*3.2.3. System operation*

When testing, the receiving probe array is coupled to the test object medium with a liquid couplant such as oil or jelly. The test object is insonified with a short pulse of ultrasound and the delay of the stroboscopic light source is set appropriately such that the acoustic images are optically frozen in time within the optical imaging medium (schlieren cell) at the instant of best focus.

The resulting optical image of the whole object field can be seen either by naked eye or can be captured with a video camera. Since the time required to form a complete image field is now only limited by the time of flight of the acoustic signals the frame rate could reach the theo‐ retically possible maximum limits (e.g. in excess of 1000 frames per second).

#### *3.2.4. Results from the first prototype*

#### *Image linearity and Isochronicity*

images on the left along the optical axis. For experimentation, only 15 of the 30 available

channels were used in this prototype.

**Figure 8.** Sonoptical assembly including the imaging cell

280 Advancements and Breakthroughs in Ultrasound Imaging

**Figure 9.** The active acoustooptical imaging system (1st prototype)

Although the system was designed with 30 channels, only 15 were used because of the difficulty of making elements of the arrays with sufficiently close characteristics as required for this imaging topology. However, as can be seen from the images, they are exceptionally of high quality in terms of image clarity and resolution. Figure 10 shows a test block with side drilled holes and the resulting image, clearly demonstrating image linearity and isochronicity as expected from the system design.

**Figure 10.** Images of 2mm side drilled holes.

Notice the axial magnification as depicted in equation 10 in appendix. The targets central to the array obviously have stronger return echoes, so they are somewhat saturated due to relatively low dynamic range of the channel amplifiers used. Figure 11 below show a test object, again with side drilled holes and the respective image with lateral and axial magnifications equalised. This image demonstrates exceptional image quality with object-to-image spatial relationship correctly maintained.

narrow in practice since wider coverage requires moving the receiving array over the test surfaces, which are typically uneven. Since the image reconstruction is heavily dependent on the amplitude and phase information of the signals, unevenness influences the image quality to a greater extent than conventional imaging methods; although in ideal conditions, excellent

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

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

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**4. Advancements in non-conventional methods — Development of a high-**

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

**1.** Ability to scan a test object for off-axis imaging, thus overcoming the limitation of small

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

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

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 sector-

**2.** Dynamic focusing along specified paths and sites to improve image quality.

results could be achieved as shown in the results section above.

**speed, computer-controlled hybrid scanner**

**3.** Means of providing a degree of image enhancement.

insonifying beam need to be steered to illuminate the object sector.

characteristics would be very desirable.

field of view.

is possible.

dependent focusing delay control.

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

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

#### *3.2.5. Practical limitations of the first prototype*

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

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 narrow in practice since wider coverage requires moving the receiving array over the test surfaces, which are typically uneven. Since the image reconstruction is heavily dependent on the amplitude and phase information of the signals, unevenness influences the image quality to a greater extent than conventional imaging methods; although in ideal conditions, excellent results could be achieved as shown in the results section above.
