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At least 973 records · Page 54Linked to original sources
PubMed1992

The effect of measurement conditions on MCG inverse solutions.

A magnetic inverse solution that uses a single current dipole in a homogeneous volume conductor with realistic torso shape was tested numerically to establish the effect of magnetic noise, number of measurement points, and torso size on the localization accuracy. Seven different sites of cardiological interest were selected as locations for the source dipole. The three components of the magnetic field were calculated as if measured by second order gradiometers, Gaussian noise was added, and Monte Carlo tests performed for inverse solutions using a single field component, or all three combined. It was found that for any of the single component solutions, and a signal-to-noise ratio of 100, 25 measuring points are sufficient for good accuracy; just 12 points are needed if all three components are used together. If, however, the torso size of the inverse solution is different from that of the field data by 10 or 20%, a larger error occurs, even for 56 measurement points and no noise. In this case, the field component orthogonal to the measurement grid, Bz, yields better results than the other two components, or even all three combined. We conclude that a multichannel system measuring the z component of the magnetic field in about 30 locations would be the best choice to locate a dipolar source, provided the torso of the field data is closely matched by the model used in the inverse solution. To this effect, scaling of the torso model can easily be included in the computation. Imaging techniques could be used to accommodate different torso shapes.

Bias↗
PubMed2000

The boundary element method in the forward and inverse problem of electrical impedance tomography.

In this paper, a new formulation of the reconstruction problem of electrical impedance tomography (EIT) is proposed. Instead of reconstructing a complete two-dimensional picture, a parameter representation of the gross anatomy is formulated, of which the optimal parameters are determined by minimizing a cost function. The two great advantages of this method are that the number of unknown parameters of the inverse problem is drastically reduced and that quantitative information of interest (e.g., lung volume) is estimated directly from the data, without image segmentation steps. The forward problem of EIT is to compute the potentials at the voltage measuring electrodes, for a given set of current injection electrodes and a given conductivity geometry. In this paper, it is proposed to use an improved boundary element method (BEM) technique to solve the forward problem, in which flat boundary elements are replaced by polygonal ones. From a comparison with the analytical solution of the concentric circle model, it appears that the use of polygonal elements greatly improves the accuracy of the BEM, without increasing the computation time. In this formulation, the inverse problem is a nonlinear parameter estimation problem with a limited number of parameters. Variants of Powell's and the simplex method are used to minimize the cost function. The applicability of this solution of the EIT problem was tested in a series of simulation studies. In these studies, EIT data were simulated using a standard conductor geometry and it was attempted to find back this geometry from random starting values. In the inverse algorithm, different current injection and voltage measurement schemes and different cost functions were compared. In a simulation study, it was demonstrated that a systematic error in the assumed lung conductivity results in a proportional error in the lung cross sectional area. It appears that our parametric formulation of the inverse problem leads to a stable minimization problem, with a high reliability, provided that the signal-to-noise ratio is about ten or higher.

Algorithms↗
PubMed2004

Solving the inverse problem of electrocardiography using a Duncan and Horn formulation of the Kalman filter.

Numeric regularization methods most often used to solve the ill-posed inverse problem of electrocardiography are spatial and ignore the temporal nature of the problem. In this paper, a Kalman filter reformulation incorporated temporal information to regularize the inverse problem, and was applied to reconstruct left ventricular endocardial electrograms based on cavitary electrograms measured by a noncontact, multielectrode probe. These results were validated against in situ electrograms measured with an integrated, multielectrode basket-catheter. A three-dimensional, probe-endocardium model was determined from multiplane fluoroscopic images. The boundary element method was applied to solve the boundary value problem and determine a linear relationship between endocardial and probe potentials. The Duncan and Horn formulation of the Kalman filter was employed and was compared to the commonly used zero- and first-order Tikhonov spatial regularization as well as the Twomey temporal regularization method. Endocardial electrograms were reconstructed during both sinus and paced rhythms. The Paige and Saunders solution of the Duncan and Horn formulation reconstructed endocardial electrograms at an amplitude relative error of 13% (potential amplitude) which was superior to solutions obtained with zero-order Tikhonov (relative error, 31%), first-order Tikhonov (relative error, 19%), and Twomey regularization (relative error, 44%). Likewise, activation time error in the inverse solution using the Duncan and Horn formulation (2.9 ms) was smaller than that of zero-order Tikhonov (4.8 ms), first-order Tikhonov (5.4 ms), and Twomey regularization (5.8 ms). Therefore, temporal regularization based on the Duncan and Horn formulation of the Kalman filter improves the solution of the inverse problem of electrocardiography.

Algorithms↗
PubMed2006

Effects of geometric head model perturbations on the EEG forward and inverse problems.

We study the effect of geometric head model perturbations on the electroencephalography (EEG) forward and inverse problems. Small magnitude perturbations of the shape of the head could represent uncertainties in the head model due to errors on images or techniques used to construct the model. They could also represent small scale details of the shape of the surfaces not described in a deterministic model, such as the sulci and fissures of the cortical layer. We perform a first-order perturbation analysis, using a meshless method for computing the sensitivity of the solution of the forward problem to the geometry of the head model. The effect on the forward problem solution is treated as noise in the EEG measurements and the Cramér-Rao bound is computed to quantify the effect on the inverse problem performance. Our results show that, for a dipolar source, the effect of the perturbations on the inverse problem performance is under the level of the uncertainties due to the spontaneous brain activity. Thus, the results suggest that an extremely detailed model of the head may be unnecessary when solving the EEG inverse problem.

Brain↗
PubMed2006

Parallelized Bayesian inversion for three-dimensional dental X-ray imaging.

Diagnostic and operational tasks based on dental radiology often require three-dimensional (3-D) information that is not available in a single X-ray projection image. Comprehensive 3-D information about tissues can be obtained by computerized tomography (CT) imaging. However, in dental imaging a conventional CT scan may not be available or practical because of high radiation dose, low-resolution or the cost of the CT scanner equipment. In this paper, we consider a novel type of 3-D imaging modality for dental radiology. We consider situations in which projection images of the teeth are taken from a few sparsely distributed projection directions using the dentist's regular (digital) X-ray equipment and the 3-D X-ray attenuation function is reconstructed. A complication in these experiments is that the reconstruction of the 3-D structure based on a few projection images becomes an ill-posed inverse problem. Bayesian inversion is a well suited framework for reconstruction from such incomplete data. In Bayesian inversion, the ill-posed reconstruction problem is formulated in a well-posed probabilistic form in which a priori information is used to compensate for the incomplete information of the projection data. In this paper we propose a Bayesian method for 3-D reconstruction in dental radiology. The method is partially based on Kolehmainen et al. 2003. The prior model for dental structures consist of a weighted l1 and total variation (TV)-prior together with the positivity prior. The inverse problem is stated as finding the maximum a posteriori (MAP) estimate. To make the 3-D reconstruction computationally feasible, a parallelized version of an optimization algorithm is implemented for a Beowulf cluster computer. The method is tested with projection data from dental specimens and patient data. Tomosynthetic reconstructions are given as reference for the proposed method.

Algorithms↗
PubMed1999

Methods for rapid cloning and detection for sequencing of cloned inverse PCR-generated DNA fragments adjacent to known sequences in bacterial chromosomes.

Since the invention of PCR, many adaptation techniques have been developed for sequencing DNA fragments flanking known sequences. Of them, inverse PCR is a matter of interest because of the simplicity of its principle. However, the protocols for inverse PCR introduced so far consist of some time-consuming procedures, and with them, we cannot "walk" chromosomes too far since the number of suitable restriction enzymes is limited. Our experiments led to confirming simpler technical approaches applicable to the case of bacterial chromosomes, that is, designing two end-specific "contextual" sequences with which we can quickly detect the desired clones of targeted DNA fragments by simply analyzing PCR products, employing "the minimum value of the desired fragments" as a "discriminating minimum" value to decrease contaminant DNA fragments, and creating a new tandem of two cleaved end fragments of a known sequence ("reordering") for PCR amplification in combination with cloning of the inverse PCR-generated DNA. With the improvements, we could both simplify the procedures and broaden the capacity of the inverse PCR in "walking" chromosomes.

Chromosomes, Bacterial↗
PubMed2006

Comparison of the binding pockets of two chemically unrelated allosteric antagonists of the mGlu5 receptor and identification of crucial residues involved in the inverse agonism of MPEP.

Fenobam [N-(3-chlorophenyl)-N'-(4,5-dihydro-1-methyl-4-oxo-1H-imidazole-2-yl)urea], a clinically validated non-benzodiazepine anxiolytic, has been shown to be a potent and non-competitive metabotropic glutamate (mGlu)-5 receptor antagonist. In the present study, we have used the site-directed mutagenesis coupled with three-dimensional receptor-based pharmacophore modelling to elucidate the interacting mode of fenobam within the seven-transmembrane domain (7TMD) of mGlu5 receptor and its comparison with that of 2-methyl-6-(phenylethynyl)pyridine (MPEP), the prototype antagonist. The common residues involved in the recognition of MPEP and fenobam include Pro654(3.36), Tyr658(3.40), Thr780(6.44), Trp784(6.48), Phe787(6.51), Tyr791(6.55) and Ala809(7.47). The differentiating residues between both modulators' interacting modes are Arg647(3.29), Ser657(3.39) and Leu743(5.47). Our data suggest that these chemically unrelated mGlu5 antagonists act similarly, probing a functionally unique region of the 7TMD. Using [3H]inositol phosphates accumulation assay, we have also identified the critical residues involved in the inverse agonist effect of MPEP. The mutation W784(6.48)A completely blocked the inverse agonist activity of MPEP; two mutations F787(6.51)A and Y791(6.55)A, caused a drastic decrease in the MPEP inverse agonism. Furthermore, these three mutations led to an increased efficacy of quisqualate without having any effect on its potency. The fact that the residues Trp784(6.48) and Phe787(6.51) are essential equally in antagonism and inverse agonism effects emphasizes again the key role of these residues and the involvement of a common transmembrane network in receptor inactivation by MPEP.

Binding Sites↗
PubMed2004

Catheter inversion: a technique to complete isthmus ablation and cure atrial flutter.

Cure of typical atrial flutter (AFL) by catheter ablation to produce bidirectional block across the tricuspid annulus-inferior vena cava isthmus (IS) is highly effective, but failures may occur. We describe a technique that may allow creation of bidirectional block where a conventional strategy has failed. AFL ablation was performed using the conventional approach with a mapping/ablation (ablation) catheter introduced via the right femoral vein (RFV) to create a line of bidirectional block across the IS. If this was not achieved after five passes of the ablation catheter from the tricuspid annulus to the inferior vena cava (IVC) a catheter inversion technique was used. This allowed stable positioning of the ablation catheter at the IVC end of the isthmus. In 11 patients, a mean of 17 (range 3 to 45) radiofrequency (RF) applications was given before the catheter inversion technique was applied. Following catheter inversion a mean of 4 (1 to 14) further RF applications achieved bidirectional isthmus block in every patient. No complications occurred. Catheter inversion provides a simple, safe, and effective means of achieving bidirectional isthmus conduction block in cases where a conventional ablation strategy might have failed.

Aged↗
PubMed1995

Application of an electrocardiographic inverse solution to localize ischemia during coronary angioplasty.

Localization of Ischemia. This study demonstrates the utility of an electrocardiographic inverse solution, coupled with body surface potential mapping (BSPM), in localizing acute ischemia in patients undergoing percutaneous transluminal coronary angioplasty (PTCA). PTCA balloon inflations produce complete occlusion and acute transient ischemia, which can be detected electrocardiographically with BSPM. Comparisons between maps recorded both during and before the inflation of the PTCA balloon allow patient- and artery-specific characterizations of the resulting ischemia. Knowledge of the patient's coronary anatomy and the location of the occlusion site by coronary angiography permit an estimation based on cardiac hemodynamics of the region of myocardium most likely to suffer from PTCA-induced ischemia. Electrocardiographic inverse solutions provide a means of predicting cardiac potentials from body surface maps. In this study, we describe an inverse solution we have developed to localize the transient ischemia produced by PTCA. To validate the procedure, we compared the locations of predicted ischemia in seven patients with a qualitative estimate of the perfusion region based on fluoroscopic examination of each patient's coronary anatomy and PTCA balloon location. In each case, the region of ischemia predicted by the model included the perfusion zone determined fluoroscopically. These results suggest that electrical changes induced by acute ischemia can be localized with an electrocardiographic inverse solution.

Angioplasty, Balloon, Coronary↗
PubMed1998

Creation of a constitutively activated state of the 5-HT2A receptor by site-directed mutagenesis: revelation of inverse agonist activity of antagonists.

Constitutively active GPCR have revealed novel properties of drugs that exhibit classical competitive antagonism at the native forms of GPCR. These drugs reverse basal levels of constitutive activity, indicating that they have inverse agonist activity. We were interested in determining if competitive antagonists of the native 5-HT2A receptor, in particular, antipsychotic drugs, exhibit inverse agonist activity at the constitutively active 5-HT2A receptor. All of the drugs tested reduced basal IP production of constitutively active 5-HT2A receptors, indicating that they all exhibited inverse agonist activity. Risperidone and ketanserin produced the greatest inhibition of basal IP production resulting in a reduction of basal activity in the C322K mutant receptor of 82% and 80%, respectively. Antipsychotic drugs display inverse agonist activity, indicating that stabilization of the inactive conformation of the 5-HT2A receptor may be a key component of their mechanism of action.

Amino Acid Substitution↗
PubMed1997

Optimization of the chiral inversion of 2-phenylpropionic acid by Verticillium lecanii.

Previous studies have demonstrated that Verticillium lecanii might be used as a microbial model of the inversion of 2-arylpropionic acids in man. This paper describes the optimization of the inversion process in respect of culture medium, pH, cell density and substrate concentration. The study demonstrates that optimum inversion occurs in Sørensen's phosphate buffer at pH 5.5. The extent and rate of inversion were also shown to be dependent on substrate concentration and cell density. This study will form the basis of the development of a microbial model of the metabolism of 2-arylpropionic acids which might be suitable for the in-vitro screening of new compounds in this class.

Buffers↗
PubMed1988

Upside-down faces: a review of the effect of inversion upon face recognition.

Several studies have found that face recognition is disproportionately impaired by stimulus inversion when compated to recognition of other classes of visual stimuli. This effect has been interpreted as evidence that face recognition benefits from a 'special' process which is not engaged by an inverted face. This paper reviews studies of the effect of inversion on face recognition in recognition memory tasks, matching tasks and upon cerebral hemisphere asymmetries. Evidence is drawn from developmental studies and from studies of brain-injured and normal adult subjects. It is concluded that the evidence that inverted faces are processed differently from upright faces is far from compelling, and therefore the effect of inversion provides little or no evidence of a unique process in face recognition. The inversion effect is interpreted in terms of expertise in face processing and the highly homogeneous nature of faces as a stimulus class.

Attention↗
PubMed2004

Reconstruction of the temperature field for inverse ultrasound hyperthermia calculations at a muscle/bone interface.

An inverse algorithm with Tikhonov regularization of order zero has been used to estimate the intensity ratios of the reflected longitudinal wave to the incident longitudinal wave and that of the refracted shear wave to the total transmitted wave into bone in calculating the absorbed power field and then to reconstruct the temperature distribution in muscle and bone regions based on a limited number of temperature measurements during simulated ultrasound hyperthermia. The effects of the number of temperature sensors are investigated, as is the amount of noise superimposed on the temperature measurements, and the effects of the optimal sensor location on the performance of the inverse algorithm. Results show that noisy input data degrades the performance of this inverse algorithm, especially when the number of temperature sensors is small. Results are also presented demonstrating an improvement in the accuracy of the temperature estimates by employing an optimal value of the regularization parameter. Based on the analysis of singular-value decomposition, the optimal sensor position in a case utilizing only one temperature sensor can be determined to make the inverse algorithm converge to the true solution.

Acoustics↗
PubMed2004

Inverse treatment planning with adaptively evolving voxel-dependent penalty scheme.

In current inverse planning algorithms it is common to treat all voxels within a target or sensitive structure equally and use structure specific prescriptions and weighting factors as system parameters. In reality, the voxels within a structure are not identical in complying with their dosimetric goals and there exists strong intrastructural competition. Inverse planning objective function should not only balance the competing objectives of different structures but also that of the individual voxels in various structures. In this work we propose to model the intrastructural tradeoff through the modulation of voxel-dependent importance factors and deal with the challenging problem of how to obtain a sensible set of importance factors with a manageable amount of computing. Instead of letting the values of voxel-dependent importance to vary freely during the search process, an adaptive algorithm, in which the importance factors were tied to the local radiation doses through a heuristically constructed relation, was developed. It is shown that the approach is quite general and the EUD-based optimization is a special case of the proposed framework. The new planning tool was applied to study a hypothetical phantom case and a prostate case. Comparison of the results with that obtained using conventional inverse planning technique with structure specific importance factors indicated that the dose distributions from the conventional inverse planning are at best suboptimal and can be significantly improved with the help of the proposed nonuniform penalty scheme.

Algorithms↗
PubMed2005

Improving IMRT delivery efficiency using intensity limits during inverse planning.

Inverse planned intensity modulated radiotherapy (IMRT) fields can be highly modulated due to the large number of degrees of freedom involved in the inverse planning process. Additional modulation typically results in a more optimal plan, although the clinical rewards may be small or offset by additional delivery complexity and/or increased dose from transmission and leakage. Increasing modulation decreases delivery efficiency, and may lead to plans that are more sensitive to geometrical uncertainties. The purpose of this work is to assess the use of maximum intensity limits in inverse IMRT planning as a simple way to increase delivery efficiency without significantly affecting plan quality. Nine clinical cases (three each for brain, prostate, and head/neck) were used to evaluate advantages and disadvantages of limiting maximum intensity to increase delivery efficiency. IMRT plans were generated using in-house protocol-based constraints and objectives for the brain and head/neck, and RTOG 9406 dose volume objectives in the prostate. Each case was optimized at a series of maximum intensity ratios (the product of the maximum intensity and the number of beams divided by the prescribed dose to the target volume), and evaluated in terms of clinical metrics, dose-volume histograms, monitor units (MU) required per fraction (SMLC and DMLC delivery), and intensity map variation (a measure of the beam modulation). In each site tested, it was possible to reduce total monitor units by constraining the maximum allowed intensity without compromising the clinical acceptability of the plan. Monitor unit reductions up to 38% were observed for SMLC delivery, while reductions up to 29% were achieved for DMLC delivery. In general, complicated geometries saw a smaller reduction in monitor units for both delivery types, although DMLC delivery required significantly more monitor units in all cases. Constraining the maximum intensity in an inverse IMRT plan is a simple way to improve delivery efficiency without compromising plan objectives.

Body Burden↗
PubMed2006

An inversion formula for the exponential radon transform in spatial domain with variable focal-length fan-beam collimation geometry.

Inverting the exponential Radon transform has a potential use for SPECT (single photon emission computed tomography) imaging in cases where a uniform attenuation can be approximated, such as in brain and abdominal imaging. Tretiak and Metz derived in the frequency domain an explicit inversion formula for the exponential Radon transform in two dimensions for parallel-beam collimator geometry. Progress has been made to extend the inversion formula for fan-beam and varying focal-length fan-beam (VFF) collimator geometries. These previous fan-beam and VFF inversion formulas require a spatially variant filtering operation, which complicates the implementation and imposes a heavy computing burden. In this paper, we present an explicit inversion formula, in which a spatially invariant filter is involved. The formula is derived and implemented in the spatial domain for VFF geometry (where parallel-beam and fan-beam geometries are two special cases). Phantom simulations mimicking SPECT studies demonstrate its accuracy in reconstructing the phantom images and efficiency in computation for the considered collimator geometries.

Algorithms↗
PubMed2000

Tomographic inversion for sediment parameters in shallow water

This article discusses inversions for bottom geoacoustic properties using broadband acoustic signals obtained from explosive sources. The experimental data used for the inversions are SUS charge explosions acquired on a vertical hydrophone array during the Shelf Break Primer Experiment conducted south of New England in the Middle Atlantic Bight in August 1996. The SUS signals were analyzed for their time-frequency behavior using wavelets. The group speed dispersion curves were obtained from the wavelet scalogram of the SUS signals. A genetic algorithm (GA) was used for the inversion of sound speeds in the water column and compressional wave speeds in the sediment layers. The variations in the sound speeds in the water column were represented using empirical orthogonal functions (EOFs). A range-independent normal mode routine was used to construct the replica fields corresponding to the parameters. Comparison of group speeds for modes 1 to 9 and for a range of frequencies 8 to 200 Hz was used to arrive at the best parameter fit. An efficient hybrid optimization scheme using the GA and a Levenberg-Marquardt algorithm is presented. Linear perturbation methods were also used to "fine tune" the inversions and to obtain resolution and variance estimates. Analysis was also done to compute the degree of convergence of each of the parameters by explicitly calculating the Hessian matrices numerically. A posteriori estimation of mean and covariance was also done to obtain error estimates. Group speeds for the inverted sound speed fields provide an excellent match to the experimental data. The inverted sediment compressional speed profile compares well with in situ measurements.

Journal Article↗
PubMed2001

An evaluation of the accuracy of shallow water matched field inversion results.

In this article the accuracy of geo-acoustic and geometric parameter estimates obtained through matched field inversion (MFI) was assessed. Multi-frequency MFI was applied to multi-tone data (200-600 Hz) received at a 2-km source/receiver range. The acoustic source was fixed and the signals were received at a vertical array. Simultaneously with the acoustic transmissions, a CTD (conductivity, temperature and depth)-chain was towed along the acoustic track. A genetic algorithm was used for the global optimization, whereas a normal mode model was applied for the forward acoustic calculations. Acoustic data received at consecutive times were inverted and the stability of the inverted parameters was determined. Also, the parameter estimates were compared with independent measurements, such as multi-channel seismic surveys (for geo-acoustic parameters). The obtained uncertainty in the inversion results was assumed to have two distinct origins. The first origin is the inversion method itself, since each optimization will come up with some solution close to the exact optimum. Parameter coupling and the fact that some parameters hardly influence the acoustic propagation further contribute to this uncertainty. The second is due to oceanographic variability. Both contributions were evaluated through simulation. The contribution of oceanographic variability was evaluated through synthetic inversions that account for the actual sound speed variations as measured by the towed CTD-chain.

Journal Article↗