Acoustic noise suppression
In SURF imaging, complex dual-frequency band ultrasound pulses are transmitted simultaneously into the tissue. The pulse complex consists of a low-frequency (LF) manipulation pulse and a high-frequency (HF) imaging pulse overlapping in time and space, where the purpose of the LF pulse is to manipulate both propagation and scattering of the HF imaging pulse in tissue. This is of particular interest for suppression of multiple reflections between tissue structures and the ultrasound probe itself, so called pulse reverberations.
A transmitted ultrasound signal that is reflected only once from tissue structures inside the body, is termed first order scattering. If this was the only scattering present in an ultrasound image, this would constitute the "true" or ideal anatomical image. In reality, ultrasound waves are scattered several times between structures, and this is termed higher order scattering or multiple scattering (reverberations). This scattering is unwanted and adds to the image as noise and may deteriorate both spatial (by interference) and contrast resolution in the image. Except for bone and air, the ultrasound probe itself is the strongest reflector compared to tissue, and it is mainly multiple scattering between tissue structures and the ultrasound probe surface that are strong enough to cause reverberations that become visible as artifacts.
Harmonic imaging, developed during the 90'thies, has shown to reduce reverberation artifacts in a range of clinical applications. The basic principle behind the method is that the energy in the harmonic beam is low close to the ultrasound probe surface, thereby reducing the impact of multiple reflections between tissue structures and the ultrasound probe occurring close to the probe surface.
In SURF imaging, combining transmission of the dual-frequency band pulse complex with advanced receive signal processing, it is possible to separate fundamental scattering from higher order scattering, thereby obtaining strong suppression of reverberations independent of the depth at which they occur. This is possible due to the presence of the LF manipulation pulse, enabling separation of first and higher order scattering in the HF imaging pulse. This is different from HI, and provides SURF imaging with a clear advantage in its ability for reverberation suppression.