Multiple HIFU beams are precisely focused on a small region of diseased tissue to locally deposit high levels of energy. Focused ultrasound can generate localized heating. Focusing can be guided by
Magnetic Resonance Imaging (MRgFUS). These procedures generally use lower frequencies than diagnostic ultrasound (0.7 to 2 MHz), but the higher frequency means lower focusing energy.
Temperature The temperature of tissue at the focus can be increased to between 65 and 85 °C. This induces
coagulative necrosis, destroying the tissue. Tissue heated above 60 °C for longer than 1 second becomes irreversibly damaged. Each
sonication (individual ultrasound energy deposition) treats a precisely defined portion of tissue. Multiple sonications cover a larger area, creating a volume of incompressible material, such as tap water. \mathit{CEM} = \int_{t_o}^{t_f} R^{T_{\mathrm{reference}}-T} dt with the
integral over the treatment time, R=0.5 for temperatures over 43 °C and 0.25 for temperatures between 43 °C and 37 °C, a reference temperature of 43 °C, and time T is in minutes. The equations and methods represent an approach for thermal dose estimation in an incompressible material such as tap water. An ultrasound acoustic wave cannot propagate through compressible tissue, such as rubber or human tissues. In that case the ultrasound energy is converted to heat. Using focused beams, a small region of heating can be achieved deep in tissues (usually on the order of 2~3 mm). Tissue changes as a function of the subtle shaking from the heated water within and the duration of this heating according to the thermal dose metric. Focusing at more than one place or by scanning, a volume can be ablated. Thermal doses of 120-240 min at 43 °C coagulate cellular protein and lead to irreversible tissue destruction.
Cavitation Inertial At high enough acoustic intensities,
cavitation (microbubbles forming and interacting with the ultrasound field) can occur. Microbubbles produced in the field oscillate and grow (due to factors including rectified
diffusion), and can eventually implode (inertial or transient cavitation). During inertial cavitation, temperatures increase inside the bubbles. The ultimate collapse during the rarefaction phase is associated with a
shock wave and jets that can mechanically damage tissue.
Stable Stable cavitation creates microstreaming, which induces high shear forces on cells and leads to
apoptosis. Bubbles produced by the vaporization of water due to acoustic forces oscillate under a low-pressure acoustic field. Strong streaming may cause cell damage, but also reduces tissue temperature via convective heat loss.
Theory Ultrasound can be focused in several ways—via a lens (for example, a
polystyrene lens, parabola curve
transducer, or a
phased array). This can be calculated using an exponential model of
ultrasound attenuation. The ultrasound intensity profile is bounded by an exponentially decreasing function where the decrease in ultrasound is a function of distance traveled through tissue: I=I_o {e}^{-2\alpha \mathrm{z}} I_o is the initial intensity of the beam, \alpha is the
attenuation coefficient (in units of inverse length), and z is the distance traveled through the attenuating medium (e.g. tissue). In this ideal model, \frac{-\partial I}{\partial \mathrm{z}} = 2\alpha I= Q is a measure of the
power density of the heat absorbed from the ultrasound field. This demonstrates that tissue heating is proportional to intensity, and that intensity is inversely proportional to the area over which an ultrasound beam is spread. Therefore, narrowly focusing the beam or increasing the beam intensity creates a rapid temperature rise at the focus. The ultrasound beam can be focused in several ways: • Geometrically, with a
lens or with a spherically curved
transducer. • Electronically, by adjusting the relative phases of elements in an array of transducers (a "
phased array"). This steers the beam to different locations. Aberrations in the ultrasound beam due to tissue structures can be corrected. This assumes no reflection, no absorption and no diffusion in intermediate tissue. The ultrasound itself can penetrate incompressible materials such as water, but compressible materials such as air, rubber, human tissue, fat, fiber, hollow bone, and fascia reflect, absorb, and diffuse the energy. ==Beam delivery==