Design and Micro-Fabrication of Focused High-Frequency Needle Transducers for Medical Imaging

Abstract

In this study, we report an advanced fabrication technique to develop a miniature focused needle transducer. Two different types of high-frequency (100 MHz) transducers were fabricated using the lead magnesium niobate-lead titanate (PMN-0.3PT) and lithium niobate (LiNbO₃) single crystals. In order to enhance the transducer's performance, a unique mass-spring matching layer technique was adopted, in which gold and parylene play the roles of the mass layer and spring layer, respectively. The PMN-0.3PT transducer had a 103 MHz center frequency with a -6 dB bandwidth of 52%, and a signal-to-noise ratio (SNR) of 42 dB. The center frequency, -6 dB bandwidth, and SNR of the LiNbO₃ transducer were 105 MHz, 66%, and 44 dB, respectively. In order to compare and evaluate the transducers' performances, an ultrasonic biomicroscopy (UBM) imaging on the fish eye was performed. The results showed that the LiNbO₃ transducer had a better contrast resolution compared to the PMN-0.3PT transducer. The fabricated transducer showed an excellent performance with high-resolution corneal epithelium imaging of the experimental fish eye. These interesting findings are useful for the future biomedical implementation of the fabricated transducers in the field of high-resolution ultrasound imaging and diagnosis purpose.

Keywords: biomedical imaging; focused transducer; high-frequency transducer; needle transducer.

Figure 1

(a) A cross-sectional view of the focused needle transducer. (b) A digital photograph of the fabricated focused needle transducer.

Figure 2

(a) A digital photograph of the press-fit system, (b) cross-sectional view of the press-fit system, and (c) the fabrication stages of press-focused needle transducer.

Figure 3

Schematic diagram of the experimental setup for ultrasound imaging using transducer.

Figure 4

Pulse–echo response (solid line) and frequency spectra (dashed line) for a focused needle transducer with 1.5 mm of focal length. (a) Simulated signals and (b) measured signals of 103 MHz PMN-0.3PT transducer with 52 % BW at −6 dB and a 10 µm thickness. (c) Simulated signals and (d) measured signals of 105 MHz LiNbO₃ with 66% BW at −6 dB and a 28 µm thickness.

Figure 5

Measured electrical impedance magnitude (solid line) and phase angle (dashed line) (a) 103 MHz PMN-0.3PT transducer and (b) 105 MHz LiNbO₃ transducer.

Figure 6

(a,b) Measured axial and lateral resolution of the PMN-0.3PT transducer. (c,d) Measured axial and lateral resolution of the LiNbO₃ transducer.

Figure 7

(a) Structure of the fish eye. (b) A digital photograph of the fish specimen.

Figure 8

Before performing the mass–spring matching layer technique, an ultrasound biomicroscopy image (UBM) of the fish eye obtained with (a) the 103 MHz PMN-0.3PT transducer and (b) the 105 MHz LiNbO₃ transducer. (c,d) Enlarged view of the center of the cornea using the filter to remove the lower-frequency signal. The cornea (A), upper cornea surface (A₁), lower cornea surface (A₂), left iris (B₁), right iris (B₂), and lens (C) are visible in both images.

Figure 9

After performing the mass–spring matching layer technique, a UBM of the fish eye obtained was obtained with (a) the 103 MHz PMN-0.3PT transducer and (b) the 105 MHz LiNbO₃ transducer. (c,d) Enlarged view of the center of the cornea. The cornea (A), upper cornea surface (A₁), lower cornea surface (A₂), iris (B), and lens (C) are visible.