**4. Advancements in non-conventional methods — Development of a highspeed, computer-controlled hybrid scanner**

From the performance of the acousto-optical imaging system shown in Figure 9, it was evident that to meet practical requirements and to advance the potential benefits, the following characteristics would be very desirable.


**Figure 11.** Imaging side-drilled holes in a test block

282 Advancements and Breakthroughs in Ultrasound Imaging

**Figure 12.** Image of a micro crack in a T-weld

*3.2.5. Practical limitations of the first prototype*

the test block.

Figure 12 below shows an actual T-weld being tested with the prototype system and the image of a crack in the weld. This micro-crack was actually visible from the ground side surface of

The results from the first prototype have clearly demonstrated the potential of direct ultrasonic imaging reaching performance close to theoretical limits. These include: maximum possible speed of imaging, maximum lateral resolution for the size of the arrays used, forming focused

However, there were still practical limitations. This system also had a small field of view approximately that covered laterally by the array aperture; in this case ~3 cm wide. This is too

images of the whole object field covered by the transducer aperture.

The first property adds a much greater degree of freedom to test uneven surfaces from selected or prepared locations. The resulting images would be similar to a B-mode sector scan, but the imaging modality is such that it produces image zones for required sectors as opposed to individual line serial scanning, with just a few overlapping sectors enabling much higher frame rates to be achieved. Since there is no requirement for focusing in the image medium, as that is already taken care of by the sonoptical design, the scanning simply means that only the insonifying beam need to be steered to illuminate the object sector.

Since the image reconstruction is implemented with a sonoptical system designed with paraxial ray equations, when imaging a wide field like a sector, there is likely to be a significant degree of peripheral geometrical aberration in the image field. It should be possible to correct these dynamically, since for each image zone, implementation of an appropriately precalculated delay in the firing of the stroboscopic light source, channel gain/phase manipulation is possible.

Inclusion of the above properties is the basis of the development [17-19] described below. Figure 13 shows the basic block diagram of the hybrid prototype developed, incorporating scanning hardware (SCH) controlled by a microcomputer (PC). Dynamic control hardware (DCH) is intended to provide gain equalisation during off-axis imaging and time varying gain (TVG) to produce a uniform image field. Field Focus Control (FFC) ensures that the strobo‐ scopic illumination is synchronised such that the image field is optically frozen at the instant of best focus. It is also intended to provide a degree of off-axis aberration correction with sectordependent focusing delay control.

Figure 14 shows a more detailed block diagram of the improved design with a possible use in

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**Figure 14.** A hybrid imaging Topology with computer controlled scanning and acoustooptical image reconstruction

Since the principle of acousto-optical imaging is very different to that of the conventional methods, some of the requirements of electronics and ultrasonic hardware is very different to that of the conventional systems in many ways. Since the received signals are converted back into ultrasound signals and retransmitted into an acoustic modulator to produce optical effects,

(Expanded block diagram)

**Figure 15.** The first prototype of the hybrid scanner (GB2278443B)

testing of welds. Figure 15 shows the first prototype hybrid scanner developed.

**Figure 13.** Basic block diagram of the hybrid scanner

#### **4.1. Principle of operation**

As mentioned previously, in contrast to conventional imaging, this system also operates on the basis of forming an ultrasonic map of the object field from received echoes inside a visible medium. Once an ultrasonic pulse illuminated the object medium, the acoustic signals intercepted by the receiving array is retransmitted into the image medium through the acoustic focusing system after conditioning and boosting signal power using signal conditioning hardware (SC). The re-transmitting section consists of an identical set of transducer array (except for the shape) to that of the receiving array, coupled to a specially designed ultrasonic focusing arrangement according to the requirements described in 8.1.1 (Appendix). Two important characteristics of this focusing lens design are that all the target echoes are brought into focus at the same instant of time irrespective of the target depth while maintaining image spatial linearity, so that a complete image frame is produced by just one insonifying pulse in real-time.

When the acoustic image field is at the time of best focus, an ultra-short stroboscopic light pulse of the order of a few nanoseconds is emitted, which produces a visible image. This can be viewed directly or captured by a video camera. Since the maximum repetition rate is now only determined by the time-of-flight of the sound pulse, the system achieves the highest speed of operation theoretically possible in ultrasonic imaging, eliminating the problem of temporal artefacts inherent with the existing systems.

Figure 14 shows a more detailed block diagram of the improved design with a possible use in testing of welds. Figure 15 shows the first prototype hybrid scanner developed.

**Figure 14.** A hybrid imaging Topology with computer controlled scanning and acoustooptical image reconstruction (Expanded block diagram)

**Figure 15.** The first prototype of the hybrid scanner (GB2278443B)

**Figure 13.** Basic block diagram of the hybrid scanner

284 Advancements and Breakthroughs in Ultrasound Imaging

artefacts inherent with the existing systems.

As mentioned previously, in contrast to conventional imaging, this system also operates on the basis of forming an ultrasonic map of the object field from received echoes inside a visible medium. Once an ultrasonic pulse illuminated the object medium, the acoustic signals intercepted by the receiving array is retransmitted into the image medium through the acoustic focusing system after conditioning and boosting signal power using signal conditioning hardware (SC). The re-transmitting section consists of an identical set of transducer array (except for the shape) to that of the receiving array, coupled to a specially designed ultrasonic focusing arrangement according to the requirements described in 8.1.1 (Appendix). Two important characteristics of this focusing lens design are that all the target echoes are brought into focus at the same instant of time irrespective of the target depth while maintaining image spatial linearity, so that a complete image frame is produced by just one insonifying pulse in

When the acoustic image field is at the time of best focus, an ultra-short stroboscopic light pulse of the order of a few nanoseconds is emitted, which produces a visible image. This can be viewed directly or captured by a video camera. Since the maximum repetition rate is now only determined by the time-of-flight of the sound pulse, the system achieves the highest speed of operation theoretically possible in ultrasonic imaging, eliminating the problem of temporal

**4.1. Principle of operation**

real-time.

Since the principle of acousto-optical imaging is very different to that of the conventional methods, some of the requirements of electronics and ultrasonic hardware is very different to that of the conventional systems in many ways. Since the received signals are converted back into ultrasound signals and retransmitted into an acoustic modulator to produce optical effects, the technique relies on the preservation of amplitude and phase characteristics of the signals to a high degree. Also, adequate signal power is required to produce acousto-optical modu‐ lation in the image medium to make the acoustic map of the object field visible.

essentially behaving as an active, acousto-optical imaging device operating in real time. Therefore, even in the scanning mode, this allows very high temporal resolution to be achieved since one beam produces a complete image field or image zone. Furthermore, the whole of the effective aperture is utilized for each image frame and therefore diffraction-limited lateral

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If the scanning angles are small, the system could produce images without the need for any compensation. This is the simplest mode of scanning operation. For larger beam angles in the scanning mode, compensation to account for the reduction in sensitivity and field uniformity may be applied by the control of insonifying energy, receiver gain and scan derived electronic

As mentioned above, although the prototype was equipped with 30 channels, the number of channels used for the feasibility study was just 8 with an effective aperture of only 1.6 cm (as marked on the receiving array in Figure 17a). This was because of a problem of some of the channels breaking into oscillations due to excessive capacitive feedback at the time of experi‐ ments. Nonetheless, it can be seen from Figure 17 that the images produced are still very good

As mentioned above, in the coaxial mode, the system resembles in operation to that shown in Figure 10 to 12. The speed of imaging is very high, e.g. in a test block of steel 40 cm deep, a complete image is formed within 0.15ms and the frame rate can be as high as 1kHz or more. Estimation of resolution capabilities previously obtained using the coaxial system of Figure 9 showed that the images can be produced to within one wavelength resolution in the axial direction and about 1.5 wavelengths laterally at about 60mm below the surface of the test block;

In order to test the operation in the scanning mode; a test block with side drilled holes covering a wide sector was prepared as shown in Figure 17(a). Figure 17 (b), (c) & (d) shows the images when the insonifying beam was steered statically (i.e. in manually selected angles) to image the 3 holes in the left, two in the centre and the two on the right-hand side of the test block. Dynamic scanning was also verified. However, since the scan sector required for the above

statically scanned, aberration can be controlled for each angle manually by selecting strobo‐

Although the prototype system was designed for NDT applications, attempts to image biological moving tissues, such as heart valves, revealed excellent temporal resolution as expected. However, since the transducer arrays used did not have the required element spacing characteristics and matching properties, and the sonoptics of the prototype was designed for industrial material, the prototype could not obviously produce true B-mode images for biological tissues although the potential for advancement in this area was clearly

scopic firing delays to achieve best focus. For smaller sectors (e.g. < 600

) peripheral geometrical aberration was significant. When

) aberration was not a

resolution approaches the theoretical maximum.

phase delays, which is an area for further development.

**4.2. Results from the 1st hybrid scanner prototype**

for the aperture used.

thus approaching theoretical limits.

test object was very large (~1200

significant factor.

evident.

For the first hybrid prototype system, the excitation pulses generated were 2MHz single sinusoidal pulses of the order of 120Vpp as shown in Figure 16 below. These pulses were shaped to obtain the near-ideal response from the transducer elements.

The front-end receiving section of the hardware consists of wideband amplifiers (15 MHz) with a variable gain of up to 60dB. The output impedance is of the order of 50 Ohms with a maximum output swing capability of 120V pp. Input surge protection up to 1kV was provided with a very low recovery time to achieve minimal dead-zone. Although the above prototype scanner was equipped with 30 channels, only 8 channels were used for the feasibility study. This represented just 1.6 cm acoustic aperture. No dynamic compensation was used. However, the performance as can be seen from statically scanned images was still very good.

#### *4.1.1. Operational modes*

The system may be operated in the axial mode for very high-speed imaging or be used in the scanning mode to cover a wide field. In the coaxial mode, the system resembles in operation to that shown in Figure 10 -12, producing narrow-field linear array B-mode images covered by the aperture of the array. In the scanning mode the system is under the control of the computer and can be used to produce wide-field, high resolution B-mode sector scan images, or be programmed to scan along any specific areas of the target. In order to achieve field uniformity for off-axis imaging, scan-angle derived field-focus compensation (FFC) and dynamic compensation (DCH) may be applied. In the scanning mode, wideband insonifying pulses are beamed to target areas by phased array beam steering. However, when receiving the echoes from the targets, there is no requirement for beam forming at all, as the system essentially behaving as an active, acousto-optical imaging device operating in real time. Therefore, even in the scanning mode, this allows very high temporal resolution to be achieved since one beam produces a complete image field or image zone. Furthermore, the whole of the effective aperture is utilized for each image frame and therefore diffraction-limited lateral resolution approaches the theoretical maximum.

If the scanning angles are small, the system could produce images without the need for any compensation. This is the simplest mode of scanning operation. For larger beam angles in the scanning mode, compensation to account for the reduction in sensitivity and field uniformity may be applied by the control of insonifying energy, receiver gain and scan derived electronic phase delays, which is an area for further development.

#### **4.2. Results from the 1st hybrid scanner prototype**

the technique relies on the preservation of amplitude and phase characteristics of the signals to a high degree. Also, adequate signal power is required to produce acousto-optical modu‐

For the first hybrid prototype system, the excitation pulses generated were 2MHz single sinusoidal pulses of the order of 120Vpp as shown in Figure 16 below. These pulses were

The front-end receiving section of the hardware consists of wideband amplifiers (15 MHz) with a variable gain of up to 60dB. The output impedance is of the order of 50 Ohms with a maximum output swing capability of 120V pp. Input surge protection up to 1kV was provided with a very low recovery time to achieve minimal dead-zone. Although the above prototype scanner was equipped with 30 channels, only 8 channels were used for the feasibility study. This represented just 1.6 cm acoustic aperture. No dynamic compensation was used. However,

The system may be operated in the axial mode for very high-speed imaging or be used in the scanning mode to cover a wide field. In the coaxial mode, the system resembles in operation to that shown in Figure 10 -12, producing narrow-field linear array B-mode images covered by the aperture of the array. In the scanning mode the system is under the control of the computer and can be used to produce wide-field, high resolution B-mode sector scan images, or be programmed to scan along any specific areas of the target. In order to achieve field uniformity for off-axis imaging, scan-angle derived field-focus compensation (FFC) and dynamic compensation (DCH) may be applied. In the scanning mode, wideband insonifying pulses are beamed to target areas by phased array beam steering. However, when receiving the echoes from the targets, there is no requirement for beam forming at all, as the system

the performance as can be seen from statically scanned images was still very good.

lation in the image medium to make the acoustic map of the object field visible.

shaped to obtain the near-ideal response from the transducer elements.

**Figure 16.** Single sinusoidal excitation pulse

286 Advancements and Breakthroughs in Ultrasound Imaging

*4.1.1. Operational modes*

As mentioned above, although the prototype was equipped with 30 channels, the number of channels used for the feasibility study was just 8 with an effective aperture of only 1.6 cm (as marked on the receiving array in Figure 17a). This was because of a problem of some of the channels breaking into oscillations due to excessive capacitive feedback at the time of experi‐ ments. Nonetheless, it can be seen from Figure 17 that the images produced are still very good for the aperture used.

As mentioned above, in the coaxial mode, the system resembles in operation to that shown in Figure 10 to 12. The speed of imaging is very high, e.g. in a test block of steel 40 cm deep, a complete image is formed within 0.15ms and the frame rate can be as high as 1kHz or more. Estimation of resolution capabilities previously obtained using the coaxial system of Figure 9 showed that the images can be produced to within one wavelength resolution in the axial direction and about 1.5 wavelengths laterally at about 60mm below the surface of the test block; thus approaching theoretical limits.

In order to test the operation in the scanning mode; a test block with side drilled holes covering a wide sector was prepared as shown in Figure 17(a). Figure 17 (b), (c) & (d) shows the images when the insonifying beam was steered statically (i.e. in manually selected angles) to image the 3 holes in the left, two in the centre and the two on the right-hand side of the test block.

Dynamic scanning was also verified. However, since the scan sector required for the above test object was very large (~1200 ) peripheral geometrical aberration was significant. When statically scanned, aberration can be controlled for each angle manually by selecting strobo‐ scopic firing delays to achieve best focus. For smaller sectors (e.g. < 600 ) aberration was not a significant factor.

Although the prototype system was designed for NDT applications, attempts to image biological moving tissues, such as heart valves, revealed excellent temporal resolution as expected. However, since the transducer arrays used did not have the required element spacing characteristics and matching properties, and the sonoptics of the prototype was designed for industrial material, the prototype could not obviously produce true B-mode images for biological tissues although the potential for advancement in this area was clearly evident.

allows true signals from targets to be coherently re-constructed in the image medium while the noise and out of phase signals are being largely rejected as evident from Figure 18 below. However, just like all ultrasound equipment multiple reflections between closely spaced targets could still cause artefacts. Therefore, inclusion of selective insonification of the object medium as in item No. 2 above should enable the system to achieve even higher SNR while

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Another important factor as mentioned in section 2 for any imaging technique is the dynamic range. In this respect, one of the typical bottlenecks is the limited dynamic range of display equipment. The present development can provide a greater display dynamic range when images are viewed with the naked eye as the acousto-optical modulator can provide a higher display dynamic range than images captured by a camera and presented on a typical conven‐

Conventional ultrasonic imaging systems have inherent limitations such as low speed leading to temporal artefacts and in some cases limited lateral resolution; these being the result of lineserial scanning, lengthy processing and other limitations arising from the particular techniques used. The extent to which the above deficiencies affect performance has been analytically

In this respect, it has been shown that an alternative hybrid approach to imaging using acoustooptical image reconstruction could give clear advantages; reaching theoretical limits of performance in speed and resolution unachievable with the existing methods. This is mainly due to the combination of electronic and sonoptical image reconstruction, avoiding line-serial scanning and lengthy processing required by the conventional systems. The hybrid system reaches almost ultimate speed and resolution in the coaxial imaging mode. In the B-scan mode,

suppressing artefacts when imaging near-field targets.

**Figure 18.** Noise suppression

tional VDU.

investigated.

**6. Conclusions**

**Figure 17.** a) - Test Block. Statically scanned images of the test block
