акустика / gan_ws_acoustical_imaging_techniques_and_applications_for_en

S-scans and electronic scans both provide a global image and rapid information about the component and any possible discontinuation detected in the ultrasonic range at all angles and positions.

S-scans have the following benefits:

1.Image display during scanning

2.True-depth interpretation

3.2D volumetric reconstruction

Advanced imaging can be achieved using a combination of linear and sectional scanning with multiple-angle scans during probe movement. S-scan displays in combination with other views lead to a new type of defect imaging. A combination of longitude wave and shear wave scans can be very useful for detection and sizing with little probe movement. Cylindrical, elliptical or spherical focused beams have a better SNR (discrimination capability) and a narrower beam spread than divergent beams.

Real-time scanning can be combined with probe movement and defect plotting into a 3-D drafting package. This method offers

1.high redundancy;

2.defect location;

3.accurate plotting;

4.defect imaging;

5.high-quality reports for customers and regulators;

6.good understanding of detection and sizing principle as well as multibeam visualization for technician training.

The limitations of phased array ultrasonic technology are as follows:

1.The equipment is too expensive as hardware is 10–20 times more expensive than conventional ultrasonic testing. Also, spare parts are expensive and too many software upgrades make the system costly.

2.The probes are too expensive, and with long lead delivery the prices are 12–20 times more expensive than conventional probes.

3.Very skilled operators are required, with advanced ultrasonic knowledge.

4.Calibration is time consuming and very complex.

9.6.6Advantages of Phased Array Testing as Compared with Conventional UT

Ultrasonic phased array system can potentially be employed in almost any test where conventional ultrasonic flaw detection has traditionally been used. Weld inspection and crack detection are the most important applications, and these tests are done across a wide range of industries, including aerospace, power generation, petrochemical, metal billet and tubular goods suppliers, pipeline construction and maintenance, structural metals and general

manufacturing. Phased arrays can also be effectively used to profile remaining wall thickness in corrosion survey applications.

The benefits of phased array technology over conventional ultrasonic testing come from its ability to use multiple elements to steer, focus and scan beams with a single transducer assembly. Beamsteering, commonly referred to as sectorial scanning, can be used for mapping components at appropriate angles. This can greatly simplify the inspection of components that have a complex geometry. The small footprint of the transducer, and the ability to sweep the beam without moving the probe, also aids the inspection of such components in situations where there is limited access for mechanical scanning. Sectorial scanning is also typically used for weld inspection. The ability to test welds with multiple angles from a single probe greatly increases the probability of detection of anomalies. Electronic focusing permits optimizing the beam shape and size at the expected defect location, as well as further optimizing the probability of detection. The ability to focus at multiple depths also improves the ability for sizing critical defects for volumetric inspections. Focusing can significantly improve the SNR in challenging applications, and electronic scanning across many groups of elements allows C-scan images to be produced very rapidly.

The potential disadvantages of a phased array system are a somewhat higher cost and a requirement for operator training. However, these costs are frequently offset by the system’s greater flexibility and a reduction in the time required to perform a given inspection.

References

[1]Vadder de, D. and Dosso, M. (1984) Caracterisation ultrasonore des bords de fissure par traitement numerique du signal. 3rd Europ. Conf. NDT Florence, vol. 5, pp. 362–374.

[2]Wustenberg,¨ H. and Mundry, E. (1972) An approach to a system-theoretical description of information sources in ultrasonic testing, Abstracts 9th Conf. NDT, Loughborough, p. 11.

[3]Baborovski, V.M., Marsh, D.M. and Slater, E.A. (1973) Schlieren and computer studies of the interaction of ultrasound with defects. Non. Destr. Test, 6, 200–207.

[4]Thompson, R.B., Smith, J.F. and Lee, S.S. (1983) Review of Progress in Quantitative Nondestructive Testing, vol. 2B (eds D.O. Thompson and D.E. Chimenti), Plenum Press, New York, p. 1339.

[5]King, R.B. and Fortunko, C.M. (1983) Determination of in-plane residual stress states in plates using horizontally polarised shear waves. J. Appl. Phys., 54, 3027–3035.

[6]Kino, G.S. et al. (1979) Acoustoelastic imaging of stress fields. J. Appl. Phys., 50(4), 2607–2613.

[7]Dike, J.J. and Thomson, G.C. (1990) Residual stress determination using acoustoelasticity. J. Appl. Mech., 57, 12–17.

[8]Drescher-Krasicka, E. (1993) Scanning acoustic imaging of stress in the interior of solid materials. J. Acoust. Soc. Am., 94, 453–364.

[9]Weglein, R.D. (1979) A model for predicting acoustic materials signatures. Appl. Phys. Lett., 34, 179–181.

[10]Parmon, W. and Bertoni, H.L. (1979) Ray interpretation of the material signature in the acoustic microscope.

Electron. Lett., 15, 684–686.

[11]Wong, E.H. (2009) Adhesive bond characterization. Powerpoint presentation.

[12]Smart TriCam Transverse Scanning System, USNR Inc, USA.

[13]Wu, Y.C. (1988) Waves generated by an inclined-plate wave generator. Intern. J. Numer. Meth. Fluids, 8(7), 803–811.

[14]Holland, M.R., and Miller, J.G. (1988) Phase-insensitive and phase-sensitive quantitative imaging of scattered ultrasound using a two-dimensional pseudo-array. Proceedings of 1988 Ultrasonics Symposium, pp. 815– 819.

[15]Buckley, J. (2000) Air-coupled ultrasound, a millennial review. Proceedings of WCNDT, Rome, 2000.

[16]Dunlap, W.L. (1996) Recent advances in piezocomposite materials for ultrasonic transducers, G.E. Inspection Technology, Lewistown, PA, USA.

[17]Meyer, P.A. (1995) The evolution of a piezocomposite transducer design. Presented at Canadian Society for Nondestructive Testing, Niagara Falls, Ontario, Canada, 15–18 May 1995.

[18]Shuxiang J. and Wong, B.S. (2004) Development of an automated ultrasonic testing system, http://www

.ndt.net/Papers/184/184.htm.

[19]Krautkramer,¨ J. and Krautkramer,¨ H. (1990) Ultrasonic Testing of Materials, Springer-Verlag, Berlin, p. 35–43.

[20]Egle, D.M. and Bray, D.E. (1975) Nondestructive measurement of longitudinal rail stresses, Report FRA-ORD- 76-270, PB-272061, Federal Railroad Administration, NTIS, Springfield, VA.

[21]Bray, D.E. and Stanley, R.K. (1997) Nondestructive Evaluation, CRC Press, Florida, USA, p. 79.

[22]Briggs, A. (1992) Acoustic Microscopy, Clarendon Press, Oxford, p. 105–107.

[23]Advances in Phased Array Ultrasonic Technology Applications (2007) http://www.olympus-ims.com/en/pdf- library/157-catld.268435479.html.

[24]Introduction to Phased Array Ultrasonic Technology Applications (2004) http://www.olympus-ims.com/en/pdf- library/157-catld.268435479.html.

10

Medical Ultrasound Imaging

10.1Introduction

Medical ultrasound imaging is catching up in the market share with X-ray imaging. Because of the advance in transducer technology, in electronic instrumentation and in digital processing teechniques, new and sophisticated medical ultrasound imaging systems have been developed. They are now complimentary to X-ray imaging and nuclear magnetic resonance (MR) imaging. The particular advantage of ultrasound compared with X-ray and nuclear MR is that they are nonionizing and are then less risk to both patients and examiners. Extensive research into the biological effects of ultrasound are in progress and so far no deleterious effects have been found. Hence, ultrasound is frequently used for imaging adult reproductive system and monitoring foetal viability in addition to its more common use, for example imaging the valve motion of the heart and the internal organ of the abdomen. These images are unique because they are obtained by ultrasound waves interacting with the mechanical properties of tissue. Hence, this modality has become complimentary to other diagnostic tools.

There are many similarities between acoustical image formation and optical image formation. Both are limited in image resolution by diffraction effects, both employ refractive and reflective elements such as lens, prisms or mirrors to control the shape and direction of the beam. Also, both rely on changes in absorption or impedance to provide image content and both have phase contrast schemes available to provide additional image contrast when absorptive or impedance variation are insufficient to distinguish object structures and both have developed holographic schemes for recording image data.

There are also significant differences between acoustical and optical image formation. First light wave is unable to penetrate solids and optical images are not able to provide interior information or images of solid objects. Sound wave on the other hand can penetrate solids through vibration and provide images of internal structure of solids. Also one cannot see sound directly. Hence, it is a characteristic of all acoustic imaging schemes that some means be provided for converting the acoustical information to visible form. Although other physical phenomena have been employed, the most common scheme is to convert the acoustic signal

Acoustical Imaging: Techniques and Applications for Engineers, First Edition. Woon Siong Gan. © 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.

to an electronic signal with an electromechanical transducer. The image is then processed and displayed in a manner very similar to television signals.

10.2 Physical Principles of Sound Propagation

10.2.1Propagation of Sound Wave in Solids

The principle of medical ultrasound imaging system depends on the propagation of sound energy through the human body. When sound propagates through solid, two types of waves are generated, the longitudinal wave and the transverse wave. In longitudinal wave, the particle motion is in the same directions as the wave propagation. In transverse wave, the particle motion is perpendicular to the direction of wave propagation. Transverse waves have not been used for medical diagnostics because of their extremely high attenuation in biological media and so is very difficult for experimental detection.

Sound waves are generated and detected by a piezoelectric transducer [1] which is a device capable of converting electrical energy to acoustical energy and vice versa.

A large number of piezoelectric materials of natural or synthetic nature have been discovered or developed [2]. Hence, good efficiency in the transduction process can be achieved.

The velocity of longitudinal waves in solid is given by the elastic properties of the medium as

v = B/ρ (10.1)

where B is the bulk modulus and ρ is the mean density of medium. The effect of dispersion (i.e. the frequency dependence of sound velocity) is small for biological materials and is not important for most ultrasound imaging systems. Table 10.1 gives a list of the important parameters of sound propagation in solids and in fluids.

Table 10.1 List of measured sound speeds in some typical biological media

Source: Havlice and Taenzer [3] © IEEE.

Sound propagation in solids and in fluids also suffers attenuations with sound intensity diminishes with direction of propagation, given by

where I0 is the initial intensity and α is the attenuation coefficient. Unlike the sound velocity, the attenuation coefficient is highly frequency dependent [4]. This has significant impact on the design and performance of acoustical imaging system. The attenuation coefficient increases approximately linearly with frequency. Hence, the penetration depth of sound wave in solids depends greatly on frequency and there is a trade-off between image resolution and the penetration depth needed. There is a guidelines imposed on the design of medical ultrasound imaging system. For instance, 3 MHz or lower frequencies are needed to image human tissues depth in the body, whereas higher frequencies are used for imaging small structures like in ophthalmology and for human tissues lying under the skin.

In a diffraction limited system, the image resolution is limited by the Rayleigh criterion giving resolution = λ/2, where λ is the sound wavelength.

10.2.2Contrast

In ultrasound imaging system design, usually these three sources of contrast are considered: attenuation, reflection and texture. For instruments providing transmission images, attenuation difference between various body structures are used. Reflectivity is the contrast used in instruments providing reflection images of the body. For structures larger than a few wavelengths, reflectivity is given by the acoustic impedance of the two adjoining layers [5]. The concept of acoustic impedance is analogous to the concept of impedance used in electricity. It is given by Z = ρc, where ρ is the material density and c is the sound velocity.

The power reflection coefficient R [6] for a normally incident sound beam travelling from a medium with impedance Z1 into a medium with impedance Z2 is given by

Hence. the greater the difference of the acoustic impedance of the adjoining tissues, the greater the amount of energy reflected from the boundary in soft tissues, the reflection coefficient varies from −20 dB (between fat and muscle) to −45 dB (between kidney and spleen). These are low-level reflections (less than 0.5%) so that most of the acoustic energy is transmitted through the interface and is available for imaging deeper structures. However, when a very high-level reflection takes place, for instance in a bone/muscle interface which has a reflection coefficient of –4 dB. In such cases, considerably less energy is transmitted and not very much energy is available for imaging deeper structures and a shadow appears in the reflection mode image. This shadowing is an important indicator of abnormality. It can be used to distinguish between soft and calcified atherosclerotic plague in the carotid arteries [7, 8] and to identify stones in the gall bladder or kidney.

In an inhomogeneous medium such as human tissue, the amount of sound and its spatial distribution reflected from an object depends not only on the difference between the acoustic impedance of the object and its surrounding but also on the physical size, orientation and shape of the object. Objects much smaller than an acoustical wavelength will reflect sound according

to the Rayleigh scattering theory. That is, they exhibit a fourth power frequency dependence with a wide angular field distribution [9]. On the other hand, objects with dimensions larger than an acoustic wavelength (specular reflection) reflect sound, independent of frequency, towards a direction which is dependent on the orientations of the object and with an angular field distribution which is dependent on the incident sound field and the shape of the object.

Coming to deal with the last source of contrast, which is texture, the theory of sound scattering and reflection from biological structure will need to be based on the theory of elasticity and the usual inhomogeneous Helmholtz wave equation cannot be used. So far this theory is yet to be properly developed. However, experimentally it has been observed that some body structures produce spatial echo patterns that have a different texture appearance than others and this difference shows the contrast. For instance, the wall of a blood vessel has a characteristic smooth specular appearance whereas a thyroid gland has a characteristic granular appearance.

10.3 Imaging Modes

10.3.1B-Scan

Ultrasound images usually are divided into two main categories: B-scan images and C-scan images [1]. Each of these types of images can be subdivided into classification based on scan technique and scan modalities. Then a further subdivision of each category into real-time or nonreal-time scanning, water bath or contact scanner and reflection or transmission mode and so on.

B-scan means brightness mode scan and it provides a front view, two-dimensional (2D), cross-sectional reflection image of the object that is scanned [10]. A B-scan image is formed by sweeping a narrow acoustic beam through a plane and positioning the received echoes on a display such that there is a correspondence between the display scan time and the direction of acoustic propagation in the tissue. Generally, the same transducer is used to both send and receive the acoustic signals. A fundamental feature of a B-scan image is that one of the dimensions is inferred from the arrival time of echoes of a short acoustic pulse, as they reflect from structures along a presumed straight-line path. Signals received from structure close to the transducer arrive earlier than signals received from structures far from the transducer [11]. The other transverse dimension is obtained by moving the transducer (either physically by mechanical means or apparently by electronic means) so that a different straight-line path through the object is interrogated by another short acoustic pulse. This process is continued until the entire object region of interest is scanned. Some means of tracking the propagation path through the object is required in order to continuously define the image. A block diagram of a generalized B-scan system is shown in Figure 10.1.

In this block diagram, it shows that an electronic pulser excites a transducer so that a short burst of ultrasound is generated. Acoustic signals reflected from the objects in the acoustic path impinge on the transducer are converted to electronic signals and processed for display. Usually, time gain compensation is used, whereby the amplifier gain is increased with time in order to partially compensate for the attenuation experienced by signals reflected from deeper part of the body. The position and angular direction of the ultrasound beam are determined by position monitoring electronics which keep track of where on the monitor the image signals should be displayed.

SYNC

Pulser

Transducer

Figure 10.1 A block diagram of a simple B-scan system (Havlice and Taenzer [3] © IEEE)

As the echoes are received by the transducer, they are amplified, rectified, filtered and the resulting signal is used to brightness modulate the display.

One of the most important developments in acoustic imaging was the introduction of greyscale display [12]. In a greyscale display, there are usually 10 or more distinct brightness levels. The imaging system assigns a given brightness level to a small range of echo intensities and distributes the brightness levels such that, for example, strong echoes are displayed brightest and weaker ones at progressively lower brightness levels. Thus, greyscale display produces B-scan images which are easier to interpret and improves image repeatability.

Figure 10.2 shows the three basic image formats of linear sector and are for mechanical scanning. In a linear scan, the transducer moves in a straight line. The field of view in this direction is limited by the length of travel of the transducer. However, in the time (or depth) dimension, the field of view is limited only by the depth of penetration (i.e. the frequency and attenuation or the physical slice of the object being scanned). One advantage of this technique is that the image may consist of a uniform line density which results in a constant spatial sampling rate of the object and a good display on the monitor.

In the sector scan, the transducer position remains fixed at a point on or above the object but is swept through an angular sector [13]. In this case, the field-of-view increases with depth of penetration. However, the line density diminishes as the field-of-view expands. This type of scan is particularly well suited for imaging through narrow apertures such as for imaging the heart through the ribs. In the arc scan, a transducer is moved along the arc of a circle, which gives rise to an image format that is the inverse of the sector scan [14]. Note that the field of view is largest near the transducer and decreases with depth of penetration. This type of scan is usually used for the manual scan of the abdomen with the surface of which resembles the arc of a rib.

An example of the next type of scan, the compound scan [15] is a combination of the sector scan with either a linear scan or an arc scan. It is illustrated in Figure 10.3. The sector is