3D Synthetic Aperture Imaging Using a Water-Jet Coupled Large-Aperture Single Transducer

3D Synthetic Aperture Imaging Using a Water-Jet Coupled Large-Aperture Single

Transducer

Miguel Casta˜ no Arranz

? Johan E. Carlson

Matti Rantatalo

Robert Risberg

Miles Weston

∗

Div. of Signals and Systems, Lule˚ a University of Technology, SE-971 87 Lule˚ a, Sweden

∗∗

Div. Operation, Maintenance, and Acoustics Lule˚ a University of Technology, SE-971 87 Lule˚ a, Sweden

∗∗∗

Creo Dynamics AB, Westmansgatan 37, 582 16 Link¨ oping, Sweden

∗∗∗∗

TWI Technology Centre Wales, Harbourside Business Park, Harbourside Road, Port Talbot, SA13 1SB UK

1. BACKGROUND AND MOTIVATION

There are several approaches for ultrasonic imaging of metal structures, depending on the geometry, accessibility, etc. For small structures, arrays can be used which are coupled directly to the surface. With the increase of the roughness and size of the surface, the quality of the coupling with a contact transducer decreases, and the wearing of the transducer increases. In such scenarios immersion tests are often preferred, which are performed in a water tank or using a water jet coupling when the immersion in a tank is not possible.

The traditional fixed focus approach gives the best resolu- tion only at the focal zone. For applications where a good resolution has to be attained at a larger range of vertical distance from the transducer, the synthetic aperture focus- ing technique (SAFT) introduced by Prine (1972) provides a dynamic focusing with uniform resolution. In the basic SAFT approach, a synthetic focus is obtained in the post- processing of the pulse-echo signals acquired by scanning with a single-element transducer.

When performing immersion tests for SAFT imaging, using a large transducer improves the SNR but reduces the quality of the image due to refraction effects. A solution is to focus the ultrasound beam at the surface of the material, and consider the focal point of the transducer as the source, leading to the the virtual source (VS) concept for synthetic aperture processing introduced by Passmann and Ermert (1996).

2. STATEMENT OF CONTRIBUTION AND METHODS

In this paper we present a novel technique that uses a transducer that focuses the sound at the surface of the sample, thus generating a diverging sound field in the sample. The novelty is in successfully applying the VS concept using water jet coupling for a large transducer.

By focusing the sound field, the water jet probe can

? Corresponding author: Miguel Casta˜no (miguel.castano@ltu.se).

be built with a small nozzle opening, limiting the water consumption and making it viable for field applications.

The annular geometry of the large transducer ensures the spherical wavefront assumed in the application of the SAFT algorithm, which usually limits the size of the transducer.

The frequency domain implementation of the SAFT algo- rithm discussed by Nagai (1984) requires that the sound speed is constant through the entire propagation medium.

Therefore, the time domain implementation has been se- lected since different methods have been introduced for adapting the algorithm to the refraction which occurs at the interface between layers. Even if ray-tracing methods have been introduced by Johnson and Barna (1983)for the calculation of the travel time of the signal, it is preferred to use simpler approximations due to the reduced com- putational cost. The selection of the used approximation is usually influenced by the difference of sound velocities between the layers, and the Taylor approximation intro- duced by Schneider (1984) has been successfully used in the described setup.

3. EXPERIMENTAL RESULTS

Fig. 1 shows the result of imaging a block of aluminum with a side-drilled flat bottom hole of diameter 3.2 mm.

The probe was positioned at the top of the sample at 57 mm distance and measurements were made in a 100 mm by 6.5 mm grid, with a spacing of 1 mm in the X axis and the following measuring positions in the Y axis:

[0, 1.45, 2.3, 3.72, 5.1, 6.75].

We demonstrated a water jet coupling system in combina-

tion with a focused transducer suitable for imaging using

SAFT algorithms. The system operates at 2.5 MHz and

uses only 1-2 liters of water/minutes.

Fig. 1. Image reconstruction of an aluminum piece with a flat bottom hole. Top: 2D slice of the defect (at y=0). Bottom: 3D volume recon- struction.

ACKNOWLEDGEMENTS

The research leading to these results has received funding from the European Union’s Seventh Framework Program managed by REA Research Executive Agency ([FP7/2007- 2013] [FP7/2007-2011]) under grant agreement no 314949.

REFERENCES

Johnson, J. and Barna, B. (1983). The effects of surface mapping corrections with synthetic-aperture focusing techniques on ultrasonic imaging. Sonics and Ultra- sonics, IEEE Transactions on, 30(5), 283–294. doi:

10.1109/T-SU.1983.31423.

Nagai, K. (1984). Fourier domain reconstruction of syn- thetic focus acoustic imaging system. Proceedings of the IEEE, 72(6), 748–749.

Passmann, C. and Ermert, H. (1996). A 100-mhz ultra- sound imaging system for dermatologic and ophthalmo- logic diagnostics. Ultrasonics, Ferroelectrics and Fre- quency Control, IEEE Transactions on, 43(4), 545–552.

doi:10.1109/58.503714.

Prine, D. (1972). Synthetic aperture ultrasonic imaging.

Proc. Engineering Applications of Hologtaphy Symp., 287–288.

Schneider, W. (1984). The common depth point stack.

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