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Ultrasound Imaging Systems

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1.1 INTRODUCTION

An ultrasound scans also known as ultrasonography. Ultrasound will form the image by scanning using the high frequency sound waves. This device suitable to evaluate some part inside of the body. In physics, ultrasound is a sound with a frequency humans cannot hear. In diagnostic sonography, the ultrasound is usually between 2 and 18 MHz. (Anon 2012)

2.0 THE ULTRASOUND IMAGING SYSTEM

Figure 1 : The principal functional components of an ultrasound imaging system.(Perry Sprawls n.d.)

2.1 TRANSDUCER

The ultrasound transducer converts an electrical signal into the ultrasound beam. The signal transmitted into the patient’s body, and then alters the returning echo into an electrical signal for processing and display. It use single-element circular disk to both transmit and receive ultrasound. (Hedrick et al. 2005)

2.1.1 CONSTRUCTION OF TRANSDUCER

Crystal of piezoelectric material with electrodes is the main part of the transducer. The electrodes are formed by plating a thin film of gold or silver on the crystal surface. The matching layer is located adjacent to the electrodes. The function is to improve the transfer of energy to and from the patient. All this part of the transducer is placed in an electrically insulating casing. This casing will give structural support. An acoustic insulator is made of rubber or cork it works to prevents the transmission of ultrasound energy into the casing. (Hedrick et al. 2005)

2.1.1.1 PIEZOELECTRIC MATERIALS

When we change the transducer it will change the frequency too. A higher frequency transducer that produces a shorter wavelength has a thinner crystal. Normally the material that always almost used in transducer is lead zirconate titanate (PZT). PZT represents a piezoelectric ceramics with various extracts. It will change the properties to equal a particular application. In medical, PZT-5 is used because it has the properties of high electromechanical coupling coefficient, high dielectric constant, and ability to be formed in a particular size and shape. (Hedrick et al. 2005)

2.1.1.2 BACKING MATERIAL

The functions of backing material will deliver the maximum amount of energy in the form of heat to the patient. This is will give a continuous output of ultrasound waves from the transducer. The other function is to absorb all the energy except for the one cycle of sound. Meaning’s that one that produced from the front face of the transducer. Backing materials should have acoustic impedance so that maximum energy transfer will occur. Besides that, backing material should have a high absorption coefficient. This is to prevent ultrasonic energy from reentering the crystal. In the backing material, they will use an epoxy resin and tungsten powder combination to damp the ultrasonic pulse. Next, the rear surface of the backing materials is slanted to prevent reflection of sound energy into the crystal. (Hedrick et al. 2005)

2.1.1.3 MATCHING LAYER

The matching layer is placed in the transducer on the exit side of the crystal. This material with acoustic impedance is placed between the crystal and the patient. The function of the acoustic impedance to produced ultrasonic energy to be largely reflected at this interface. This creates a long pulse and reduces the beam intensity that enters the patient, which is we did not want it in the ultrasound. The reason why we need the matching layer is to shorten the pulse and the improve energy transfer across the crystal-tissue interface. However, the matching layer must have low-loss properties since high attenuation would stabilize the desired effect of high transmission. (Hedrick et al. 2005)

2.2 PULSE GENERATOR

The pulse generator produces the electrical pulses. The size of the electrical pulses can be used to change the intensity and energy of the ultrasound beam. (Perry Sprawls n.d.)

2.3 AMPLIFICATION

After the echo is received by the transducer, amplification is used to intensification the size of the electrical pulses. Gain setting will controls the amount of amplification. The time gain compensation function is to alter the increase in relationship to the distance of echo sites inside the body. (Hedrick et al. 2005)

2.4 SCAN GENERATOR

Controlling the scanning of the ultrasound beam is done by the scan generator. The way is by control the procedure when electrical pulses are functional to the piezoelectric elements in the transducer. (Perry Sprawls n.d.)

2.5 IMAGE PROCESSOR

The digital imageis to produce the chosenforms for display. This includes giving it specific contrast characteristics and reformatting the image. (Perry Sprawls n.d.)

2.6 DISPLAY

The digital ultrasound images are observed on the monitor and transmitted to work station. The other part of the ultrasound system is the digital storage device. The function is to store images for later viewing.(Perry Sprawls n.d.)

3.0 THE ULTRASOUND PULSE

Figure 3 : The production of the ultrasound pulse. (Perry Sprawls n.d.)

4.0 ULTRASOUND CHARACTERISTIC

4.1 FREQUENCY

Frequency is the number of wave cycles passing a given point in a given increase of time. The unit is cycles/ second or hertz. Frequency is the inverse of the period. (Hedrick et al. 2005)

Figure 4 : The ultrasound pulse frequency. (Perry Sprawls n.d.)

4.2 VELOCITY

Velocity is the rate and direction at which sound propagates through a medium. The average velocity of sound in soft tissue is 1540 m/s. (Hedrick et al. 2005)

Figure 5 : The ultrasound of velocity. (Perry Sprawls n.d.)

4.3 WAVELENGTH

Wavelength is a physical characteristic of a wave that is the distance for one complete wave cycle. (Hedrick et al. 2005)

Figure 6 : The wavelength of the ultrasound. (Perry Sprawls n.d.)

4.4 AMPLITUDE

Amplitude used to refer to the particle displacement, particle velocity or acoustic pressure of a sound wave. Amplitude also show the strength of the detected echo or the voltage induced in a crystal by a pressure wave. (Hedrick et al. 2005)

5.0 INTENSITY AND POWER

Intensity is a physical parameter that describes the amount of energy flowing through a unit cross-sectional area of a beam each second. This is the rate at which the wave transmits the energy over a small area. The unit of intensity is the watt per square centimeter or joule per second per square centimeter. (Hedrick et al. 2005)

Power is a measure of the total energy transmitted summed over the entire cross-sectional area of the beam per unit time. The unit of power is the watt. (Hedrick et al. 2005)

5.1 TEMPORAL CHARACTERISTICS

As the transducer emits pulses, it causes large instabilities of intensity in the region through which the pulse move. Each pulse consists of multiple cycles that produce intensity variations within the pulse itself-the maximum intensity, designated temporal peak (TP). Pulse average (PA) will controls the intensity averaged over the duration of a single pulse. Temporal average (TA) will controls the intensity averaged over the longer interval of the pulse repetition period. The TA intensity is related to the PA intensity by the duty factor (DF):

TA = DF×PA or by the pulse duration (PD) and pulse repetition frequency (PRF):TA =PD × PRF × PA.(Hedrick et al. 2005)

5.2 SPATIAL CHARACTERISTICS

The maximum intensity of all measured values within the sound field is designated as the spatial peak (SP). The designation of spatial peak is not well-defined. In some applications it refers to the maximum intensity in a plane perpendicular to the beam axis at a particular distance from the transducer. The maximum intensity throughout the ultrasonic field which usually occurs along the beam axis. The focusing of the transducer is the most important determinant of spatial peak.(Hedrick et al. 2005)

5.3 TEMPORAL/SPATIAL COMBINATION

Spatial averaging over the cross-sectional area of the beam for each temporal intensity is also specified. A cutoff point of 0.25 times the SP intensity has been established to the limit area over which the intensity is averaged. These three combinations are possible to happen are I(SATP)-spatial average, temporal peak intensity, I(SAPA)-spatial average, pulse average intensity and I(SATA)-spatial average, temporal average intensity.(Hedrick et al. 2005)

6.0 INTERACTIONS OF ULTRASOUND

Figure 7: The interaction within a body of ultrasound (Perry Sprawls n.d.).

6.1 ABSORPTION AND ATTENUATION

Absorption is the procedure whereby energy is placed in a medium by converting ultrasonic energy into other energy forms, primarily heat. It is an exponentially decreasing function and is the major factor in the total attenuation of the beam. (Hedrick et al. 2005)

Attenuation is the decrease in intensity as a sound beam travels through the medium. Attenuation depends on all the interactions of ultrasound with tissues which include scattering, divergence, and absorption. (Hedrick et al. 2005) Scattering is the rerouting of sound energy resulting from the sound beam striking an interface whose physical dimension is less than one wavelength. It is also called non specular reflection. (Hedrick et al. 2005)

6.2 REFLECTION

Reflection is an interaction that results when the sound being redirected into the medium after striking an acoustic interface. The angle of incidence equals the angle of reflection. The intensity of the reflected wave is depends on the composition of the interface. (Hedrick et al. 2005)

6.3 REFRACTION

Refraction is a process whereby sound enters one medium from another that will result in a bending or deviation of a sound beam from the predictable straight-line path. Refraction obeys Snell’s law, which is based on the ratio of the velocity of the sound in the respective media. Refraction will make artifacts in the image by the misregistration of structures (Hedrick et al. 2005)

7.0 PULSE DIAMETER AND BEAM WIDTH

A low-Q transducer has a short pulse length and a broad bandwidth while a high-Q transducer has a long pulse length and narrow bandwidth. The objectives beam width is to transmit a beam that would be directional with a narrow beam width. An echo is created anyway of the lateral position of the object in the ultrasonic field. The lateral dimension of the object in the image is defined as the same size as the beam width. Multiple small objects equidistant from the transducer are not resolved when encompassed by the beam. Focusing reduces the beam width at specific depth to enhance the spatial mapping of received echoes.(Ding et al. 2014) Sampling is restricted laterally by the width of the beam. Objects located outside the beam do not contribute signals. (Small 1971)

7.1 TRANSDUCER FOCUSSING

The focusing transducer made-up with an indented active element exhibits much broader bandwidth and higher sensitivity. To fabricate focusing transducers, we can add a lens and shaping the piezoelectric element. Among the focusing transducer designing methods, the shaping element used in transducers was reported to be much effective for fabricating high sensitivity device. Hard pressing and pressure defection techniques are the usual ways to shape transducer elements. For the flexible composite and polymer materials, the focusing transducer can be easily fabricated using those techniques.(Chen et al. 2013)

Figure 8: The width and pulse diameter characteristics of both unfocused and focused transducer. (Perry Sprawls n.d.)

7.2 ADJUSTABLE TRANSMIT FOCUS

Transmit focusing happen when the depth of the focal zone is altered by varying the delay times between crystal excitations. (Wright 1997)The scanning of the region of interest is conducted with a depth of focus selected by the operator. After review of the real-time image, a new focal zone may be certain to rescan the same area with dissimilar focusing in the scan plane. The beam is focused to a new depth simply by changing the delay times. The transducers that have the capabilities of this focusing are phased linear arrays. (Kossoff & Eng 2000)Electronic phasing of the elements allows variable focusing along the scan line which in turn controls beam width in the plane direction. High resolution images with multiple focal zones throughout the images are also possible using this adjustment delay lines. Multi zone transmit focusing reduces the frame rate, because the data must be composed for all the lines of sight across the array with a set focal zone depth before the lines of sight are repetitive with a different focal zone depth.

7.3 DYNAMIC RECEIVE FOCUS

Dynamic focusing is in the receive mode. It does will reduce the effective sampling volume.(Kossoff & Eng 2000) Dynamic focusing will operate at all depths. The wave front from the object appears to be in phase for all the crystals resulting in a focused beam from the depth of interest. Beam formation is the delay and sum of strategy. The master synchronizer sends timing messages to the receiver-delay lines to indicate the elapsed time from transmission to reception. The elapsed time determines the delay times for each crystal. The depth for receive focus is always known, and thus receive-delay times are constantly changed to yield continually focused beam at all depths. During acquisition of image data the receive times delays are varied dynamically to sweep the focal zone to each point along the scan line. (Hedrick et al. 2005)

8.0 CONCLUSION

In ultrasound, high frequencies provide better quality images, but cannot penetrate through skin and organ deeply. Low frequencies can penetrate deeper, but the image quality is poor. Ultrasound is useful to view part inside of the body. They may also be useful in helping the surgeon when carrying out some types of biopsies. Ultrasound is a one of the safe procedure in imaging department.


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