Biomedical Engineering Reference
In-Depth Information
The use of echo shifts has received the most attention in the
last decade. By tracking scattering volumes and measuring
the time shift of received echoes, investigators have been able
to estimate temperature with encouraging preliminary
in vivo
studies. Acoustic attenuation is dependent on temperature, but
with significant changes occurring only at temperatures above
50°C. This property may lead to further development of its use
in high-temperature thermal ablation therapy. Minimal change
in attenuation, however, below this temperature range reduces
its attractiveness for use in clinical hyperthermia.
The change in backscattered energy is scatterer dependent.
Taking advantage of scatterer-dependent behavior enhances
the thermal signal. This behavior has been matched with novel
simulation methods for diverse scatterer populations and can be
enhanced with stochastic signal processing methods. Monotonic
thermal dependence of the change in backscattered energy has
been measured to 60°C. Temperature maps with 1-2°C accuracy
and 0.5 cm
2
spatial resolution can be produced routinely during
nonuniform heating
in vitro
.
All of the ultrasonic thermometry methods, just like tempera-
ture imaging from MRI, must be able to cope with motion of
the image features on which temperature estimates are based.
Echo shift methods track and exploit that motion. Motion must
be compensated in attenuation and CBE thermometry. Motion
tracking and compensation are usually the most computation-
ally intensive components of ultrasonic temperature imaging
and limit frame rates for temperature imaging.
Thermal therapies are poised for rapid development and
advancement due in part to a shift to volumetric temperature
imaging from sparse invasive thermometry, offering improved
monitoring as well as feedback for improved therapy control.
Noninvasive temperature imaging with ultrasound could better
(1) monitor and guide both hyperthermia treatment and high-
temperature ablation and (2) deepen our understanding of tissue
changes during hyperthermia and in ablation border zones now
performed blindly, with limited invasive thermometry, or more
expensive fixed-installation MRI thermometry that may limit
options for heating sources. A crucial step in identifying a viable
ultrasonic approach to temperature estimation is its performance
during
in vivo
tests. The potential for significant clinical impact
of ultrasonic thermometry is imminent with minimal addition, if
any, to the existing hardware of ultrasonic imaging systems.
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