Biomedical subjects

R Martin Arthur

Publications and source records attributed to R Martin Arthur.

5 recordsLinked to original sources

PubMed2005

Temperature dependence of ultrasonic backscattered energy in motion-compensated images.

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 degrees C in 0.5 degrees C steps. Envelopes of motion-compensated image regions were found then smoothed with a 3 x 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 degrees C from its value at 37 degrees C. Relatively noise-free CBE curves from tissue volumes of less than 1 cm3 supports the use of CBE for temperature estimation.

Algorithms↗

PubMed2004

Estimation of heart-surface potentials using regularized multipole sources.

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 +/-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.

Adult↗

PubMed2004

Estimation of surface pose with a physically-based ultrasonic image model.

State-of-the-art approaches to shape analysis in medical images use a variety of sophisticated models for object shape. We have developed an image model that permits the application of these approaches to ultrasonic images, with detailed methods for representing rough surfaces. Our physically-based, probabilistic image model incorporates the combined effects of the system point-spread function (PSF), the tissue microstructure, and the gross tissue shape. At each image pixel, the amplitude mean and variance are computed directly from the model, characterizing the combined influence of shape, microstructure, and system PSF. Calculation of the SNR0 is used to further classify each pixel as Rayleigh- or non-Rayleigh-distributed. This characterization was used here to generate a data likelihood representing any set of images of a given surface by a probability density conditioned on the surface pose, or rotation and translation. The utility of this likelihood was demonstrated by applying maximum likelihood estimation to infer the pose of a cadaveric vertebra from simulated images of its surface. Successful results were achieved using derivative-based optimization algorithms for a data set of only three images. With a quasi-Newton BFGS algorithm, error in 15 of 20 trials was less than 0.4 degrees in rotation and 0.2 mm in translation. Estimation was inaccurate in only 1 of 20 trials. These results illustrate the potential of a physically-based image model in a rigorous approach to image analysis and also serve as an example of quantitative assessment of the model via performance in a specific application.

Algorithms↗

PubMed2003

Detection of the fingerprint of the electrophysiological abnormalities that increase vulnerability to life-threatening ventricular arrhythmias.

Reduction of sudden death requires accurate identification of patients at risk for ventricular tachycardia (VT) and effective therapies. The Multicenter Unsustained Tachycardia Trial and Multicenter Automatic Defibrillator Implantation Trials demonstrate that the implantable cardioverter defibrillator impacts favorably on the incidence of VT in patients with myocardial infarction, underscoring the need to detect the electrophysiologic abnormalities required for the development of VT. Methods used for this purpose include: Holter monitoring, ejection fraction, signal-averaged ECG, heart rate variability, T-wave alternans, baroreflex sensitivity, and programmed stimulation. Performance of each method alone has demonstrated high-negative but low-positive predictive values. Recent studies confirm that their use in combination augments performance.A second approach for improving performance has been to reexamine how well each method detects the electrophysiological derangements that lead to VT. Our recent work has focused on the signal-averaged ECG. Judging from transmural maps of ventricular activation during VT and sinus rhythm obtained from patients, late potentials fail to detect completely signals from myocardium responsible for VT. To obviate this limitation we developed an approach based on inferred epicardial potentials in the frequency domain from 190-surface ECGs using individualized heart-torso models. Torso geometry and electrode positions are measured with a 3-armed digitizer. The location of cardiac structures is determined using echocardiography. The pericardial surface is approximated by a sphere that encloses the heart. Epicardial potentials are inferred using the boundary element method with zero-order Tikhonov regularization and the Composite Residual Smoothing Operator over the QRS complex. Studies are underway to determine if analysis of bioelectrical signals enveloping arrhythmogenic tissue improves identification of patients vulnerable to VT.

Arrhythmias, Cardiac↗

PubMed2003

Noninvasive temperature estimation based on the energy of backscattered ultrasound.

Hyperthermia has been used as a cancer treatment in which tumors are elevated to cytotoxic temperatures to aid in their control. A noninvasive method for volumetrically determining temperature distribution during treatment would greatly enhance the ability to uniformly heat tumors at therapeutic levels. Ultrasound is an attractive modality for this purpose. We investigated changes in backscattered energy (CBE) from pulsed ultrasound with temperature. Our predicted changes in backscattered energy were matched by in vitro measurements in samples of bovine liver, turkey breast, and pork rib muscle. We studied CBE in tissue regions with multiple scatterers, of isolated individual scatterers, and in collections of individual scatterers. The latter appears to have the most potential. We measured the CBE with a focused circular transducer with a center frequency of 7.5 MHz. The standard deviation of the CBE of 75-125 scattering regions from 0.3 to 0.5 cm3 volumes increased nearly monotonically from 37 degrees C to 50 degrees C in each tissue type. Although the slopes were different, the curve for each type of tissue was well matched by a second-degree polynomial, with a correlation coefficient of 0.99 in each case. Thus the use of the CBE of ultrasound for temperature estimation may have clinical promise with a convenient, low cost modality. Because our approach exploits the inhomogeneities present in tissue, we believe that if it is successful in vitro, it holds promise for in vivo application.

Animals↗