**2. Limitations of the conventional technology**

## **2.1. Basic limitations**

*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

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 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

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

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

an image frame takes significant time.

272 Advancements and Breakthroughs in Ultrasound Imaging

in this chapter.

scanning.

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

come into existence [4, 5].

instead of the fundamental [9].

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 only give improvements with largely diminishing returns.

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‐ rations.

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

**Figure 1.** Line-serial scanning

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 and time for acquisition, storage and display of data.

$$\begin{aligned} \mathbf{t}\_{\text{f}} &= \begin{pmatrix} \mathbf{N} \cdot \mathbf{N}\_{\text{G}} \end{pmatrix} \begin{bmatrix} \left(\frac{2\mathbf{d}}{\mathbf{c}}\right) & + & \mathbf{t}\_{\text{rest}} \end{bmatrix} \\\\ \text{1. } & \text{Frame rate} \quad \approx \quad \frac{1}{\mathbf{t}\_{\text{f}}} \end{aligned} \tag{2}$$

The other limitation with a linear array scanner, as mentioned above, is the degradation of lateral resolution arising from the size of the aperture formed by the group of elements firing at any instant of time. The lateral resolution is limited by the beam width of the transducer or the group of elements. The diffraction limited beam width for a given transducer of aperture

δθ <sup>=</sup> <sup>1</sup>.22 λ

to that achievable with the fully available aperture is not realised.

**2.2. Ideal properties of an imaging system**

where, λ is the wavelength of sound in the medium. As can be clearly seen, for each scan line, D is determined by the number of elements within the group fired and this is clearly very much less than the total aperture size (A) of the array. Hence, the lateral resolution corresponding

From the above discussion, it is clearly evident that two key properties of an ideal real-time ultrasonic imaging system is that it should be able to produce a complete image frame of the object volume from one insonifying pulse, while utilizing the full aperture available for each frame of the image. Additionally, an ideal system should produce focused images of the whole object field at the same instant of time irrespective of the distances of targets from the surface (Isochronicity) while maintaining accurate object-to-image spatial resemblance (image linearity). It is also clear that the existing technology utilising line-serial scanning cannot achieve these ideal properties and therefore alternative techniques may be needed to improve speed and high resolution capabilities beyond that feasible with conventional technology. This requires investigating non-conventional imaging modalities that can possess those capabili‐ ties, and the ways of overcoming limitations that may have precluded their use in practice. The next section presents some early developments that have some of the above key properties achieved through direct ultrasonic image reconstruction. Beyond academic interest, these techniques have not realised in large-scale use except in very specific applications due to other

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

<sup>D</sup> (4)

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

275

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

(D) could be expressed as:

inherent problems.

**3. Non-conventional methods**

**3.1. Direct ultrasonic imaging**

With reference to Figure 1, time (tf ) taken to produce one frame is:

where, NG is the number of elements in the group firing at any one time and c is the speed of sound. Hence, as an example, when using an array with 150 elements, coupled to a medium in which the velocity of sound is 1.54 mm/μs, thickness 20 cm, NG = 50, and neglecting trest, it can be seen that the maximum frame-rate achievable is about 75. This will be even less when trest is applied.

However, the above frame rate may sound adequate, and indeed so for many cases, but the line-serial scanning introduces a basic limitation in the case of fast moving targets such as heart valves or machinery. During the scanning time within a frame, one part of the target may have displaced significantly relative to another, causing spatial distortion, thus a frozen image of the target at any time may differ significantly from its actual position or shape. This is illustrated in Figure 2.

**Figure 2.** Formation of temporal artefacts

With reference to Figure 2, spatial distortion (Δs) could be written as:

$$\Delta \mathbf{s}\_{\{\mathbf{x}\}} = \begin{array}{c} \mathbf{x} \\ \hline \mathbf{A} \end{array} \times \begin{array}{c} \mathbf{t}\_{\{\mathbf{f}\}} \times \begin{array}{c} \mathbf{v}\_{\mathbf{x}} \\ \end{array} \tag{3}$$

where, vX is the velocity of a point (x) on target.

The other limitation with a linear array scanner, as mentioned above, is the degradation of lateral resolution arising from the size of the aperture formed by the group of elements firing at any instant of time. The lateral resolution is limited by the beam width of the transducer or the group of elements. The diffraction limited beam width for a given transducer of aperture (D) could be expressed as:

$$
\delta\Theta = \begin{array}{c c}
\frac{1.22\ \lambda}{D} \\
\end{array}
\tag{4}
$$

where, λ is the wavelength of sound in the medium. As can be clearly seen, for each scan line, D is determined by the number of elements within the group fired and this is clearly very much less than the total aperture size (A) of the array. Hence, the lateral resolution corresponding to that achievable with the fully available aperture is not realised.

#### **2.2. Ideal properties of an imaging system**

( ) é ù æ ö = + ê ú ç ÷

c

f

where, NG is the number of elements in the group firing at any one time and c is the speed of sound. Hence, as an example, when using an array with 150 elements, coupled to a medium in which the velocity of sound is 1.54 mm/μs, thickness 20 cm, NG = 50, and neglecting trest, it can be seen that the maximum frame-rate achievable is about 75. This will be even less when

However, the above frame rate may sound adequate, and indeed so for many cases, but the line-serial scanning introduces a basic limitation in the case of fast moving targets such as heart valves or machinery. During the scanning time within a frame, one part of the target may have displaced significantly relative to another, causing spatial distortion, thus a frozen image of the target at any time may differ significantly from its actual position or shape. This is

A

x

( ) D = ´´ <sup>x</sup> f x x s t v

Vx

Δs

<sup>A</sup> (3)

Direction of Scan

V1

With reference to Figure 2, spatial distortion (Δs) could be written as:

t

f G rest

2d t N - N t

\ »

With reference to Figure 1, time (tf

274 Advancements and Breakthroughs in Ultrasound Imaging

trest is applied.

illustrated in Figure 2.

**Figure 2.** Formation of temporal artefacts

where, vX is the velocity of a point (x) on target.

1 Frame rate

ë û è ø

) taken to produce one frame is:

(2)

From the above discussion, it is clearly evident that two key properties of an ideal real-time ultrasonic imaging system is that it should be able to produce a complete image frame of the object volume from one insonifying pulse, while utilizing the full aperture available for each frame of the image. Additionally, an ideal system should produce focused images of the whole object field at the same instant of time irrespective of the distances of targets from the surface (Isochronicity) while maintaining accurate object-to-image spatial resemblance (image linearity). It is also clear that the existing technology utilising line-serial scanning cannot achieve these ideal properties and therefore alternative techniques may be needed to improve speed and high resolution capabilities beyond that feasible with conventional technology. This requires investigating non-conventional imaging modalities that can possess those capabili‐ ties, and the ways of overcoming limitations that may have precluded their use in practice. The next section presents some early developments that have some of the above key properties achieved through direct ultrasonic image reconstruction. Beyond academic interest, these techniques have not realised in large-scale use except in very specific applications due to other inherent problems.
