Biomedical Engineering Reference
In-Depth Information
FOV = 3 cm
44
Scanning path
42
40
38
Focused transducer
FIGURE 14.7
(Left) Illustration of mechanical circular scanning with a spherically focused transducer. (Right) Average spatial temperature
distribution of during a 20-minute ultrasound hyperthermia exposure in a VX2 tumor in rabbit thigh using an FN = 2, 5 cm aperture transducer
operating at 2.787 MHz. Eight control points are used to control temperature rise.
Ultrasound hyperthermia allows deep seated tumors to be
treated since the sound energy can permeate farther into the tis-
sue. Ultrasound hyperthermia was investigated in combination
with radiation therapy starting in the 1970s [Marmor and Hahn,
1978], and much research into the field continued through-
out the 1980s and early 1990s. Clinically, ultrasound-induced
hyperthermia has been used with radiation to treat brain can-
cer [Shimm et al., 1988; Guthkelch et al., 1991] through a bone
window. However, interest in mild hyperthermia as an adjuvant
to radiation therapy in the brain has decreased since the 1990s,
and the focus has shifted toward higher intensity thermal abla-
tion procedures.
in FUS research [Hynynen et al., 1993a; Hynynen et al., 1993b;
Cline et al, 1993]. The safety and precision afforded by MRI-
guidance and monitoring, combined with the noninvasiveness
of through-skull treatments, has resulted in strong clinical inter-
est in thermal ablations in the brain. MR-thermometry can be
used to control thermal exposures real time during treatments
[Vanne and Hynynen, 2003], and MRI can confirm lesion for-
mation [Hynynen et al., 1994; Hynynen et al., 1997]. With
specific application in the brain, MR-thermometry has been
shown to be able to measure temperature rises below the ther-
mal damage threshold [Hynynen et al., 1997], and the recorded
temperature elevations can be related to the level of tissue dam-
age [Vykhodtseva et al., 2000]. To further improve safety, it has
been shown that the induced temperature elevations next to the
skull can be accurately recorded using MR-thermometry dur-
ing ultrasound heating of the brain [McDannold et al., 2004a].
The clinical prototype focused ultrasound brain system devel-
oped by InSightec (Haifa, Israel) described earlier has produced
promising clinical ablation results in two centers [Martin et al.,
2009; McDannold et al., 2010].
14.3.2 thermal ablation
In contrast to hyperthermia, in which modest temperature
rises are sought, thermal ablation employs higher intensities to
quickly induce high temperatures (55-60°C) and rapidly necrose
a tissue volume. Early experiments with ultrasound exposures
ranging in intensity and duration found that at lower intensities
(100 -1500 W/cm
2
), thermal effects dominate lesion formation,
whereas at high intensities (>2000 W/cm
2
) [Fry et al., 1970] cavi-
tation effects seem to be primarily responsible.
The first ultrasound brain ablations performed in humans
were conducted in the 1950s and 1960s for a range of neuro-
logical disorders including Parkinson's disease and phantom
limb pain [Fry and Fry, 1960]. These treatments were performed
through a craniotomy window, requiring the patient to undergo
traditional surgery to remove a portion of the skull. For this rea-
son, thermal ablation in the brain did not advance as quickly as
it might have. In fact, it was not until the late 1990s when ultra-
sound surgery through an intact skull was shown to be feasible
[Hynynen and Jolesz, 1998] that interest in ultrasound-induced
thermal ablation was revived. The introduction of MRI to guide
and monitor treatments was another important development
14.3.3 Cavitation Effects
The formation of vapor-filled cavities in a liquid was first
described by Reynolds over a century ago [Reynolds, 1894]. The
specific case of acoustic cavitation, vapor-filled cavities nucle-
ated during the negative pressure phase of an ultrasonic wave,
was described half a century later by Noltingk and Neppiras
[Noltingk and Neppiras, 1950; Neppiras and Noltingk 1951].
Cavitation can be either stable or inertial, and can refer to either
vapor- or gas-filled cavities [Neppiras 1980].
Inertial cavitation describes the formation and violent col-
lapse of bubbles within a liquid, while stable cavitation refers to
the oscillation of bubbles under a changing pressure field. The
collapse of bubbles during inertial cavitation produces high
