Acoustical properties of the human skull.

The acoustical properties insertion loss, reflection loss, and sound speed were measured on a series of fresh and subsequently formalin immersed human skulls. Measurements were made in the frequency range from 0.25 to 6 MHz. Most studies were restricted to an upper frequency limit of 2.2 MHz. An axisymmetric focused beam configuration was used as the sound source for the measurements and the receivers were small disk-type (3-mm-diam) piezoelectric ceramics. The geometric and temporal character of the focused beam was studied as a consequence of passage through the skull sections. Some skulls were sectioned so that their individual layer components could be studied. A simple three-layer analytical model seems to explain the major aspects of insertion and reflection loss. The dominant feature in determining human adult skull losses is the middle layer (diploe) of cancellous bone. This study corroborates previous work on insertion loss as a function of frequency for composite skull. The study provides new quantitative information on the acoustic scattering properties of diploe, sound velocity, and dispersion in composite skull and its components, attenuation coefficients in skull components and sound-beam distortion and shifts after transmission through composite skull. We conclude that with selection of appropriate frequencies (0.5-1.0 MHz) and beam configuration it will be possible to perform clinically significant transkull diagnostic imaging and interrogation in the adult human brain.