Superharmonic imaging based on chirps

In medical ultrasound harmonic images of biological tissue are commonlyobtained by analyzing the reflected echoes from the 2nd harmonicband. A new modality dubbed super-harmonic imaging (SHI) targets a combinationof the 3rd-5th harmonics. SHI is expected to yieldenhanced spatial resolution and thus to increase the quality of echographicimages. On the other hand, those images obtained using short imaging pulses aresusceptible to so-called multiple axial reflection artifacts, stemming from thetroughs in between harmonics in the frequency domain. The recently proposeddual-pulse frequency compounding method suppresses these artifacts but reducesthe frame rate by a factor of 2. In this work we research the feasibility ofemploying a chirp protocol to perform SHI without compromising the frame rate.The chirp protocol was implemented using an interleaved phased array transducer(44 elements tuned at 1 MHz, 44 elements at 3.7 MHz) in combination with a fullyprogrammable ultrasound system. The transducer was mounted in the side of awater-filled tank. Linear chirps with a center frequency of 1 MHz and abandwidth of 40% were used as excitation pulses. Radio frequency traces wererecorded at the focal plane along the lateral axis using a hydrophone, filteredover the superharmonic band and convolved with a decoding signal to obtain pointspread functions (PSFs). The decoding signal was acquired by simulating theemitted beam using the KZK method for a rectangular aperture. The decodedsuperharmonic chirp had an SNR of 35-40 dB. Comparing to a the 3 rdharmonic produced by a 2.5 cycle 1 MHz Gaussian apodized sine burst transmissionthe lateral beam width of the superharmonic chirp signal is 0.8 and 0.9 timesthat of the 3rd harmonic at the 6 dB and 20 dB levels respectively.Regarding the axial beam width, the superharmonic chirp signal has 0.9 and 0.8times the axial beam width of the 3rd harmonic at the 6 dB and 20 dBlevels respectively. The superharmonic chirp PSF is virtually free from imagingartifacts. Based on the SNR measurements the chirp protocol yields a sufficientdynamic range. The PSF has increased spatial resolution in comparison with the3rd harmonic. The first in-vitro images show promise, but thedecoding pulse requires improvement.

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