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Principles and Methods of Diagnostic Ultrasound

Name of College student: Nur Atikah Bt Ibrahim

Introduction

Ultrasound is the mechanised wave involves high regularity above human reading which is greater than 20 KHz. In diagnostic ultrasound, occurrence of 2 to 20 MHz was found in imaging certain structure which provides less unsafe but effective method especially for 'in-vivo' analysis.

Sound is a physical disruption which required medium for propagation. During propagation, it transmits the vitality forward ensuing compression (high pressure) and rarefaction (low pressure) to make a wave. Sound influx is longitudinal influx that propagate through medium causes the particle of medium to oscillate backwards and forwards in a collection with path of wave travel called simple harmonic motion.

The sound influx has its properties. Period (T) is enough time considered for particle to travel in a medium to make one complete oscillation. Rate of recurrence (f) is the amount of oscillation per second (cycles/sec = Hz). Wave speed or speed (c) is the rate of a influx that propagates through medium. It determine by denseness and compressibility. Wavelength is the distance between two consecutive in influx. Amplitude (A) is the change of magnitude of an physical entity or the quantity of energy in a reasonable wave. Vitality (W) is the rate of energy movement through confirmed area while strength (I) is the energy per product area.

A account behind the forming of diagnostic image ultrasound

  1. Basic transducer construction

Basically, the forming of ultrasound image is start from the generation of ultrasound pulses. To be able to create this pulse, the utilization of transducer is greatly necessary. Let's take a glance on the engineering of the transducer to further understand how it works.

Ultrasonic transducer is a device that changes one form of energy into another. It functions both to generate and also discover the ultrasound. The most important component in a transducer is the piezoelectric crystal, usually used business lead zirconate titanate that attach to the electrode on opposing side to create the changing polarity. Thin film serve as an electrode layered the crystal and hook up it to the electric powered connector so the potential difference will be offered to the crystal for pulsing. Matching coating is used to improve the energy transfer into and out of the patient as well concerning reduce the pulse. Then, backing material will absorb the transmitted ultrasound energy while diminish the crystal calling. It's important in pulse-echo rule where transducer directs a short burst of energy followed by 'listening phased' to wait for the coming back echoes. Damping also will suppress the ringing and hence control the pulse span that influences image image resolution. This complete component housed in casing to provide support and insulate with acoustic insulator that prevent the transmission of ultrasound energy in to the casing (Hedrick).

  1. Generation of ultrasound pulse

The production of ultrasound influx depends on the change piezoelectric effect while the detection of echoes is dependant on the piezoelectric effect (C2). This effect occurs in crystalline materials that have dipole (positive and negative demand) on each molecule. Change piezoelectric effect occur when crystals are heated by the use of electric pulses cause the substances to move freely and the dipole to align. The enlargement and contraction of crystal cause by the movements of dipole endeavoring to align with applied electric powered pulses creates the sensible waves. Meanwhile, piezoelectric result occurs when crystal are being fired up by the returning echoes causing the change of mechanical energy into the electricity.

By sending electric pulses to the transducer, each crystal produces sound wave. The summation of reasonable wave forms the audio beam. Sound influx is produced in pulse by applying a short period of electrical current to the transducer. Audio is a mechanised energy that sent by pressure influx through medium such as gas, water, or solid. Sensible influx is a longitudinal influx brought on by compression (high energy) and rarefaction (low energy). Ultrasound is a mechanical influx with higher frequencies above individual ability to hear which is greater than 20, 000 Hz or 20 kHz. For diagnostic ultrasound, occurrence with 2 to 20 MHz is required to produce an ultrasound image.

  1. Production of ultrasound beam that propagate through tissue

The ultrasound beam will be sent from transducer in to the patient and propagate through tissues by moving their energy during conversation with the cells molecule. So, the molecule will vibrate and oscillate backwards and forwards about their rest position in a range with route travel (simple harmonic action) and then interact with neighboring molecule. The propagations of ultrasound beam show two distinct routine which are Fresnel zone (close to field) and fraunhofer zone (much field). Fresnel area is next to the transducer face has a converging beam that occur because of constructive and harmful interference of reasonable wave from transducer surface while fraunhofer zone is an area of the beam that is diverged.

  1. Ultrasound beam interact with tissue in plane of its origin

As ultrasound beam propagate through cells, several kind of interaction will take place. However, the major conversation that plays a part in the formation of ultrasound image is representation which responsible for the major organ outline seen in diagnostic ultrasound examination. For reflection to occur, the ultrasound beam that propagates through tissue must undergo user interface, the junction between tissues of different acoustic impedance. In essence, the ultrasound beam come across with software will be shown, scattered or sent into second medium. However, if the sound beam directed at right position (normal occurrence) to a even interface bigger than the width of the beam, it'll be partially reflected back again to the audio source or transducer. This user interface is named specular reflector. On the other hand, non-specular reflector signifies the interface that have small physical dimensions (significantly less than several wavelength in proportions). The mirrored beam from program go back to the transducer can be registered as an echo. If there is no interface is available, no ultrasound beam is mirrored and hence no echo diagnosed. This structure reported to be anechoic which will appear black on the ultrasound image. Furthermore, acoustic impedance, the momentum of ultrasound also takes on important role during connections of the ultrasound beam with the tissue especially reflection. Reflection only occurs if there is difference acoustic impedance, impedance mismatch between adjacent tissues because when there is same acoustic impedance in one medium as in another, audio will be transmitted from one to other.

Scattering or non-specular reflection is also an important discussion between ultrasound and muscle which responsible for providing internal surface of body organ in image. It is caused by discussion with an extremely small reflector or an extremely rough interface resulting in redirection of the sound-wave in several guidelines. So, only a portion of the sound-wave comes back to the scanhead. In addition, it known as diffuse reflection.

When ultrasound beam attacks an user interface between two mass media at 90‹ perspective, refraction will happen. A percentage will be mirrored back again to the first medium and the others will be sent in to the second medium without a change in route. When the beam attacks the interface at an viewpoint apart from 90‹, the transmitted part will be refracted or bent from straight line way. It obeys the snell's legislations.

  1. Reflected echo with different strength

However, take note that for every of the connection not all the sound-wave is reflected during their first user interface, therefore a few of the wave remains or transmitted deeper in to the body. It will finally reflected back to the transducer from software with deeper structure structures to provide the echoes indicate to form ultrasound image. The brightness of the pixel on image is immediately proportional to the effectiveness of the echo received at the transducer. It is is determined by the reflector power which in turn depend on how much both materials are different in term of acoustic impedance. The better the reflector, the greater sound is return to the transducer, the whiter the pixel on the screen and vise versa. However, the echo returning to the transducer is a lot smaller than the original pulse made by the transducer as the sound beam has been attenuated by both attenuation and absorption.

  1. The echo received by the crystal in transducer

The transducer again takes on its work by have the echo signal. It'll convert the echoes indicate into electronic current after the echoes striking and exiting the crystal. Then, this electrical power current will be send to ultrasound machine for processing and display it as an ultrasound image. This technique involves the mapping of the echo routine reflected from acoustic boundaries within tissue. Different tissue show their own characteristic echo pattern hinge how big is the echo and the distance of echo origin from the transducer. A couple of signal across the scan line (beam route) that signifies single-dimensional information will be registered matching to reflecting limitations laying at different distance from transducer. Two-dimensional 2D image is made by sweeping the ultrasound beam across the at the mercy of produce many scan collection.

  1. Generation of electrical power signal

As I mention above, the crystal will have the going back echo and convert it into electric powered signal by piezoelectric effect. This process occurs when the coming back echo in form of mechanical energy striking the crystal in the transducer cause the crystal to fired up. So, the crystal will convert this mechanical energy into electricity that will be process to form noticeable image.

  1. Electrical signal converted into varying level of grayscale depending on its strength

Before display on cathode ray pipe (CRT), the sign is electronically refined and structured in computer recollection. During the process, the signs are amplified to increase their size. It is because the sign received by the transducer from returning echo is relatively lower than the ultrasound influx sent by transducer scheduled to attenuation and absorption during discussion with tissue. As a result, the electric energy produce by the interaction of echo indication and piezoelectric crystal is quite small. Then, the coming back echoes from different tissue depth must be subjected to reimbursement for attenuation different. As the transmission level received depending on relative ranges from reflective user interface to the transducer, it could be disadvantageous to the reflector which have similar size, condition and representation coefficient. So, Time Gain Payment was used to apply differential amplification to indicators received from different muscle depth. In this process, the echoes from longer distance undertake better amplification than those from shorter distance so that similar cells boundaries give equivalent size sign regardless their depth in tissues. The amount of amplification is chosen by the term gain. From then on, the number of transmission size is compressed by using logarithmic amplifiers as the vibrant range of indication size is quite wide. The transmission sizes which are extremely small may be electronically declined because it can improve the probability to produce the artefacts. Only sign whose magnitudes below than certain threshold level are eradicated through process called rejection. In contrast, the accepted alerts are structured in computer ram before send it to CRT for screen. Each echo indication is associated with its own strength level and anatomical position in cells. Intensity and geometrical coordinates must therefore be assigned to each accepted transmission. This information is the read aloud of storage and displayed as a graphic on CRT. Hard copies of a graphic may be captured using thermal printing newspaper.

  1. The signal viewed as a noticeable image

After the sign has been process, it requires to be screen for looking at and recording. There will vary methods used to display the information obtained from ultrasound evaluation. The first one is A-mode or amplitude method. On this setting, the sign is displayed by means of spike. The position of spikes along horizontal screen axis denotes the depth of the program, while the vertical screen axis denotes the effectiveness of the echo. The amplitude of the spike measure the echo size, as the position of spike along the time base measure the distance of reflecting boundary from transducer. This setting have a lot of drawbacks such as it screen only 1D information, the image does not constitute a graphic, and it need a whole lot of space to display an image on CRT scheduled to amount of information it provided along beam avenue.

In B-mode or lighting mode, signals display as a spots of varying intensities. These small dots changed the spikes of A-mode which lead to less space required for screen on CRT. The brightness of the dots measures the depth or echo size which means that large echo display as glowing dots on CRT and vice versa. The positioning of dots along enough time base is a way of measuring the length of the associated reflector from transducer. The dots positioned in each scan lines match 1D information. If the beam is swept across preferred section of subject, different dots collection created for every scan brand. So, different dot collection displayed at different position on CRT displace laterally in one to another resulting the merged information to produce 2D image by which the beam swept.

Then, M-mode or movement mode can be used to generate a moving object along avenue of ultrasonic beam by positioning the transducer in a set position in relation to the moving structure. Such as B-mode, the echoes also screen by means of dots of varying intensity along a period base. For fixed reflector, the dots will stay in the same position along the time base. In contrast, the reflectors that move in the course of scan lines will change their position along the time base. M-mode provide 1D information over the beam course which useful in analyzing cardiac movement.

In real-time mode, a rapid B-mode scanning is utilized to generate image of a picked subject within object repetitively at high rate to create film. Meanwhile, Doppler function was used in the study of blood flow and cardiac motion. In this mode, the ultrasound beam with continuous frequency will interact with moving acoustic boundary and shown back as an echo. As the boundary is moving, the transducer will identify an echo with Doppler switch in consistency. The regularity will be higher when the user interface is getting close while lower when the program moving away. The move frequency is related with speed of moving reflector and the course of motion.

Whatever function of screen was used, each of them have their own functions in showing a visible image from returning echo signal which gives valuable diagnostic information for the organised being examine in the ultrasound exam.

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