Typical diagnostic sonographic scanners operate in the frequency range of 2 to 18 megahertz, though frequencies up to 50-100 megahertz has been used experimentally in a technique known as biomicroscopy in special regions, such as the anterior chamber of eye.
[[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]] The above frequencies are hundreds of times greater than the limit of human hearing, which is typically accepted as 20 kilohertz. The choice of frequency is a trade-off between spatial resolution of the image and imaging depth: lower frequencies produce less resolution but image deeper into the body.
Sonography (ultrasonography) is widely used in
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]. It is possible to perform both
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] and therapeutic procedures, using ultrasound to guide interventional procedures (for instance
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] or drainage of fluid collections).
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] are medical
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] who perform scans for diagnostic purposes. Sonographers typically use a hand-held probe (called a transducer) that is placed directly on and moved over the patient.
Sonography is effective for imaging soft tissues of the body. Superficial structures such as
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط],
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط],
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط],
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] and the
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] brain are imaged at a higher
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] (7-18 MHz), which provides better axial and lateral
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]. Deeper structures such as liver and kidney are imaged at a lower frequency 1-6 MHz with lower axial and lateral resolution but greater penetration.
Therapeutic applications
Therapeutic applications use ultrasound to bring heat or agitation into the body. Therefore much higher energies are used than in diagnostic ultrasound. In many cases the range of frequencies used are also very different.
From sound to image
The creation of an image from sound is done in three steps - producing a
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط], receiving
echoes, and interpreting those echoes.
Producing a sound wave
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] Medical sonographic instrument
A sound wave is typically produced by a
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] encased in a housing which can take a number of forms. Strong, short electrical pulses from the ultrasound machine make the transducer ring at the desired frequency. The
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] can be anywhere between 2 and 18
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]. The sound is focused either by the shape of the transducer, a lens in front of the transducer, or a complex set of control pulses from the ultrasound scanner machine (
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]). This focusing produces an arc-shaped sound wave from the face of the transducer. The wave travels into the body and comes into focus at a desired depth.
Older technology transducers focus their beam with physical lenses. Newer technology transducers use
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] techniques to enable the sonographic machine to change the direction and depth of focus. Almost all piezoelectric transducers are made of
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط].
Materials on the face of the transducer enable the sound to be transmitted efficiently into the body (usually seeming to be a rubbery coating, a form of
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]). In addition, a water-based gel is placed between the patient's skin and the probe.
The sound wave is partially reflected from the layers between different tissues. Specifically, sound is reflected anywhere there are density changes in the body: e.g. blood cells in blood plasma, small structures in organs, etc. Some of the reflections return to the transducer
Receiving the echoes
The return of the sound wave to the transducer results in the same process that it took to send the sound wave, except in reverse. The return sound wave vibrates the transducer, the transducer turns the vibrations into electrical pulses that travel to the ultrasonic scanner where they are processed and transformed into a digital image.
Forming the image
The sonographic scanner must determine three things from each received echo:
- How long it took the echo to be received from when the sound was transmitted.
- From this the focal length for the phased array is deduced, enabling a sharp image of that echo at that depth (this is not possible while producing a sound wave).
- How strong the echo was. It could be noted that sound wave is not a click, but a pulse with a specific carrier frequency. Moving objects change this frequency on reflection, so that it is only a matter of electronics to have simultaneous [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط].
Once the ultrasonic scanner determines these three things, it can locate which pixel in the image to light up and to what intensity and at what
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] if frequency is processed (see
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] for a natural mapping to hue).
Transforming the received signal into a digital image may be explained by using a blank spreadsheet as an analogy. First picture a long, flat transducer at the top of the sheet. Send pulses down the 'columns' of the spreadsheet (A, B, C, etc.). Listen at each column for any return echoes. When an echo is heard, note how long it took for the echo to return. The longer the wait, the deeper the row (1,2,3, etc.). The strength of the echo determines the brightness setting for that cell (white for a strong echo, black for a weak echo, and varying shades of grey for everything in between.) When all the echoes are recorded on the sheet, we have a greyscale image.
Displaying the image
Images from the sonographic scanner can be displayed, captured, and broadcast through a computer using a
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] to capture and digitize the analog video signal. The captured signal can then be post-processed on the computer itself
Sound in the body
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] Linear Array Transducer
Ultrasonography (
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]) uses a probe containing one or more acoustic
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] to send pulses of sound into a material. Whenever a sound wave encounters a material with a different density (acoustical impedance), part of the sound wave is reflected back to the probe and is detected as an echo. The time it takes for the
echo to travel back to the probe is measured and used to calculate the depth of the tissue interface causing the echo. The greater the difference between acoustic impedances, the larger the echo is. If the pulse hits gases or solids, the density difference is so great that most of the acoustic energy is reflected and it becomes impossible to see deeper.
The frequencies used for medical imaging are generally in the range of 1 to 18 MHz. Higher frequencies have a correspondingly smaller wavelength, and can be used to make sonograms with smaller details. However, the attenuation of the sound wave is increased at higher frequencies, so in order to have better penetration of deeper tissues, a lower frequency (3-5 MHz) is used.
Seeing deep into the body with sonography is very difficult. Some acoustic energy is lost every time an echo is formed, but most of it (approximately
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]) is lost from acoustic absorption.
The speed of sound is varies as it travels through different materials, and is dependent on the
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] of the material. However, the sonographic instrument assumes that the acoustic velocity is constant at 1540 m/s. An effect of this assumption is that in a real body with non-uniform tissues, the beam becomes somewhat de-focused and image resolution is reduced.
To generate a
2D-image, the ultrasonic beam is swept. A transducer may be swept mechanically by rotating or swinging. Or a 1D
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] transducer may be use to sweep the beam electronically. The received data is processed and used to construct the image. The image is then a 2D representation of the slice into the body.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] images can be generated by acquiring a series of adjacent 2D images. Commonly a specialised probe that mechanically scans a conventional 2D-image transducer is used. However, since the mechanical scanning is slow, it is difficult to make 3D images of moving tissues. Recently, 2D phased array transducers that can sweep the beam in 3D have been developed. These can image faster and can even be used to make live 3D images of a beating heart.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] ultrasonography is used to study blood flow and muscle motion. The different detected speeds are represented in color for ease of interpretation, for example leaky heart valves: the leak shows up as a flash of unique color. Colors may alternatively be used to represent the amplitudes of the received echoes
Modes of sonography
Several different modes of ultrasound are used in medical imaging.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] These are:
- A-mode: A-mode is the simplest type of ultrasound. A single transducer scans a line through the body with the echoes plotted on screen as a function of depth. Therapeutic ultrasound aimed at a specific tumor or calculus is also A-mode, to allow for pinpoint accurate focus of the destructive wave energy.
- B-mode: In B-mode ultrasound, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image on screen.
- M-mode: M stands for motion. In m-mode a rapid sequence of B-mode scans whose images follow each other in sequence on screen enables doctors to see and measure range of motion, as the organ boundaries that produce reflections move relative to the probe.
- Doppler mode: This mode makes use of the Doppler effect in measuring and visualizing blood flow
- Color doppler: Velocity information is presented as a color coded overlay on top of a B-mode image
- Continuous doppler: Doppler information is sampled along a line through the body, and all velocities detected at each time point is presented (on a time line)
- Pulsed wave (PW) doppler: Doppler information is sampled from only a small sample volume (defined in 2D image), and presented on a timeline
- Duplex: a common name for the simultaneous presentation of 2D and (usually) PW doppler information. (Using modern ultrasound machines color doppler is almost always also used, hence the alternative name Triplex
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Doppler sonographySee also: [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]
Spectral Doppler of Common Carotid Artery
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]
Colour Doppler of Common Carotid Artery
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]
Computer-enhanced [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط].
Sonography can be enhanced with Doppler measurements, which employ the [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] to assess whether structures (usually blood) are moving towards or away from the probe, and its relative velocity. By calculating the frequency shift of a particular sample volume, for example flow in an artery or a jet of blood flow over a heart valve, its speed and direction can be determined and visualised. This is particularly useful in cardiovascular studies (sonography of the vascular system and heart) and essential in many areas such as determining reverse blood flow in the liver vasculature in [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]. The Doppler information is displayed graphically using spectral Doppler, or as an image using [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] (directional Doppler) or [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] (non directional Doppler). This Doppler shift falls in the audible range and is often presented audibly using stereo speakers: this produces a very distinctive, although synthetic, pulsating sound.
Most modern sonographic machines use pulsed Doppler to measure velocity. Pulsed wave machines transmit and receive series of pulses. The frequency shift of each pulse is ignored, however the relative phase changes of the pulses are used to obtain the frequency shift (since frequency is the rate of change of phase). The major advantages of pulsed Doppler over continuous wave is that distance information is obtained (the time between the transmitted and received pulses can be converted into a distance with knowledge of the speed of sound) and gain correction is applied. The disadvantage of pulsed Doppler is that the measurements can suffer from [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]. The terminology "Doppler ultrasound" or "Doppler sonography", has been accepted to apply to both pulsed and continuous Doppler systems despite the different mechanisms by which the velocity is measured.
It should be noted here that there are no standards for the display of color Doppler. Some laboratories insist on showing arteries as red and veins as blue, as medical illustrators usually show them, even though, as a result, a tortuous vessel may have portions with flow toward and away relative to the transducer. This can result in the illogical appearance of blood flow that appears to be in both directions in the same vessel. Other laboratories use red to indicate flow toward the transducer and blue away from the transducer which is the reverse of 150 years of astronomical literature on the [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]. Still other laboratories prefer to display the sonographic Doppler color map more in accord with the prior published physics with the [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] representing longer waves of echoes (scattered) from blood flowing away from the transducer; and with blue representing the shorter waves of echoes reflecting from blood flowing toward the transducer. Because of this confusion and lack of standards in the various laboratories, the sonographer must understand the underlying acoustic physics of color Doppler and the physiology of normal and abnormal blood flow in the human body
Contrast media
The use of microbubble contrast media in medical sonography to improve ultrasound signal [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] is known as [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]. This technique is currently used in [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط], and may have future applications in molecular imaging and drug delivery
Compression ultrasonography
Compression ultrasonography is a technique used for diagnosing [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] and combines ultrasonography of the deep veins with venous compression.[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] The technique can be used on deep veins of the upper and lower extremities, with some laboratories limiting the examination to the [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] and the [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط], whereas other laboratories examine the deep veins from the inguinal region to the calf, including the calf veins.[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]
Compression ultrasonography in B-mode has both high [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] for detecting proximal deep vein thrombosis in symptomatic patients. The sensitivity lies somewhere between 90 to 100% for the diagnosis of symptomatic deep vein thrombosis, and the specificity ranges between 95 to 100%.[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]
Attributes
As with all imaging modalities, ultrasonography has its list of positive and negative attributes
Strengths
It images [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط], [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط], and bone surfaces very well and is particularly useful for delineating the interfaces between solid and fluid-filled spaces.
It renders "live" images, where the operator can dynamically select the most useful section for diagnosing and documenting changes, often enabling rapid diagnoses. Live images also allow for ultrasound-guided biopsies or injections, which can be cumbersome with other imaging modalities.
It shows the structure of organs.
It has no known long-term side effects and rarely causes any discomfort to the patient.
Equipment is widely available and comparatively flexible.
Small, easily carried scanners are available; examinations can be performed at the bedside.
Relatively inexpensive compared to other modes of investigation, such as [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط], [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] or [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط].
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] is better in high frequency ultrasound transducers than it is in most other imaging modalities.
Through the use of an [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط], an ultrasound device can offer a relatively inexpensive, real-time, and flexible method for capturing data required for special research purposes for tissue characterization and development of new image processing techiniques
Weaknesses
Sonographic devices have trouble penetrating [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]. For example, sonography of the adult brain is very limited though improvements are being made in transcranial ultrasonography.
Sonography performs very poorly when there is a gas between the transducer and the organ of interest, due to the extreme differences in [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]. For example, overlying gas in the gastrointestinal tract often makes ultrasound scanning of the [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] difficult, and lung imaging is not possible (apart from demarcating pleural effusions).
Even in the absence of bone or air, the depth penetration of ultrasound may be limited depending on the frequency of imaging. Consequently, there might be difficulties imaging structures deep in the body, especially in obese patients.
Body habitus has a large influence on image quality, image quality and accuracy of diagnosis is limited with obese patients, overlying subcutaneous fat attuates the sound beam and a lower frequency tranducer is required (with lower resolution)
The method is operator-dependent. A high level of skill and experience is needed to acquire good-quality images and make accurate diagnoses.
- There is no scout image as there is with CT and MRI. Once an image has been acquired there is no exact way to tell which part of the body was imaged.
- Risks and side-effects
Ultrasonography is generally considered a "safe" imaging modality.[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] However slight detrimental effects have been occasionally observed (see below). Diagnostic ultrasound studies of the foetus are generally considered to be safe during pregnancy. This diagnostic procedure should be performed only when there is a valid medical indication, and the lowest possible ultrasonic exposure setting should be used to gain the necessary diagnostic information under the "as low as reasonably achievable" or [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] principle.
World Health Organizations technical report series 875(1998).[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] supports that ultrasound is harmless: "Diagnostic ultrasound is recognized as a safe, effective, and highly flexible imaging modality capable of providing clinically relevant information about most parts of the body in a rapid and cost-effective fashion". Although there is no evidence ultrasound could be harmful for the foetus, US Food and Drug Administration views promotion, selling, or leasing of ultrasound equipment for making "keepsake foetal videos" to be an unapproved use of a medical device.
Studies on the safety of ultrasound
- A study at the [ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] found a correlation between prolonged and frequent use of ultrasound and abnormal neuronal migration in mice.[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] A meta-analysis of several ultrasonography studies found no statistically significant harmful effects from ultrasonography, but mentioned that there was a lack of data on long-term substantive outcomes such as neurodevelopment
- Regulation
Diagnostic and therapeutic ultrasound equipment is regulated in the USA by the FDA, and worldwide by other national regulatory agencies. The FDA limits acoustic output using several metrics. Generally other regulatory agencies around the world accept the FDA-established guidelines.
Currently New Mexico is the only state in the USA which regulates diagnostic medical sonographers. Certification examinations for sonographers are available in the US from three organizations: The American Registry of Diagnostic Medical Sonography,Cardiovascular Credentialing International and the American Registry of Radiological Technologists.
The primary regulated metrics are MI ([ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]) a metric associated with the cavitation bio-effect, and TI (Thermal Index) a metric associated with the tissue heating bio-effect. The FDA requires that the machine not exceed limits that they have established. This requires self-regulation on the part of the manufacturer in terms of the calibration of the machine. The established limits are reasonably conservative so as to maintain diagnostic ultrasound as a safe imaging modality.[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط]
In India, lack of social security and consequent preference for a male child has popularized the use of ultrasound technology to identify and abort female foetuses. India's Pre-natal Diagnostic Techniques act makes use of ultrasound for ****** selection illegal,[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذا الرابط] but unscrupulous Indian doctors and would-be parents continue to discriminate against the girl child
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موضوع: diagnostic application of ultrasonography 
الثلاثاء 16 نوفمبر 2010 - 19:34
