Focused Ultrasound
Chapter 15
Chapter 15: Focused Ultrasound
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15
Introduction
Focused Ultrasound (FUS) has many potential and actual medical
applications ranging from creating transient cellular membrane
permeability (for targeted drug delivery) to thermal heating
(for tumor and diseased tissue ablation). This chapter covers the
following topics:
• FUS and other common abbreviations
• The history of FUS research and development for
medical applications
• Relevant physics and biological aspects for focusing
acoustic energy
• Variations of FUS (high intensity vs. low intensity and
thermal vs. mechanical effects)
• Different guidance systems used in conjunction with
FUS
• Clinical indications for FUS therapy
• Parameters of technology adoption
• A brief discussion of the future for this technology over
the next decade
1. Focused Ultrasound (General)
FUS refers to the deposition of ultrasound energy into a patient’s
body, using a beam that converges, starting out larger at the trans-
ducer surface, and becoming smaller as it reaches the focal point
(much like how a magnifying glass narrows a beam of light to a
single point). For example, an FUS procedure for the treatment of
uterine fibroids with a transducer diameter (D) of 12 cm creates
an ellipsoidal volume of effect with a diameter of Δx = 0.5 cm and
length of Δz = 2 cm (
Figure 1
).
Fig. 1
Salient features of a focused ultrasound beam. The
transducer acts much like a magnifying glass that can focus light
to a single point.
1.1 Definitions
As suggested by the name,high intensity focused ultrasound (HIFU)
and high intensity therapeutic ultrasound (HITU)—as opposed to
low intensity focused ultrasound (LIFU)—refer to focused ultra-
soundwith high power (Watts) transmission. MRgFUS orMR-HIFU
procedures refer to FUS procedures where magnetic resonance
imaging (MRI) is used to provide guidance and treatment control.
A transcranial MRgFUS (tcMRgFUS) procedure is a procedure
in which energy is directed to brain tissue through the skull. An
USgFUS procedure is a focused ultrasound procedure performed
with ultrasound guidance.
1.2 History of Focused Ultrasound
In 1935, Gruetzmacher published the first report on focusing ultra-
sound.He was able to focus ultrasound energy using a piezoelectric
transducer with a concave surface (a focusing technique discussed in
Chapter 5).From 1942 to 1955,both Lynn et al.and the Fry brothers
(Francis and William) were able to produce FUS thermal lesions
in bovine liver and cat brain, respectively. Clinical studies began
in 1959 when FUS was used to produce lesions in the pallidofugal
and nigral complexes of the brains of patients with hyperkinetic
(excessive muscle movement) and hypertonic (excessive muscle
tension) disorders.
Arik Hananel, MD, MBA, BsCs, Neal F. Kassell, MD, Jean-François Aubry, PhD
