Name of Student: Nur Atikah Bt Ibrahim
Ultrasound is the mechanical wave consists of high frequency above human hearing which is greater than 20 KHz. In diagnostic ultrasound, frequency of 2 to 20 MHz was used in imaging certain structure which provides less harmful but effective method especially for ‘in-vivo’ diagnosis.
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Sound is a physical disturbance which required medium for propagation. During propagation, it transmits the energy forward resulting compression (high pressure) and rarefaction (low pressure) to create a wave. Sound wave is longitudinal wave that propagate through medium causes the particle of medium to oscillate back and forth in a line with direction of wave travel called simple harmonic motion.
The sound wave has its own properties. Period (T) is the time taken for particle to travel in a medium to make one complete oscillation. Frequency (f) is the number of oscillation per second (cycles/sec = Hz). Wave speed or velocity (c) is the speed of a wave that propagates through medium. It determine by density and compressibility. Wavelength is the distance between two consecutive in wave. Amplitude (A) is the change of magnitude of a physical entity or the amount of energy in a sound wave. Power (W) is the rate of energy flow through a given area while intensity (I) is the power per unit area.
A story behind the formation of diagnostic image ultrasound
- Basic transducer construction
Basically, the formation of ultrasound image is start from the generation of ultrasound pulses. In order to create this pulse, the use of transducer is greatly necessary. Let’s have a look on the construction of the transducer to further understand how it works.
Ultrasonic transducer is a device that converts one form of energy into another. It functions both to generate and also detect the ultrasound. The most important component in a transducer is the piezoelectric crystal, usually used lead zirconate titanate that attach to the electrode on opposite side to create the changing polarity. Thin film serve as an electrode coated the crystal and connect it to the electrical connector so that the potential difference will be supplied to the crystal for pulsing. Matching layer is used to improve the energy transfer into and out of the patient as well as to shorten the pulse. Then, backing material will absorb the transmitted ultrasound energy while diminish the crystal ringing. It is important in pulse-echo principle where transducer sends a short burst of energy followed by ‘listening phased’ to wait for the returning echoes. Damping also will suppress the ringing and hence control the pulse length that affects image resolution. This entire component housed in casing to provide support and insulate with acoustic insulator that prevent the transmission of ultrasound energy into the casing (Hedrick).
- Generation of ultrasound pulse
The production of ultrasound wave relies on the reverse piezoelectric effect while the detection of echoes is based on the piezoelectric effect (C2). This effect occurs in crystalline materials that have dipole (positive and negative charge) on each molecule. Reverse piezoelectric effect occur when crystals are heated by the application of electric pulses cause the molecules to move freely and the dipole to align. The expansion and contraction of crystal cause by the movement of dipole trying to align with applied electrical pulses creates the sound waves. Meanwhile, piezoelectric effect occurs when crystal are being excited by the returning echoes resulting the conversion of mechanical energy into the electrical energy.
By sending electrical pulses to the transducer, each crystal produces sound wave. The summation of sound wave forms the sound beam. Sound wave is generated in pulse by applying a short duration of electrical current to the transducer. Sound is a mechanical energy that transmitted by pressure wave through medium such as gas, liquid, or solid. Sound wave is a longitudinal wave caused by compression (high energy) and rarefaction (low energy). Ultrasound is a mechanical wave with higher frequencies above human hearing which is greater than 20, 000 Hz or 20 kHz. For diagnostic ultrasound, frequency with 2 to 20 MHz is needed to produce an ultrasound image.
- Production of ultrasound beam that propagate through tissue
The ultrasound beam will be transmitted from transducer into the patient and propagate through tissue by transferring their energy during interaction with the tissue molecule. So, the molecule will vibrate and oscillate back and forth about their rest position in a line with direction travel (simple harmonic motion) and then interact with neighboring molecule. The propagations of ultrasound beam exhibit two distinct pattern which are Fresnel zone (near field) and fraunhofer zone (far field). Fresnel zone is adjacent to the transducer face has a converging beam that occur because of constructive and destructive interference of sound wave from transducer surface while fraunhofer zone is an area of the beam that is diverged.
- Ultrasound beam interact with tissue in plane of its origin
As ultrasound beam propagate through tissue, several type of interaction will take place. However, the major interaction that contributes to the formation of ultrasound image is reflection 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 interface, the junction between tissues of different acoustic impedance. Basically, the ultrasound beam encounter with interface will be reflected, scattered or transmitted into second medium. However, if the sound beam directed at right angle (normal incidence) to a smooth interface larger than the width of the beam, it will be partially reflected back to the sound source or transducer. This interface is called specular reflector. In contrast, non-specular reflector represents the interface that have small physical dimension (less than several wavelength in size). The reflected beam from interface return to the transducer can be recorded as an echo. If there is no interface exists, no ultrasound beam is reflected and hence no echo detected. This structure said to be anechoic which will appear black on the ultrasound image. In addition, acoustic impedance, the momentum of ultrasound also plays important role during interaction of the ultrasound beam with the tissue especially reflection. Reflection only occurs if there is difference acoustic impedance, impedance mismatch between adjacent tissues because if there is same acoustic impedance in one medium as in another, sound will be transmitted from one to other.
Scattering or non-specular reflection is also an important interaction between ultrasound and tissue which responsible for providing internal texture of organ in image. It is caused by interaction with a very small reflector or a very rough interface resulting in redirection of the sound-wave in several directions. So, only a portion of the sound-wave returns to the scanhead. It also known as diffuse reflection.
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When ultrasound beam strikes an interface between two media at 90Ëš angle, refraction will occur. A percentage will be reflected back to the first medium and the rest will be transmitted into the second medium without a change in direction. If the beam strikes the interface at an angle other than 90Ëš, the transmitted part will be refracted or bent away from straight line path. It obeys the snell’s law.
- Reflected echo with different strength
However, take note that for each of the interaction not all the sound-wave is reflected during their first interface, therefore some of the wave continues or transmitted deeper into the body. It will finally reflected back to the transducer from interface with deeper tissue structures to give the echoes signal to form ultrasound image. The brightness of the pixel on image is directly proportional to the strength of the echo received at the transducer. It is depends on the reflector strength which in turn depend on how much the two materials differ in term of acoustic impedance. The stronger the reflector, the more sound is return to the transducer, the whiter the pixel on the screen and vise versa. However, the echo returning to the transducer is much smaller than the original pulse produced by the transducer as the sound beam has been attenuated by both attenuation and absorption.
- The echo received by the crystal in transducer
The transducer again plays its work by receive the echo signal. It will convert the echoes signal into electrical current after the echoes striking and exiting the crystal. Then, this electrical current will be send to ultrasound machine for processing and display it as an ultrasound image. This process involves the mapping of the echo pattern reflected from acoustic boundaries within tissues. Different tissues show their own characteristic echo pattern depend the size of the echo and the distance of echo origin from the transducer. A set of signal along the scan line (beam path) that represents single-dimensional information will be recorded corresponding to reflecting boundaries lying at different distance from transducer. Two-dimensional 2D image is generated by sweeping the ultrasound beam across the subject to produce many scan line.
- Generation of electrical signal
As I mention above, the crystal will receive the returning echo and convert it into electrical signal by piezoelectric effect. This process occurs when the returning echo in form of mechanical energy striking the crystal in the transducer cause the crystal to excited. So, the crystal will convert this mechanical energy into electrical energy that will then be process to form visible image.
- Electrical signal converted into varying level of grayscale depending on its strength
Before display on cathode ray tube (CRT), the signal is electronically processed and organized in computer memory. During the process, the signals are amplified to increase their size. It is because the signal received by the transducer from returning echo is somewhat lower than the ultrasound wave transmitted by transducer due to attenuation and absorption during interaction with tissue. As a result, the electric current produce by the interaction of echo signal and piezoelectric crystal may be very small. Then, the returning echoes from different tissue depth must be subjected to compensation for attenuation different. As the signal level received depending on relative distances from reflective interface to the transducer, it may be disadvantageous to the reflector that have similar size, shape and reflection coefficient. So, Time Gain Compensation was used to apply differential amplification to signals received from different tissue depth. During this process, the echoes originating from longer distance undergo greater amplification than those from shorter distance so that similar tissues boundaries give equal size signal regardless their depth in tissue. The amount of amplification is designated by the term gain. After that, the range of signal size is compressed by using logarithmic amplifiers as the dynamic range of signal size may be very wide. The signal sizes which are extremely small may be electronically rejected because it can increase the probability to produce the artefacts. Only signal whose magnitudes below than certain threshold level are eliminated through process called rejection. In contrast, the accepted signals are organized in computer memory before send it to CRT for display. Each echo signal is associated with its own intensity level and anatomical position in tissue. Intensity and geometrical coordinates must therefore be assigned to each accepted signal. This information is the read out of memory and displayed as an image on CRT. Hard copies of an image may be captured using thermal printing paper.
- The signal displayed as a visible image
After the signal has been process, it needs to be display for viewing and recording. There are different methods used to display the information acquired from ultrasound examination. The first one is A-mode or amplitude mode. On this mode, the signal is displayed in the form of spike. The position of spikes along horizontal display axis denotes the depth of the interface, while the vertical display axis denotes the strength of the echo. The amplitude of the spike measure the echo size, while the position of spike along the time base measure the distance of reflecting boundary from transducer. This mode have a lot of disadvantages such as it display only 1D information, the image does not constitute an image, and it need a lot of space to display an image on CRT due to amount of information it provided along beam path.
In B-mode or brightness mode, signals display as a dots of varying intensities. These small dots replaced the spikes of A-mode which result in less space required for display on CRT. The brightness of the dots measures the intensity or echo size which means that large echo display as bright dots on CRT and vice versa. The position of dots along the time base is a measure of the distance of the associated reflector from transducer. The dots positioned in each scan line correspond to 1D information. When the beam is swept across selected section of subject, different dots line created for each scan line. So, different dot line displayed at different position on CRT displace laterally from one to another resulting the combined information to produce 2D image through which the beam swept.
Then, M-mode or motion mode is used to generate a moving object along path of ultrasonic beam by placing the transducer in a fixed position in relation to the moving structure. As in B-mode, the echoes also display in the form of dots of varying intensity along a time base. For stationary reflector, the dots will remain in the same position along the time base. In contrast, the reflectors that move in the direction of scan line will change their position along the time base. M-mode provide 1D information along the beam path which useful in examining cardiac motion.
In real time mode, a rapid B-mode scanning is used to generate image of a selected subject within object repetitively at high rate to create motion picture. Meanwhile, Doppler mode was used in the study of blood flow and cardiac motion. In this mode, the ultrasound beam with constant frequency will interact with moving acoustic boundary and reflected back as an echo. As the boundary is moving, the transducer will detect an echo with Doppler shift in frequency. The frequency will be higher when the interface is approaching while lower when the interface moving away. The shift frequency is related with velocity of moving reflector and to the direction of motion.
Whatever mode of display was used, each of them have their own functions in displaying a visible image from returning echo signal which gives valuable diagnostic information for the structured being examine in the ultrasound examination.
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