Brain tissue motion estimation: 2D speckle tracking using synthetic lateral phase technique

It is well known that the pulsatile motion of brain parenchyma results from cardiac and breathing cycles. During systole, the brain displaces in three directions, mediolateral, anteroposterior and cephalocaudal; followed by a slow return to the initial configuration during diastole. In a previous study we measured the mediolateral displacement (along the ultrasound beam direction) using traditional ID speckle tracking method. To a better description of the complex and imperfectly understood three-dimensional motion, we propose in this work to measure 2D displacement vector (mediolateral and anteroposterior component) of an imaged area of the brain through the right temporal window. A two dimensional speckle tracking based on the phase zero crossing detection of 2D correlation was applied to RadioFrequency (RF) signals to estimate the displacement vector. However, unlike the axial direction, the fundamental limitation for lateral tracking motion is the lack of RF phase information. The synthetic lateral phase (SLP) technique was then used to create synthetic phase in this direction. Zero crossing detection was then applied to the 2D complex phase of the synthetic RF signal. The amplitude of the anteroposterior displacement was greater (~60μm) than of the mediolateral one (~30μm) for the same region of interest. We can also notice a phase delay of about 24° between curves. The advantage of SLP technique is that there is no need to modify the emission/reception ultrasound process to create synthetic phase in the lateral direction.