Page 12 - Volume 8, Issue 4 - Winter 2012

APPLICATIONS OF TRANSCRANIAL FOCUSED ULTRASOUND SURGERY
Daniel Pajek and Kullervo Hynynen
Physical Sciences Platform, Sunnybrook Research Institute Toronto, Ontario, Canada
and
Department of Medical Biophysics, University of Toronto Toronto, Ontario, Canada
“It was not until the adoption of magnetic resonance imaging and thermometry for the guidance of focused ultrasound that transcranial focused ultrasound surgery became a clinical reality.”
Introduction to transcranial focused ultrasound
Focused ultrasound is capable of
delivering energy into tissue, non-
invasively and without the use of
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ionizing radiation. The ability of
focused ultrasound to generate heat in tissue was demonstrated in the brain decades ago, with the creation of lesions in the mammalian central nerv-
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ous system. A spherically focused
ultrasonic transducer causes emitted
ultrasound waves to superimpose con-
structively at a focus, leading to very
high energy deposition within a small volume, of a size pro- portional to the wavelength. Focused ultrasound is an emerging non-invasive alternative to surgery and an alter- native to radiation therapy. This has led to the use of ultra- sound in non-invasive hyperthermia and ablative applica-
which can emit ultrasound waves with independently set phases and ampli- tudes. Phased arrays can be used to elec- tronically steer an ultrasound focus within a volume by applying appropriate delays to the RF-signals driving the transducer elements. With a priori knowledge of the transmission delay faced along each beam path, phases of the RF-signals driving each of the trans- ducer elements can be set to ensure that emitted waves all arrive coherently at the focus. There are a number of tech- niques for measuring the phase correc-
tions or time delays required for transcranial focusing. These include placing a hydrophone at the intended focus and emit- ting from each element independently4 or placing an acoustic point source at the focus and measuring the received signal at
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each array element. Although potentially less invasive than a
full craniotomy, these techniques still require some form of surgery to achieve the measurements required for transcra- nial focusing.
It was not until the development of CT-based correction
techniques that completely non-invasive transcranial focused
ultrasound became possible. CT-based phase correction
models rely on spatially varying skull thickness and density
information derived from CT images, which can be used to
estimate both the speed of sound and attenuation along each
tions, such as the treatment of uterine fibroids.
3
Hurdles preventing the use of focused ultrasound in the skull
For many years, the brain remained an elusive target and
it was believed that ultrasonic brain therapy could only be
1
accomplished through a cranial window. The increased
speed of sound in the skull results in severe distortion of the ultrasonic focus, the large impedance mismatch between water and bone results in much of the acoustic energy being reflected away from the skull, and the high attenuation of ultrasound in bone greatly reduces the intensity of the
4,5
beam path.
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Similarly, inverse calculations using time-
acoustic wave after it passes through the skull. Furthermore, with standard transducers, the high absorption of ultrasound in bone meant that undesired heating of the skull could occur before sufficient heating of the underlying tissue would be achieved.6 These issues were not reasonably addressed until the 1990s, with the development of large aperture phased arrays,7,8 illustrated in Fig. 1. This enabled the incident ultrasonic energy to be spread over a large sur- face area producing large gains, which were sufficient for safe heating. The use of a multi-element phased array allows one to electronically correct for the skull’s distortion to restore a strong focus within the brain (Fig. 2).
Development of computer tomography (CT) -based correction
reversal methods have been proposed for precise through-
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skull focusing. Transcranial therapy has been further
refined through the use of amplitude correction, which can be optimized to either maintain a spatially uniform focus or
Phased arrays are composed of many small transducer elements, each connected to independent driving amplifiers,
Although CT imaging enables non-invasive focusing in theory, in practice, difficulties related to image registration have lead to targeting inaccuracies, making open-loop treat- ments risky. It was not until the adoption of magnetic reso- nance (MR) imaging and thermometry for the guidance of focused ultrasound that transcranial focused ultrasound sur- gery became a clinical reality. With the addition of image guidance and feedback, focused ultrasound grew into a safe and potentially viable treatment alternative.13 These systems relied first on MR imaging to register previously acquired CT data to patient position and determine the proper phase cor- rection values required for transcranial focusing. Second,
8 Acoustics Today, October 2012
minimize localized hotspots on the skull.
Treatment monitoring
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