Ultrasonography
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Diagnostic sonography (ultrasonography) is an ultrasound-based diagnostic imaging technique used for visualizing subcutaneous body structures including tendons, muscles, joints, vessels and internal organs for possible pathology orlesions. Obstetric sonography is commonly used during pregnancy and is widely recognized by the public.
In physics, the term “ultrasound” applies to all sound waves with a frequency above the audible range of human hearing, about 20 kHz. The frequencies used in diagnostic ultrasound are typically between 2 and 18 MHz.Diagnostic applications Urinary bladder (black butterfly-like shape) and hyperplastic prostate (BPH) visualized by medical ultrasonography technique. Typical diagnostic sonographic scanners operate in the frequency range of 2 to 18 megahertz, though frequencies up to 50–100 megahertz have been used experimentally in a technique known as biomicroscopy in special regions, such as the anterior chamber of the eye.[citation needed] 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. Higher frequency sound waves have a smaller wavelength and thus are capable of reflecting or scattering from smaller structures. Higher frequency sound waves also have a larger attenuation coefficient and thus are more readily absorbed in tissue, limiting the depth of penetration of the sound wave into the body. Sonography (ultrasonography) is widely used in medicine. It is possible to perform both diagnosis and therapeutic procedures, using ultrasound to guide interventional procedures (for instance biopsies or drainage of fluid collections). Sonographers are medical professionals who perform scans which are then typically interpreted by Radiologists, physicians who specialize in the application and interpretation of a wide variety of medical imaging modalities, or by Cardiologists in the case of cardiac ultrasonography (echocardiography). 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 muscles, tendons, testes,breast and the neonatal brain are imaged at a higher frequency (7–18 MHz), which provides better axial and lateralresolution. 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. Medical sonography is used in the study of many different systems: System Description Anesthesiology Ultrasound is commonly used by anesthesiologists (Anaesthetists) to guide injecting needles when placing local anaesthetic solutions near nerves Gastroenterology In abdominal sonography, the solid organs of the abdomen such as thepancreas, aorta, inferior vena cava, liver, gall bladder, bile ducts,kidneys, and spleen are imaged. Sound waves are blocked by gas in the bowel and attenuated in different degree by fat, therefore there are limited diagnostic capabilities in this area. The appendix can sometimes be seen when inflamed (as in e.g.: appendicitis). Gynecology Neonatology for basic assessment of intracerebral structural abnormalities, bleeds,ventriculomegaly or hydrocephalus and anoxic insults (Periventricular leukomalacia). The ultrasound can be performed through the soft spots in the skull of a newborn infant (Fontanelle) until these completely close at about 1 year of age and form a virtually impenetrable acoustic barrier for the ultrasound. The most common site for cranial ultrasound is the anterior fontanelle. The smaller the fontanelle, the poorer the quality of the picture. Neurology for assessing blood flow and stenoses in the carotid arteries (Carotid ultrasonography) and the big intracerebral arteries Obstetrics Obstetrical sonography is commonly used during pregnancy to check on the development of the fetus. Ophthalmology Urologyto determine, for example, the amount of fluid retained in a patient’s bladder. In a pelvic sonogram, organs of the pelvic region are imaged. This includes the uterus and ovaries or urinary bladder. Males are sometimes given a pelvic sonogram to check on the health of their bladder, the prostate, or their testicles (for example to distinguishepididymitis from testicular torsion). In young males, it is used to distinguish more benign testicular masses (varicocele or hydrocele) fromtesticular cancer, which is still very highly curable but which must be treated to preserve health and fertility. There are two methods of performing a pelvic sonography – externally or internally. The internal pelvic sonogram is performed either transvaginally (in a woman) or transrectally (in a man). Sonographic imaging of the pelvic floor can produce important diagnostic information regarding the precise relationship of abnormal structures with other pelvic organs and it represents a useful hint to treat patients with symptoms related to pelvic prolapse, double incontinence and obstructed defecation. It is used to diagnose and, at higher frequencies, to treat (break up) kidney stones or kidney crystals (nephrolithiasis). Musculoskeletal tendons, muscles, nerves, ligaments, soft tissue masses, and bone surfaces Cardiovascular systemTo assess patency and possible obstruction of arteries Arterial sonography, diagnose DVT (Thrombosonography) and determine extent and severity of venous insufficiency (venosonography) Other types of uses include:
A sonogram (not to be confused with an ultrasound scan) uses the reflections of high-frequency sound waves to construct an image of a body organ. 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 sound wave, receiving echoes, and interpreting those echoes. Producing a sound wave A sound wave is typically produced by a piezoelectrictransducer 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 frequencies can be anywhere between 2 and 18 MHz. 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 (Beamforming). 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 phased arraytechniques to enable the sonographic machine to change the direction and depth of focus. Almost all piezoelectric transducers are made of ceramic. 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 impedance matching). 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:
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 frame grabber to capture and digitize the analog video signal. The captured signal can then be post-processed on the computer itself.[6] For computational details see also: Confocal laser scanning microscopy, Radar, Sound in the body Ultrasonography (sonography) uses a probe containing multiple acoustic transducers 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 ) is lost from acoustic absorption. The speed of sound varies as it travels through different materials, and is dependent on the acoustical impedance 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 phased array transducer may be used 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. 3D 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. Doppler 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. These are:
An additional expansion or additional technique of ultrasound is biplanar ultrasound, in which the probe has two 2D planes that are perpendicular to each other, providing more efficient localization and detection. Furthermore, anomniplane probe is one that can rotate 180° to obtain multiple images. In 3D ultrasound, many 2D planes are digitally added together to create a 3-dimensional image of the object. In contrast-enhanced ultrasound, microbubble contrast agents enhance the ultrasound waves, resulting in increased contrast. Doppler sonography Sonography can be enhanced with Doppler measurements, which employ the Doppler effect 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 portal hypertension. The Doppler information is displayed graphically using spectral Doppler, or as an image using color Doppler (directional Doppler) or power Doppler (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 aliasing. 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 show arteries as red and veins as blue, as medical illustrators usually show them, even though some vessels may have portions flowing towards and portions flowing away from the transducer. This results in the illogical appearance of a vessel being partly a vein and partly an artery. Other laboratories use red to indicate flow toward the transducer and blue away from the transducer. Still other laboratories prefer to display the sonographic Doppler color map more in accord with the prior published physics with the red shift 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 backscatter is known ascontrast-enhanced ultrasound. This technique is currently used in echocardiography, and may have future applications in molecular imaging and drug delivery. Compression ultrasonography Compression ultrasonography is a technique used for diagnosing deep vein thrombosis and combines ultrasonography of the deep veins with venous compression. The technique can be used on deep veins of the upper and lower extremities, with some laboratories limiting the examination to the common femoral vein and the popliteal vein, whereas other laboratories examine the deep veins from the inguinal region to the calf, including the calf veins. Compression ultrasonography in B-mode has both high sensitivity and specificity 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% and 100%. Attributes As with all imaging modalities, ultrasonography has its list of positive and negative attributes. Strengths
Risks and side-effects Ultrasonography is generally considered a safe imaging modality. Diagnostic ultrasound studies of the fetus 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 ALARA principle. World Health Organizations technical report series 875 (1998). 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 fetus, US Food and Drug Administration views promotion, selling, or leasing of ultrasound equipment for making “keepsake fetal videos” to be an unapproved use of a medical device. Studies on the safety of ultrasound
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 (Mechanical Index) 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. 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 fetuses. India’s Antenatal (US: Prenatal) Diagnostic Techniques act makes use of ultrasound for sex selection illegal, but unscrupulous Indian doctors and would-be parents continue to discriminate against the girl child. |
