Quasi-tomographic ultrasound imaging of the brain
BEP/MEP project – april 2018
Deep brain stimulation (DBS) is a technique in which brain function is modified locally by means of electrical signals from an implanted electrode. Electrodes are usually placed deep in the brain and are connected to a pulse generator similar to a heart pacemaker. Precise localization of the DBS electrodes inside the brain is a key to success of such a treatment. Transcranial ultrasound imaging could be an easy-to-use, accurate and cheap modality for detecting the position of the electrodes in DBS procedure. However, transcranial ultrasonic brain imaging on adults is currently limited by the strong aberrating effect of the skull bone. In order to obtain a good ultrasound image, it is necessary to focus ultrasound beams inside the brain and reduce the sidelobes of the ultrasound beam to increase resolution and contrast respectively. The skull bone is absorbing the ultrasonic wave and the speed of sound is not homogeneous inside the skull, so that the phase of the wave front is locally shifted. As the speed of sound and absorption inside the skull are spatially dependent and are a priori unknown, it is necessary to use adaptive methods to focus through the skull.
Figure 1: Demonstration of the experimental setup for this project
The ultimate goal of this master project is achieving image quality sufficient to reliably differentiate various anatomical structures in the brain as well as the location of the DBS electrodes in respect to those anatomical structures (e.g. subthalamic nucleus).
Various techniques have been proposed for aberration correction in skull such as: using an alternative imaging modality (MRI or CT) to obtain information about the skull to compute the expected aberrations (Sun Hynynen 1998 and 1999, Clement and Hynynen 2002a and 200b, Aubry et al. 2003, Hynynen et al. 2006); Point-target based aberration correction (Flax and O’Donnell 1988, Zhao and Trahey 1991, Fink 1992, Pernot et al. 2006, Kripfgans et al. 2002, Psychoudakis et al. 2004); Obtaining the approximate aberration of the skull using two arrays that are placed on opposing sides of the skull, one on each of the parietal (or temporal) bones (Vignon et al 2006).
Following the approach proposed by Vignon et al. we have developed a setup with 2 phased array transducers (P4-1) positioned opposite to each other. Each transducer has a centre frequency of 2.5 MHz, 96 elements of 16 mm long, and a pitch of 0.3 mm. These clinical transducers are hardwired to an research ultrasound module (Verasonic) (figure 1).
- Literature review of the proposed approaches and creating a systematic comparison between the methods and their pros. and cons.
- Calibration of the system: large realization of the transmit and receive data (96 × 96) in this setup gives the opportunity to calculate the translational misalignments
(up-down and left-right) as well as the angular misalignments (tilt and rotation) independently. The arrival time of the acoustic wave can be used to detect such misalignments and correct for them in the rest of the measurements.
- Realization of the state of the art aberration correction technique based on the dual transducer approach for phased array transducers.
- Implementing the calibration, imaging and phase correction sequences on the Verasonics system.
- Performing in-vitro experiments on simplified situation (water thank).
- Performing in-vitro experiments on human skull model.
- Performing in-vivo experiments on volenteers.
Overview of the literature
Aubry JF, Tanter M, Pernot M, Thomas JL, Fink M. Experimental demonstration of noninvasive trans-skull adaptive focusing based on prior computed tomography scans. J Acoust Soc Am 2003;113: 84–93.
Clement GT, Hynynen K. Correlation of ultrasound phase with physical skull properties. Ultrasound Med Biol 2002a;28:617– 624.
Clement GT, Hynynen K. A noninvasive method for focusing ultrasound through the human skull. Phys Med Biol 2002b;47:1219– 1236.
Fink M. Time-reversal of ultrasonic fields - part I: Basic principles. IEEE Trans Ultrason Ferroelec Freq Control 1992;39:555–566.
Flax S, O’Donnell M. Phase-aberration correction using signals from point reflectors and diffuse scatterers: Basic principles. IEEE Trans Ultrason Ferroelec Freq Control 1988;35:758 –767.
Haworth KJ, Fowlkes JB, Carson PL, Kripfgans OD. Towards aberration correction of transcranial ultrasound using acoustic droplet vaporization. Ultrasound in Medicine and Biology. 2008 Mar 1;34(3):435-45.
Hynynen K, McDannold N, Clement GT, Jolesz FA, Zadicario E, Killiany R, Moore T, Rosen D. Preclinical testing of a phased array ultrasound system for MRI-guided noninvasive surgery of the brain—a primate study. Eur J Radiol 2006;59:149 –156.
Kripfgans OD, Fowlkes JB, Woydt M, Eldevik OP, Carson PL. In vivo droplet vaporization for occlusion therapy and phase aberration correction. IEEE Trans Ultrason Ferroelec Freq Control 2002;49: 726–738.
Pernot M, Montaldo G, Tanter M, Fink M. “Utrasonic stars” for time-reversal focusing using induced cavitation bubbles. Appl Phys Lett 2006;88:1–3.
Psychoudakis D, Fowlkes JB, Volakis JL, Carson PL. Potential of microbubbles for use as point targets in phase aberration correction. IEEE Trans Ultrason Ferroelec Freq Control 2004;51: 1639 –1648.
Sun J, Hynynen K. Focusing of therapeutic ultrasound through a human skull: A numerical study. J Acoust Soc Am 1998;104: 1705–1715.
Sun J, Hynynen K. The potential of trans-skull ultrasound therapy and surgery using the maximum available surface area. J Acoust Soc Am 1999;105:2519 –2527.
Vignon F, Aubry JF, Tanter M, Fink M. Adaptive focusing for transcranial ultrasound imaging using dual arrays. J Acoust Soc Am 2006;120:2737–2745.
Zhao D, Trahey GE. Comparisons of image quality factors for phase aberration correction with diffuse and point targets: Theory and experiments. IEEE Trans Ultrason Ferroelec Freq Control 1991;38: 125–132.