**1.1. Origin, expansion and applications of ultrasonic imaging**

History and the discoveries of the use of ultrasound can be traced back to late 18th century, but one of the major steps toward the practical use of ultrasound may be attributed to Lewis Nixon who invented the very first sonar type listening device in 1906 as a way of detecting icebergs [1]. Rapid developments of the use of ultrasound occurred since then, particularly after world war II; initially for underwater (sonar) and industrial uses, followed by developments for medical applications [2, 3]. Today, the technology is widespread in Medicine, Non Destructive Testing (NDT) and Sonar in many specialised areas such as: industrial and medical imaging, study and classification of the properties of material and biological tissues, seismic explora‐ tions high-intensity applications etc. and the applications are growing. These rapid advances are directly related to the parallel advancements in electronics, computing, and transducer technology together with sophisticated signal processing techniques.

Irrespective of the field of applications, arguably one of the most important applications of ultrasound is "Imaging", which is the chosen subject of this chapter. However, it is important to note that imaging as applied to the three main application areas mentioned above; namely, NDT, Sonar and Medicine have fundamental similarities and also differences. For example, the speed of sound in water in the case of sonar and biological tissues are comparable (~1450m/ s) but sonar is extremely long range.

In the case of NDT, the speeds of sound in industrial materials are generally very much higher, although the penetration distances are comparable to that for medical imaging. On the other hand, sonar and medical imaging primarily relies on one form of ultrasound propagation, namely, compressional waves while in solid material, there are multiple propagational modes,

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such as compressional, shear, surface, creep, lamb and torsional waves etc. These different modalities have widely different characteristics and they can coexist with possible mode conversions depending on the particular geometry and test scenario. In NDT, the co-existence or co-generation of this multimode propagation can be both advantages in some situations, and equally become a nuisance in other cases.

of the following section. The discussion does not include a comprehensive treatment of imaging techniques in use, but the ones that represent properties central to the understanding

Breaking Through the Speed Barrier — Advancements in High-Speed Imaging

http://dx.doi.org/10.5772/56378

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The simplest form of ultrasonic visualisation is what is known as the A-scan. Here an ultrasonic transducer generates a short pulse of high frequency sound which is coupled to the test medium. Echoes generated from any acoustic discontinuities within the path of the ultrasound beam are received, usually by the same transducer, and are displayed on a screen or a monitor as intensity modulated signals. The technique essentially provides (a) Target-depth informa‐ tion (b) An indication of the extent of the discontinuity and to a much lesser extent information on its orientation or shape. Since the time required for displaying the echoes is virtually equal to the time of flight of the acoustic pulse within the object medium, it gives the maximum temporal resolution for moving targets. However, the interpretation and information obtained from an A-scan (1- spatial dimension) or that derived from it is very limited, highly operator-

However, a collection of A-scans in a given plane in the test object could provide a twodimensional map (or an image) of the acoustic discontinuities. This requires acquisition, storage and display of successive A-scan lines requiring digital data storage and processing. These line-serial scans may be generated in a number of ways: mechanically moving the transducer along a given direction; using an array of transducers which are switched on in succession to mimic a mechanical movement (linear or curvilinear array scanning). Images thus generated represent a two-dimensional (2-D) view of the target medium and are com‐

Alternatively, the ultrasound beam may be electronically steered using a transducer array (phased array scanning) or by a mechanically rotating transducer. The 2-D images thus

One of the confusing terminologies in imaging (or any other form of data presentation) is *"Realtime"*. The naturally implied meaning of the term real-time is: *"as it happens".* This meaning is incompatible with laws of Physics since nothing can be observed at the same instant as something happens. So, image presentation can only approach this ideal of real-time, provided that what an observer sees in an image is close enough to what was happening to the object as a whole, both in time and spatial resemblance. Then the expression "real-time" can be considered appropriate. For example, an observer seeing a trajectory of a meteoroid views this in real-time because the velocity of light is at least 6 to7 orders of magnitude higher than the moving object. But this is not the case, for example, seeing a moving heart valve in an ultra‐ sound image taking a few milliseconds to form just one frame. Obviously, the image is temporally and spatially distorted. Therefore a target that appears to be moving in a succession of image frames is not seen "as it happens" and therefore not true real-time. The term *"Pseudo*

dependent and therefore of limited use in diagnostic applications.

obtained are normally referred to as sector B-scans.

of primary limitations.

**1.2. General principles**

monly known as B-scans.

*1.2.1. Real-time versus determinism*

In processing and assessing an ultrasound image, one of the most striking differences between industrial NDT and medical imaging is that in the case of the latter, there is good preanatomical knowledge of the part of the anatomy being examined; often with the possibility of supplementary data obtained from other forms of imaging such as MRI, CT scans etc. However, for NDT inspection, the target features (shapes, sizes and orientation etc.) are almost always largely unknown, thus the interpretation is largely based on the reliability of the images being produced. This is a major challenge in industrial imaging applications. On the positive side, the targets being imaged in NDT applications e.g. a crack in a structure generally tends to appear as a good acoustic discontinuity yielding good Signal-to-noise ratio (SNR) compared to that from a tissue boundary, since the latter is dependent on small impedance contrast.

#### *1.1.1. Dynamic range considerations*

For all imaging applications, the signal dynamic range is an extremely important considera‐ tion. Usually this could be very high; far above that may be accommodated by display equipment. For medical Imaging, the dynamic range of signals could be of the order of order of 100dB covering both backscattered signals from tissues and specular reflections. (However, the range of interest is of the order of 40 to 50 dB, which is the range covered by backscattered signals). Although impedance contrast is generally high in NDT and sonar, dynamic range of signals can still be very large depending on the application, not necessarily because of high attenuation as in the case of biological tissues but because of the size of the targets such as micro-defects in NDT or beam divergence in the case of sonar. In the case of sonar, the range is extreme and the signals of interest could be of the order of several hundred millivolts to submicro volts. The dynamic range can be evaluated for homogeneous media, such as water, by considering signal loss between targets of same strength placed at different axial distances (R1 & R2) form equation 1 below, which helps formulating compensating strategies.

$$\text{Loss}(dB) = 20 \log \frac{R\_2}{R\_1} + 2 \propto \left( R\_2 \cdot R\_1 \right) \tag{1}$$

Where ∝ is the attenuation coefficient of the medium at the frequency of interest. Accommo‐ dating large dynamic range and depth gain compensation are therefore features common to all imaging systems; albeit for different reasons. Once the ranges and the loss characteristics are known, accommodation can be handled electronically, e.g. by using logarithmic compres‐ sion and Time-varying-gain (TVG) functions to accommodate the signals within the much limited display dynamic range.

In order to understand the potential of imaging and areas of improvement, it is necessary to examine the fundamental characteristics of common imaging modalities, which is the subject of the following section. The discussion does not include a comprehensive treatment of imaging techniques in use, but the ones that represent properties central to the understanding of primary limitations.
