Amplitude modulated chirp excitation to reduce grating lobes and maintain ultrasound intensity at the focus of an array

Abstract

During application of high intensity focused ultrasound (HIFU) with therapy arrays, the existence of grating lobes can cause heating at unintended tissue regions. Therefore, the reduction of grating lobes in therapeutic arrays is an important goal. One way to reduce the grating lobes in therapy arrays is to excite the arrays with broadband signals (defined here as >10% fractional bandwidth). To achieve a reduction in grating lobe levels in an ultrasonic array, coded waveforms can be utilized that reduce the grating lobe levels while maintaining the spatial peak temporal average intensity. In this study, a 5-MHz, 9-element, 1.25 mm inter-elemental spacing linear array was excited by a sinusoidal waveform, a conventional linear chirp, and a modified linear chirp. Both chirps spanned the -3-dB bandwidth of the transducer. The conventional chirp was a broadband signal with a linear sweep of frequencies between 2.5 and 7.5 MHz, with all frequency components excited with equal amplitude. The modified chirp signal also swept the frequencies between 2.5 and 7.5 MHz, but the amplitude was weighted such that the edges (low and high frequencies of the band) were excited with more energy than the center of the band. In simulations, the field patterns for the sinusoidal, conventional chirp and modified chirp excitations were produced from the array using Field II and compared. For experiments, the beam pattern from a 5-MHz single-element transducer was mapped using a hydrophone for the sinusoidal, conventional chirp and modified chirp excitation. Each field from the transducer was repeated and summed to produce a field from an array of 9 elements. The difference in the time averaged intensity (in dB) in the main lobe and grating lobes were estimated for each excitation and compared. The results demonstrated that the chirp signals resulted in decreases in grating lobe levels compared to the main lobe, i.e. 10 dB down for focusing and 6 dB down for focusing and steering. A further 1 dB decrease in grating lobe levels was observed for the modified chirp excitation compared to the conventional chirp excitation, which corresponds to ~21% reduction in energy deposition at the grating lobe location.

Figure 1

Experimental setup showing the 5 MHz transducer, hydrophone and related electronics.

Figure 2

Impulse response and bandwidth of the 5 MHz single element transducer.

Figure 3

Depiction of the transducer and the field plane scanned by the hydrophone.

Figure 4

Conventional chirp (2.5-7.5 MHz) and modulated chirp (2.5-7.5 MHz) transducer excitation inputs and corresponding signals recorded by the hydrophone.

Figure 5

Bandwidths (−3 dB) for chirp signal before and after convolution with modified impulse response.

Figure 6

Comparison of simulated fields and sidelobe levels for sinusoidal (top left), conventional chirp (top right) and modified chirp (bottom left) excitation for focusing at 70 mm from the source on axis. (Color on web only)

Figure 7

Comparison of simulated fields and sidelobe levels for sinusoidal (top left), conventional chirp (top right) and modified chirp (bottom left) excitation for focusing at 70 mm from the source and −5 mm laterally from the axis. (Color on web only)

Figure 8

Improvement in grating lobe reduction for conventional chirp. The values refer to differences in dB of levels calculated from the intensities.

Figure 9

Improvement in grating lobe reduction for conventional chirp and modified chirp excitation for range of array pitch values compared to sinusoid excitation. The values refer to differences in dB of levels calculated from the intensities.

Figure 10

Comparison of beamformed fields and sidelobe levels for sinusoidal (top left), conventional chirp (top right) and modified chirp (bottom left) excitation for focusing at 70 mm from the source on axis. (Color on web only)

Figure 11

Comparison of beamformed fields and sidelobe levels for sinusoidal (top left), conventional chirp (top right) and modified chirp (bottom left) excitation for focusing at 70 mm from the source and −5 mm laterally from the axis. (Color on web only)