Quantitative ultrasonic assessment of normal and ischaemic myocardium with an acoustic microscope: Relationship to integrated backscatter

Purpose of investigation - The aim was to study ultrasonic propagation properties of normal and ischaemic myocardium with a scanning laser acoustic microscope and to correlate these changes with ultrasonic backscatter.Design - Myocardial ischaemia was produced by total occlusion of left anterior descending coronary artery in anaesthetised open chest dogs. Myocardium supplied by left circumflex coronary artery served as normal control. IBR5, an optimum weighted frequency average (4-6.8 MHz) of the squared envelope of diffraction corrected backscatter, was measured in vivo. Ultrasonic attenuation coefficient, an index of loss per unit distance, the propagation speed and heterogeneity index were measured from nonnal and ischaemic regions with a scanning laser acoustic microscope which operates at 100MHz in vitro. Myocardial water content of nonnal and ischaemic myocardium was also estimated.Subjects - Were five anaesthetised mongrel dogs.Results - Attenuation coefficient of 33.8(SD4.2) dB·mm^-1 in the ischaemic tissue was lower than 63.8(17.2) dB·mm^-1 in the normal tissue (p<0.01). Ultrasonic speed was lower in ischaemic than normal myocardium at 1584(25) v 1612(35) m·s^-1(p<0.05). Heterogeneity index of 11(7) m·s^-1 in the ischaemic region was lower than 14(8) m·s^-1 in the normal region (27% reduction, p<0.05). IBR5 and myocardial water content were higher in the ischaemic than the normal myocardium: -37.2(SEM1.8) dB v -46.6(0.6) dB, (p<0.01) and 80.9(0.0)% v 78(0.2)%, (p<0.05) respectively.Conclusion - Ultrasonic properties of the myocardium are significantly altered during acute ischaemia.