Model of electrical conductivity of skeletal muscle based on tissue structure

Recent experiments carried out in our laboratory with the four-electrode method showed that the electrical conductivity of skeletal muscle tissue depends on the frequency of the injected current and the distance between the current electrodes. A model is proposed in order to study these effects. The model takes into account the structure of the tissue on the scale of individual fibres. It discerns three main components with respect to electrical properties: (a) extracellular medium with electrical conductivity σ_e; (b) intracellular medium with electrical conductivity σ_i; (c) muscle fibre membrane with impedance Z_m. The model results show an apparent frequency dependence of the electrical conductivity of skeletal muscle tissue, as well as the way the conductivity is affected by the length the current is conducted.     

Conductance Method for the Measurement of Cross-Sectional Areas of the Aorta

A modified conductance method to determine the cross-sectional areas (CSAs) of arteries in piglets was evaluated in vivo. The method utilized a conductance catheter having four electrodes. Between the outer electrodes an alternating current was applied and between the inner electrodes the induced voltage difference was measured and converted into a conductance. CSA was determined from measured conductance minus parallel conductance, which is the conductance of the tissues surrounding the vessel times the length between the measuring electrodes of the conductance catheter divided by the conductivity of blood. The parallel conductance was determined by injecting hypertonic saline to change blood conductivity. The conductivity of blood was calculated from temperature and hematocrit and corrected for maximal deformation and changes in orientation of the erythrocytes under shear stress conditions. The equations to calculate the conductivity of blood were obtained from in vitro experiments. In vivo average aortic CSAs, determined with the conductance method CSA _(G) in five piglets, were compared to those determined with the intravascular ultrasound method CSA_(IVUS). The regression equation between both values was CSA _(G) =–0.09+1.00·CSA_(IVUS) r=0.97, n=53. The mean difference between the values was –0.29% · 5.57% (2 standard deviations). We conclude that the modified conductance method is a reliable technique to estimate the average cross-sectional areas of the aorta in piglets. © 1999 Biomedical Engineering Society. PAC99: 8780-y, 8437+q, 8719Nn     

Ex vivo biomechanical behavior of abdominal aortic aneurysm: Assessment using a new mathematical model

Knowledge of the biomechanical behavior of abdominal aortic aneurysm (AAA) as compared to nonaneurysmal aorta may provide information on the natural history of this disease. We have performed uniaxial tensile testing of excised human aneurysmal and nonaneurysmal abdominal aortic specimens. A new mathematical model that conforms to the fibrous structure of the vascular tissue was used to quantify the measured elastic response. We determined for each specimen the yield σ_y and ultimate σ_u strengths, the separate contribution to total tissue stiffness by elastin (E _E) and collagen (E _C) fibers, and a collagen recruitment parameter (A), which is a measure of the tortuosity of the collagen fibers. There was no significant difference in any of these mechanical properties between longitudinal and circumferential AAA specimens, nor inE _E andE _C between longitudinally oriented aneurysmal and normal specimens.A, σ_y, and σ_u were all significantly higher for the normal than for the aneurysmal group:A=0.223±0.046versus A=0.091±0.009 (mean ± SEM;p<0.0005), σ_y versus σ_y (p<0.05), and σ_u versus σ_u (p<0.0005), respectively. Our findings suggest that the AAA tissue is isotropic with respect to these mechanical properties. The observed difference inA between aneurysmal and normal aorta may be due to the complete recruitment and loading of collagen fibers at lower extensions in the former. Our data indicate that AAA rupture may be related to a reduction in tensile strength and that the biomechanical properties of AAA should be considered in assessing the severity of an individual aneurysm.     

APCO2 surface electrode working on the principle of electrical conductivity

APCO₂ electrode working on the principle of electrical conductivity is described. The calibration curve can be linearized according to the formula . This linearity has been tested in thePCO₂ range of 0.93–9.33 kPa (7–70 Torr). For the experiments electrodes are used which have conductivity values of about 50 nS and drifts of maximally 5%/h at aPCO₂ of 5.33 kPa (40 Torr). The response time (T ₉₀) is about 20 s. The temperature sensitivity is 2.4 nS/1 K between 298K–310K. The standard error of the measurements is =0.33 nS. With these electrodes tissuePCO₂ can be measured on the surface of various organs.     

Effects of physical parameters on the cylindrical model for volume measurement by conductance

Despite its undisputed utility for determining changes in ventricular pressure-volume relationships, the conductance catheter technique has not been proven reliable for measuring absolute volume. This limitation is due to violations of the assumptions inherent in the cylindrical model on which the method is based (i.e., homogeneous electric field and no leakage current). The purpose of this investigation was to relate cylindrical model correction factors to the physical environment of the catheter and to the cylindrical equation. Physical measurements of saline-filled, nonconductive cylinders using a four-electrode conductance catheter were compared with a three-dimensional finite element model of the physical apparatus. These measurements were incorporated into a parallel conductance model to relate physical parameters to corrections in the cylindrical equation for volume measurement. Excellent agreement between measured and modeled data was found. Results demonstrated a nonlinear relationship between the field nonhomogeneity correction factor (α) and cylinder diameter. The relationship between α and diameter was consistent with a theoretical extrapolation of cylinder diameter toward infinity. An inverse relationship between α and the parallel conductance volume (V _P) was also clarified. The parallel conductance model was able to demonstrate opposite effects of the physical presence of the catheter body and electrodes, which tended to cancel out any net effect on measured conductance. Results of this investigation and the developed finite element model clarify the nature of the correction terms in the cylindrical model and may lead to greater application of the conductance technique.     

Effect of intracellular anisotropy on electrical source determination in a muscle fibre

Expressions are available for describing, quantitatively, the source associated with an action potential propagating along an excitatble fibre. For a nerve fibre one such expression defines an equivalent volume dipole density function τ(x) = − ∂/∂x (σ_i φ_i(x) − σ_e φ_e(x))a_x (where x is the axial co-ordinate, i is the intracellular and e the extracellular region, σ_i and σ_e are isotropic conductivities, ϕ the potential at the membrane, while axial symmetry is assumed), and this source fills the intracellular region. This source, as distinct from transmembrane current formulations, lies in a uniform, isotropic, extracellular, medium. Consequently, for a fibre bundle a simple superposition of sources, all lying in a uniform, isotropic, extracellular space, can be accomplished. However, for muscle fibres the presence of non-conducting myofibrils causes the intracellular space to be anisotropic. The paper describes the modification in the aforementioned expressions for the case of longitudinal and transverse propagation and extrapolation to an arbitrary angle of propagation. The resultant source continues to be expressed relative to a uniform, isotropic, extracellular medium.     

Electric impedance cuff for the indirect measurement of blood pressure and volume elastic modulus in human limb and finger arteries

A new plethysmograph, the electric impedance cuff, was designed for the indirect measurement of blood pressure, volume elastic modulus E_v and compliance C_a in human limb arteries. This comprises a compression chamber filled with electrolyte solution and a tetrapolar electric impedance plethysmograph whose electrodes are placed inside the chamber; the former for controlling transmural arterial pressure P_t, and the latter for detecting total limb volume V_o, mean arterial volume and its variation ΔV_a. Systolic and mean arterial pressure in the upper arms, forearms and fingers were measured by detecting pulsatile impedance variation during the gradual (3–5 mm Hg per heart beat) increase (or decrease) in chamber pressure by the volume oscillometric technique. Diastolic and pulse pressure ΔP were calculated from these pressure values. Compliance C_a=ΔV/ΔP and volume elastic modulus were recorded at various P_t levels, controlled by the compression pressure. Although this is a kind of impedance plethysmograph, the volume change in a limb segment can be detected by this method without passing electric current through the limb.     

Cardiac assessment mechanics: 1 Left ventricular mechanomyocardiography,a new approach to the detection of diseased myocardial elements and states

The inadequacies of currently employed methods for assessment of cardiac mechanics are discussed, and the need for development of more intrinsic assessment parameters is emphasised. To this end, a new technique is presented to enable determination of regional mechanical constitutive properties of the myocardium during diastole; this technique has been originally named left-ventricular mechanomyocardiography (or l.v.-m.m.c.g.). The data required for implementation of the techniques consist of left-ventricular sequential dynamic geometry and associated recorded chamber pressure. The method entails matching of the inner-boundary deformations of the instantaneous finite-element model of the left ventricle (which is loaded by the recorded instantaneous incremental pressure) with the actual instantaneous endocardial deformations (as derived from either cineangiocardiography or 2-dimensional echocardiography), to determine the regional distribution of the Young's modulus E_ne and the incremental stresses Δσ_ne (and hence the total stress σ_ne=∑_nΔσ_ne) of the myocardial elements. The mechanical constitutive properties of the myocardial elements can be then characterised by the constitutive relation E_ne=a+bσ. The constitutive parameters a and b have typical ranges for normal and pathological (ischaemic and infarcted) myocardial elements and hence can be employed to distinguish diseased elements. The values of a and b are calculated for normal and pathological subjects and their normal and pathological ranges are presented.     

Hydrogel basedin vivo reference electrode catheter

A hydrogel basedin vivo reference electrode catheter has been developed. A simple diffusion model of ion transport was applied to study chloride ion transport through polyhydroxyethylmethacrylate (pHEMA) membranes. Based on an experimentally derived effective diffusion coefficient of D_eff=4·04±0·5×10⁻⁸ cm²s⁻¹, a reference electrode catheter was fabricated featuring a dimensionally appropriate pHEMA porous liquid junction, a gelled Ringer's solution internal electrolyte compartment and a Ag/AgCl internal half cell. The reference electrode potential is not a function of pH from pH6 to pH9 and is linearly related to temperature by 0·33 m V^oC⁻¹. In animal trials, the intravascular catheter electrodes exhibit an average stability of ±0·92mV for 6–8h. Stability in blood can be attributed to the haemocompatibility and transport properties of pHEMA.     

Kinematic Modeling-based Left Ventricular Diastatic (Passive) Chamber Stiffness Determination with <Emphasis Type="Italic">In-Vivo</Emphasis> Validation

The slope of the diastatic pressure–volume relationship (D-PVR) defines passive left ventricular (LV) stiffness K. \mathcal{K}. Although K \mathcal{K} is a relative measure, cardiac catheterization, which is an absolute measurement method, is used to obtain the former. Echocardiography, including transmitral flow velocity (Doppler E-wave) analysis, is the preferred quantitative diastolic function (DF) assessment method. However, E-wave analysis can provide only relative, rather than absolute pressure information. We hypothesized that physiologic mechanism-based modeling of E-waves allows derivation of the D-PVR_E-wave whose slope, K_{\textE-\textwave} \mathcal{K}_{{\text{E-}}{\text{wave}}} , provides E-wave-derived diastatic, passive chamber stiffness. Our kinematic model of filling and Bernoulli’s equation were used to derive expressions for diastatic pressure and volume on a beat-by-beat basis, thereby generating D-PVR_E-wave, and K_{\textE-\textwave} \mathcal{K}_{{\text{E-}}{\text{wave}}} . For validation, simultaneous (conductance catheter) P–V and echocardiographic E-wave data from 30 subjects (444 total cardiac cycles) having normal LV ejection fraction (LVEF) were analyzed. For each subject (15 beats average) model-predicted K_{\textE-\textwave} \mathcal{K}_{{\text{E-}}{\text{wave}}} was compared to experimentally measured K_\textCATH \mathcal{K}_{\text{CATH}} via linear regression yielding as follows: K_{\textE-\textwave} = aK_\textCATH + b  (R² = 0.92), \mathcal{K}_{{\text{E-}}{\text{wave}}} = \alpha {\mathcal{K}}_{\text{CATH}} + b\;(R^{2} = 0.92), where, α = 0.995 and b = 0.02. We conclude that echocardiographically determined diastatic passive chamber stiffness, K_{\textE-\textwave} \mathcal{K}_{{\text{E-}}{\text{wave}}} , provides an excellent estimate of simultaneous, gold standard (P–V)-defined diastatic stiffness, K_\textCATH \mathcal{K}_{\text{CATH}} . Hence, in chambers at diastasis, passive LV stiffness can be accurately determined by means of suitable analysis of Doppler E-waves (transmitral flow).     

Radio-frequency ring applicator: energy distributions measured in the CDRH phantom

SAR distributions were measured in the CDRH phanton, a 1 cm fatequivalent shell filled with an abdomen-equivalent liquid (σ=0.4−1.0 S m⁻¹; dimensions 22×32×57 cm) to demonstrate the feasibility of the ring applicator to obtain deep heating. The ring electrodes were fixed in a PVC tube; diameter 48 cm, ring width 20 cm and gap width between both rings 31.6 cm. Radio-frequency energy was fed to the electrodes at eight points. The medium between the electrodes and the phantom was deionised water. The SAR distribution in the liquid tissue volume was obtained by a scanning E-field probe measuring the E-field in all three directions. With equal amplitude and phase applied to all feeding points, a uniform SAR distribution was measured in the central cross-section at 30 MHz. With RF energy supplied to only four adjacent feeding points (others were connected to a 50 ω load), the feasibility to perform amplitude steering was demonstrated; SAR values above 50% of the maximum SAR were measured in one quadrant only. SAR distributions obtained at 70 MHz showed an improved focusing ability; a maximum at the centre exists for an electric conductivity of the abdomen-equivalent tissue of 0.6 and 0.4 S m⁻¹.     

A new pulse duplicator with a passive fill ventricle for analysis of cardiac dynamics

A new pulse duplicator was designed for evaluation of the performance of ventricular assist devices through pressure–volume (P–V) diagrams of the native heart. A linear drive system in combination with a pusher-plate mechanism was designed as a drive system to implement the passive fill mechanism during diastole of the mock ventricle. The compliances of the native heart during both diastole and systole were simulated by placing a ventricle sack made of soft latex rubber in a sealed chamber and by varying the air-to-fluid volume ratio inside the chamber. The ratio of the capacities of the systemic venous and pulmonary circuits was adjusted to properly reflect the effects of volume shift between them. As the air-to-fluid volume ratio was varied from 1:12.3 to 1:1.58, the contractility of the ventricle expressed by E _max varied from 1.75 to 0.56 mmHg/ml with the mean V ₀ of 4.58 ml closely mimicking those of native hearts (p < 0.05). Because the E _max value of the normal human heart ranges from 1.3 to 1.6, with a value below 1.0 indicating heart failure, the mock ventricle is applicable in simulating the dynamics of the normal heart and the sick heart. The P–V diagram changes seen with rotary blood pump assistance revealed changes similar to those reported by other workers. The effects of the ventricular assist device, either pulsatile or continuous flow, on cardiac dynamics can be easily simulated with this system to derive design criteria for clinical circulatory assist devices.     

A model for simulation and patient-specific visualization of the tissue volume of influence during brain microdialysis

Microdialysis can be used in parallel to deep brain stimulation (DBS) to relate biochemical changes to the clinical outcome. The aim of the study was to use the finite element method to predict the tissue volume of influence (TVI_max) and its cross-sectional radius (r _TVImax) when using brain microdialysis, and visualize the TVI_max in relation to patient anatomy. An equation based on Fick’s law was used to simulate the TVI_max. Factorial design and regression analysis were used to investigate the impact of the diffusion coefficient, tortuosity and loss rate on the r _TVImax. A calf brain tissue experiment was performed to further evaluate these parameters. The model was implemented with pre-(MRI) and post-(CT) operative patient images for simulation of the TVI_max for four patients undergoing microdialysis in parallel to DBS. Using physiologically relevant parameter values, the r _TVImax for analytes with a diffusion coefficient D = 7.5 × 10⁻⁶ cm²/s was estimated to 0.85 ± 0.25 mm. The simulations showed agreement with experimental data. Due to an implanted gold thread, the catheter positions were visible in the post-operative images. The TVI_max was visualized for each catheter. The biochemical changes could thereby be related to their anatomical origin, facilitating interpretation of results.     

Biomechanical and Microstructural Properties of Common Carotid Arteries from Fibulin-5 Null Mice

Alteration in the mechanical properties of arteries occurs with aging and disease, and arterial stiffening is a key risk factor for subsequent cardiovascular events. Arterial stiffening is associated with the loss of functional elastic fibers and increased collagen content in the wall of large arteries. Arterial mechanical properties are controlled largely by the turnover and reorganization of key structural proteins and cells, a process termed growth and remodeling. Fibulin-5 (fbln5) is a microfibrillar protein that binds tropoelastin, interacts with integrins, and localizes to elastin fibers; tropoelastin and microfibrillar proteins constitute functional elastic fibers. We performed biaxial mechanical testing and confocal imaging of common carotid arteries (CCAs) from fibulin-5 null mice (fbln5 ^−/−) and littermate controls (fbln5 ^+/+) to characterize the mechanical behavior and microstructural content of these arteries; mechanical testing data were fit to a four-fiber family constitutive model. We found that CCAs from fbln5 ^−/− mice exhibited lower in vivo axial stretch and lower in vivo stresses while maintaining a similar compliance over physiological pressures compared to littermate controls. Specifically, for fbln5 ^−/− the axial stretch λ = 1.41 ± 0.07, the circumferential stress σ _θ  = 101 ± 32 kPa, and the axial stress σ _z  = 74 ± 28 kPa; for fbln5 ^+/+ λ = 1.64 ± 0.03, σ _θ  = 194 ± 38 kPa, and σ _z  = 159 ± 29 kPa. Structurally, CCAs from fbln5 ^−/− mice lack distinct functional elastic fibers defined by the lamellar structure of alternating layers of smooth muscle cells and elastin sheets. These data suggest that structural differences in fbln5 ^−/− arteries correlate with significant differences in mechanical properties. Despite these significant differences fbln5 ^−/− CCAs exhibited nearly normal levels of cyclic strain over the cardiac cycle.     

Association of muscle hardness with muscle tension dynamics: a physiological property

This study aimed to investigate the relationship between muscle hardness and muscle tension in terms of length–tension relationship. A frog gastrocnemius muscle sample was horizontally mounted on the base plate inside a chamber and was stretched from 100 to 150% of the pre-length, in 5% increments. After each step of muscle lengthening, electrical field stimulation for induction of tetanus was applied using platinum-plate electrodes positioned on either side of the muscle submerged in Ringer’s solution. The measurement of muscle hardness, i.e., applying perpendicular distortion, was performed whilst maintaining the plateau of passive and tetanic tension. The relationship between normalised tension and normalised muscle hardness was evaluated. The length–hardness diagram could be created from the modification with the length–tension diagram. It is noteworthy that muscle hardness was proportional to passive and total tension. Regression analysis revealed a significant correlation between muscle hardness and passive and total tension, with a significant positive slope (passive tension: r = 0.986, P < 0.001; total tension: r = 0.856, P < 0.001). In conclusion, our results suggest that muscle hardness depends on muscle tension in most ranges of muscle length in the length–tension diagram.     

In vivo measurement of carbon dioxide tension with a miniature electrode

A commercially available catheter type electrode with whichP _CO2 can be continuously measured in vivo and in vitro gave progressively less accurate results the longer the measuring period was extended. This proved to be due to temperature effects and a change in sensitivity with time. A correction procedure for these effects was developed which was based on two observations. 1. The relationship between temperature and the logarithm of the sensitivity of the electrodeamplifier combination was linear and virtually identical for 9 electrodes: 8% change in sensitivity for a deviation of 1° C from the temperature during calibration. 2. The change in sensitivity due to drift of the electrode output is approximately a logarithmic function of time: 1 h after calibration all electrodes exhibited a decreased sensitivity, varying between 0.3 and 16.7%. The drift effect can be dealt with by repeated calibrations, preferably at 1^1/2 h intervals.The adequacy of the correction procedure was assessed in in vivo measurements in cats and dogs. The meanP _CO2 difference between the in vivo measurement, corrected for temperature and drift, and samples analyzed with a conventional electrode, was 0.005 kPa (0.04 mm Hg) with a standard deviation of 0.187 kPa (1.39 mm Hg).     

Genetic Covariation Between Brain Volumes and IQ,Reading Performance,and Processing Speed

Although there has been much interest in the relation between brain size and cognition, few studies have investigated this relation within a genetic framework and fewer still in non-adult samples. We analyzed the genetic and environmental covariance between structural MRI data from four brain regions (total brain volume, neocortex, white matter, and prefrontal cortex), and four cognitive measures (verbal IQ (VIQ), performance IQ (PIQ), reading ability, and processing speed), in a sample of 41 MZ twin pairs and 30 same-sex DZ twin pairs (mean age at cognitive test = 11.4 years; mean age at scan = 15.4 years). Multivariate Cholesky decompositions were performed with each brain volume measure entered first, followed by the four cognitive measures. Consistent with previous research, each brain and cognitive measure was found to be significantly heritable. The novel finding was the significant genetic but not environmental covariance between brain volumes and cognitive measures. Specifically, PIQ shared significant common genetic variance with all four measures of brain volume (r _g = .58–.82). In contrast, VIQ shared significant genetic influence with neocortex volume only (r _g = .58). Processing speed was significant with total brain volume (r _g = .79), neocortex (r _g = .64), and white matter (r _g = .89), but not prefrontal cortex. The only brain measure to share genetic influence with reading was total brain volume (r _g = .32), which also shared genetic influences with processing speed.     

In Vivo Measurement of Real-Time Aortic Segmental Volume Using the Conductance Catheter

The goal of this investigation was to determine if the conductance catheter technique for chamber volume measurement could be applied in vivo to determine real-time phasic aortic segmental volume. A four-electrode conductance catheter was used to measure time-varying resistance of the descending thoracic aorta in open-chest, anesthetized dogs. Resistance was converted to segmental volume and the slope correction factor () and parallel conductance volume (V _P ) were determined. The results showed excellent linear correlation between conductance and sonomicrometric segmental volume. The correction factors and V _P were found to be empirically related to average vessel diameter. The relatively high values for the slope correction factor (=4.59±0.17 SEM) were found to be primarily related to low-resistivity shunt paths probably originating in the periadventitial aortic wall and to a lesser extent to changes in flow-induced increases in blood resistivity, hematocrit, catheter position, and other adjacent tissue resistivity. The results demonstrate that correction factors empirically derived from measurements of mean aortic diameter could be used to determine absolute real-time phasic segmental volume, cross-sectional area, or diameter. The conductance technique may possess the same potential for determining aortic mechanical properties which has already been demonstrated for determining ventricular mechanical properties.     

Intravascular mean blood velocity measurements using a crosscorrelation technique

In order to answer the need for a method of measuring mean blood velocities in intact animals and in human beings we have developed a catheter tip velocimeter. In this design two pairs of electrodes are mounted on the same 6F catheter, which has an injection channel allowing the introduction of a bolus with a conductivity slightly different from that of the transporting fluid. The mean transit time of the tracer between the two measuring zones is determined by calculating the crosscorrelation function of two signals detected at the two measuring zones by the two low-frequency impedance bridges (10 kHz, 100 μA and frequency response 0·1–20 Hz). Each pair of electrodes is employed as one arm of each impedance bridge. This probe has been tested in model experiments simulating different kinds of flows in rigid and viscoelastic tubes. The calibration curves presented here show the linear relationship between the mean velocity of flow and the inverse of the maximum of the crosscorrelation functions. The difference between the theoretical and experimental results is discussed. The maximum error was found to be ±2% of fullscale deflection and is due to the unknown position of the catheter in relation to the vascular axis. The accuracy of this technique depends not on the nature of the tracer but only on the detector.

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Resumé La vitesse moyenne de l'écoulement sanguin, pulsé ou continu, est déterminée par la mesure du temps de transit τ_p d'un embole (de conductivité différente de celle du sang) entrè deux paires d'électrodes portées par le même cathéter et séparées par une distance connue L, ( ). On montre que le maximum de la fonction d'intercorrelation des deux signaux conductimétriques représente le temps de transit le plus probable de l'embole entre les points de mesure. Des séries expérimentales,in vitro, en simulant les différents régimes de circulation, ont permis l'étalonnage de la sonde dont la réponse est linéaire dans tout le domaine des vitesses mesurées (avec un coefficient moyen de corrélation de 0,99). Cette sonde a été utilisée chez l'animal (chien) pour mesurer les vitesses moyennes dans la veine cave inférieure.

     

Computational Model of Interstitial Transport in the Spinal Cord using Diffusion Tensor Imaging

Local drug delivery methods, including convection-enhanced delivery (CED), are being used to increase distribution in selected regions of nervous tissue. There is a need for 3D models that predict spatial drug distribution within these tissues. A methodology was developed to process magnetic resonance microscopy (MRM) and diffusion tensor imaging (DTI) scans, segment gray and white matter regions, assign tissue transport properties, and model the interstitial transport of macromolecules. Fiber tract orientation was derived from DTI data and used to assign directional dependence of hydraulic conductivity, K, and tracer diffusivity, D _t , transport tensors. Porous media solutions for interstitial fluid pressure, velocity, and albumin distribution were solved using a finite volume method. To test this DTI-based methodology, a rat spinal cord transport model was developed to simulate CED into the dorsal white matter column. Predicted distribution results correspond well with small volume (∼1 μl) trends found experimentally, although albumin loss was greater at larger infusion volumes (>2 μl). Simulations were similar to those using fixed transport properties due to the bulk alignment of white matter fibers along the cord axis. These findings help to validate the DTI-based methodology which can be applied to modeling regions where fiber tract organization is more complex, e.g., the brain.