**1.2. General principles**

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,

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.

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.

*R*2

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

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

*<sup>R</sup>*<sup>1</sup> <sup>+</sup> <sup>2</sup>∝(*R*<sup>2</sup> - *<sup>R</sup>*1) (1)

*Loss*(*dB*)= 20log

and equally become a nuisance in other cases.

270 Advancements and Breakthroughs in Ultrasound Imaging

*1.1.1. Dynamic range considerations*

limited display dynamic range.

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 operatordependent and therefore of limited use in diagnostic applications.

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‐ monly known as B-scans.

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 obtained are normally referred to as sector B-scans.

#### *1.2.1. Real-time versus determinism*

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* *Real-time"* is a more meaningful definition in such cases where the image is seen to be live, while the velocity ratio (i.e. the speed of the target to that of sound) is significant, or forming an image frame takes significant time.

It is also important to note that there has been renewed interest in some of the early develop‐ ments that uses direct imaging technology such as ultrasonic cameras and other imaging

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

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

273

A fundamental limitation with all of the above existing technology, which arises from the need for line-serial scanning, is the loss of temporal resolution. Attempts to improve this aspect by increasing computing power alone cannot give the full-potential of real-time imaging and may

The second limitation, for example when using linear-array technology, is the use of smaller effective aperture compared to the size of the total array aperture, hence affecting the lateral resolution achievable. These two limitations are analysed below in order to reveal the extent to which they affect potential performance in ultrasonic imaging. Although the discussion is based on using linear arrays for clarity, the same considerations are valid for other configu‐

Figure 1 below shows a linear array transducer coupled to a test medium. The test object is assumed to be homogeneous and have a depth (d). The array consists of (N) elements with a total aperture size (A). The elements are usually switched on as a group (as shown) rather than single elements to improve lateral resolution, since this depends on the effective aperture, as depicted in equation 3. The next group in the firing sequence advance by only one element

Test medium

Group fired

Between individual scanning lines, a rest period (trest) is applied before firing the second consecutive group of elements as a practical requirement to allow multiple echoes to die down

A elements

Transducer Array (N)

thus keeping the line resolution equal to the spacing of the array elements.

**2. Limitations of the conventional technology**

only give improvements with largely diminishing returns.

modalities [10, 11].

**2.1. Basic limitations**

rations.

d

**Figure 1.** Line-serial scanning

Direction of Scan

and time for acquisition, storage and display of data.

On the other hand, *"Determinism"* can be an important concept in defining a key aspect of image integrity. This can be a predominant requirement – for example in quantitative image analysis where accurate spatial information is important. The use of the term "determinism" in this context does not really mean "as it happens" (although the time interval between a moving target and its presentation as an image is usually small). Determinism essentially means that the spatial integrity of a moving target is persevered in the image to a high degree. In the case of ultrasound the theoretical limit of spatial determinism can only be reached if a single ultrasound pulse emitted by an imaging device produces a complete image field of the object within its time of flight. This is clearly not achievable with the existing line-serial imaging technology, but it is one of the main features of the new hybrid imaging system described later in this chapter.

#### **1.3. State-of-the-art techniques**

In late 1980's, the techniques for generating 3-D images from a collection of 2-D images have been demonstrated [4]. Producing a single frame of such an image may have taken about 20 minutes then, but with the advancement of computing power and processing techniques, live 3-D and 4-D volume imaging reaching pseudo-real time volumes of surface features have now come into existence [4, 5].

One of the fairly recent additions to B-mode imaging is the so called Zone-sonography [6]. A main feature in this system is the acquisition of data from a relatively low number of zone sectors, as opposed to line-by-line acquisition, thereby significantly improving the temporal artefacts inherent with the conventional methods. It has been claimed that in some cases, this approach could produce speed improvements of up to 10 times compared to conventional lineserial imaging systems [6], although the zones in themselves are produced using serial scanning.

In addition, in the case of medical imaging, various echo enhancement techniques such as Coded Excitation (CE) and Digitally Encoded Ultrasound (DEU) which improve sensitivity, penetration and contrast are now been widely used [7, 8]. CE is a pulse compression technique which is designed to differentiate and boost weak return signals from deep within the body. This is done by transmitting an encoded pulse sequence, isolating the coded return signal and amplifying only this signal, while regaining longitudinal resolution which is distributed within the insonifying pulse train [7]. Some systems using CE can also show blood flow together with the body tissue as a B-mode image, without the need for overlay as would be the case with Doppler imaging. Tissue Harmonic Imaging (THI) is another widely used technique which reduces haze, clutter and image artefacts. In this approach, higher harmonic components of the echoes generated by the tissues due to non-linear propagation are used instead of the fundamental [9].

It is also important to note that there has been renewed interest in some of the early develop‐ ments that uses direct imaging technology such as ultrasonic cameras and other imaging modalities [10, 11].
