# Iterative 2D Tissue Motion Tracking in Ultrafast Ultrasound Imaging
## Materials and Methods
### Cine Loops  
The phantom cine loops were collected in Japan at a pulse repetition frequency of 5208 Hz by a 96-channel ultrasound scanner (RSYS0002, Microsonic, Tokyo, Japan) equipped with two linear array ultrasonic transducers with a center frequency of 7.5 MHz. The transducers had a pitch of 100 \(\upmu\)m and 200 \(\upmu\)m, respectively, which corresponds to \(\lambda/2\) and \(\lambda\). The pixel densities in the cine loops were 40.6 mm\({}^{-1}\) axially and either 10.0 mm\({}^{-1}\) or 5.0 mm\({}^{-1}\) laterally depending on the pitch of the transducer. Each transducer was moved diagonally repeatedly back and forth at constant velocity with short stops at each turning point (Figure 2) using automatic stages (ALS-6012-G0M and ALV-600H0M, Chuo Precision Industrial, Tokyo, Japan). A sponge was used to create realistic speckle. Two different velocities were used: 2.0 mm/s laterally and 1.0 mm/s axially with displacements of 1.0 mm laterally and 0.5 mm axially; and 1.0 mm/s laterally and 0.5 mm/s axially with displacements of 0.5 mm laterally and 0.25 mm axially. In the lateral direction, this corresponds to a displacement of 1.5 \(\upmu\)m/frame and 0.77 \(\upmu\)m/frame, respectively. The collected radio frequency data were beamformed using delay-and-sum [13]. In the present study, one frame was obtained from four plane wave transmissions resulting in a frame rate of 1302 Hz. A plane wave is transmitted with 96 active elements, and echo signals were received by the same elements. Each receiving beam was created using the echo signals obtained from 72 of 96 elements. Consequently, 24 receiving beams were created in one transmit event. Then, the active aperture was translated laterally by 24 elements, and the same procedure was repeated four times to obtain 96 receiving beams. In receive, a Hanning apodization was used. Cine loops were also collected at a lower frame rate (41 Hz) with a conventional linear scan scheme. These cine loops used the same transducers, transducer movements, and kernel size, while the motion estimations were conducted in every frame (\(k=1\), see above).  
To evaluate the feasibility of using the proposed 2D tissue motion estimation tracking scheme in vivo, a limited in vivo study was conducted in Sweden. The in vivo experiment was performed using a 64-channel ULA-OP system [29; 30] equipped with a 192-element LA435 linear array transducer  
Figure 2: Scheme for the set motion of the transducer and the frames used for determining displacement and velocity. The displacement of each kernel was calculated as the difference between its position in the start and end frames. The velocity was estimated as the difference between two consecutive frames.  
(Esaote SpA, Florence, Italy) with a 200 \(\upmu\)m pitch. The cine loops were collected at 1500 Hz using a single plane wave transmission; the 64-line frames were beamformed using delay-and-sum with dynamic apodization having the f-number equal to 2. The line distance between the 64 lines was 200 \(\upmu\)m. ECG was not available. To be able to compare with state-of-the-art estimation of the longitudinal movement of the arterial wall, low frame rate cineloops in vivo were collected using a Philips Epiq 7 (Philips Medical Systems, Bothell, WA, USA) equipped with a linear array transducer (model L18-5, Philips Medical Systems, Bothell, WA, USA). The right common carotid artery was scanned in the longitudinal direction, oriented horizontally in the image 2-3 cm proximal to the bifurcation. The healthy volunteers gave informed consent according to the Helsinki Declaration and the study was approved by the Ethics Committee, Lund University.