Design factors of intravascular dual frequency transducers for super-harmonic contrast imaging and acoustic angiography

Abstract

Imaging of coronary vasa vasorum may lead to assessment of the vulnerable plaque development in diagnosis of atherosclerosis diseases. Dual frequency transducers capable of detection of microbubble super-harmonics have shown promise as a new contrast-enhanced intravascular ultrasound (CE-IVUS) platform with the capability of vasa vasorum imaging. Contrast-to-tissue ratio (CTR) in CE-IVUS imaging can be closely associated with low frequency transmitter performance. In this paper, transducer designs encompassing different transducer layouts, transmitting frequencies, and transducer materials are compared for optimization of imaging performance. In the layout selection, the stacked configuration showed superior super-harmonic imaging compared with the interleaved configuration. In the transmitter frequency selection, a decrease in frequency from 6.5 MHz to 5 MHz resulted in an increase of CTR from 15 dB to 22 dB when receiving frequency was kept constant at 30 MHz. In the material selection, the dual frequency transducer with the lead magnesium niobate-lead titanate (PMN-PT) 1-3 composite transmitter yielded higher axial resolution compared to single crystal transmitters (70 μm compared to 150 μm pulse length). These comparisons provide guidelines for the design of intravascular acoustic angiography transducers.

Figure 1

Side view of the layout design of the dual frequency transducer with: (a) interleaved configuration, (b) stacked configuration, (c) stacked configuration with acoustic filter, (d) stacked configuration with different apertures and acoustic filter, and (e) Final configuration of the transducer including a matching layer and electrical connections. Abbreviations HF, LF and AF denote high frequency, low frequency and acoustic filter, respectively.

Figure 2

Cartoon illustrating the setup of the acoustic angiography phantom tested in vitro.

Figure 3

The transducer prototypes made at different frequencies with different piezoelectric materials. The lower right inset shows the transducer mounted inside a commercial sheathing.

Figure 4

Beam profile of the (a) receiving beam, and the transmitting beam of (b) the interleaved configuration and (c) the stacked configuration with an acoustic filter.

Figure 5

Transmission efficiency of the prototype transducers for 1 cycle (left) and 2 cycle (right) burst excitation waves. Horizontal lines are drawn to indicate the peak negative pressure of the time series waveform and the vertical lines indicate the -6 dB beginning and end of the pulse for measurement of the pulse length. All values are normalized to input voltage amplitudes

Figure 6

Pulse-echo response of the 30 MHz reception element.

Figure 7

Comparison of contrast to tissue ratio with excitations at frequencies of (a) 6.5 MHz and (b) 5 MH with 2-cycle excitation.

Figure 8

Comparison of axial resolution with excitations from transducers made of (a) 6.5 MHz PMN-PT single crystal, (b) 5 MHz single crystal and (c) 5 MHz PMN-PT 1-3 composite with 1-cycle excitation.