**3. Non-conventional methods**

#### **3.1. Direct ultrasonic imaging**

Direct ultrasonic image visualization using alternative methods, such as ultrasonic hologra‐ phy, Bragg diffraction imaging etc. have been documented by many researchers in the past [12, 13, 14]. One such technique that has the above ideal properties is the Direct Ultrasonic Visualization of Defects (DUVD) system demonstrated by Hansted in the 1970's [12]. As schematically shown in Figure 3(a), this is a passive acousto-optical configuration which uses a pair of ultrasonic lenses with a common focus to form an ultrasonic image of the object field in a transparent medium from echoes received from a test object; much in the same way as an

However, two main problems with the DUVD system shown above are: very low sensitivity and its design is such that the test objects virtually becomes part of the system; thus severely limiting flexibility as can be seen from the block diagram representation of the DUVD below

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**Figure 4.** DUVD block schematic (Passive system). The insonifying transducer is bonded at the boundary between me‐

Further attempts to improve this approach have been reported by others in the late 1970's [13, 14]. But these were also passive systems and apart from theoretical interest the performance

Because of the attractive features of direct ultrasonic imaging without the need for line serial scanning, a significant development was undertaken in the mid 1980's [15]. For initial feasi‐ bility studies, an active 2D version of the DUVD concept was considered. A major advance‐ ment was the introduction of amplification between a set of transmitting and receiving arrays of transducers; thereby solving the problems of low sensitivity and inflexibility inherent with the DUVD approach. This decouples the test object from the rest of the system as shown in

Since the system is now transformed from passive sonoptics to an active sampling and reconstruction technique, the design specifications were derived by detailed computer simulations and practical investigations to achieve satisfactory image quality. It should be emphasised that the requirements for image reconstruction for this system is very different to conventional imaging. It essentially involves image formation utilizing amplitude and phase

> *exp*- *<sup>j</sup>*(*k ri* <sup>+</sup>∅*<sup>i</sup>* )

> > *ri*

is the distance to the point (x,y) from the ith

where, p(x, y) represent the acoustic pressure at a point x, y in the image space as in Figure 6

is the relative phase of the ith element, and k is the wave number.

(5)

of signals as represented by equation 5 and Figure 6 below.

is the normalised signal amplitude, ri

 *P*(*x*, *<sup>y</sup>*)= ∑ *i*=1 *N ai*

(Figure 4).

dium 1 & 2.

was practically inadequate.

*3.2.1. Conceptual development*

Figure 5 below.

below, ai

element, φ<sup>i</sup>

**3.2. A key development in the 1980's**

**Figure 3.** Direct Ultrasonic Imaging (DUVD) (a) DUVD Basic configuration; (b) A liquid coupled version of DUVD

optical lens producing an optical image. The insonifying transducer is physically bonded or intimately coupled via a liquid medium to the test object as part of Lens 1. Returned echoes are brought into focus by the acoustic lens arrangement in the image medium. The ultrasonic images thus produced is made visible by stroboscopic light which is synchronized to the transmitted insonifying ultrasound pulse, but with a fixed delay to allow the image to be formed and viewed at the point of best focus.

One of the DUVD realisations with liquid coupled lenses and a schlieren acoustooptical visualisation system developed by Hansted [12] is shown in Figure 3(b) above. It is interesting to note that the DUVD has many ideal features: It operates in Real-time; producing 3D images of the complete object field with every single insonifying pulse. 9

However, two main problems with the DUVD system shown above are: very low sensitivity and its design is such that the test objects virtually becomes part of the system; thus severely limiting flexibility as can be seen from the block diagram representation of the DUVD below (Figure 4).


**Figure 4.** DUVD block schematic (Passive system). The insonifying transducer is bonded at the boundary between me‐ dium 1 & 2.

Further attempts to improve this approach have been reported by others in the late 1970's [13, 14]. But these were also passive systems and apart from theoretical interest the performance was practically inadequate.

#### **3.2. A key development in the 1980's**

#### *3.2.1. Conceptual development*

optical lens producing an optical image. The insonifying transducer is physically bonded or intimately coupled via a liquid medium to the test object as part of Lens 1. Returned echoes are brought into focus by the acoustic lens arrangement in the image medium. The ultrasonic images thus produced is made visible by stroboscopic light which is synchronized to the transmitted insonifying ultrasound pulse, but with a fixed delay to allow the image to be

**Figure 3.** Direct Ultrasonic Imaging (DUVD) (a) DUVD Basic configuration; (b) A liquid coupled version of DUVD

(b)

One of the DUVD realisations with liquid coupled lenses and a schlieren acoustooptical visualisation system developed by Hansted [12] is shown in Figure 3(b) above. It is interesting to note that the DUVD has many ideal features: It operates in Real-time; producing 3D images

9

formed and viewed at the point of best focus.

of the complete object field with every single insonifying pulse.

(a)

Object medium

Defect

276 Advancements and Breakthroughs in Ultrasound Imaging

Lens 1 Lens 2

Common focus

Image of Defect

Transparent Image medium

Because of the attractive features of direct ultrasonic imaging without the need for line serial scanning, a significant development was undertaken in the mid 1980's [15]. For initial feasi‐ bility studies, an active 2D version of the DUVD concept was considered. A major advance‐ ment was the introduction of amplification between a set of transmitting and receiving arrays of transducers; thereby solving the problems of low sensitivity and inflexibility inherent with the DUVD approach. This decouples the test object from the rest of the system as shown in Figure 5 below.

Since the system is now transformed from passive sonoptics to an active sampling and reconstruction technique, the design specifications were derived by detailed computer simulations and practical investigations to achieve satisfactory image quality. It should be emphasised that the requirements for image reconstruction for this system is very different to conventional imaging. It essentially involves image formation utilizing amplitude and phase of signals as represented by equation 5 and Figure 6 below.

$$P\_{\{\mathbf{x}\_i, y\}} = \sum\_{i=1}^{N} a\_i \frac{\exp^{-j\{k\_{r\_i} + \mathcal{O}\_i\}}}{r\_i} \tag{5}$$

where, p(x, y) represent the acoustic pressure at a point x, y in the image space as in Figure 6 below, ai is the normalised signal amplitude, ri is the distance to the point (x,y) from the ith element, φ<sup>i</sup> is the relative phase of the ith element, and k is the wave number.

Therefore in order to achieve good image quality, stringent control on the uniformity of the transducer elements in terms of their amplitudes and phase responses were essential requiring special fabrication techniques, since achieving close elemental uniformity in transducer arrays

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The details of the prototype design while achieving the ideal features described in section 4.1 above namely: linearity, isochronicity and maximum lateral resolution corresponding to the

For the feasibility study of the active direct imaging concept, a 2-D version of the 3-D DUVD sonoptics was chosen. However, it is important to note that as for the 3-D sonoptics, the same acoustooptical relationships could be maintained by using cylindrical lenses to give an image field similar to a B-scan. Because of the flexibility introduced by the sampling and retransmit‐ ting acoustic arrays, the operation of the lenses could be emulated by a number of different ways; e.g. by electronic focusing to represent both lenses, using solid cylindrical lenses, or by a combination of both. It should be emphasized that once the lens characteristics required are implemented, there is no further special requirement for dynamic focusing as the entire image field will be in focus at one instant of time requiring only one excitation pulse to produce the

After investigating different possibilities, the following configuration shown in Figure 7 below

**Figure 7.** Active system configuration with conversion of DUVD sonoptics to maintain image linearity, Isochronicity

Figure 8 below shows the construction of the retransmitting section of the system with retransmitting array, cylindrical lens and image medium. The design parameters were chosen to image a section of steel of depth up to 40 cm, which is well in excess of typical applications.

Figure 9 show the trolley-mounted 1st prototype system designed in accordance with the simplified block diagram shown in Figure 5. The optical assembly is equipped with an ultrashort stroboscopic light source [16] mounted on the right and a video camera for capturing

is a challenging task.

*3.2.2. First prototype system*

image of the whole object field.

was used for the first prototype.

and resolution.

total acoustic aperture are presented in the Appendix.

**Figure 5.** (a) – Active system block diagram with arrays (b) – Active system schematic block diagram

**Figure 6.**

Therefore in order to achieve good image quality, stringent control on the uniformity of the transducer elements in terms of their amplitudes and phase responses were essential requiring special fabrication techniques, since achieving close elemental uniformity in transducer arrays is a challenging task.

The details of the prototype design while achieving the ideal features described in section 4.1 above namely: linearity, isochronicity and maximum lateral resolution corresponding to the total acoustic aperture are presented in the Appendix.
