Super Resolution Ultrasound using Recursive Imaging of Highly Dense Scatterers

Microbubbles (MBs) tracking is an integral part of super-resolution ultrasound imaging (SRI). In SRI, MBs are tracked to make intensity and velocity maps. These MBs must be sparse and separable to make MBs tracking reliable for a desired intensity and velocity maps. However, the sparse distribution of the MBs necessitates several minutes long acquisitions to go through the whole circulation. Because of abundance of targets in SUper Resolution ultrasound imaging with Erythrocyte (SURE), it is possible to make a super resolution image in just seconds instead of several minutes in SRI with MBs. That means increasing the number of scatterers could reduce the acquisition time, however tracking of high density scatterers is quite challenging. This paper hypothesizes that non-separable target traveling can yield super resolution using a fast recursive synthetic aperture imaging technique. A phantom with four tube pairs are simulated using Field II with 175 scatterers per resolution cell and also with realistic tissue motion measured in-vivo from a rat kidney. For simulation, the impulse response and characteristics of GE L8-18iD probe is used. The wavelength is 154 $\mu \mathrm{m}$ . A synthetic aperture imaging sequence is consisting of 12 emissions. Two seconds of data is acquired by a frame rate of 208.3 Hz. Recursive imaging is used to increase the frame rate to 2500 Hz. Motion compensation and echo-cancelling is applied to the beamformed data to tracking detected peaks for creating the velocity map. The velocity maps and profiles showed estimated directions and velocities for the four tube pairs in two opposite flow directions. The velocity map and profiles showed that all tube pairs with 200, 100, 50, and 25 $\mu \mathrm{m}$ are separated correctly for 2 seconds instead of several minutes. Errors between actual and estimated vessel center distances are 6.6, 5.1, 2.6, and 1.7 $\mu \mathrm{m}$ , respectively. Recursive imaging thus makes it is possible to obtain a super-resolution image of highly dense scatterers not only for tube distances less than the diffraction limit of half of a wavelength but also less than a quarter of a wavelength in just 2 seconds.