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PubMed2004

Haemodynamic resistance model of monochorionic twin pregnancies complicated by acardiac twinning.

An acardiac twin is a severely malformed monochorionic twin fetus that lacks most organs, particularly a heart. It grows during pregnancy, because it is perfused by its developmentally normal co-twin (called the pump twin) via a set of placental arterioarterial and venovenous anastomoses. The pump twin dies intrauterine or neonatally in about 50% of the cases due to congestive heart failure, polyhydramnios and prematurity. Because the pathophysiology of this pregnancy is currently incompletely understood, we modified our previous haemodynamic model of monochorionic twins connected by placental vascular anastomoses to include the analysis of acardiac twin pregnancies. We incorporated the fetoplacental circulation as a resistance circuit and used the fetal umbilical flow that perfuses the body to define fetal growth, rather than the placental flow as done previously. Using this modified model, we predicted that the pump twin has excess blood volume and increased mean arterial blood pressure compared to those in the acardiac twin. Placental perfusion of the acardiac twin is significantly reduced compared to normal, as a consequence of an increased venous pressure, possibly implying reduced acardiac placental growth. In conclusion, the haemodynamic analysis may contribute to an increased knowledge of the pathophysiologic consequences of an acardiac body mass for the pump twin.

Biophysics↗
PubMed2005

Low resolution brain electromagnetic tomography in a realistic geometry head model: a simulation study.

It is of importance to localize neural sources from scalp recorded EEG. Low resolution brain electromagnetic tomography (LORETA) has received considerable attention for localizing brain electrical sources. However, most such efforts have used spherical head models in representing the head volume conductor. Investigation of the performance of LORETA in a realistic geometry head model, as compared with the spherical model, will provide useful information guiding interpretation of data obtained by using the spherical head model. The performance of LORETA was evaluated by means of computer simulations. The boundary element method was used to solve the forward problem. A three-shell realistic geometry (RG) head model was constructed from MRI scans of a human subject. Dipole source configurations of a single dipole located at different regions of the brain with varying depth were used to assess the performance of LORETA in different regions of the brain. A three-sphere head model was also used to approximate the RG head model, and similar simulations performed, and results compared with the RG-LORETA with reference to the locations of the simulated sources. Multisource localizations were discussed and examples given in the RG head model. Localization errors employing the spherical LORETA, with reference to the source locations within the realistic geometry head, were about 20-30 mm, for four brain regions evaluated: frontal, parietal, temporal and occipital regions. Localization errors employing the RG head model were about 10 mm over the same four brain regions. The present simulation results suggest that the use of the RG head model reduces the localization error of LORETA, and that the RG head model based LORETA is desirable if high localization accuracy is needed.

Algorithms↗
PubMed2005

Ellipsoidal electrogastrographic forward modelling.

The theoretical and computational study of the electromagnetic forward and inverse problems in ellipsoidal geometry is important in electrogastrography because the geometry of the human stomach can be well approximated using this idealized body. Moreover, the anisotropies inherent to this organ can be highlighted by the characteristics of the electric potential associated with current dipoles in an ellipsoid. In this paper, we present a forward simulation for the stomach using an analytic expression of the gastric electric potential that employs a truncated expansion of ellipsoidal harmonics; we then demonstrate that an activation front of dipoles propagating along the body of an ellipsoid can simulate gastric electrical activity. In addition to the usefulness of our model, we also discuss its limitations and accuracy.

Algorithms↗
PubMed2005

Passive vortex currents in magneto- and electrocardiography: comparison of magnetic and electric signal strengths.

Vortex currents may be of importance in the early diagnosis of myocardial infarction caused by an occlusion of a coronary artery. We investigated the influence of a passive vortex current distribution, modelled by different conductivities in a hollow cylinder, on the localization error and on the signal strength in both the magnetocardiogram and the electrocardiogram. The hollow cylinder was mounted in a realistically shaped physical torso phantom. A platinum dipole was inserted into the cylinder. The compartment boundaries were modelled with two special ionic exchange membranes. The conductivity ratio of the cylinder compartment to the torso compartment was varied from 0.25 to 100. We compared the simultaneously measured magnetic and electric signal strengths as a function of this conductivity ratio. We found that an increasing conductivity ratio causes only a slight increase (about 19%) of the magnetic signal strength but a strong decrease (about 81%) of the electric signal strength. Using a homogeneous torso model, the dipole localization errors were, depending on the conductivity ratio, up to 16 mm. In conclusion, passive vortex currents might partially explain the differences between magnetocardiographic and electrocardiographic recordings observed both experimentally and clinically.

Body Surface Potential Mapping↗
PubMed2006

Frequency-dependent complex modulus of the uterus: preliminary results.

The frequency-dependent complex moduli of human uterine tissue have been characterized. Quantification of the modulus is required for developing uterine ultrasound elastography as a viable imaging modality for diagnosing and monitoring causes for abnormal uterine bleeding and enlargement, as well assessing the integrity of uterine and cervical tissue. The complex modulus was measured in samples from hysterectomies of 24 patients ranging in age from 31 to 79 years. Measurements were done under small compressions of either 1 or 2%, at low pre-compression values (either 1 or 2%), and over a frequency range of 0.1-100 Hz. Modulus values of cervical tissue monotonically increased from approximately 30-90 kPa over the frequency range. Normal uterine tissue possessed modulus values over the same range, while leiomyomas, or uterine fibroids, exhibited values ranging from approximately 60-220 kPa.

Adult↗
PubMed2006

A technique to consider mismatches between fMRI and EEG/MEG sources for fMRI-constrained EEG/MEG source imaging: a preliminary simulation study.

fMRI-constrained EEG/MEG source imaging can be a powerful tool in studying human brain functions with enhanced spatial and temporal resolutions. Recent studies on the combination of fMRI and EEG/MEG have suggested that fMRI prior information could be readily implemented by simply imposing different weighting factors to cortical sources overlapping with the fMRI activations. It has been also reported, however, that such a hard constraint may cause severe distortions or elimination of meaningful EEG/MEG sources when there are distinct mismatches between the fMRI activations and the EEG/MEG sources. If one wants to obtain the actual EEG/MEG source locations and uses the fMRI prior information as just an auxiliary tool to enhance focality of the distributed EEG/MEG sources, it is reasonable to weaken the strength of fMRI constraint when severe mismatches between fMRI and EEG/MEG sources are observed. The present study suggests an efficient technique to automatically adjust the strength of fMRI constraint according to the mismatch level. The use of the proposed technique rarely affects the results of conventional fMRI-constrained EEG/MEG source imaging if no major mismatch between the two modalities is detected; while the new results become similar to those of typical EEG/MEG source imaging without fMRI constraint if the mismatch level is significant. A preliminary simulation study using realistic EEG signals demonstrated that the proposed technique can be a promising tool to selectively apply fMRI prior information to EEG/MEG source imaging.

Biophysical Phenomena↗
PubMed2006

Left ventricular motion reconstruction with a prolate spheroidal B-spline model.

Tagged cardiac magnetic resonance (MR) imaging can non-invasively image deformation of the left ventricular (LV) wall. Three-dimensional (3D) analysis of tag data requires fitting a deformation model to tag lines in the image data. In this paper, we present a 3D myocardial displacement and strain reconstruction method based on a B-spline deformation model defined in prolate spheroidal coordinates, which more closely matches the shape of the LV wall than existing Cartesian or cylindrical coordinate models. The prolate spheroidal B-spline (PSB) deformation model also enforces smoothness across and can compute strain at the apex. The PSB reconstruction algorithm was evaluated on a previously published data set to allow head-to-head comparison of the PSB model with existing LV deformation reconstruction methods. We conclude that the PSB method can accurately reconstruct deformation and strain in the LV wall from tagged MR images and has several advantages relative to existing techniques.

Algorithms↗
PubMed2006

Validation of a personalized dosimetric evaluation tool (Oedipe) for targeted radiotherapy based on the Monte Carlo MCNPX code.

Dosimetric studies are necessary for all patients treated with targeted radiotherapy. In order to attain the precision required, we have developed Oedipe, a dosimetric tool based on the MCNPX Monte Carlo code. The anatomy of each patient is considered in the form of a voxel-based geometry created using computed tomography (CT) images or magnetic resonance imaging (MRI). Oedipe enables dosimetry studies to be carried out at the voxel scale. Validation of the results obtained by comparison with existing methods is complex because there are multiple sources of variation: calculation methods (different Monte Carlo codes, point kernel), patient representations (model or specific) and geometry definitions (mathematical or voxel-based). In this paper, we validate Oedipe by taking each of these parameters into account independently. Monte Carlo methodology requires long calculation times, particularly in the case of voxel-based geometries, and this is one of the limits of personalized dosimetric methods. However, our results show that the use of voxel-based geometry as opposed to a mathematically defined geometry decreases the calculation time two-fold, due to an optimization of the MCNPX2.5e code. It is therefore possible to envisage the use of Oedipe for personalized dosimetry in the clinical context of targeted radiotherapy.

Computer Simulation↗
PubMed2006

Optimization of the artificial urinary sphincter: modelling and experimental validation.

The artificial urinary sphincter should be long enough to prevent strangulation effects of the urethral tissue and short enough to avoid the improper dissection of the surrounding tissue. To optimize the sphincter length, the empirical three-parameter urethra compression model is proposed based on the mechanical properties of the urethra: wall pressure, tissue response rim force and sphincter periphery length. In vitro studies using explanted animal or human urethras and different artificial sphincters demonstrate its applicability. The pressure of the sphincter to close the urethra is shown to be a linear function of the bladder pressure. The force to close the urethra depends on the sphincter length linearly. Human urethras display the same dependences as the urethras of pig, dog, sheep and calf. Quantitatively, however, sow urethras resemble best the human ones. For the human urethras, the mean wall pressure corresponds to (-12.6 +/- 0.9) cmH2O and (-8.7 +/- 1.1) cmH2O, the rim length to (3.0 +/- 0.3) mm and (5.1 +/- 0.3) mm and the rim force to (60 +/- 20) mN and (100 +/- 20) mN for urethra opening and closing, respectively. Assuming an intravesical pressure of 40 cmH2O, and an external pressure on the urethra of 60 cmH2O, the model leads to the optimized sphincter length of (17.3 +/- 3.8) mm.

Adult↗
PubMed1989

Experimental fluid dynamics of aortic stenosis in a model of the human aorta.

Aortic stenosis has been modelled in an in vitro, pulsatile mock circulatory system (MCS) using a porcine valvular prosthesis, and studied with a laser Doppler anemometer (LDA). The MCS incorporated an acrylic model of the human aorta made from a cadaveric casting in situ. A Carpentier-Edwards aortic valve prosthesis was placed in the MCS after being rendered stenotic by suturing of the valve cusps. Flow velocity profiles across the lumen of the aorta in the presence of aortic stenosis were determined using LDA at two preselected sites in the ascending aorta, and at one preselected site in the brachiocephalic artery. Results indicate that a strong systolic jet bordered by transient vortices with intensely reversed flows is produced distal to severely stenotic aortic valves, becoming less intense with a lesser degree of stenosis. Peak fluid velocities in the systolic jet were determined by LDA at distances of 2.6 and 5.6 cm from the valve inlet for a mean flow rate of 5.2 l min-1. Peak systolic pressure gradients and peak turbulent axial stresses were also determined and found to increase dramatically with stenosis. Furthermore, increasing degrees of stenosis also resulted in more severely disturbed flows in the brachiocephalic artery. Peak fluid velocities and their associated turbulent axial stresses in the systolic jets produced by aortic valvular stenosis are remarkably sensitive to even small changes in the calculated valve orifice areas, and can therefore be very useful in assessing the severity and progression of valvular disease. In addition, increasing degrees of aortic stenosis cause more turbulence to be transported into the brachiocephalic artery.

Aorta↗
PubMed1989

The error due to fat inhomogeneity in lumbar spine bone mineral measurements.

Bone mineral mass measurements using dual photon absorptiometry rely on the assumption that fat thickness is constant within the region scanned by the photon beam. This assumption has been tested using both single and multiple slices from archived CT scans. In 26 patients, the difference in fat content between the bone and baseline measurement sites, the fat deviation, was found to vary from -2.7 to 18.7%. The median fat deviation was 4.4% which can lead to errors in measured lumbar spine bone mineral of 6 to 11%. Approximately one third of patients will have errors greater than 10%.

Adipose Tissue↗
PubMed1991

Magnetocardiographic localisation and modelling.

In our magnetocardiographic (MCG) localisation studies, two modelling approaches have been applied: (a) modelling the sources with dipole and quadrupole moments in a general multipole expansion and using a homogeneous, semi-infinite volume conductor, and (b) using a single current dipole source in a homogeneous, realistically shaped torso. Both approaches have been successfully applied in localising the premature ventricular excitation site in patients suffering from the Wolff-Parkinson-White syndrome. In addition, we have participated in developing a model of propagation of electrical activation in the ventricles. Anisotropic conductivity properties and spiral arrangement of myocardial fibres are included in the model.

Heart Function Tests↗
PubMed1991

Modelling the fetal magnetocardiogram.

During gestation the abdominally recorded fetal ECG (FECG) changes as a result of changes in the conductive medium. In particular, a minimum occurs in the FECG amplitude around 30 weeks of gestation. Based on the results of an extensive FECG study, the effect of these changes on the fetal MCG are simulated. It is shown that the FMCG amplitude is not greatly affected by the changes in the conductive medium. The FMCG waveform, however, is.

Fetal Heart↗
PubMed1991

Experiences in data analysis and modelling with a multichannel biomagnetic system.

Evaluation of MEG/MCG data, measured with the Siemens biomagnetic multichannel system KRENIKON, in patients with epilepsy, infarction, Wolff-Parkinson-White (WPW) syndrome or extra systoles are in good agreement with the results of different investigation techniques. The evaluations have been performed using an equivalent current dipole model within a sphere or a half-space with homogeneous conductivity. In cases where the current dipole model is not adequate, multiple dipoles or complete distributions of current sources have to be considered. Results from simulations and applications to in vivo data and the influence of geometries better adjusted to realistic geometries are discussed.

Brain↗
PubMed1991

Integrated imaging of neuromagnetic reconstructions and morphological magnetic resonance data.

New neuromagnetic imaging methods provide spatial information about the functional electrical properties of complex current distributions in the human brain. For practical use in medical diagnosis a combination of the abstract neuromagnetic imaging results with magnetic resonance (MR) or computed tomography (CT) images of the morphology is required. The biomagnetic images can be overlayed onto three-dimensional morphological images with spatially arbitrary selectable slices, calculated from conventional 2D data. For the current reconstruction the 3D images furthermore provide a priori information about the conductor geometry. A combination of current source density calculations and linear estimation methods for handling the inverse magnetic problem allows quick imaging of impressed current source density in arbitrary volume conductors.

Acoustic Stimulation↗
PubMed1991

The use of the asymptotic expansion to speed up the computation of a series of spherical harmonics.

When a function is expressed as an infinite series of spherical harmonics the convergence can be accelerated by subtracting its asymptotic expansion and adding it in analytically closed form. In the present article this technique is applied to two biophysical cases: to the potential distribution in a spherically symmetric volume conductor and to the covariance matrix of biomagnetic measurements.

Brain↗
PubMed1991

The finite element method for a realistic head model of electrical brain activities: preliminary results.

In order to model the brain's electrical activity realistically, the finite element method has been used to compute the potential distribution due to a current dipole. This approach has the advantage over the boundary element method of being able to consider anisotropies of the different conducting sub-volumes. The forward solution has been evaluated in the particular case of a three-layer concentric sphere isotropic head model of the head where an analytical formula is known. The errors on the dipole position and orientation have been estimated in the inverse problem procedure.

Brain↗