акустика / gan_ws_acoustical_imaging_techniques_and_applications_for_en

(a)

(b)

Figure 10.4 (a) Linear scan and image of a specular cylindrical reflector. (b) Linear scan with compounding of the same object. Compounding ‘fills in’ the nonimaged segments of the linear scan (Havlice and Taenzer [3] © IEEE)

usually much smaller in compound scanning than in simple sector scanning where angles as large as ±45◦ are used. For illustrative purposes, only two positions in the linear travel and the respective sectors are shown in Figure 10.3. In compound scanning, object points are imaged by more than one acoustic pulse along different ray paths. Compound scanning is used to overcome a major problem in B-scan imaging, namely the difficulties of imaging specular reflectors and objects lying behind specular reflection. It is known that a specular reflector reflects sound towards a direction that is dependent on its orientation to the transducer. Hence, it is possible for any incident sound beam to reflect from a specular reflector in a direction such that the reflected sound beam does not reach to the transducer. The imaging system falsely interprets this as the absence of a reflector and does not display a signal even though a very strong reflecting interface may have been present. This is shown in Figure 10.4(a) for a simple linear scan of a cylindrical object (a blood vessel, for example). The sound that impinges on the side of the object is reflected away from the transducer so that it is never received. In this simple case, it is possible to mentally connect the two areas to form a mental image of the time object shape. However, in a complex biological medium, this is not always possible. The compound scan helps to ‘paint in’ that part of the specular surface that was not imaged in the simple scan. This is illustrated in Figure 10.4(b). Noted that only two positions of the

transducers are shown for the linear travel along with the particular sector angle that images part of the side of the vessel. The compound scan is also useful for imaging behind highly reflecting or attenuating structure (i.e. ribs), since hidden object points can be imaged from an unobstructed direction.

10.3.1.1Resolution

Resolution here means spatial resolution. There are two types of spatial resolution in a B-scan:

(1) lateral or transverse resolution, resolution in the direction of transducer motion, and (2) axial resolution, resolution in the direction of a wave pulse propagation.

We consider first the lateral or transverse resolution. In a focused optical system, the resolution is defined by the Rayleigh criterion [16] which is determined by the light wavelength and the numerical aperture of the focusing elements through

where F is the focal length of the system and D is the diameter of the circular entrance pupil. For two incoherent point sources this criterion places the centre of the airy disc [16] of one source onto the first zero of the airy disc of the second source. The resulting intensity pattern has a 19% dip midway between the centres of the images of the two sources. Bringing the sources closer will cause this dip to fill in until only a central maximum is present and no obvious feature of the intensity pattern allows one to distinguish the presence of one source

from other.

The original purpose of Rayleigh formulated this resolution criterion was to predict the ability of an optical system to distinguish two self-luminous incoherent point sources (stars). For the optical system, it was operating in a receiver-only mode, whereas for the ultrasound B-scan systems, it operates in a transmit/receive mode. This mean that the effective spatial response of the ultrasound system to a point source reflector is the product of the transmitter field pattern with the receiver field pattern. Because the same transducer is usually employed for both transmit and receive, the effective spatial response pattern for a B-scan system is not an airy pattern, but the square of the airy pattern. This is illustrated in Figure 10.5. The zeros of the two functions still coincide, but the squared response function is sharper than the unsquared response.

This will affect resolutions. If one defines the criterion for resolution as the distance to the first zero of the propose function, then the resolution is identical to that calculated by Rayleigh [6]. As an example, for a 19-mm diameter, 2.25 MHz focused transducer with a 12-cm focal length, the Rayleigh resolution in a homogeneous medium such as water is about 5 mm at the focal distance. So far is lateral or transverse resolution. For axial resolution which is in the direction of acoustic pulse propagation, it is inferred from the arrival time of sequentially reflected acoustic pulses. It is relatively unaffected by the presence or absence of focusing elements but is determined principally by the bandwidth of the transducer [17]. The larger the bandwidth, the shorter the acoustic pulse that can be generated and received and the finer the definition along the axis of propagation.

In the presence of a wide bandwidth signal, the application of Rayleigh’s criterion is not straightforward. Rather than simply having a single wavelength λ, there is a wide spectrum of

The system shown in Figure 10.7 could also be used for reflection mode C-scan imaging by using transducer No. 1 scanned mechanically as before to obtain the 2D image. In this case, range gating not only removes multipath reverberations but also determines the distance of the image plane from the transducer.

Although the reflection mode and transmission mode C-scan techniques are similar, the resulting images are quite different. The reflection images, for instance, depend for their contrast primarily on acoustic impedance variations. They are particularly susceptible to specular reflections effects – small changes in object orientation often result in significantly different images. The transmission mode images depend for their contrast primarily on the differential attenuation properties of tissue. They are independent of specularity but are susceptible to coherent interference effects [27, 28].

The resolution of a C-scan system generally relies on the focusing properties of an acoustic lens for both displayed dimensions, the Rayleigh criterion given by equation (10.4) is a good estimate for definitions. In a C-scan system, the effective point response function may or may not be the square of the airy function, depending on the type of system used. This is in contrast to B-scans where the response function is almost always squared. In a C-scan image, both dimensions are lateral dimensions; hence, the bandwidth of the transducer is not a factor in resolution. Depth of focus is not a major, direct factor in C-scan resolution, but it has some significant indirect effects. For example, out of the focal plane objects may appear as out-of- focus artefacts in the images. As in B-scan, C-scan resolution also infers whenever ultrasound passes through tissue due to the frequency-dependent absorption coefficient [29].

10.4B-scan Instrumentation

B-scan instruments can be generally be classified into the following types.

10.4.1Manual Systems

The manual compound contact B-scan system has been the main story of diagnostic ultrasound imaging for many years. This form of ultrasound imaging system has evolved into sophisticated equipment capable of producing images with a significant degree of diagnostic information. Often manual B-scan equipment is used for diagnosing ailments in the region of the abdomen such as cystic and solid lesions [30], kidney and gall stones [31], carcinoma of the liver and uteral cirrhosis of the liver and for obstetrical applications such as placental localization [32] and the measurement of foetal biparietal diameter (BPD).

It is also beginning to be used for cardiac studies, imaging the thyroid gland, and in the pancreas and stomach. Indeed, as time progresses and equipment improves, the number of uses for the manual contact B-scan imaging system is ever expanding. Details of the operation of the compound contact B-scan system is given in Wells’ book [6].

Contact B-scan imaging system consists of three main parts: (1) a scanning arm to control the travel of an ultrasound transducer so that the ultrasound beam is always maintained in a single plane, (2) appropriate electronics for amplifying and detecting the returning echoes, monitoring the position and angle of the transducer and driving and deflecting a display device, and (3) a display to convert the electronic signals into an image on a CRT device. A block diagram of a typical manual contact B-scan system is shown in Figure 10.9. Ultrasonic coupling

Because of its basic design, the manual contact scanner does give rise to certain problem. Since it works in contact with the patient, the skin and organ close to the skin are generally imaged poorly or not imaged at all because the receiver circuits require some time to recover from the large overload that occurs when the transmitter pulses the transducer. Typically, the first centimetre of the image is artefact not actually related to the tissues that are present. Since the scan is manually controlled, image quality varies with the expertise of the operator, and this operator dependence can be a significant problem. Not only must operator be trained before they can produce quality images, but manual scanning is slow and relatively tedious. A patient procedure takes a considerable length of time considering the few diagnostic images that are produced. During the relatively long time (1–10 s) that it takes to scan out a single image, organs can move causing the image to be distorted which, in turn, may confuse the diagnosis. Lastly, manual contact B-scanners do not display organ motions in real time.

10.4.2Real-Time System

Ultrasound image system which can produce image rapidly enough to display motion are called real-time system. In certain diagnostic procedures, the accurate display of tissue motion can be important for a proper diagnosis, such as the detection of diseased heart valves or the determination of foetal viability. In addition to being able to display organ motion, patient procedures can be accomplished very rapidly since little time is wasted in locating the organ or tissue of interest and the operator has nearly instantaneous positional feedback. In real-time system, the ultrasound beam is either mechanically or electronically scanned.

On the basis of the constraint that only one acoustic probe should be travelling in the field of interest at any instant in time, there is a constraint on real-time equipment given by

where

R = maximum frame rate (s−1) D = depth of field (m)

N= number of scan lines

v = velocity of sound (m/s)

The velocity of sound is not significantly different for the various soft tissues of the body so that the product of frame rate, depth of field and number of scan lines is essentially a constant. For example, to get more scan lines in the image, either the frame rate or the depth of field must be decreased. Therefore, high-quality, real-time images are difficult to achieve for those organs such as the liver that require a large field of view.

10.4.3Mechanical Scan

Mechanical scan is the simplest technique for obtaining real-time images. The mechanical system replaces the human hand and moves the transducer automatically. Here, a motorized mechanism automatically reads or rotates the transducer while it is in contact with the patient’s skin [33, 34]. Position sensors continuously detect the angle of the transducer and produce a