Ultrasound myocardial integrated backscatter signal processing: Frequency domain versus time domain

In the literature, different forms of measuring the ultrasound power returned by myocardial tissue are reported. Frequency domain methods will give the maximum frequency information, whereas time domain methods are limited in bandwidth, but more practical to realize. It was the purpose of this study to compare the various methods of signal processing. High frequency ultrasound signals from a pig's myocardium, digitally recorded during normal contractile performance, were analyzed by six different methods of signal processing to obtain estimates of backscatter power. The myocardial tissue characterization parameters studied were the integrated power as well as its cyclic variation during the cardiac cycle. A total number of 8109 ultrasound traces obtained in 16 pigs were processed. The study included three signal processing methods in the frequency domain: frequency compensated integrated backscatter calculated over both a large (4 MHz, method 1) as well as a small frequency bandwidth (2 MHz, method 2) and uncompensated integrated backscatter (method 3), and three methods in the time domain: high frequency signal squared and integrated (method 4), mean rectified signal level (method 5) and mean signal level after logarithmic compression and envelope detection (method 6). The random measurement variation (including beat-to-beat variation) was analyzed as well as the paired differences of the backscatter parameters obtained by the respective methods as compared with the only theoretically correct method in the time domain (method 4). The magnitudes of the random measurement variation expressed as a standard deviation (SD) were comparable (range 0.93-1.2 dB) except for method 6 (0.61 dB), where the measurement variation is decreased by the logarithmic compression. Analysis of the variation (SD) of the paired differences of absolute backscatter levels, compared with the "gold" standard, resulted in 1.8 dB (method 2), 1.2 dB (method 3), 1.2 dB (method 4), 1.3 dB (method 5) and 1.5 dB (method 6). Comparison with method 4 yielded 1.2 dB (method 1), 0.8 dB (method 2), 0.0 dB (method 3), 0.4 dB (method 5) and 1.5 dB (method 6). The paired differences of cyclic variation showed the same tendencies. From these results, it can be concluded that a limited bandwidth of high frequency signals produce only small differences in the measurements of cyclic variation, even if the applied processing method is not strictly valid theoretically. Signal processing in the frequency domain offers the advantage of an extended frequency bandwidth. Using time domain methods with a limited bandwidth, single measurements of cyclic variation may deviate as much as ±2 dB (±2 SD) from those obtained in the frequency domain using the full frequency bandwidth. However, in normal myocardial tissue these differences appear to cancel out

The relationship between myocardial integrated backscatter, perfusion pressure and wall thickness during isovolumic contraction: An isolated pig heart study

The independent effect of myocardial wall thickness and myocardial perfusion pressure on integrated backscatter was investigated through experiments wherein integrated backscatter of normally perfused myocardial tissue was measured while changes in wall thickness during the cardiac cycle were reduced to a minimum. Results of the experiments show that integrated backscatter is mainly determined by myocardial wall thickness if only wall thickness and perfusion pressure are involved

Ultrasonic myocardial integrated backscatter and myocardial wall thickness in animal experiments

The purpose of this study was to distinguish between normal and ischemic myocardium using ultrasonic integrated backscatter (IB) measurements and to relate IB with myocardial wall thickness. IB was measured in 9 open-chested Yorkshire pigs (24-30 kg) before, after 30 minutes of partial occlusion of the proximal left anterior descending coronary artery (LADCA), and after 60 minutes of subsequent reperfusion. The ultrasound transducer (4 MHz) was sutured onto the epicardial surface perfused by the LADCA. IB measurements were made with a repetition rate of 50 times per heart rate simultaneously with a left ventricular pressure signal. Myocardial wall thickness was measured off-line. The measurements of integrated backscatter, left ventricular pressure and wall thickness were based on mean values of ten subsequent cardiac cycles. End-systolic IB measurements were 5.3 dB higher during occlusion as compared to the reference measurements (7.1 ± 3.2 dB versus 1.8 ± 2.6 dB; p = 0.002). No statistically significant differences were found in end-systolic IB measurements. End-systolic wall thickness was 5 mm smaller during occlusion as compared to the reference measurements (7.2 ± 1.4 mm versus 12.2 ± 1.2 mm; p < 0.001). Simple linear regression analysis showed a statistically significant inverse relationship between IB measurements and wall thickness in 21 out of the 23 sequences in which wall thickness could be measured. End-systolic IB measurements are favourable to distinguish acute ischemic myocardium from normal myocardium. There is a distinct inverse relationship between IB and myocardial wall thickness