5 / Control and 3D real-time correction of motions for ultrasound therapy.

One of the fundamental limits of the H.I.F.U. (High Intensity Focused Ultrasound) in abdominal organs (liver, kidney, …) is the patient motions (mainly breathing motions) during treatment. Most of the actual system are based on a technique using a single-transducer spherical dish. So the focal geometrical spot is fixed during treatment. Breathing motions could lead to 1 cm displacement around the fixed point which induce a decrease of the treatment effectiveness. Moreover, we are not sure in this special case that the targeted zone is completely necrosed. At this time, we are the only team to suggest and validate an innovative technique of real-time correction of the ultrasound beam during a movement of the targeted zone [1].

Fig. 1: Correction of motion system for the focused ultrasound therapy. Four groups of transducers of the 200 singles transducers implanted on the system are used to assess the motions between two heats in the four different directions.

Fig. 2: Necrosis obtained by ultrasound therapy on liver samples during motions : (a,b) in the case of an horizontal motion of 10 mm, a) without correction motion. b) same experiment with correction motion.

In this concept of motion correction, the principle is to use some group of transducers of a therapy network for estimate in real-time (every 1/20 seconds) the motion of the targeted zone in different directions (Fig. 1). The displacements are assesses by emitting from each group of transducers some ultrasound pulse and by recording the successive speckle of the tissues. By crosscorrelation of successive speckles it is possible to estimate the local displacement of the tissues between to shot. Like this, one can get instantaneously the 3D zone motion and correct in real-time focused ultrasound beam in order to necrosed only the targeted zone. Heating time and correction assessment are interlaced in time. This technique is very fast and robust, since it can be executed in thousands of seconds. Thus, during a treatment, more than 95 % of the time could be used to burn tissues. This technique, the first in the world, has been implemented on our multi-elements electronic and tested in vitro (Fig. 2) on liver samples [1].