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![]() Journal of Applied Mathematics and Physics, 2013, 1, 71-78 Published Online November 2013 (http://www.scirp.org/journal/jamp) http://dx.doi.org/10.4236/jamp.2013.15011 Open Access JAMP Flow Dynamics in Restricted Geometries: A Mathematical Concept Based on Bloch NMR Flow Equation and Boubaker Polynomial Expansion Scheme Omotayo Bamidele Awojoyogbe1*, Oluwaseun Michael Dada1, Ka r e m Bou b ak e r2, Omoniyi Adewale Adesola1 1Department of Physics, Federal University of Technology, Minna, Nigeria 2UPDS/ESSTT/63 Rue Sidi Jabeur 5100, Mahdia, Tunisia Email: *[email protected] Received August 4, 2013; revised September 25, 2013; accepted October 2, 2013 Copyright © 2013 Omotayo Bamidele Awojoyogbe et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT Computational techniques are invaluable to the continued success and development of Magnetic Resonance Imaging (MRI) and to its widespread applications. New processing methods are essential for addressing issues at each stage of MRI techniques. In this study, we present new sets of non-exponential generating functions representing the NMR transverse magnetizations and signals which are mathematically designed based on the theory and dynamics of the Bloch NMR flow equations. These signals are functions of many spinning nuclei of materials and can be used to obtain information observed in all flow systems. The Bloch NMR flow equations are solved using the Boubaker polynomial expansion scheme (BPES) and analytically connect most of the experimentally valuable NMR parameters in a simpli- fied way for general analyses of magnetic resonance imaging with adiabatic condition. Keywords: Bloch NMR Flow Equations; Boubaker Polynomial Expansion Scheme (BPES); Magnetic Resonance Imaging (MRI); Adiabatic Condition 1. Introduction Flow through porous media represents a vast field of study with many scientific and engineering applications [1-7]. A great number of experimental and theoretical studies on flow in restricted motion using NMR are avail- able in the literature [1-18]. Most of these studies are based on either numerical or approximation solutions of Bloch NMR equations. However, it will be fundamental and ideal if the theoretical and experimental application of MRI for flow analysis in restricted geometry is based on the ana- lytical solutions of Bloch NMR equations. This has been claimed over the years to be the best approach for ob- taining fundamental information to accurately access fluid dynamical properties in porous media/restricted geome- try. It is possible to derive necessary relationships ana- lytically for free motion. However, in the case of re- stricted motion for which porous media are defined, the macroscopic approach becomes mathematically intracta- ble. Thus, in general case, one is forced to use different method to find mathematical relation for the MRI signal in terms of NMR experimental parameters [19-21]. In this investigation, we solved the Bloch NMR flow equation which is transformable to Bessel equation of order zero using the Boubaker Polynomial Expansion Scheme to obtain the NMR transverse magnetization for the analysis of flow in anisotropic fluid flow. The rela- tionships between fluid velocity, the NMR relaxation rates and the path length x for cerebrospinal fluid, white and gray matter of human cerebrum are demonstrated. The Boubaker Polynomials Expansion Scheme BPES is a resolution protocol which has been successfully applied to several applied-physics and mathematics problems. Solutions have been proposed through the BPES in many fields such as numerical analysis [22-27], theoretical physics [24-29], mathematical algorithms [26], heat transfer [30,31], homodynamics [28,29], material char- acterization [32], fuzzy systems modeling [31] and boil- ogy [32,33]. *Corresponding author. ![]() O. B. AWOJOYOGBE ET AL. 72 2. Mathematical Analysis The BPES protocol ensures the validity of the related boundary conditions regardless of the main features of the equation. The BPES is mainly based on Boubaker polynomials first derivatives properties 4 10 4 1 20 0; q N q qx N q qxr Bx N Bx ; (1) and 4 10 4 11 22 4 13 4 41 d0 d d d 42 with: 4 q Nq qx NN q q qq xr n nn qn q nnn n n n Bx x Bx H x rr Br H Br r Br (2) In this investigation, the Boubaker Polynomials Ex- pansion Scheme BPES has been applied to the bound- ary-valued second order Bloch NMR flow differential equation through setting the expression: 0 4 1 0 1 2 N yk k kk M xB N xr (3) where My(x) are the time independent NMR transverse magnetizations, 4k are the 4k-order Boubaker poly- nomials, B 0,x1 is the normalized variable, k are minimal positive roots, is a prefixed integer r 4k B0 N and 0 1, , kkN are unknown pondering real coefficients. Based on the conditions which may conform to the real-time experimental arrangements, we obtained a sec- ond order non homogeneous differential equation from the Bloch NMR flow equation [34,35] at Larmor fre- quency: 0 o fB The x, y, z components (in the rotating frame) of the magnetization of a fluid moving with spatially varying velocity v is given by the Bloch equations which may be written as follows: 2 d d x x x 1 2 d d yy yz y M MM vgradMMB x tt T (5) 0 1 1 d d z zz zy MM MM v gradMM Bx tt T (6) Subject to the following conditions: 1) Mo Mz a situation which holds well in general and in particular when the RF B1(x) field is strong say of the order of 1.0 G or more. 2) Before entering signal detector coil, fluid particles has magnetization. Mx = 0, My = 0. 3) If B1(x) is large; 11GBx or more so that My of the fluid bolus changes appreciably from the equilib- rium magnetization Mo. denotes the gyromagnetic ratio of fluid spins; 2 is the RF excitation frequency; 0 f is the off-reso- nance field in the rotating frame of reference. T1 and T2 are the spin-lattice and spin-spin relaxation times respec- tively, the reciprocals of T1 and T2 are defined as relaxa- tion rates. Mo is the equilibrium magnetization and RF B1 is the spatially varying magnetic field [35] which may be designed as 1 Bx gx (7) where g is the field gradient. Equations (5) and (6) give a second order non-homogenous differential equation called the Bloch NMR flow equation: 2 00 1 222 1 dd d d yy y MM Sx TM M Bx vx xvvT (8) where 22 01 12 12 11 1 ,TSxBx TT TT In NMR systems, when the RF B1 field is applied, My has a maximum value when RF B1 has maximum ampli- tude and Mo ≈ 0. In biological systems especially at the molecular level we need to solve Equation (8) to provide velocity profiles for different tissues materials such that o v x vnTR T (9) o TTR n (10) where n is the number of pulses, TR is the repetition time. If is the time between two pulses, we write: 0 TR nT (11) x M MM v gradM tt T (4) For adiabatic condition, Equation (8) becomes: Open Access JAMP ![]() O. B. AWOJOYOGBE ET AL. 73 2 2 22 2 dd 0 d d yy y o MM x xT x g xM (12) where 22 1 12 1 Bx TT Equations (13) and (14) can be solved using the Bou- baker polynomial expansion scheme [21-23] with bound- ary conditions based on traditional NMR procedures. 2 2 2;1;2,3,4,5 dd 0 d d d0 01; 0; d yy n y n y y MM xxM x x M Mx (13) where β is a constant which is unique to the NMR system being described. For this system, the gradient field is chosen (under the condition 2 ) such that 2 o T g We define n as a dimensionless variable 2 o n fTx (14) where is a special flow property of the fluid (for this analysis 1 ), α is dimenssioless constant and f is a pro- perty of the medium. Equations (13) and (14) reduce to: 00 0 2 4 ,, 2 11 00 2 ,4 1 0 d 11 2 d 1 NN kk kk kn kn kk N knkk k Bxr Bxr NxN x Bxr N 4 d dx (15) The BPES solution is obtained by determining the non-null set of coefficients 0 1, , kkN that minimizes the absolute difference : 0 N 00 0,, 11 00 11 22 NN Nknkkn kk NN k (16) with: 0 2 1 24 2 0 2 1 4 4 1 00 dd d d 21 d d k kk k N k kkkk k k B rxrx x B rxrBxr xx N x (17) The final solution is: 0 ,, 2,3,4,5 1 0 1 2 N ynknk k nk 3. Analysis of Results From Equations (7), (9)-(13), we obtain for the value of α = 2, the following 2 o v xT (19) 2 2 o o nT n fTx v 2 (20) Tables 1-4 show how the fluid velocity and relaxation parameters changes with x for different human tissues materials at 1.5 T. The tables show the usefulness of BPES to different tissues on MRI scan. They can also be used to observe the same tissue materials at different locations. Figure 1 shows the NMR transverse magnetization when the value of x is small, high and very high for the Boubaker polynomial expansion scheme (BPES). The number of pulses n have more influence on the NMR signal when the value of x is small that when it is high. This can be useful to determine the number of pulses needed for a particular NMR experiment. Figures 2 and 3 show velocity profiles for different tissues materials. The color bands represent the different magnitude of the fluid velocity for different tissue. For example in Figure 2, the velocity profile is 0.030 m/s in cerebrospinal fluid while it is 0.25 m/s in gray matter for the same color band with the T1 and T2 relaxation rates providing tissue contrast. Table 1. Values of the path length, velocity and the relaxa- tion rate for cerebrospinal fluid at 1.5 T. 4 M xB N xr (18) Cerebrospinal Fluid x x2 τ = α/T0 T0 v 0.046114 0.002127 0.332266 6.019278 0.138786 0.044374 0.001969 0.307664 6.500591 0.144229 0.042634 0.001818 0.284009 7.042029 0.150115 0.040894 0.001672 0.261300 7.654041 0.156502 0.039154 0.001533 0.239537 8.349447 0.163457 0.037414 0.001400 0.218720 9.144115 0.171059 0.035674 0.001273 0.198849 10.05788 0.179402 0.033934 0.001152 0.179924 11.11578 0.188601 0.032194 0.001036 0.161946 12.34980 0.198795 0.030454 0.000927 0.144913 13.80134 0.210153 0.028714 0.000824 0.128827 15.52468 0.222888 0.026974 0.000728 0.113687 17.59216 0.237266 0.025234 0.000637 0.099493 20.10193 0.253626 0.023494 0.000552 0.086245 23.18975 0.272410 0.021754 0.000473 0.073943 27.04779 0.294199 0.000000 0.000000 0.000000 ∞ ∞ Open Access JAMP ![]() O. B. AWOJOYOGBE ET AL. 74 Table 2. Values of the path length, velocity and the relaxa- tion rate for gray matter of the cerebrum at 1.5 T. Gray Matter x x2 τ = α/T0 T0 v 0.046114 0.002127 1.329063 1.504819 0.034697 0.044374 0.001969 1.230657 1.625148 0.036057 0.042634 0.001818 1.136036 1.760507 0.037529 0.040894 0.001672 1.045200 1.913510 0.039126 0.039154 0.001533 0.958147 2.087362 0.040864 0.037414 0.001400 0.874880 2.286029 0.042765 0.035674 0.001273 0.795396 2.514469 0.044851 0.033934 0.001152 0.719698 2.778944 0.047150 0.032194 0.001036 0.647784 3.087451 0.049699 0.030454 0.000927 0.579654 3.450335 0.052538 0.028714 0.000824 0.515309 3.881169 0.055722 0.026974 0.000728 0.454748 4.398041 0.059316 0.025234 0.000637 0.397972 5.025483 0.063407 0.023494 0.000552 0.344980 5.797437 0.068102 0.021754 0.000473 0.295773 6.761946 0.159695 0.000000 0.000000 0.000000 ∞ ∞ Table 3. Values of the path length, velocity and the relaxa- tion rate for white matter of the cerebrum at 1.5 T. White Matter x x2 τ = α/T0 T0 v 0.046114 0.002127 1.772084 1.128615 0.026022 0.044374 0.001969 1.640877 1.218861 0.027043 0.042634 0.001818 1.514715 1.320380 0.028147 0.040894 0.001672 1.393599 1.435133 0.029344 0.039154 0.001533 1.277530 1.565521 0.030648 0.037414 0.001400 1.166506 1.714522 0.032074 0.035674 0.001273 1.060529 1.885852 0.033638 0.033934 0.001152 0.959597 2.084208 0.035363 0.032194 0.001036 0.863711 2.315588 0.037274 0.030454 0.000927 0.772872 2.587751 0.039404 0.028714 0.000824 0.687078 2.910877 0.041791 0.026974 0.000728 0.606331 3.298531 0.044487 0.025234 0.000637 0.530629 3.769112 0.047555 0.023494 0.000552 0.459973 4.348078 0.051077 0.021754 0.000473 0.394364 5.07146 0.055162 0.000000 0.000000 0.000000 ∞ ∞ Table 4. Values of the path length, velocity and the relaxa- tion rate for white matter of cystic tumor at 1.5 T. Cystic tumor x x2 τ = α/T0 T0 v 0.046114 0.002127 0.189866 10.53374 0.242876 0.044374 0.001969 0.175808 11.37603 0.252400 0.042634 0.001818 0.162291 12.32355 0.262701 0.040894 0.001672 0.149314 13.39457 0.273879 0.039154 0.001533 0.136878 14.61153 0.286050 0.037414 0.001400 0.124983 16.0022 0.299353 0.035674 0.001273 0.113628 17.60129 0.313954 0.033934 0.001152 0.102814 19.45261 0.330052 0.032194 0.001036 0.092541 21.61216 0.347891 0.030454 0.000927 0.082808 24.15235 0.367768 0.028714 0.000824 0.073616 27.16819 0.390054 0.026974 0.000728 0.064964 30.78629 0.415215 0.025234 0.000637 0.056853 35.17838 0.443846 0.023494 0.000552 0.049283 40.58206 0.476717 0.021754 0.000473 0.042253 47.33363 0.514848 0.000000 0.000000 0.000000 ∞ ∞ Based on Equation (19) and Tables 1-4, the analysis of fluid velocity, relaxation rates and the path length x can be described within the following three limits: 21 o n fTx (21) 21 o n fTx (22) 21 o n fTx (23) Equation (21) is the short time limit where the particle does not flow far enough during time o nT to feel the effect of parameter . When f 1, as shown in equation (22) some of the particles feel the effects of restriction and the value of measured within this time scale will be a function of o nT . Equation (23) shows that the time is long enough for all the particles to feel the effects of restriction and the displacement of the particle depends not on time o nT , but only on path length x. This indi- Open Access JAMP ![]() O. B. AWOJOYOGBE ET AL. Open Access JAMP 75 Figure 1. Plots of the NMR transverse magnetization against (a) Small values of x; (b) Higher values of x; (c) Much higher values of x for the Boubaker polynomial expansion scheme (BP ES). (a) (b) Figure 2. Plots of the fluid velocity against the relaxation rate and the path length x for cerebrospinal fluid and gray matter within the human brain at a static magnetic field of 1.5 T. ![]() O. B. AWOJOYOGBE ET AL. 76 (a) (b) Figure 3. Plots of the fluid velocity against the relaxation rate and the path length x for white matter of human cerebrum and cystic tumor at a static magnetic field of 1.5 T. cates that the value of n and not , in Equations (13) and (14) as solved by the Boubaker polynomial expansion scheme is very significant for the analysis of flow in re- stricted geometry where the measured fluid velocity de- pends of the relaxation parameters as shown in Figures 2 and 3. We may conclude that f, has a memory of the chemical differences within the spin’s immediate envi- ronment or the magnitude of the static magnetic field Bo. Therefore, selecting a particular value of x may corre- spond to selecting certain magnitude of Bo field or the molecular imprints of the tissue containing a flowing spin. The values of used in this study, is for computa- tional purposes. 4. Conclusion A mathematical concept of magnetic resonance imaging for flow analyses in restricted geometries has been pre- sented by solving the Bloch flow equation using the Boubaker polynomial expansion scheme (BPES). These demonstrate the usefulness of Bloch NMR flow equation and the Boubaker polynomial expansion scheme for studying fluid flow in restricted geometries to obtain the NMR transverse magnetization for the analyses of flow in anisotropic fluid flow. The relationship between fluid velocity, the NMR relaxation rates and the path length x for cerebrospinal fluid, white and gray matter of human cerebrum as demonstrated provides tissue contrast for different tissues materials. This can prove to be a very good starting point for building more sensitive and less expensive magnetic resonance imaging sequences. 5. Acknowledgements The authors acknowledge the support from Federal Uni- versity of Technology, Minna, Nigeria through the STEP B research programme of the World Bank. 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