Читать книгу Intracranial Gliomas Part III - Innovative Treatment Modalities - Группа авторов - Страница 63

To understand how ultrasound can be used to treat tumors in the brain, it is useful to first make definitions and review some general concepts, although a detailed description of the physics is beyond the scope of this chapter. Ultrasound generally refers to compressional waves at frequencies above the upper threshold for human hearing (>20 kHz). Similar to other wave phenomena, ultrasound can be focused by manipulating the source geometries, and carry energy. As the waves travel through the conducting medium (tissue), energy is lost, and the way in which it is transferred to the tissue governs the nature of the resultant biological effect. For example, for a given frequency, source diameter and excitation amplitude, a concave, spherically curved source produces a focal spot that is roughly the same size as that produced by a planar source but with a significantly greater intensity, correspondingly resulting in a greater rate of energy transfer. While considering ultrasound sources, therapeutic ultrasound typically involves frequencies in the range of hundreds kHz to several MHz. A higher frequency and larger aperture result in a tighter focus; however, the frequency must be balanced with the intended depth of the target, as penetration significantly decreases with increasing frequency.

In discussing the thermal ablation of tumors, attenuation is the most pertinent ultrasound interaction with tissue. It results from dissipation of energy due to both scattering and absorption, and dictates the depth of penetration. The contribution of scattering to the total attenuation is relatively small; the scattered energy is eventually also absorbed, but over a larger region. Absorption of ultrasound energy can occur as a result of properties of the medium itself or the boundaries of the medium; eventually all acoustic energy is converted to thermal energy. Losses take the form of viscous losses, heat conduction losses, and losses due to internal molecular processes. Viscous losses occur when there is relative motion between parts of the medium, as occurs with the compression and rarefaction associated with the propagating pressure wave. Losses due to heat conduction occur via the transfer of thermal energy from higher temperature compressive regions to lower temperature regions of expansion. Losses that occur at the molecular level may be a result of the conversion of kinetic energy to potential energy, absorption into rotational or vibrational energy, and even the association and dissociation of different ionic species and complexes in solution [4]. In effect, with every cycle of the ultrasound wave, energy is transferred from the wave to the medium but returns slightly out of phase due to the finite time required for this transfer to occur, resulting in absorptive losses [5].