evaluation of results from ultrasound process …...2016/01/27 · 7th international symposium on...

7th International Symposium on Process Tomography

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Evaluation of Results from Ultrasound Process Tomography of Multiphase Media

Sascha Langener1, Michael Vogt2, Helmut Ermert2, and Thomas Musch1

1Institute of Electronic Circuits, Ruhr-University Bochum, 44780 Bochum, Germany

[email protected] 2High Frequency Engineering Research Group, Ruhr-University Bochum, 44780 Bochum, Germany

ABSTRACT The measurement of volume fractions of different components in multiphase flows is of great interest in many industrial applications. Ultrasound process tomography is well-suited for the differentiation of gaseous and liquid phases. Simulations and measurements were conducted for a measurement concept using a ring of 32 ultrasound transducers on a measurement pipe. A reflection-mode reconstruction technique was used to obtain the system’s point spread function and examine the performance on inhomogeneous configurations of water containing multiple gaseous objects. Furthermore, the possibility of reducing the number of transducers used as transmitters and its effect on the quality of the reconstructed images were investigated. Keywords Ultrasound process tomography, fan-shaped backprojection, multiphase media,

ray-tracing

1 INTRODUCTION In industrial applications, where multiphase flows are involved, the volume fractions of the different phases and their spatial distribution are of particular interest. As gaseous and liquid phases differ significantly in their acoustical properties, ultrasound-based measurement techniques are especially well-suited for their differentiation. A number of tomographic approaches for two-phase media have been developed (e.g., [UFFC1989], [RTI1997], [UFFC1999]). A typical measurement technique uses a ring of ultrasound transducers along the circumference of a measurement pipe containing the multiphase flow. All, or a subset of the transducers are sequentially used as transmitters to insonify the cross-section, i.e. fill it with ultrasound waves, from multiple directions, while all transducers are used as receivers of the transmitted and scattered ultrasound waves. Based on these measurements, the spatial distribution of the phases can be reconstructed by backprojecting echo signals in the so called reflection-mode. Usually, a thresholding operation is applied to the received signals prior to the backprojection process. In this contribution, results of a reflection-mode reconstruction technique, which uses the full amplitude information for reconstruction, are discussed. The results are based on signals obtained from simulations and measurements of configurations with a wire phantom, representing a point-like object in the cross-section, and also with multiple gaseous objects.

2 MEASUREMENT SYSTEM AND SIMULATION TECHNIQUE

Figure 1: Experimental setup: a) Measurement pipe with ultrasound transducers and cylindrical phantom, b) two-dimensional cross-section.


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Figure 1 illustrates the experimental setup, which has been used for the acquisition of measurement data on static configurations [IWPT5]. A set of 32 ultrasound transducers is attached along the circumference of the measurement pipe (material: Low-density polyethylene, diameter: 63 mm). Test measurements have been performed using cylindrical phantoms, which have been positioned along the axis of the pipe, which was filled with water. The transducers have a fan-shaped beam pattern, i.e. diverging across the cross-section while narrow along the axis of the pipe. Consequently, the transducers insonify a large area of the cross-section with each excitation. This reduces the number of transmitters necessary for the coverage of the whole cross-section and, accordingly, the measurement time. While the system is only suited for measurements on static configurations, it is intended for the evaluation of measurement concepts for multiphase flows with rapidly changing phase distributions where the measurement time is an important parameter.

Figure 2: Block diagram of the measurement system.

Figure 2 shows a block diagram of the realized measurement system, which is an improved version of a previously presented system [IWPT5]. Two 1 to 32 multiplexers (MUX) sequentially connect the ultrasound transducers to the transmit- and receive-electronics to acquire complete measurement sets of 32 times 32 signals. During transmission, one transducer is excited with a well-defined short pulse, generated using a digital-to-analog converter (DAC), which shares a common clock source with the analog-to-digital converter (ADC). This is done, to achieve precise and synchronized timing conditions in the measurement electronics. For two-dimensional simulations in the case of two-phase media, consisting of gaseous objects in a liquid, a ray-tracer has been implemented [IUS2014] to determine results under idealized and well-defined conditions. For excitation, a number of 220 rays originate from the transmitting transducer, equally distributed over a beam-angle θ = 100°. The path of each ray is then calculated under the assumption of specular reflections on the surface of gaseous objects. The calculation is finished if a receiver is reached or if a maximum number of 8 reflections have occurred. For every ray which path ends on a receiver, an individual broadband transfer function is determined. The propagation phase results from the time of flight (TOF), calculated from the length of the path and a constant speed of sound, and the number of reflections at gaseous objects, each causing a phase shift of 180°. The transfer functions between the transmitters and receivers are then determined as superposition of the individual transfer functions and multiplied with the transfer function of the ultrasound transducers as shown in figure 3 b). The comparably large number of rays is necessary to minimize discretization errors. Finally, the corresponding band-limited impulse responses, needed as input for the reconstruction technique, are calculated by means of the inverse Fourier transform.

Figure 3: Simulation of transducer: a) Impulse response, b) transfer function.


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The transfer function of the transducers was modelled using a tapered cosine-function with a -6 dB bandwidth of 400 kHz and a center frequency of 500 kHz. For the calculation of the time of flight (TOF), a constant speed of sound of 1430 m/s was used. Reflections at the pipe’s wall and the transducer’s beam pattern have not been included in the simulation. In the case of a point object a uniform scattering pattern was assumed.

3 REFLECTION-MODE RECONSTRUCTION TECHNIQUE

Figure 4: Measurement result: Signal received for angular separation of 7 transducer positions (φ = 78.75°), with no phantom inside the measurement pipe.

The implemented reflection-mode reconstruction technique is based on a linear backprojection of the envelopes of the received signals. The envelope signals are backprojected to elliptic arcs corresponding to the TOF under the assumption of a constant speed of sound. The length of the backprojected arcs is limited by the transducer’s beam-angle θ = 100°, as shown in figure 1 b) for the backprojection of an echo acquired with an angular separation of 7 transducer positions (φ = 78.75°) between transmitter (Tx) and receiver (Rx). The aspects to be considered are multiple reflections inside the transducers and influences of the pipe’s wall on the measurement signals. As an example, figure 4 shows a signal received with an angular separation φ = 78.75° for a measurement pipe filled with water only. Just behind the signal transmitted along the direct path between the transmitter and the receiver, an undesired signal is visible. To prevent artifacts and reconstruction errors caused by this signal, an appropriate time (or range) window, is applied to the signal used for the reconstruction, as illustrated in figure 4. This reduces the number of available projections in the vicinity of the straight paths between transmitters and receivers.

Figure 5: Number of projections per pixel: a) 1 transmitter and 32 receivers, b) 32 transmitters and 32 receivers.

In figure 5 a), the numbers of projections per pixel inside the reconstructed image are shown for one transmitter and 32 receivers. The number of projections decreases to zero in the vicinity of the transmitter, where the direct paths to the receivers are closer together. Figure 5 b) shows the superposition for 32 transmitters and 32 receivers. Due to the limited beam-angle θ of the transmitters and the receivers, the number of projections per pixel is smallest at the boundary of the measurement pipe. A maximum of 594 projections per pixel is reached on a ring formed by the overlapping edges of the beam patterns.


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4 EVALUATION The point-spread function (PSF) describes the response of an imaging system to an infinitely small object and is often used as a measure of quality, especially regarding resolution. Hereby, the width of its amplitude and its invariance over the image plane are major criteria. For a characterization of the measurement system and the reconstruction technique, point spread functions were determined from simulations and measurements.

Figure 6: Simulation results: PSF, point object at different positions along the x-axis: a) xobj = 0 mm, b) xobj = -10 mm, c) xobj = -15 mm, d) xobj = -25 mm.

Figure 6 shows the PSF of the simulated system for positioning an infinitely small point-object at the center of the pipe (xobj = 0 mm), and along the x-axis at xobj = -10 mm, xobj = -15 mm, and xobj = -25 mm distance to the center. The green crosses indicate the positions of the receiving transducers, and the red crosses transmitters. It can be seen that the PSF becomes shift-variant in the vicinity of the pipe’s wall, as it spreads tangentially to the wall the closer the point-object is positioned to it.

Figure 7: Simulation result: Profiles of PSF, point object at different positions along the x-axis: a) y-direction, b) x-direction.

The variations of the shape of the PSF are visible in greater details in the profiles shown in figure 7, exhibiting the pattern of the amplitude along the x- and y- direction for different positions of the object. They all show a blurring along both directions, which is typically obtained with unfiltered linear backprojection techniques [UI1979]. Filtering techniques are able to reduce this blurring, but they depend on assumptions like for example weak scattering [UFFC1999], which are not fulfilled in the case of the intended application of reconstructing also strongly inhomogeneous configurations. For this reason, here no techniques for a deblurring of the reconstructed images were employed. While the variation of the number of projections has little effect on the PSF, a nonuniform angular distribution of the projected arcs is given, what causes a directional variation of the blurring. Especially for the position xobj = -25 mm, which is outside the beam of the majority of the transducers, almost only the projections from the opposite side of the pipe are considered in the reconstruction. Here, in the x-direction, the PSF is almost reduced to the envelope of the transducers impulse response, while it is significantly widened in the y-direction.


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Figure 8: Measurement results: Wire phantom at different positions along x-axis:

a) xobj = 0 mm, b) xobj = -15 mm, c) xobj = -20 mm.

For the assessment of the point spread function of the realized measurement system, a wire phantom (material: steel, diameter: 0.3 mm) was positioned in the cross-section of the measurement pipe. As the diameter of the wire equals approximately a tenth of the wavelength at the center frequency, it can be assumed that Rayleigh scattering is given. Figure 8 shows reconstruction results for positions of the wire at the center (xobj = 0 mm), and along the x-axis with xobj = -15 mm and xobj = -20 mm. The large image amplitudes at the circumference result from reflections and disturbances at the pipe’s wall, which significantly exceed the comparably weak scattering by the wire phantom. This makes an analysis of the PSF in the vicinity of the pipe’s wall, for comparison to the simulated results, difficult. As mentioned above, the measurement time is an important parameter when dealing with rapidly changing phase distributions. The reduction of the measurement time by reducing the number of excitations is a feasible approach, but two limiting conditions are to be considered. Firstly, in the case of strongly inhomogeneous configurations, some parts of phantoms or entire areas of the cross-section might not or only insufficiently be insonified, if shadowed by larger objects. Secondly, reconstruction artifacts occur due to a less equal distribution of projections and coverage by fewer projections.

Figure 9: Simulation results: PSF for different numbers of transmitters: a) 4 transmitters, b) 8 transmitters, c) 16 transmitters, d) 32 transmitters.

Figure 9 shows a comparison of simulated point spread functions for a number of 4 to 32 transmitters used for the reconstruction. Especially in the case of using only 4 transmitters, a significant formation of artifacts due to the nonuniform distribution of projections is visible. As these artifacts are rather small, the use of more than 4 and less than 32 transmitters seems to be sufficient for the imaging of point-like objects in the case of weakly scattering media.


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Figure 10: Measurement results: PSF by wire phantom for different numbers of transmitters:

a) 4 transmitters, b) 8 transmitters, c) 16 transmitters, d) 32 transmitters.

The images reconstructed from measurement data shown in figure 10 confirm this conclusion, as reconstruction artifacts are barely visible. But the surrounding noise is reduced due to the averaging over a greater number of projections when using a larger number of transmitters.

Figure 11: Measurement configurations: Gaseous objects inside water-filled measurement pipe.

As the system is intended to be used for measurements on multi-phase flows, configurations with larger objects representing gaseous inclusions were examined both, in simulations and measurements. Figure 11 shows two exemplary configurations with gaseous objects, each 3 mm in diameter. These inhomogeneities are symmetrically arranged in the configuration A, and asymmetric in the configuration B. In the measurements, the objects are realized by means of air-filled, thin-walled plastic tubes.

Figure 12: Simulation results: Configuration A for different numbers of transmitters:


Figure 12 shows reconstructed images for configuration A, obtained from simulations with different numbers of transmitters. It can be seen in figure 12 d), that in the case of 32 transducers used for both, transmission and reception, the outer surfaces of the objects, i.e. the surfaces facing towards the transducers, have higher amplitudes than those oriented towards the center of the image. This is caused by the shadowing of the inner surfaces. To achieve results of acceptable quality, a number of at least 16 transmitters should be used.


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Figure 13: Simulation results: Configuration B for different numbers of transmitters:


The reconstructed images for configuration B and different numbers of transmitters in figure 13 show a significantly smaller effect of using a small number of transmitters. Artifacts are largely reduced when using 8 or more transmitters. The smaller distances between phantoms lead to a stronger shadowing effect than in configuration A. For this configuration, the difference between using 16 and 32 transmitters appears to be negligible.

Figure 14: Measurement results: Configuration A for different numbers of transmitters:


Figure 15: Measurement results: Configuration B for different numbers of transmitters:


Measurements have also been performed with both configurations A and B, and results have been compared with the simulation results above. In figure 14, reconstruction results for different numbers of transmitters are shown for configuration A, and for configuration B in figure 15. While relatively strong artifacts and noise are visible, the agreement with the results from the simulated data can be seen. Diffraction effects, which are not included in the simulations, are one major cause of artifacts here, as the diameter of the phantoms equals the wavelength at the center frequency. While shadowing effects are reduced by diffraction, especially in configuration B, the time-of flight information becomes inaccurate and echo signals are projected to wrong positions. Additionally, artifacts are introduced by multiple reflections between objects and the wall of the measurement pipe. In the case of 4 or 8 transmitters, multiple artifacts occur widely distributed in the cross-section. By utilizing more transducers as transmitters, their amplitude is reduced due to the averaging over a larger number of projections. Therefore, at least 16 transmitters are necessary to achieve an acceptable quality for these configurations.


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5 CONCLUSION In this paper, results of an ultrasound process tomography system have been discussed. The point spread function (PSF) of the system and reconstruction technique has been evaluated by means of measurements and simulations. The PSF was determined to be largely shift-invariant, except in regions near the wall of the measurement pipe, where the angular distribution of projections becomes significantly nonuniform due to the limited beam-angle of the utilized ultrasound transducers. To achieve a shorter measurement time, the possibility of using a small number of transducers as transmitters has been evaluated. While it has small effects on the PSF, the quality of reconstructed images is significantly influenced in the case of largely inhomogeneous configurations. As concluded from the investigated configurations, at least 16 transducers should be used as transmitters, in order to achieve a reasonable good quality.

6 REFERENCES NORTON S. J., LINZER M., (1979), Ultrasonic Reflectivity Tomography: Reconstruction with Circular Transducer Arrays, Ultrasonic Imaging, 1 154-184, [UI1979]. WIEGAND F., HOYLE B. S., (1989), Simulations for parallel processing of ultrasound reflection-mode tomography with applications to two-phase flow measurement, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 36, 6, 652-660, [UFFC1989]. YANG M., SCHLABERG H.I., HOYLE B.S., BECK M.S. AND LENN C., (1997), Parallel image reconstruction in real-time ultrasound process tomography for two-phased flow measurements, Real-Time Imaging, 3, 295-303, [RTI1997]. YANG M., SCHLABERG H.I., HOYLE B.S., BECK M.S. AND LENN C., (1999), Real-time ultrasound process tomography for two-phase flow imaging using a reduced number of transducers, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 46, 3, 492-501, [UFFC1999]. LANGENER S., MUSCH T., MALLACH M., ERMERT H., AND VOGT M., (2014), A Non-Invasive System for Ultrasound Process Tomography of Multiphase Flows, Proc. 5th International Workshop on Process Tomography, [IWPT5].

LANGENER S., MUSCH T., ERMERT H., AND VOGT M., (2014), Simulation of Full-Angle Ultrasound Process Tomography with Two-Phase Media Using a Ray-Tracing Technique, Proc. IEEE Ultrasonics Symposium, [IUS2014].

evaluation of results from ultrasound process …...2016/01/27 · 7th international symposium on...

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