Super-Resolution Mapping of Wave Field Using a Receiver from a Far Distance

Recently, a PSF-modulation super-resolution imaging method was developed by the author (IEEE TUFFC, January 2024). In this paper, the method was used to obtain super-resolution images of pulse ultrasound wave field (super-resolution means higher than diffraction-limited resolution or a higher resolution than that limited by the highest spatial frequency component of the point spread function (PSF) of the imaging system). This method can not only be used to map ultrasound field, but also be applied to map any other wave fields such as electromagnetic and optical fields. In principle, this method can work well with any linear shift-invariant (LSI) system. Even if the ultrasound wave field is disturbed or scattered by objects such as biological soft tissues, this method is still applicable. Compared to the conventional method where a small hydrophone or a small scatterer was used to map ultrasound wave fields, this method has the advantages of the small scatterer method that is simple and can withstand a higher ultrasound intensity while being flexible in terms of receiver positioning and the existence of other scattering objects in the ultrasound field.In the study, a 2.25-MHz center frequency, 61.3% -6-dB pulse-echo relative bandwidth, and 25.4-mm diameter PZT (lead zirconate titanate) transducer (transmitter) focused at about 33.5 mm was used to produce a short ultrasound pulse in water (the transducer was driven by a one-cycle electrical pulse signal). The produced pulse ultrasound field was then mapped by another PZT transducer that was used as a receiver and had the same center frequency, bandwidth, and diameter as the transmitter except that the receiver had a focal length of about 50 mm (an f-number of about 2). The axis of the receiver was in parallel with that of the transmitter and the receiver was scanned mechanically in the direction that was perpendicular to the wave propagation to map the pulse ultrasound field at the focal distance of the transmitter (33.5 mm). Because the receiver was placed at a far distance from the ultrasound field measured, the spatial resolution of the image of the pulse ultrasound field was low. However, when a small modulator that was a glass bead of about 0.7-mm diameter was placed at the focal point of the receiver to map the ultrasound field together with the receiver, high spatial frequency components were introduced to the image of the ultrasound field. Subtracting the images of the ultrasound fields mapped with and without the modulator, a super-resolution image of the wave field was produced.To evaluate the quality of image of the ultrasound field mapped with the PSF-modulation super-resolution imaging method, a broadband (1-20 MHz) needle hydrophone of 0.6-mm diameter was used to map the same ultrasound field. The results show that the super-resolution image obtained with the PSF-modulation super-resolution imaging method was very close to those obtained by the hydrophone.Finally, to demonstrate the flexibility of the super-resolution imaging method in mapping the ultrasound field, the receiver was rotated 90° from the axis of the transmitter to obtain the super-resolution image of the pulse ultrasound field. The results show that the ultrasound field mapped after the rotation of the receiver was very close to that mapped before the rotation.