Measurements of the anisotropy of ultrasonic velocity in freshly excised and formalin-fixed myocardial tissue

Abstract

The objective of this study was to quantify the anisotropy of ultrasonic velocity in freshly excised myocardial tissue and to examine the effects of formalin-fixation. Through-transmission radio-frequency-based measurements were performed on ovine and bovine myocardial specimens from 24 different hearts. A total of 81 specimens were obtained from specific locations within each heart to investigate the possibility of regional differences in anisotropy of velocity in the left ventricular wall and septum. No regional differences were observed for either lamb or cow myocardial specimens. In addition, no specific species-dependent differences were observed between ovine and bovine myocardium. Average values of velocity at room temperature for perpendicular and parallel insonification were 1556.9 +/- 0.6 and 1565.2 +/- 0.7 m/s (mean +/- standard error), respectively, for bovine myocardium (N=45) and 1556.3 +/- 0.6 and 1564.7 +/- 0.7 m/s for ovine myocardium (N=36). Immediately after measurements of freshly excised myocardium, ovine specimens were fixed in formalin for at least one month and then measurements were repeated. Formalin-fixation appears to increase the overall velocity at all angles of insonification and to increase the magnitude of anisotropy of velocity.

Fig. 1

Experimental set-up for performing measurements of the anisotropic properties of ultrasonic velocity in myocardium.

Fig. 2

Comparison of ultrasonic velocities versus angle of insonification relative to the predominant direction of the myofibers for freshly excised ovine (N = 36 cylindrical specimens obtained from 12 ovine hearts) and freshly excised bovine (N = 45 cylindrical specimens obtained from 12 bovine hearts) myocardial tissue. Representative standard errors are shown.

Fig. 3

Effects of formalin-fixation on measurements of ultrasonic velocity in ovine myocardial tissue (N = 36 cylindrical specimens obtained from 12 ovine hearts). The top panel represents absolute measurements of ultrasonic velocity and the bottom panel shows the relative anisotropy with mean values subtracted. Formalin-fixation appears to result in an overall increase in ultrasonic velocity at all angles of insonification as well as a greater magnitude of anisotropy of ultrasonic velocity.

Fig. A1

Radiofrequency traces for water reference (top panel) and a representative myocardial specimen at perpendicular insonification (minimum attenuation) (middle panel) and at parallel insonification (maximum attenuation) (bottom panel).