Encyclopedia
Medical ultrasonography is an
ultrasound-based diagnostic imaging technique used to visualize muscles and internal organs, their size, structures and any pathological lesions. Obstetric
sonography is commonly used during
pregnancy.
Ultrasound is also used medically for theraputic uses.
In physics the term "ultrasound" applies to all acoustic energy with a frequency above human hearing . Typical diagnostic sonography scanners operate in the frequency range of 2 to 13 megahertz, hundreds of times greater than this limit. The choice of frequency is a trade-off between the image spatial resolution and the penetration depth into the patient, with lower frequencies giving less resolution and greater imaging depth.
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Diagnostic applications
Sonography is widely utilized in
medicine. It is possible to perform diagnosis or therapeutic procedures with the guidance of sonography . Sonographers are medical professionals who perform scans for diagnostic purposes; they work with specialized doctors called sonologists who provide reports of the images obtained. Sonographers typically use a hand-held probe that is placed directly on and moved over the patient. A water-based gel is used to couple the ultrasound between the probe and patient.
Ultrasound is effective for imaging soft tissues of the body. Superficial structures such as
muscles,
tendons,
testes,
breast and
neonatal brain are imaged at a higher
frequency , which provides better axial and lateral resolution. 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, for example:
...
- Gastroenterology
- Gynaecology; see gynecologic ultrasonography
- Obstetrics; see obstetric ultrasonography
- Ophthalmology; see A-scan ultrasonography, B-scan ultrasonography
- Urology
- Musculoskeletaltendons, muscles nerves
- Vascular, arteries and veins
- Intravascular ultrasound
- Intervenional
- Contrast enhanced ultrasound
A general-purpose ultrasound machine may be able to be used for most imaging purposes. Usually specialty applications may be served only by use of a specialty transducer.
Echocardiography is a major sub-specialty of diagnostic ultrasound that is different. The dynamic nature of cardiac studies generally requires specialized features in an ultrasound machine for it to be effective.
Obstetrical ultrasound is commonly used during
pregnancy to check on the development of the
fetus.
In a pelvic ultrasound, organs of the pelvic region are imaged. This includes the uterus and
ovaries or urinary bladder. Men are sometimes given a pelvic ultrasound to check on the health of their bladder and
prostate. There are two methods of performing a pelvic ultrasound - externally or internally. The internal pelvic ultrasound is perfomed either trans
vaginally or transrectally . See:-
- Gynecologic ultrasonography
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In an abdominal ultrasound, the solid organs of the abdomen such as the
pancreas, aorta, inferior vena cava,
liver,
gall bladder,
bile ducts,
kidneys, and
spleen are imaged. Sound waves are blocked by gas in the bowel, therefore there are limited diagnostic capabilities in this area. The appendix can sometimes be seen when inflammed eg:
appendicitis.
Therapeutic applications
Theraputic applications of ultrasound usually make use of sound to bring heat or agitation into the body. Therefore it uses much higher energies are used than in diagnostic ultrasound. And in many cases the range of frequencies used are also very different.
- Treating benign and malignant tumors and other disorders, via a process known as Focused Ultrasound Surgery or High Intensity Focused Ultrasound . These procedures generally use lower frequencies than medical diagnostic ultrasound , but significantly higher energies. The treatment is often guided by MRI, as in Magnetic Resonance guided Focused Ultrasound.
- Ultrasound may be used to clean teeth in dental hygiene.
- Ultrasound sources may be used to generate local heating in biological tissue, e.g. in occupational therapy, physical therapy and cancer treatment.
- Focused ultrasound may be used to break up kidney stones by lithotripsy.
- Ultrasound may be used for cataract treatment by phacoemulsification.
- Additional physiological effects of low-intensity ultrasound have recently been discovered, e.g. the ability to stimulate bone-growth and its potential to disrupt the blood-brain barrier for drug delivery.
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
In medical
ultrasonography, a sound wave is typically produced by creating short, strong pulses of sound from a
phased array of
piezoelectric transducers . The electrical wiring and transducers are encased in a probe. The electrical pulses vibrate the ceramic to create a series of sound pulses from each. The
frequencies present in this sound wave can be anywhere between 2 and 13 MHz. The goal is to produce a single focused arc-shaped sound wave from the sum of all the individual pulses emitted by the transducer.
To make sure the sound is transmitted efficiently into the body , the transducer face has a
rubber coating. In addition, a water-based gel is placed between the probe and the patient's skin.
The sound wave is partially reflected from the interface between different tissues and returns to the transducer. This returns an echo. Sound that is scattered by very small structures also produces echoes.
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, just in reverse. The return sound wave vibrates the transducer's elements and turns that vibration into electrical pulses that are sent from the probe to ultrasound scanner where they are processed and transformed into a digital image.
Forming the image
The ultrasound scanner must determine three things from each received echo: 1.) The direction of the echo. 2.) How strong the echo was. 3.) How long it took the echo to be received from when the sound was transmitted. Once the ultrasound scanner determines these three things, it can locate which pixel in the image to light up and to what intensity. Transforming the received signal into a digital image can be best explained by using a blank spreadsheet as an analogy. The transducer elements receiving the impulse determines the 'column' in our spreadsheet . The time that it took to receive the echo determines the 'row' , and the strength of the echo determines the brightness that the cell should change to
Sound in the body
Ultrasonography uses a probe containing one or more acoustic transducers to send pulses of sound into a material. Whenever a sound wave encounters a material with a different density , 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 13 MHz. Higher frequencies have a correspondingly smaller wavelength, and can be used to make images 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 is used.
Seeing deep into the body with ultrasound is very difficult. Some acoustic energy is lost every time an echo is formed, but most of it is lost from acoustic absorption. A common model of this loss is 0.3 dB /cm of depth / MHz.
The speed of sound is different in different materials, and is dependent on the acoustical impedance of the material. However, the ultrasound scanner 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 become somewhat de-focused and image resolution is reduced.
To generate a 2D-image, the ultrasound beam is swept. A transducer may be swept mechanically by rotating or swinging. Or a 1D
phased array 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.
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.
Most ultrasound machines can also produce color images. The colors are usually used to represent movement and is used to study blood flow and muscle motion. As a usage example, this representation makes it easy to detect leaky heart valves because the leak shows up as a flash of unique color. Colors may alternatively be used to represent the amplitudes of the received echoes.
Doppler sonography
Sonography can be enhanced with Doppler measurements, which employ the
Doppler effect to assess whether structures are moving towards or away from the probe, and its relative velocity. By calculating the frequency shift of a particular sample volume, for example 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 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 colour Doppler or power Doppler . This Doppler shift falls in the audible range and is often presented audibly using stereo speakers: this produces a very distinctive, although synthetic, pulsing sound.
Strictly speaking, most modern ultrasound machines do not use the Doppler effect to measure velocity, as they rely on pulsed wave Doppler . Pulsed wave machines transmit pulses of ultrasound, and then switch to receive mode. As such, the reflected pulse that they receive is not subject to a frequency shift, as the insonation is not continuous. However, by making several measurements, the phase change in subsequent measurements can be used to obtain the frequency shift . To obtain the phase shift between the received and transmitted signals, one of two algorithms is typically used: the Kasai algorithm or the crosscorrelation. Older machines, that use continuous wave Doppler, exhibit the Doppler effect as described above. To do this, they must have separate tranmission and reception transducers. The major drawback of CW machines, is that no distance information can be obtained .
In the ultrasound community , the terminology "Doppler" ultrasound, has been accepted to apply to both PW and CW Doppler systems despite the different mechanisms by which the velocity is measured.
Microbubbles
The use of microbubble contrast media in medical sonography to improve ultrasound signal backscatter is known as contrast enhanced ultrasound. This technique is currently used in
echocardiography, and may have future applications in molecular imaging and drug delivery.
Strengths of sonography
- It images muscle and soft tissue 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.
- 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 .
Weaknesses of ultrasound imaging
- Large body habitus, obese patients limit image quality as the overlying adipose tissue scatters the sound and greater depth the sound waves need to travel attenuate or weaken the signal on transmission and relection back to the transducer. A fetus close to the surface will be imaged at a higher resolution than those at greater distance to the skin surface.
- Ultrasound devices have trouble penetrating bone. For example, ultrasound imaging of the brain is very limited.
- Ultrasound can detect fluid surrounding the lung but the high impedance mismatch between the solid tissues and the air filled lungs limits image.
- Ultrasound performs very poorly when there is a gas between the transducer and the organ of interest, due to the extreme differences in acoustical impedance. For example, overlying gas in the gastrointestinal tract often makes ultrasound scanning of the pancreas
...
difficult, and lung imaging is not possible .
- Even in the absence of bone or air, the depth penetration of ultrasound is limited, making it difficult to image structures deep in the body, especially in obese patients.
- The method is operator-dependent. A high level of skill and experience is needed to acquire good-quality images and make accurate diagnoses. For information on education and certification in sonography see ARDMS.
- There is no scout image as there is with CT and MR. Once an image is acquired there is no exact way to tell which part of the body was imaged.
Dangers of ultrasound imaging
There have been disputes whether ultrasound is safe. Since ultrasound is energy, there are questions such as "What are the energy waves doing to my tissue?". A meta-analysis of several ultrasonography studies were performed showing that there were no statistical significant harmful effects from ultrasonography. This however does not rule out the possibility that harmful effects are present, although they must be so small as not to show up in the sample sizes of choice in the studies. In addition, the report states in its main results that there is a lack of data with regard to long-term substantive outcomes such as neurodevelopment.
There may be a number of bio-effects, including but not necessarily limited to:-
- Heat generation: Local tissue absorbs the ultrasound energy and increases their temperatures. Long-duration elevated temperatures above 41 C can damage tissue.
- Cavitation: Very high negative acoustic pressures can cause temporary microscopic vacuum pockets. When these collapse, they produce very high local temperatures that can cause damage to the immediate region.
- Bubble formation: dissolved gases come out of the solution due to local heat increases
A study at the
Yale Medical School found a correlation between prolonged and frequent use of ultrasound and abnormal neuronal migration in mice.
A study published in 2001 by a team working at the
Karolinska Institute in Stockholm found a correlation between the number of scans received by male foetuses and subsequent left-handedness. .
Regulation
Diagnostic ultrasound is regulated in the USA by the FDA, and world-wide 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.
The primary regulated metrics are MI a metric associated with the cavitation bio-effect, and TI a metric associated with the tissue heating bio-effect. The FDA requires that the machine not exceed limits that they have established. This requires the manufacturer to calibrate their machines and make them self-regulating. The established limits are reasonably conservative and therefore maintains diagnostic ultrasound as a safe imaging modality.
History
United States
Ultrasonic energy was first applied to the human body for medical purposes by Dr. George D. Ludwig at the Naval Medical Research Institute, Bethesda, Maryland in the late 1940s.
The first demonstration of color Doppler was by Geoff Stevenson, who was involved in the early developments and medical use of Doppler shifted ultrasonic energy.
Sweden
Medical ultrasonography was used 1953 at
Lund University by
cardiologist Inge Edler and Carl Hellmuth Hertz, the son of
Gustav Ludwig Hertz, who was a graduate student at the department of nuclear physics.
Edler had asked Hertz if it was possible to use
radar to look into the body, but Hertz said this was impossible. However, he said, it might be possible to use ultrasonography. Hertz was familiar with using ultrasonic
reflectoscopes for nondestructive materials testing, and together they developed the idea of using this method in medicine.
The first successful measurement of heart activity was made on October 29, 1953 using a device borrowed from the ship construction company
Kockums in
Malmö. On December 16 the same year, the method was used to generate an echo-encephalogram . Edler and Hertz published their findings in 1954.
Scotland
Parallel developments in
Glasgow,
Scotland by Professor Ian Donald and colleagues at the Glasgow Royal Maternity Hospital led to the first diagnostic
applications of the technique. Donald was an
obstetrician with a self-confessed "childish interest in machines, electronic and otherwise", who, having treated the wife of one of the company's directors,
was invited to visit the Research Department of boilermakers Babcock & Wilcox at Renfrew, where he used their industrial ultrasound equipment to conduct experiments on various morbid anatomical specimens and assess
their ultrasonic characteristics. Together with the medical physicist Tom Brown and fellow obstetrican Dr John MacVicar, Donald refined the equipment to enable differentiation of pathology in live volunteer patients. These
findings were reported in
The Lancet on 7th June 1958 as "Investigation of Abdominal Masses by Pulsed Ultrasound" - possibly one of the most important papers ever published in the field of diagnostic medical imaging.
At GRMH, Professor Donald and Dr James Willocks then refined their techniques to obstetric applications including fetal head measurement to assess the size and growth of the foetus. With the opening of the new Queen Mother's Hospital on Yorkhill in 1964, it became possible to improve these methods even further. Dr Stuart Campbell's pioneering work on
fetal cephalometry led to it acquiring long-term status as the definitive method of study of fetal growth. As the technical quality of the scans was further developed, it soon became possible to study pregnancy from start to finish and diagnose its many complications such as multiple pregnancy, fetal abnormality and
placenta praevia. Diagnostic ultrasound has since been imported into practically every other area of medicine.
References
External links
- for individual certifications by the American Registry for Diagnostic Medical Sonography
- of radiological sites
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- at various stages of fetal development
- - The radiology information resource for patients: Ultrasonography
