Molecular Mechanisms Contributing to Ultrasonic Absorption by Tissues

The interpretation of data presented by ultrasonic visualization techniques may be expected to benefit from an understanding of the mechanisms of the predominant contributions to the total ultrasonic absorption coefficients of tissues. Since aqueous solutions of hemoglobin have been most extensively investigated, analyses of specific molecular mechanisms have been undertaken with relation to the structure of this macromolecule and its environment. The contributions that could arise from reactions occurring at the charged side chains of residues of arginine, lysine, aspartic acid, and glutamic acid were considered since these residues occupy positions with highest frequency of occurrence at the hydrophilic exterior of the hemoglobin molecule. On the assumption of a structural mechanism, a family of possible single relaxation curves was calculated from which one member could be selected with knowledge of an experimental value of relaxation time. We conclude that basic side chains make significant contributions, which are larger than those of acidic side chains. [Work supported by a grant from the National Institutes of General Medical Sciences.]