Introduction to Acoustics

High intensity focused ultrasound harmonic
concentration at focus
Pressure (MPa) Axial pressure
8
6
4
2
0
Computed linear envelope
Computed nonlinear envelope
Ambient 1 atm pressure
Zero absolute pressure
–2 Linear
–4 Nonlinear
Linear envelope
–6
20 25 30 35 40
Geometric focal depth Axial distance (mm)
Fig. 21.72 Computed central beam instantaneous pressures
from a HIFU transducer. The high acoustic pressures in the
focal zone create nonlinear effects that generate harmonics
in the transmitted ultrasound beam (courtesy F. Curra)
1. stopping internal bleeding of the liver or spleen,
2. stopping bleeding due to catheter procedures,
3. reducing the size of tissues such as uterine fibroids
and prostate hypertrophy,
4. destroying cancer cells and tumors,
5. birth-control procedures such as vasectomy and
tubal ligation,
6. tissue welding, and
7. pushing tissues using radiation force to stretch or
compress them.
Thermal dose is one way to summarize hyperthermia
treatment conditions. Thermal dose implies that
tissue changes occur above 43 ◦C, and that the effect
is increased with increasing time and increasing temperature.
One empirical formula for thermal dose has been
given [21.81]as:
�t2
D = n(T) (T−43) dt ,
t1
21.8 Lithotripsy of Kidney Stones
Explosive ultrasound can be used to fracture kidney
stones in the renal pelvis where the urine is collected
from the kidney and directed down the ureter to the
bladder. Stones which form in the renal pelvis from
solutes in the urine are very painful. The pain occurs
because urine is moved down the ureter by peristaltic action,
and the peristalsis cannot handle a bolus the size of
Medical Acoustics 21.8 Lithotripsy of Kidney Stones 891
1 MPa
1.2 MPa
1.6 MPa
2.6 MPa
8 MPa
25 W/cm 2
40 W/cm 2
80 W/cm 2
200 W/cm 2
2000 W/cm 2
Fig. 21.73 Typical HIFU ultrasound beam profile. Highintensity
focused ultrasound beam profile showing increase
in beam intensity, not accounting for attenuation. The small
circles post focus represent bubbles formed by a combination
of mechanical-index-induced cavitation boiling. They
are pushed into the post-focus zone by radiation pressure.
As the exposure continues, more bubbles will be formed at
shallower depths. Post-focal bubbles may provide a safety
shield, preventing HIFU exposure of deeper, healthy tissue
where t1 and t2 are the initial and final times of the heating
profile, respectively, and n(T) is4forT < 43 ◦ C
and 2 for T > 43 ◦ C. The temperature is determined by
the rate of heat delivery per tissue volume, which is
the product of the intensity times the attenuation minus
the rate at which heat is removed from the tissue
by conduction through tissues, and convection via the
blood.
The most difficult issue with HIFU is the small treatment
volume versus the relatively large access window.
Under ideal conditions, the treatment of a single volume
of tissue 10 mm 3 takes about 2 s. A cubic region 50 mm
on a side has a volume of 125 000 mm 3 and requires
nearly 4 h to treat. These long treatment times present
a considerable economic problem for the adoption of the
technology.
a stone. One type of ultrasound lithotripter uses a 10 cmdiameter
concave reflector with a spark gap at the source
focus. The elliptical curvature forms a target focus 12 cm
from the face of the reflector. Equally intense ultrasound
pulses can be formed using focused piezoelectric transducers
or electromagnetically driven membranes with
the ultrasound focused using an acoustic lens.
Part F 21.8