**6. Conclusions**

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 investigated.

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, it produces sector images formed by overlapping zones, but each zone requiring only one pulse to produce a complete zone image, thus gaining by far the highest speed of sector scanning compared to conventional methods.

**•** Element width-to-gap ratio

**•** Signal dynamic range

below.

necessary.

**•** Input and output impedances

**Maintaining image linearity**

**Maintaining isochronicity**

**•** Element damping characteristics

**Acoustooptical design considerations**

**•** Physical size, shape and transducer material

**•** Typical and worst case input signal levels

**•** Output signal power required from each channel

Figure 19 shows the two-lens system of the DUVD.

giving the system its ability to display the whole image at once.

**•** Phase linearity and bandwidth considerations

The primary aim was to obtain an idea of the sharpness of the images and the extent of artefacts that may be formed in a given image space. Since the transducer backings were conductive, maintaining a low level of electrical cross-coupling was required for the size of the backing thus the element gap spacing was kept around 0.3mm. For visualisation of the acoustic image formed in the modulating medium, the requirements for channel characteristics were then determined. These included practical determinations of number of parameters including:

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As mentioned in section 4 above, as opposed to conventional image reconstruction, the passive DUVD system had some ideal properties for ultrasonic imaging namely, image linearity and isochronicity. Maintaining the same properties in the active system was therefore important. The requirement for image linearity as determined by Hansted [12] for the DUVD is given

Using paraxial ray analysis, it can be shown that in order to maintain image linearity through‐ out the image field, the only requirement is to make the focal point of the two acoustic lenses coincident. This however changes the lateral magnification as given in equation 10, but since this is a constant it does not affect linearity or image quality and can be easily compensated if

As stated in section 4 above, iscochronicity means that when the test object is insonified with a short acoustic pulse, all the echoes from the different targets, irrespective of their special distribution within the object field, arrive simultaneously at their respective image points thus

The above capability is in stark contrast to the need for line serial scanning in conventional imaging, giving the system its speed - the highest theoretically possible speed of imaging as it

Furthermore, the clarity and contrast of the images are high as the stroboscopic image mapping results in gating out many artefacts, problems due to reverberations and noise, to a high degree. Since the image reconstruction, by design, is based on coherent summation of signals in amplitude and phase corresponding to true targets satisfying the sonoptical focusing require‐ ments, random noise gets suppressed without the need for further processing.
