Daniel T. Barry

Muscle sounds are emitted at the resonant frequencies of skeletal muscle

The changes in mechanical resonant frequency of whole muscles during twitch and tetanic contractions are compared to changes in frequency components of the pressure wave produced by muscles during contraction. Resonant frequencies were determined by imposing sinusoidal length changes on a muscle and observing transverse standing waves when the frequency of length change matched the muscles resonant frequency or a harmonic of the resonant frequency. Acoustic signal instantaneous frequency spectra were calculated using time-frequency transformations including the Wigner transform and the exponential distribution. During a tetanic muscle contraction, the peak instantaneous frequency initially increased and then became constant as the force plateau was reached. The resonant frequencies determined during the force plateau and during relaxation spanned the same range as the peak instantaneous frequency of the acoustic signal. These results suggest that the acoustic signal may be useful as a noninvasive monitor of muscle resonant frequency during contraction.<<ETX>>

Linear signal synthesis using the Radon-Wigner transform

Thresholding the Radon-Wigner transform (RWT) followed by filtered backprojection to the time-frequency plane reduces noise and cross-term power for multicomponent linear-FM signals. Although the RWT is bilinear, it may be calculated as the magnitude-squared of two linear functions. In this paper, we derive a linear synthesis algorithm from the RWT and demonstrate its efficacy. >

Time-frequency transforms: a new approach to first heart sound frequency dynamics

The binomial joint time-frequency transform is used to test the hypothesis that first heart sound frequency rises during the isovolumic contraction period. Cardiac vibrations were recorded from eight open-chest dogs using an ultralight accelerometer cemented directly to the epicardium of the anterior left ventricle. Three characteristic time-frequency spectral patterns were evident in the animals investigated: (1) a frequency component that rose from approximately 40-140 Hz in a 30-50-ms interval immediately following the ECG R-wave, (2) a slowly varying or static frequency of 60-100 Hz beginning midway through the isovolumic contraction period, and (3) broadband peaks occurring at the time of the Ia and Ib high frequency components. The binomial transform provided much better resolution than the spectrograph or spectrogram. By revealing the onset and dynamics of first heart sound frequencies, time-frequency transforms may allow mechanical assessment of individual cardiac structures.<<ETX>>

Time-frequency analysis of skeletal muscle and cardiac vibrations

Skeletal muscle and the heart vibrate during contraction producing nonstationary signals whose time-varying frequency reflects dynamic changes in physiological properties. Consequently, pathological changes in the mechanical integrity or loading of skeletal muscle or the heart can be expected to alter their vibrations. Classic frequency analysis techniques have been inadequate to characterize these subtle changes because of rapidly varying frequency components. A poor understanding of heart and muscle sound generation has also limited investigations. This paper demonstrates how time-frequency (TF) techniques have illuminated the relationships between muscle/heart material properties and loading and frequency dynamics of heart and muscle vibrations. Studies of evoked twitches from frog skeletal muscle reveal that muscle vibrations occur as transverse oscillations at the muscles resonant frequency. Using a classic Rayleigh-Ritz model and crude estimates of the muscle geometry, muscle force can be accurately predicted from the muscle sound TF profile. First heart sound vibrations, in contrast, are shown to be a nonresonant phenomena, consisting of propagating transients superimposed upon bulk acceleration of myocardial contraction. Consequently, first heart sound frequency dynamics depend upon cardiac electrical excitation and hemodynamic loading in addition to intrinsic material properties and geometry, necessitating further work to characterize pathophysiologic correlations.

Radon transformation of time-frequency distributions for analysis of multicomponent signals

The Radon transform of a time-frequency distribution produces local areas of signal concentration that facilitate interpretation of multicomponent signals. The Radon transform can be efficiently implemented with dechirping in the time domains; however, only half of the possible projections through the time-frequency plane can be realized because of aliasing. It is shown that the frequency dual to dechirping exists, so that all of the time-frequency plane projections can be calculated efficiently. Some Radon transforms of Wigner distributions are demonstrated.<<ETX>>

Fast calculation of the Choi-Williams time-frequency distribution

The method presented allows faster calculation of any time-frequency distribution with a kernel that can be formulated in the time-lag plane. Specific examples are the Wigner and Choi-Williams distributions. The Choi-Williams distribution (CWD) uses an exponential kernel in the generalized class of bilinear time-frequency distributions to achieve a reduction in the cross-term components of the distribution. Matrix manipulations provide an intuitive approach and, when combined with parallel processing, improve the processing speed to allow real-time calculations of the CWD. The use of an outer product matrix with a weighting matrix is particularly useful when evaluating different weighting parameters. For any given signal, the outer product matrix needs to be calculated just once. The various weighting matrices can be stored and used with any signal when needed. Parallel processing architectures allow implementation of the algorithm with speeds that are appropriate for real-time, running window calculations. >

Spatial Variation Of First Heart Sound Frequency Dynamics Across The Canine Leff Ventricle: A Comparison Of Intracardiac And Epicardial Recordings

Cardiac compressional and shear wave contribution to the first heart sound was investigated by comparing the frequency and phase of epicardial and intraventricular recordings. We hypothesized that the impulse-like closure of the mitral valve would dominate intraventricular recordings in contrast to the rising frequencies observed in epicardial recordings. However, intraventricular and epicardial recordings often exhibited very similar time-frequency spectra with a powerful rising frequency component beginning prior to mitral valve closure. Time-frequency spectra varied little across the left ventricle but no simple phase relationships were observed among the recording sites. While mitral valve closure produced a prominent high frequency component, it did not profoundly alter the underlying signal dynamics. These findings suggest that myocardial contraction plays a significant role in first heart sound initiation and frequency modulation.

Time Frequency Transforms Of The Human First Heart Sound

The frequency dynamics of the human first heart sound were assessed at 27 locations across the human thorax using Reduced Interference time-frequency transforms. Time-frequency transformation demonstrated prominent impulse-like components in the superior thorax. In this region, signal dynamics were well described by the instantaneous power and the power spectrum. Spectra from the inferior thorax, however, revealed signal components whose frequency rose rapidly during the isovolumic contraction period. Statistics based upon the joint time-frequency spectrum, such as the instantaneous modal frequency, were necessary to capture heart sound frequency variation. These studies demonstrate that the externally recorded human first heart sound is a non-stationary signal and suggest that time-frequency transforms may be useful in relating human first heart sound frequency dynamics to the underlying vibrational mechanics.

Acoustic Myography Frequencies Track The Rise Of Skeletal Muscle Tetanic Tension

We tested the hypothesis that muscle sound frequencies emitted during isometric contractions can be used to determine muscle tension. Frog semitendinosus muscles were isolated, held isometric, and tetanically stimulated while acoustic signals and force were recorded. The acoustic signals obtained had a characteristic chirp-like waveform, with rapidly rising frequency beginning at -50 Hz near the onset of contraction. The muscle was modeled as a string with distributed mass. The model was combined with resonant frequencies determined from acoustic myography data to predict muscle tension. These predictions matched direct measures of muscle tension, showing that temporal variations in muscle tension can be monitored acoustically during muscle contraction.

Clinical Applications Of Acoustic Myography

Some of the problems encountered in recording muscle sounds clinically include transducer calibration, adequate transducer bandwidth, and movement artifacts. Electrically stimulated muscle contractions eliminate movement artifacts and generate reproducible acoustic waveforms with high signal-to-noise ratios. Accelerometers allow calibration in physiologic units. Lateral muscle acceleration measurements from stimulated muscle twitches reveal a decline in signal amplitude that parallels the decline in force with muscle fatigue.