Effective ultrasonic virtual sources which can be positioned independently of physical aperture focus positions

Chikayoshi Sumi, Shinya Uga
Department of Information and Communication Sciences, Faculty of Science and Technology, Sophia University, Tokyo, Japan
Abstract: In this report, newly obtained ultrasonic virtual sources, VS1 and VS2, are described. That is, both virtual sources are obtained on the basis of the new viewpoint that the position of a virtual source can also be obtained regardless of the focus position of any physical aperture. VS1 is a virtual source that is a point acoustic source model set behind physical array elements and enables an increase in the transmitted ultrasound intensity using plural transmission ultrasound elements, and increases the size of the region of interest, that is, the same virtual source as that reported solo by other groups previously. As is well known, VS1 increases the echo signal-to-noise ratio; this yields a tissue displacement/strain measurement as well as B-mode imaging, and enables associated applications, eg, shear modulus reconstruction. Alternatively, VS2 uses an arbitrary position ahead of the physical array elements as a virtual source position with a synthetic aperture method. With VS2, random scattering media (material) such as point acoustic sources can be used as virtual sources. Preliminary experimental results are presented for a lateral modulation method. In the experiments described here using an ultrasound linear array type transducer (ultrasound element pitch, 0.2 mm; ultrasound frequency, 7.5 MHz), it was possible to use a depth range of ± 3 mm for virtual source positions. Remarkably, VS2 yielded higher lateral resolutions than the physical sources. That is, VS2 mitigates the physical and electrical limitations for using smaller ultrasound array elements and a smaller pitch, such that both a high lateral resolution and a high echo signal-to-noise ratio are obtained.
Keywords: ultrasound, virtual source, lateral modulation, synthetic aperture, lateral resolution, displacement vector, strain tensor, shear modulus