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Medical ultrasound machines are some of the most sophisticated signal processing machines widely use today. This paper will discuss the equipment operation of ultrasound machine and how it's work. The topics are subdivided from the topic of the choice of transducers until the image display. The schematic blog diagrams show the overview of ultrasound system.
The transducer converts electrical signals to ultrasound waves, and then picks up the reflected waves converting it back into electrical signals. The electrical signals returned from the transducer are applied to form ultrasound image. There are two types of transducer which are;
mechanical transducer and electronic transducer.
Mechanical transducers use a combination of single element oscillation, various element rotation, or a single element and set of acoustic mirrors to generate the sweeping beam for 2D mode. This type sometimes called the 'wobbler' because of the vibration created as the mirrors rotate or oscillate inside the cover (BL Ji, Goldberg BB, 1998).
Mechanical probes produce beam projecting in different direction by moving the transducer element. This can be done by a moving single element, rotating transducer or by oscillating the element back and forth across specific arc. Another method involves a stationary element and a moving reflective mirror. The result is a pie shape or sector image which was a broad far field but narrow near field. However the major drawback is equipment reliability because there are too many moving parts (BL Ji, Goldberg BB, 1998).
Electronic transducer is performed with arrays. It consists of transducers assemblies with several transducer elements. The elements are rectangular and are arranged in a straight line or curved line.
Phased linear array
It developed to overcome certain limitations inherent in the segmental linear array (Hedrick, Hykes, Stachman, 2005) the image formation produces by a rectangular formation which the width of the field of view is determined by the physical length of the row element in the array and the maximum number of scan lines corresponds to the number of element in the array
Phased linear array is an electronically fired array of individual transducer elements arranged along a flat surface face to produce an image that is rectangular in shaped. Another type is consists of multiple elements within a single small faced transducer (BL Ji, Goldberg BB, 1998).
Activating adjacent transducer elements effectively steers the beam across the arc and produce pie shaped sector display with slight time delays with respect to each other.
The limitation is the size of the transducer face which may limit access to certain anatomic areas due to small field of view (BL Ji, Goldberg BB, 1998).
The linear array
The linear array transducer can have up to 512 elements spaced over 75-120 mm (Brunner E, 2002). The beam created by such a narrow element will diverge very quickly after the wave travels only a few millimeters (the smaller the face of the transducer, the more divergent). This would cause in degraded lateral resolution due to beam divergence and low sensitivity due to the small element size.
In order to resolve this, adjacent elements typically 8 to 16 or more in wide-aperture designs, are pulsed at the same time. In the subgroup of elements, pulsing of the inner elements is delayed with respect to the outer elements. A focused beam results from the interference of the small divergent wavelets. The time delays decide the depth of focus for the transmitted beam and can be changed during scanning. The same delay factors are also applied to the elements during the receiving phase resulting in a dynamic focusing effect on return. A single scan line in the real-time image is formed as follows. To produce the next adjacent scan line, another group of elements is formed by shifting one element position along the transducer array from the previous group. The same pattern is then repeated for this set of elements and all other sets along the array, in a sequential and repetitive manner (BL Ji, Goldberg BB, 1998).
For the linear array transducer, electronic focusing in the plane along the line of the transducer elements improves lateral resolution as well as sensitivity by increasing the amount of energy in the focal zone. Focusing in the direction at right angles to the scan plane determine the slice thickness. The ability to change the depth of the focal zone simply by changing the amount of delays applied to the individual elements and the focal zone can be scanned through a specified range of depths during the real-time exam.
A significant problem in the early linear array designs were off-axis beam artifacts. It caused by ultrasound beams that emulated at predictable angles off-axis to the main beam. Grating lobes are unique to array transducers and are caused by the regular, periodic spacing of the small array elements and produce "ghost images" blurring the main image. To correct these problems by eliminating grating lobes also improves the signal-to-noise ratio by increasing the size of the main lobe energy relative to the background energy. This further improves image contrast (Joseph Woo, 1998).
Curvilinear Arrays (Curved / Convex / Radial)
Wide range of curved array probes available with, 3 MHz to 7.5 MHz center frequencies, and 128 to 256 elements. Face of transducer convex slightly which diverges the beam effectively and result broad near field and even wider far field which produce large sector format. The radius of curvature is usually 25-100mm. the width of the field of view is extends by the radius of curvature. The physical dimensions of the crystal elements affect the beam pattern.
It sweeps the beam by firing multiple crystals in a group, stepping down after the suitable delay and the firing next group. The lines of sight are not 90o to the array surface. At the edge of the field of view there no loss of focus occurs. However, beam divergence may limit the useful depth and loss of lateral resolution i also can reduce grating lobes.
Compound Linear Arrays / Vector array
It has been developed to incorporate characteristics from both the linear array and the linear phased array. Multiple crystals are fired once to steer the beam in various directions. By directing the beam 90o to the array to get scans lines for central field of view. The ultrasound beam is steered at wide angles by phased method for extreme of a large field of view.
Relatively small surface is needed to be in contact with body tissue. It produces an image with broad field of view. The disadvantages of the sector displayed are lack of image homogeneity as the divergence beam is greater as they leave the transducer.
Lateral resolution capabilities of this transducer are progressively worse as the far structure is from the transducer face. Electronic focusing of phased array minimizes these adverse reactions.
Frequency: Gray Scale/ Color Doppler
In real time color Doppler imaging, two entirely separate beams are formed for flow and for grayscale imaging. One beam is more focused for grayscale usage (the imaging beam) and an angled, less focused beam is utilized for Doppler.
Autocorrelation analysis of the Doppler beam information is the technique used by all ultrasound manufacturers to produce color flow Doppler images. Autocorrelation color Doppler compares three or more (typically 8 to 16) samples of the same view (scan line) for each realtime image. The data from the 8 to 16 samples are compared and all stationary echoes are eliminated, leaving only the Doppler shifted echoes. Color flow images are produced with this information, which is derived only from these moving Doppler shifted echoes. Autocorrelation gives phase and Doppler frequency shift information (see the Doppler equation) that is used to create color Doppler flow images. The autocorrelation signal has another feature, which is amplitude. Power Doppler images are produced from this amplitude information.
In conventional frequency/phase color Doppler sonography, both flow direction and frequency shift (proportional to flow velocity) are color-coded. Usually, the colors red and blue are used to represent flow direction toward or away from the ultrasound beam (transducer). It is important to remember that color Doppler sonography displays average Doppler frequency shift, not true angle-corrected velocity. In the qualitative flow images produced with realtime color Doppler sonography, the average frequency shift is proportional to flow velocity.
Color Doppler generally does a good job of displaying relative blood flow velocities within the area scanned, even though the images do not display the true velocity (WR Phillips,
Diagnostic pulse echo systems using ultrasound for examination of targets deep within the body will typically produce echo signals spanning a dynamic intensity amplitude range of 100 dB or more. In any given range segment, target acoustic impedance differences from the surroundings and geometrical orientation will provide variations in echo strength of 30-50 dB. This represents the desired target information. The additional 50-70 dB variations originating from the tissue attenuation over the total path length represents an unwanted component. These additional 50-70 dB variations are taken care of by the TGC circuitry which compensates for intensity variations of the echo due to absorption.
Direct observation of signals in the 30-50 dB dynamic range is not practical with conventional display devices. Therefore, it is apparent that it is not practical to enable viewing more than a small segment of the entire dynamic range at any one time (Phillips WR). Thus, ultrasound systems generate images by converting echoes of different amplitudes into image points of different brightness. The brightness is expressed by a graduated gray scale where lower amplitude echoes are resolved as darker shades of gray and higher amplitude echoes as brighter shades. The assignment of a given gray shade to particular echo amplitude is arbitrary and it is determined by an echo to gray shade conversion curve employed during processing of the data. In fact, there are over 1,000 shades of gray in ultrasound images after the TGC. The present state of the art ultrasound systems reduce these 1,000 shades of gray by methods such as logrithmically compressing the data; i.e., a variable gain is used as a function of the signal level. The higher the signal level, the lower the gain. Thus, for example, the differential gains at high input levels may be only about 0.01 of the gain at the lower signal input levels (Freedman, Goldberg et al, 1995)
Another part It consists of:
Analog to digital converter
It produces electric voltages to drive the transducer, forming the beam that sweeps through the tissue to be imaged. The driving voltages are in form of electric pulses of a cycle or two of voltages. The transducer produces ultrasound pulses that travel into the tissue. The frequency of the voltage pulse determines the frequency of the resulting ultrasound pulse. The ranges between 2-15 MHz
The pulse repetition frequency (PRF) is the number of voltage pulses sent to the transducer each second. The PRF range is from 4-15 kHz and 5-30 kHz for Doppler application. The ultrasound PRF is equal to the voltage PRF and the ultrasound pulse repetition period is equal to the voltage pulse repetition period. This is the time from the beginning of one pulse to the next.
The pulser is adjusts the PRF approximately for the current depth. As frequency is reduced and penetration increase, the PRF must be reduced to avoid echo misplacement. The number of images that are generated each second is called frame rate. The greater the voltage amplitude produced by the pulser the greater intensity of the ultrasound pulse produced by transducer. Reduction of acoustic output reduces received echo amplitude which can compensate by increasing amplifier gain.
The pulse delay carry out and accomplished task such as sequencing, phase delays and variation in pulse amplitude that are necessary for the electronic control of beam scanning, steering, transmission focusing, aperture and apodization. As there are many complicated and series of pulse amplitude, a coded excitation and channels is used.
This approach accomplishes functions such as multiple transmission foci, separation of harmonic echo bandwidth from the transmitted pulse bandwidth, increase penetration, reduction of speckle with improve contrast resolution and gray scale imaging of blood flow.
A channel is an independent signal path consisting of a transducer element, delay, and possible other electronic components.The reason for this is because there is many elements in the array and each one of it need a different delay to form ultrasound beam properly because each independent delay and element combination constitute a transmission channel.
An increase number of channel allows more precise control of beam characteristic. Typical numbers of channel in modern ultrasound machine are 64, 128 and 192. Normally the number of channels does not exceed the number of elements in the transducer.
Transmit / Receive Switch
Its function is to direct the driving voltages from the pulser and pulse delays to the transducer when transmission and then directs the returning echo voltages from transducer to the amplifier during reception.
It does also protect the sensitive input components of the amplifiers from the large driving voltages from the pulser.
Each channel in beam former has one amplifier to increase voltage amplitude. Conversion of small voltages received from the transducer elements to larger one which is suitable for further processing and storage is called amplification. The ratio of amplifier output to input electric power is called gain.
Analog to Digital Converters
The digitizer is to convert the voltage from analog to digital form. The echo voltage has been proportional to the echo pressure. After digitize numbers representing the echo voltages and further manipulation of the echoes is accomplished as digital signal processing.
The digitizer also interrogates the incoming voltage at regular intervals and determines instantly its value at each interrogation. The interrogation rate must be two times the highest frequency involved in the interrogated voltage to maintain all harmonics all the harmonics contained in the interrogated voltage. The voltage precision of the digitizer depends on the numbers of bits of digitized binary number.
The echo voltages pass through digital delay line to archive reception dynamic focus and steering function
After all the channel signal component are delayed properly to archive the focus and steering function, they are added together in the summer to construct the resulting scan line that along with all other, will be displayed after signal processing and image processing. As part of the summing process the reception apodization and dynamic aperture function are accomplished also.
After conversion of echo data into image form and preprocessed, the image frames are stored in image memory. Each image is stored in memory as the sound beam is scanned through the anatomy allows display of a single image/ frame out of the rapid sequence of several frames acquired each sound in real time ultrasound equipment.
Freeze frame is the holding and displayed one frame out of sequence. Many of the instrument store the last several frames acquired before freezing which also known as cine loop, cine review or image review feature.
Ultrasound Imaging Modes
There is several ways to display the information delivered but one is similar to all clinical application is brightness mode. The types of imaging modes are:
A Mode = amplitude
B Mode = brightness
C Mode = color doppler
D Mode = PW doppler
P Mode = power doppler
Triplex Mode = B, C, D mode
M Mode = motion
Tissue Harmonic Imaging
A - Mode (Amplitude)
A - mode display showing a plot of echo amplitude versus time or depth.
Echoes received from different depths are displayed on a CRT. A voltage to the horizontal plates moves the time base sweep across the display at a constant rate that correlates with the speed of ultrasound in tissue. The sweep must advance the equivalent of 1cm on the distance scale for every 13µs, which is initiated by a pulse from the master synchronizer and as it progress across the display, the signal detected for each interface is amplified and sent to the vertical deflection plates to control the electron beam's vertical position.
The displayed signals remain unchanged as long as transducer is directed along the same scan line and also the scan is repeated many times each second.The pulse repetition frequency (PRF) (200 - 2000 times per second)is equal to the sampling rate the displayed signal changes in accordance with the interface encountered along this line. Thus the scan is displayed at the rate of PRF. A-mode (Amplitude-mode) ultrasound is used to judge the depth of an organ, or otherwise assess an organ's dimensions. A-mode technology has been used in midline echoencephalography for rapid screening of intracranial mass lesions and ophthalmologic scanning. A-mode ultrasound imaging is now obsolete in medical imaging. The A-mode scan had also been used for early pregnancy assessment (detection of fetal heart beat), cephalometry and placental localization.
B - Mode / Brightness (Static)
Is the imaging the amplitude of the signal or echo strength which represented by brightness of a dot. Linear array of transducers simultaneously scans a plane through the body from many different directions. A single line of sight through the patient is acquired at each position of the transducer when the pulser is activated. The scan lines consist of series of dot representing the interference and reflection encountered along the line of sight. The superimpositions of multiple scan lines create a composite 2D image that demonstrate contour of the internal organ generally
B - Mode / Brightness (Real-time)
The real time has higher PRF range as high as 5000 pulses per second to preserve spatial resolution. It requires the acquisition in very rapid fashion to give perception motion (Bhargava, Srivastiva, 2002) as motion become rapid within FOV, a faster frame rate necessary to display the structure without any jerkiness.
Slower frame rate is required to visualize deep structure.
C - Mode / Color Doppler
It produces a color coded map of Doppler shifts superimposed onto a B-mode ultrasound image. Although color flow imaging uses pulsed wave ultrasound, its processing differs from that used to provide the Doppler sonogram. Color flow imaging may have to produce several thousand color points of flow information for each frame superimposed on the B-mode image. Color flow imaging uses fewer, shorter pulses along each color scan line of the image to give a mean frequency shift and a variance at each small area of measurement. This frequency shift is displayed as a color pixel. The scanner then repeats this for several lines to build up the color image, which is superimposed onto the B-mode image. The transducer elements are switched rapidly between B-mode and color flow imaging to give an impression of a combined simultaneous image.
The pulses used for color flow imaging are typically three to four times longer than those for the B-mode image, with a corresponding loss of axial resolution. Assignment of color to frequency shifts is usually based on direction (for example, red for Doppler shifts towards the ultrasound beam and blue for shifts away from it) and magnitude (different color hues or lighter saturation for higher frequency shifts).
D - Mode / Pulse-Wave Doppler
Pulse wave Doppler uses a single element transducer that emits brief pulses of ultrasound energy. The time interval between transmitting and receiving the echoing sound can be used to calculate the depth from where the echo arises. To assess the flow the angle of insonation must be parallel to flow as possible. It gives an accurate measure of the blood flow at a specific area and allows the detection of both velocity and direction. The reception of the returning signals is timed and shows flows at specific depths.
M - Mode
The M-mode (Motion-mode) ultrasound is used for analyzing moving body parts (also called time-motion or TM-mode) commonly in cardiac and fetal cardiac imaging. The application of B-mode and a strip chart recorder allows visualization of the structures as a function of depth and time. The M-mode ultrasound transducer beam is stationary while the echoes from a moving reflector are received at varying times.
A single beam in an ultrasound scan is used to produce the one-dimensional M-mode picture, where movement of a structure such as a heart valve can be depicted in a wave-like manner. The high sampling frequency (up to 1000 pulses per second) is useful in assessing rates and motion, particularly in cardiac structures such as the various valves and the chamber walls (Deane C, 2002).
Tissue Harmonic mode
Tissue harmonic ultrasonography (US) is based on the phenomenon of nonlinear distortion of an acoustic signal as it travels through the body. Imaging begins with insonation of tissue with ultrasound waves of a specific transmitted frequency. Harmonic waves are generated within the tissue and build up with depth to a point of maximal intensity before they decrease because of attenuation. improved axial resolution due to shorter wavelength, better lateral resolution due to improved focusing with higher frequencies, and less artifact than with conventional US. Reduced artifact in harmonic imaging is due to the relatively small amplitude of the harmonic waves, which reduces detection of echoes from multiple scattering events (Choudhry S et al 2000).
Image optimization: Power Output, Gain, Time Gain Compensation
The output characteristics from diagnostic device are different depending on the design and application. Real time pulse echo imaging system acoustic power output is typically less than 50mW (Henderson J et al, 1991).
PW Doppler has the higher Spatial Peak Temporal Average Intensity value because the temporal pulse length is long compared to those generated in real time scanner. The Spatial Peak Temporal Average Intensity value less than 2W/cm2 (Henderson J et al, 1991)
The acoustic power output, Spatial Peak Temporal Average Intensity value and peak negative pressure of M -mode units are comparable to those found in B - mode scanner.
The time average intensities for CW peripheral vascular Doppler unit is much higher than that obtained from obstetric Doppler unit. The spatial average, temporal average intensity are generally 400mW/cm2 and 15mW/cm2 respectively. The pulsed Doppler used for cardiac have spatial average, temporal average intensity of 3 - 32mW/cm2 (Henderson J et al, 1991)
Most recent surveys of the acoustic output levels of diagnostic ultrasound equipment have demonstrated the trend toward higher output intensity (Hedrick, Hykes, Starchman, 2005).
Signal noise ratio is unchanged by gain. The gain control decides how much amplification is accomplished in the amplifier. Gain is used to establish the proper brightness level for echoes of varying strength, but cannot improve detection of subtle differences between scatters. Too little gain, weak echoes not imaged. Too much gain echoes appear bright and lost of differences in echo strength.
Time Gain Compensation (TGC)
TGC controls are used to increase the amplitude of processed signals with time or depth. The amplification could be reverse exponential function because the signals decrease exponentially. TGC also contributes to signal compression.
The variable TGC controls allow adjustable compensation for different frequency transducer allowing greater amplification when high frequencies are used. When frequency is increased more amplification is required to counteract the accelerated loss of beam intensity. The flexibility enables higher quality image to be obtained.
Image recording options
The image is stored in the CD ROM provides low cost option to communicate the result of the examination to another facilities. This mean the record is available to the patient. Software like viewing tools installed on the CD Rom with the image allows the user to select and view images on the monitor.
Picture Archiving & Communication System (PACS)
Is a computer system that allows digitized images from various imaging modalties to be stored electronically for lateral retrieval, display, manipulation interpretation and distribution to remote location (Hedrick, Hykes, Starchman, 2005). All the text, patient demorgraphic data must be stored in such manner where allow the appropriate information to be assessed with the corresponding image. Therefore the ultrasound image must be DICOM compatible.
A single emulsion film compatible with automatic processing is recommended for hard copies cameras (Hedrick, Hykes, Starchman, 2005). The polyester base may have light absorbing antihalation layer which prevent reflection from the base in to the emulsion, improves the sharpness of the image. The film is recorded by a multi format camera. The number of image per sheet is usually fixed as1, 2, 4, 6 or 9 depend on the model.
The color printed paper provides a color hardcopy recording of the image displayed on the monitor. This used very frequently in Doppler images. A video signal sent to computer memory and stored. 512x512 color image requires 0.75M bytes of computer memory. The color gradation are controlled by superimposing varying amount of each dye on the paper. The color level ranges up several million. Each primary color coded in 8 bits (24 bits per pixel). The number of lines printed more than 500. Therefore the video signal and not the device limit the spatial resolution (Hedrick, Hykes, Starchman, 2005).
Ultrasound technology is expended throughout the years. There are many creation and innovation to produce good quality ultrasonography. Having said so, it's still a personal dependant and the subject is an organ specific. Therefore an increase of personal's technique must be along with the advance of technology.
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