R. Martin Arthur

Theoretical estimation of the temperature dependence of backscattered ultrasonic power for noninvasive thermometry

The backscattered signal received from an insonified volume of tissue depends on tissue properties, such as attenuation, velocity, density, and backscatter coefficient and on the characteristics of the transducer at the insonified volume. Analysis of scattering in response to a burst of insonification showed that the temperature dependence of backscattered power was dominated by the effect of temperature on the backscatter coefficient. The temperature dependence of attenuation had a small effect on backscattered power. Backscattered power was independent of effects of temperature on velocity. These results were seen in the analysis of two types of inhomogeneity: 1) an aqueous scatterer in a water-based medium and 2) a lipid-based scatterer in the same water-based medium. The temperature dependence of the backscatter coefficient was inferred assuming that the backscatter coefficient was proportional to the scattering cross-section of a small scatterer. Backscattered power increased nearly logarithmically with temperature over the range from 37 degrees to 50 degrees C. Our model predicted a change of 5 dB for the lipid scatterer and a change of up to 3 db for the aqueous-based scatterer over that temperature range. For situations in which temperature dependence of the backscattered power can be calibrated, it may be possible to use the backscattered power level to track temperature distributions in tissue.

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.

Effect of Inhomogeneities on the Apparent Location and Magnitude of a Cardiac Current Dipole Source

A mathematical model was used to investigate the effects of electrical inhomogeneities introduced by the intracavitary blood mass and the lungs on the apparent location and magnitude of a current dipole source representing local activity in the myocardium. Radial and tangential current source moments were considered, and lung conductivity was taken as a variable parameter. Both the blood mass and the lungs cause a current source to appear closer to the heart center than it actually is. The effect increases as the source moves away from the endocardial surface, and maximum displacement is estimated to be of the order of 1 cm. The net effect of the inhomogeneities is to increase the magnitude of the radial component of a source dipole while diminishing its tangential component. These effects depend on source location.

Processing and analysis of cardiac optical mapping data obtained with potentiometric dyes

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.

The path of the electrical center of the human heart determined from surface electrocardiograms

Summary The path of the moving electrical center of the heart during P, QRS, and T has been derived from measurements of torso shape and surface electrocardiographic potentials alone. The locus of the path is within the heart border throughout the cardiac cycle, in the atria during P and in the ventricles during QRS and T. The general features of the path during P and QRS agree well with the known events of the activation sequence.

In vivo change in ultrasonic backscattered energy with temperature in motion-compensated images.

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.

3-D in vitro estimation of temperature using the change in backscattered ultrasonic energy

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 × 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.

Noninvasive detection of patients with ischemic and nonischemic heart disease prone to ventricular fibrillation

Abnormalities in the fast Fourier transforms of signal-averaged electrocardiograms (ECGs) obtained during sinus rhythm appear to distinguish patients with ischemic heart disease and sustained monomorphic ventricular tachycardia from those without ventricular tachycardia. This study was performed to determine the power of frequency analysis to detect patients with a history of ventricular fibrillation, to determine the extent to which spectra of signal-averaged ECGs from patients with ischemic and nonischemic heart disease are comparable and to compare results of signal-averaged ECG analysis in patients with ventricular fibrillation with results of programmed ventricular stimulation. Signal-averaged ECGs were obtained during sinus rhythm from 60 patients with sustained ventricular tachycardia (Group I) and 34 patients with ventricular fibrillation (Group II). Results of signal-averaged ECG analysis were abnormal in 92% of patients with ventricular tachycardia and 85% of patients with ventricular fibrillation (p = NS). Abnormal spectra were detected in the signal-averaged ECGs from 90% of patients with ischemic and from 86% of patients with nonischemic heart disease (p = NS). In contrast, the results of programmed stimulation differed markedly between the two patient groups. Sustained ventricular arrhythmias were induced in 91% of the patients with ventricular tachycardia compared with only 46% of those with ventricular fibrillation (p less than 0.0001). Moreover, ventricular tachycardia was inducible in 81% of patients with ischemic heart disease compared with only 50% of those with nonischemic heart disease (p less than 0.02). Thus, abnormalities in the spectra of signal-averaged ECGs were found in the majority of patients with ventricular fibrillation and were detectable even in those whose arrhythmia was not inducible by programmed stimulation. These results broaden the potential clinical application of noninvasive interrogation of signal-averaged ECGs to include the prospective identification of patients with ischemic or nonischemic heart disease prone to ventricular tachycardia or ventricular fibrillation.

Harmonic analysis of two-tone discharge patterns in cochlear nerve fibers

Discharges in cochlear nerve fibers evoked by low frequency phase-locked sinusoidal acoustic stimuli are synchronized to the stimulus waveform. Excitation and suppression regions of single units were explored using a stimulus composed of either a fixed intensity test tone at the characteristic frequency, a variable intensity interfering tone with a simple integer frequency relation to the characteristic frequency, or both. Compound period histograms were constructed from period histograms in response to normal and reversed polarity stimuli. Discharge patterns were characterized by Fourier components of the histogram envelopes. The two stimulus frequencies constituted the principal harmonics in the histogram envelopes and their combination accounted for observed rate changes. Suppression of the test tone harmonic as a function of interfering tone intensity was always seen; rate suppression was not. The harmonic was typically suppressed by 20–30 dB compared to the value for the test tone alone and often reached the 40–60 dB resolution limit of the experiment. Suppression plots were nearly linear on a power scale with an average slope of-0.8. The onset of suppression occurred for an interfering tone 9 dB greater on average than the test tone intensity. Information transfer through the peripheral system was described by the ratio of the principal harmonic amplitudes versus the ratio of the intensities of the two stimulus tones. These plots were nearly linear on a power scale with an average slope of 0.9. Neither the onset of suppression nor the slopes of the harmonic plots displayed strong dependence on characteristic frequency or interfering tone frequency. These features of harmonic behavior, however, are closely related to system nonlinearity. Comparison of measured harmonics to the predictions of two phenomenological models suggest the presence of complex nonlinear transformations in the peripheral auditory system.

Signal quality of resting electrocardiograms

Artifact may cause errors of technical origin when ECGs are interpreted by automatic methods. Baseline shift and high-frequency noise content of minimal and typical length ECG records from pediatric and adult populations were measured to allow prediction of both the likelihood of interpretation errors of technical origin and the number of reacquistions needed to obtain an artifact-free record. Ages of the 708 subjects in this study ranged from 2 weeks to 27 years. When a baseline shift of 0.25 mv (exceeded in 7% of the R-R intervals in the database) or a noise content greater than 15 muv RMS (exceeded in 6% of the R-R intervals in the database) within six seconds of three simultaneous leads was declared an unacceptable artifact, then 68% of the records from 0-4 year olds and 31% of the records from adults (greater than 19 years), were rejected on the basis of technical quality. These failure rates mean that, on the average, 3.1 tries would be needed to obtain an artifact-free record from 0-4 year olds; 1.4 tries would be needed for adults. If acquisition is done interactively, the measurement time for a 6-second, 3-lead group would be increased by 13 seconds for 0-4 years olds and by three seconds for adults in order to assure adequate signal quality for computer-assisted analysis.