Influence of attenuation on measurements of ultrasonic myocardial integrated backscatter during cardiac cycle: (an in vitro study)

The purpose of this study was to investigate the dependence of ultrasonic integrated backscatter (IB) and attenuation in myocardium on wall thickness in a state of acute ischemia. Therefore, an in vitro experiment was set up in which attenuation, IB and wall thickness of a piece of freshly excised myocardium could be measured almost simultaneously. The myocardium was taken from 11 Yorkshire pigs (25-30 kg) that were killed less than 45 min before the experiment. The myocardium was placed in the far field of an ultrasound transducer (3.2-7.2 MHz) and then compressed by a stainless steel sphere. Data were processed off-line. Backscatter and attenuation were also measured as a function of frequency at 100% and 75% wall thickness, respectively. Both attenuation and IB varied during compression. Attenuation had an initial value of 2.19 ± 0.76 dB/cm and a slope of 0.015 ± 0.017 dB/cm% wall thickness. IB had an initial value of -76.9 ± 2.7 dB and a slope of -0.12 +- 0.07 dB/% wall thickness. After subtracting the influence of the attenuation from the IB the initial value of IB was -74.0 ± 2.7 dB and the slope -0.08 ± 0.07 dB/% wall thickness. Attenuation appeared to have a linear dependency on frequency. Backscatter appeared not to increase with increasing frequency without correction of the spectrum for the frequency dependent insonified volume.