We present a method of nonlinearly imaging ultrasound contrast agents (UCA's) using a coded chirp excitation at high frequency. This method selects the desired harmonic component using chirp compression filters matched to the subharmonic, second harmonic, and ultraharmonic responses of the UCA. To evaluate harmonic imaging using chirp excitations, experiments were performed using a UCA, Definity~(TM), and compared with measurements performed on a linear scatterer, Orgasol~(TM), in a flow cell immersed in water, for both chirp and conventional (i.e. short) excitations. The linear chirp excitation ranged from 27-33 MHz with Gaussian apodization (FWHM(velence)1ms). The peak negative pressure at the focus was measured with a hydrophone to be 1.5 MPa (MI(velence)0.27). The conventional Gaussian excitation was centered at 30 MHz with a fractional bandwidth and peak negative pressure (at the focus) equal to that of the chirp. The subharmonic compression filter ranged from 13.5-16.5 MHz, the second harmonic from 54-66 MHz, and the ultraharmonic from 40.5-49.5 MHz. With the chirp excitation, a 14 dB increase in SNR was observed for the UCA subharmonic relative to that of the conventional excitation. The SNR increased by 5 dB for the second harmonic, and 8 dB for the ultraharmonic. The contrast-to-tissue ratio (CTR), found by dividing the UCA SNR by the linear scatterer SNR, showed a 10 dB enhancement of the chirp subharmonic relative to that of the conventional excitation, a 10 dB enhancement of the ultraharmonic, and no significant enhancement of the second harmonic. B-mode images of the UCA were also obtained in a tissue-mimicking vessel phantom, which revealed that the imaging resolution was similar between chirp and conventional excitations. The results indicate that coded chirp excitations offer advantages for harmonic imaging of UCAs at high frequency, though further optimization will be required to minimize agent disruption while maximizing harmonic CTR's.
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