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An image model that permits the application of state-of-art approaches to ultrasonic images, with detailed methods for representing rough surfaces is developed. This physically-based, probabilistic image model incorporates the combined effects of the system point-spread function (PSF), the tissue microstructure, and the gross tissue shape.

Direct inference of heart-surface potentials from body-surface potentials has been the goal of most recent work on electrocardiographic inverse solutions. We developed and tested indirect methods for inferring heart-surface potentials based on estimation of regularized multipole sources. Regularization was done using Tikhonov, constrained-least-squares, and multipole-truncation techniques. These multipole-equivalent methods (MEMs) were compared to the conventional mixed boundary-value method (BVM) in a realistic torso model with up to 20% noise added to body-surface potentials and [+ or -] 1 cm error in heart position and size. Optimal regularization was used for all inverse solutions. The relative error of inferred heart-surface potentials of the MEM was significantly less (p < 0.05) than that of the BVM using zeroth-order Tikhonov regularization in 10 of the 12 cases tested. These improvements occurred with a fourth-degree (24 coefficients) or smaller multipole moment. From these multipole coefficients, heart-surface potentials can be found at an unlimited number of heart-surface locations. Our indirect methods for estimating heart-surface potentials based on multipole inference appear to offer significant improvement over the conventional direct approach. Index Terms--Constrained least squares, inverse electrocardiology, multipole expansion, Tikhonov regularization.

The electrocardiographic (ECG) inverse problem offers more sophisticated approaches for assessing the electrical condition of the heart than the standard 12-lead ECG. Because of the ill-posted nature of the ECG inverse problem, regularization must be used to obtain clinically meaningful estimates of heart-surface potentials. We compared optimal inverse solutions using L1 regularization with those found using zero-order Tikhonov (ZOT) regularization in two heart-torso models with known heart-surface potentials, but different spatial detail. In the model based on the Visible Human Male with an epicardial-shell heart-model, there was no difference in relative error (RE) and correlation coefficient (CC) during activation and recovery of the ventricles (QT interval) between L1 and ZOT inferred heart-surface potentials. In the heart-torso model based on the ECGSIM package with a ventricular heart model there were significant differences in RE and CC throughout the QT interval. At the peak of the R wave, RE was reduced from 0.841 (ZOT) to 0.358 (L1). L1 regularization reduced RE compared to ZOT except during the downstroke of the R wave. These results suggest that improvement with L1 regularization is dependent on heart model complexity, not its sampling density. [ABSTRACT FROM AUTHOR]

Algorithms are described for generating a pixel-based probabilistic image model. An arbitrary surface shape and descriptions of the surface microstructure and characteristics of the imaging system are assumed as givens.

To quantify effects of torso-model construction on the accuracy of inferred heart-surface potentials, images of the Visible Human Male were segmented to create full-body, torso- and heartmodel surfaces. Known heart-surface potentials were 90-electrode sock recordings from the ventricles of an adult male. Full-body and all torso models were zonal, that is, constructed from adjacent, sampled contours. Each model contained the same 163 electrode sites. Relative errors (REs) of forward-problem solutions compared to the full-body results at the electrodes were about 2.5% for typical torso models. The REs were largest on the left side at the bottom of the torso, in the vicinity of the heart, and on the upper left back. REs of inverse solutions with 10µv peak-to-peak noise in body-surface potentials increased by 22% at the peak of the R wave and by 24% over the QRS complex. Results suggest that selection of electrode locations near body-surface truncation sites and fidelity of surface construction near the heart are critical steps in data acquisition and torso modeling needed to minimize inverseproblem errors. [ABSTRACT FROM AUTHOR]

Fourier analysis of the signal-averaged ECG (SAECG) has previously revealed significant differences in magnitude spectra that differentiate patients with ventricular tachycardia (VT) from those without VT. To determine additional distinguishing features in the frequency domain, we analyzed phase spectra of SAECG's of sinus beats from 57 patients with VT, 65 without VT, and 20 normal controls. Unwrapped phase spectra from SAECG's of the entire cardiac cycle were calculated with respect to three fiducial points: onset of the P and Q waves, and the negative of the slope of the phase (group delay) for frequencies in the band, which accounted for 97.5% of the energy in the vector magnitude of the Frank SAECG leads. Phase spectra of SAECG's from patients with VT differed from the non-VT patients at frequencies [greater than of equal to] 21 Hz (p = 0.000039) for the P-wave fiducial, at frequencies [greater than or equal to] 60 Hz (p = 0.00085) for the Q-wave fiducial, and at frequencies [less than or equal to] 62 Hz (p = 0.0035) for the 97.5% energy fiducial. Group delays in SAECG's from patients with and without VT differed from 10 to 26 Hz (p = 0.000016) for the P-wave fiducial, and from 14 to 24 Hz (p = 0.00000070) for the Q-wave fiducial. Group delays with respect to the Q-wave fiducial in the VT patients in the 14-24 Hz band were, on average, 9 ms and 5 ms longer than those of the non-VT's and normals, respectively. Thus, phase spectra of SAECG's contain previously undetected features that together with magnitude may be helpful in improving methods for stratifying the risk of VT.

Both diabetes and obesity cause cardiac dysfunction. To separate consequences of geometric changes due to obesity from electrophysiological ones, we investigated how changes in cardiac and torso geometry affected body-surface ECGs. For this study, we modified the realistic heart and torso models of the simulation package ECGSIM. ECGs were calculated from action potentials on the heart surface using our bidomain forward-problem solution. These ECGs were studied using spectral- and principal-component analyses and isopotential and energy maps. We found relative errors over the body-surface during the QT interval of 12%, 14%, and 68% for hypertrophy of the heart, extension of the abdomen, and heart displacement with obesity, respectively. The major change to the standard 12-lead set also occurred with heart displacement. The mean relative error over the QT interval in the precordial leads was 78% with heart displacement. These results demonstrate the limitations of using standard lead sets to characterize electrocardiographic changes in obese subjects and point to the need for more inclusive measures, such as body-surface mapping and inverse electrocardiography, to describe electrical remodeling in the presence of habitus changes due to obesity [ABSTRACT FROM AUTHOR]

Temperature imaging with a non-invasive modality to monitor the heating of tumors during hyperthermia treatment is an attractive alternative to sparse invasive measurement. Previously, we predicted monotonic changes in backscattered energy (CBE) of ultrasound with temperature for certain sub-wavelength scatterers. We also measured CBE values similar to our predictions in bovine liver, turkey breast muscle, and pork rib muscle in 2-D in vitro studies and in nude mice during 2-D in vivo studies. To extend these studies to three dimensions, we compensated for motion and measured CBE in turkey breast muscle. 3-D data sets were assembled from images formed by a phased-array imager with a 7.5-MHz linear probe moved in 0.6-mm steps in elevation during uniform heating from 37 to 45°C in 0.5°C increments. We used cross-correlation as a similarity measure in RF signals to automatically track feature displacement as a function of temperature. Feature displacement was non-rigid. Envelopes of image regions, compensated for non-rigid motion, were found with the Hilbert transform then smoothed with a 3 x 3 running average filter before forming the backscattered energy at each pixel. CBE in 3-D motion-compensated images was nearly linear with an average sensitivity of 0.30 dB/°C. 3-D estimation of temperature in separate tissue regions had errors with a maximum standard deviation of about 0.5°C over 1-cm³ volumes. Success of CBE temperature estimation based on 3-D non-rigid tracking and compensation for real and apparent motion of image features could serve as the foundation for the eventual generation of 3-D temperature maps in soft tissue in a non-invasive, convenient, and low-cost way in clinical hyperthermia. [ABSTRACT FROM AUTHOR]

Ultrasound is an attractive modality for non-invasive imaging to monitor temperature of tumorous regions undergoing hyperthermia therapy. Previously, we predicted monotonic changes in backscattered energy (CBE) of ultrasound with temperature for certain sub-wavelength scatterers. We also measured CBE values similar to our predictions in bovine liver, turkey breast muscle, and pork rib muscle in both 1D and 2D in in vitro studies. To corroborate those results in perfused, living tissue, we measured CBE in both normal tissue and in implanted human tumors (HT29 colon cancer line) in 7 nude mice. Images were formed by a phased-array imager with a 7.5 MHz linear probe during homogeneous heating from 37° to 45°C in 0.5°C steps and from body temperature to 43°C during heterogeneous heating. We used cross-correlation as a similarity measure in RF signals to automatically track feature displacement as a function of temperature. Feature displacement was non-uniform with a maximum value of 1 mm across all specimens during homogeneous heating, and 0.2 mm during heterogeneous heating. Envelopes of image regions, compensated for non-rigid motion, were found with the Hilbert transform then smoothed with a 3 × 3 running average filter before forming the backscattered energy at each pixel. Means of both the positive and negative changes in the BE images were evaluated. CBE was monotonic and accumulated to 4-5 dB during homogeneous heating to 45°C and 3-4 dB during heterogenous heating to 43°C. These results are consistent with our previous in vitro measurements and support the use of CBE for temperature estimation in vivo during hyperthermia. [ABSTRACT FROM AUTHOR]

Noninvasive temperature imaging would enhance the ability to uniformly heat tumors at therapeutic levels. Ultrasound is an attractive modality for this purpose. Previously, we predicted monotonic changes in backscattered energy (CBE) of ultrasound with temperature for certain subwavelength scatterers. We also measured CBE values similar to our predictions in bovine liver, turkey breast, and pork muscle in one dimension (1-D). Those measurements were corrected manually for changes in the axial position of echo signals with temperature. To investigate the effect of temperature on CBE in 2-D, we imaged 1-cm thick samples of bovine liver, turkey breast, and pork muscle during heating in a water bath. Images were formed by a phased-array imager with a 7 MHz linear probe. Using radio frequency (RF) signals permitted the use of cross correlation as a similarity measure for automatic tracking of feature displacement as a function of temperature. Feature displacement across the specimen was nonuniform with typical total displacements of 0.5 mm in both axial and lateral directions. Apparent movement in eight image regions in each specimen was tracked from 37 to 50°C in 0.5°C steps. Envelopes of motion-compensated image regions were found then smoothed with a 3 ⊗ 3 running average filter before forming the backscattered energy at each pixel. Our measure of CBE compared means of both the positive and negative changes in the backscattered energy (BE) images. CBE was monotonic and differed by about 4 dB at 50°C from its value at 37°C. Relatively noise-free CBE curves from tissue volumes of less than 1 cm³ supports the use of CBE for temperature estimation. [ABSTRACT FROM AUTHOR]

Focuses on a study which investigated the applicability of a linear systems model with a simple approximation of the three-dimensional point-spread function. How the study was conducted; Results; Discussion.

A minimum-phase function, which characterizes the velocity dispersion in tissue was calculated from measured attenuation. This function was incorporated into a causal tissue model. Predictions of attenuation using the minimum-phase function with just a single pole matched measured attenuation in the 1- to 10-MHz range within a few percent. Dispersion of phase velocity predicted by the single-pole model was comparable to measured dispersion. The frequency associated with the single pole, which is a relaxation frequency, decreased with hemoglobin concentration and collagen content but increased with temperature. The electrical equivalent circuit for this model is a delay coupled with a low-pass filter which can be configured as a resistance in series with a parallel combination of resistance and capacitance. [ABSTRACT FROM AUTHOR]

Optical mapping has become an increasingly important tool to study cardiac electrophysiology in the past 20 years. Multiple methods are used to process and analyze cardiac optical mapping data, and no consensus currently exists regarding the optimum methods. The specific methods chosen to process optical mapping data are important because inappropriate data processing can affect the content of the data and thus alter the conclusions of the studies. Details of the different steps in processing optical imaging data, including image segmentation, spatial filtering, temporal filtering, and baseline drift removal, are provided in this review. We also provide descriptions of the common analyses performed on data obtained from cardiac optical imaging, including activation mapping, action potential duration mapping, repolarization mapping, conduction velocity measurements, and optical action potential upstroke analysis. Optical mapping is often used to study complex arrhythmias, and we also discuss dominant frequency analysis and phase mapping techniques used for the analysis of cardiac fibrillation.