The acoustic characteristics of the skull

Abstract

Many investigators have stated that the difficulties of imaging with acoustical energy through the skull result from the marked attenuation of the energy by the skull. In the literature measurements of total attenuation have been confused with those for absorption. Measurements made by us show that absorption by compact bone varies between 2–3 dB cm −1 MHz −1 and, in the low megaHertz region appears to be directly proportional to frequency. It has also between shown that the convoluted inner surface of the ivory bone of the inner table of the skull may degrade the collimation and directionality of the beam by refraction. Cancellous bone, such as is present in the dipole of the skull, greatly attenuates the energy. It is postulated that this largely results from scattering. It is also postulated that the energy propagates through cancellous bone as two components, one in the soft tissues and the other partly in the bony spicules. Observations suggest that attenuation due to scattering much more markedly affects the latter of these components and scatters more greatly the higher frequencies in a pulse of broad bandwidth. The energy in each component has varying propagation paths so that the later cycles in the pulse of each component are subject to increasing interference as a result of the variations in propagation times. The two components moreover may have different propagation times so that interference may occur between the pulses of each component as well. All of these phenomena degrade the collimation, coherence, directionality, beam width, pulse length, frequency and other properties of the ultrasonic energy upon which imaging through the skull depends. The interference effects described above are least for the first cycle in the pulse which usually is not the cycle of highest amplitude. Since, in the free field, most of the energy is concentrated around the beam axis, most of the energy in the field which is deflected from its normal propagation path is deflected away from the beam axis. Thus the directionality of the beam is least degraded in the beam axis. The effects of the skull in degrading the properties of the ultrasonic pulse would therefore be lessened if the amplitude of the first cycle of the pulse and the directionality of its energy could be used for imaging.