 1–9   7

    Fig. 9.   Influence of cooling medium temperature (top) and influence of heating power (bottom) on the skin surface temperature response.

    where

     g II n(t ) =

    kαT a + qm − kαq0 f 

    Si[λnR1]

    αk

    1 − exp

    −kαρc 

    − qw f Si[λnR1]

    cosω0 f t 

    − exp

    −kαρc 

    + q pηαk

    1 − exp

    −kαρc 

       R10

    exp [−ηξ ] sin [λnξ ]ξ 

      dξ .

    Fig. 10 shows the effect of the heating power  P 0(t ) (top) and theeffect of the scattering coefficient  η  to the skin surface temperatureresponse. To generate the curves of this figure, we use the constant

    cooling medium temperature   f (t ) = 25 ◦C and the tissue propertiesshown in Table 1. The toppanelof Fig. 10 shows the transient temper-

    atures of tissues subject to four constant heating with η = 200 m−1[37].  Obviously, the larger the heating power, the higher the tem-

    perature increases. Such information is valuable for thermal comfort

    evaluation. The bottom panel of  Fig. 10 gives out the effects of the

    scattering coefficient η on the temperature transients at skin surfacefor  P 0(t ) = 250 W/m2. The plots of this figure show that the largerthe coefficient, the higher the temperature decreases. Although the

    scattering coefficient   η   seems to have insignificant effect on thetemperature distribution on the skin surface, the bottom panel of 

    Fig. 10 reveals that the tissue temperature increases as the scattering

    coefficient η  decreases. Because different heating apparatus such aslaser or microwave may have different power   P 0(t ) and scattering

    coefficient   η, we conclude that the above results are expected tobe useful for the heating-dose planning during the hyperthermia

    treatment or parameter estimation.

    Fig. 11 is the transient temperature at three positions of biological

    bodies subject to a constant spatial heating with  P 0(t ) = 250 W/m2and η = 200 m−1 [37]. Curves, A, B and  C  are the transient tempera-tures of tissues at positions r  = 0.01 m, r  = 0.015 m, and r  = 0.025 m.Plots of  Fig. 11 indicate that the tissue temperature decreases as the

    tissue depth increases; this means that the highest temperature oc-curs at the body core, while the lowest temperature occurs at the skin

    surface. This situation is due to the surface cooling by the flowing

    medium. It is also seen from this figure that the temperature at any

    given depth decreases as a function of time.

    4. Conclusion

    In this work, the bioheat transfer problems for the spherical liv-

    ing biological tissues have been investigated based on 1-D Pennes

    model, and the analytical solution of the corresponding equation

    has been found with the help of the method of separation of vari-

    ables. The obtained exact solution of the problem has been used to

    study the tissue temperature distribution in radial direction. Our re-

    sults show that the scattering coefficient  η  and the heating powers

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    8   E. Kengne, A. Lakhssassi / Mathematical Biosciences 269 (2015) 1–9

    Fig. 10.   Effects of the heating power (top) and scattering coefficient (bottom) on temperature response at skin surface for   f (t ) = 25   °C. Top:  η = 200 m−1;   Bottom:  P 0(t ) =250 W/m2.

    Fig. 11.   Transient temperatures at three positions whena constant spatial heating with P 0(t ) = 250W/m2 and η = 200m−1 was applied. The cooling medium temperature is takento be  f (t ) = 25 ◦C.

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    E. Kengne, A. Lakhssassi/ Mathematical Biosciences 269 (2015) 1–9   9

    P 0(t ) and  P 1(t ) of the spatial heating are neglected for further anal-

    ysis of the temperature distribution, though the tissue density, the

    tissue specific heat and the cooling medium temperature are to be

    take into account when investigating temperature distribution in the

    biological bodies. The solution of the bioheat problem presented in

    this paper is found to be very useful for a variety of bio-thermal

    studies.

     Acknowledgment

    This work was supported by the Natural Sciences and Engineering

    Research Council of Canada.

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