Superharmonic imaging based on chirps

Abstract

In medical ultrasound harmonic images of biological tissue are commonly obtained by analyzing the reflected echoes from the 2nd harmonic band. A new modality dubbed super-harmonic imaging (SHI) targets a combination of the 3rd–5th harmonics. SHI is expected to yield enhanced spatial resolution and thus to increase the quality of echographic images. On the other hand, those images obtained using short imaging pulses are susceptible to so-called multiple axial reflection artifacts, stemming from the troughs in between harmonics in the frequency domain. The recently proposed dual-pulse frequency compounding method suppresses these artifacts but reduces the frame rate by a factor of 2. In this work we research the feasibility of employing 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 fully programmable ultrasound system. The transducer was mounted in the side of a water-filled tank. Linear chirps with a center frequency of 1 MHz and a bandwidth of 40% were used as excitation pulses. Radio frequency traces were recorded at the focal plane along the lateral axis using a hydrophone, filtered over the superharmonic band and convolved with a decoding signal to obtain point spread functions (PSFs). The decoding signal was acquired by simulating the emitted beam using the KZK method for a rectangular aperture. The decoded superharmonic chirp had an SNR of 35–40 dB. Comparing to a the 3rd harmonic produced by a 2.5 cycle 1 MHz Gaussian apodized sine burst transmission the lateral beam width of the superharmonic chirp signal is 0.8 and 0.9 times that 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 0.8 times the axial beam width of the 3rd harmonic at the −6 dB and −20 dB levels respectively. The superharmonic chirp PSF is virtually free from imaging artifacts. Based on the SNR measurements the chirp protocol yields a sufficient dynamic range. The PSF has increased spatial resolution in comparison with the 3rd harmonic. The first in-vitro images show promise, but the decoding pulse requires improvement.