A custom-made linear array transducer for photoacoustic breast imaging

Wenfeng Xiaa, Daniele Pirasa, Michelle Heijbloma,b, Johan C. G. van Hespena, Spiridon van Veldhovenc, Christian Prinsc, Ton G. van Leeuwena,d, Wiendelt Steenbergena and Srirang Manohara

a Biomedical Photonics Imaging Group, Mira institute for Biomedical Technology and Technical Medicine, University of Twente, Enschede, The Netherlands;

b Center for Breast Care, Medisch Spectrum Twente hospital, Enschede, The Netherlands; c Oldelft Ultrasound B.V. Delft, The Netherlands;

d Biomedical Engineering and Physics, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.

Second generation of the Twente Photoacoustic Mammoscope (PAM II)

Ultrasound transducer arrays for PAM II

(Sensitive, broadband)

• Active material: CTS 3203 HD

• Element size: 5 mm x 5 mm

• Center frequency: 1 MHz

• Acoustic lens for directivity enlargement

A Prototype transducer array developed

Design consideration:

• CTS 3203 HD as active material for its high sensitivity.

• Large active area (5 mm x 5 mm) and low resonance frequency (1 MHz ) is preferable for high sensitivity.

• Acoustic lens to enlarge the narrow directivity angle caused by its large active area for each transducer element.

• Two impedance maching layers to broaden the bandwidth.

Transducer characterization:

1. Minimum Detectable Pressure (MDP) 2. Frequency response

The MDP of the detector is estimated using a substitution method. (D. Piras et al, IEEE J. Sel. Topic Quantun Electron. 17, 730-739 (2010))

• Transducer element with a preamplifier (b) has much lower MDP (8 Pa)

when compared with element without such a preamplifier ((a), 170 Pa).

• The final MDP of the transducer with a preamplifier (0.8 Pa with 100

averages) is 100 times lower than detector used in PAM I.

(S. Manohar et al, Phys. Med. Biol. 50, 2543-2557 (2005)).

In the frequency response measurements, acoustic impulse is generated using

a photoacoustic method. (S. Manohar et al, Phys. Med. Biol. 50, 2543-2557 (2005)).

• Transducer has center frequency of 1 MHz.

• 60% fractional bandwidth is not preferable and can be improved when

using optimized matching layer thickness.

Plans for improvement

• Measured electrical impedance of an element in the prototype array

shows a strong lateral resonance at around 330 KHz corresponding to its

lateral dimension. This will cause various difficulties for photoacoustic

imaging.

1. A FEM model will be built to optimize the matching layer thickness to

broaden the bandwidth.

2. Sub-dicing will be used to reduce the lateral resonance of the transducer.

3. Phantom experiments to evaluate the system when the final optimized

transducer array is used.

Obstacles to overcome

• Bandwidth is too low (60%)