Medical ultrasonography - Article
Article: Medical ultrasonography
- Sonography redirects here. For the tactile alphabet called "sonography", see Night writing.
Medical ultrasonography (sonography) is an ultrasound-based diagnostic imaging technique used to visualize muscles and internal organs, their size, structure and any pathological lesions, making them useful for scanning the organs. Obstetric sonography is commonly used during pregnancy.
The choice of frequency is a trade-off between the image spatial resolution and the penetration depth into the patient. Typical diagnostic sonography scanners operate in the frequency range of 2 to 13 megahertz.
Whilst in physics the term "ultrasound" applies to all acoustic energy with a frequency above human hearing (20,000 Hertz), its common usage as a term of medical imaging applies to just a band of frequencies hundreds of times higher.
Sonography (ultrasonography) is widely utilized in medicine. It is possible to perform diagnosis or therapeutic procedures with the guidance of sonography (for instance biopsies or drainage of fluid collections). 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 (called a transducer) that is placed directly on and moved over the patient: a water-based gel ensures good coupling between the patient and scan head. This eliminates the air between the transducer head and the skin. Ultrasound image quality is limited by the amount of overlying adipose(fat) tissue, as the fatty tissue tends to scatter the sound and greater depth leads to attenuation of the sound beam. Superficial structures such as muscles, tendons, testes, breast and neonatal brain (through the fontanelles) are imaged at a higher frequency (7-15 Mhz), 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:
- Cardiology; see echocardiography
- Gynaecology; see gynecologic ultrasonography
- Obstetrics; see obstetric ultrasonography
- Ophthalmology; see A-scan ultrasonography, B-scan ultrasonography
- Musculoskeletaltendons, muscles nerves
- Vascular, arteries and veins
- Intravascular ultrasound (e.g. ultrasound guided fluid aspiration, fine needle aspiration, guided injections)
- Contrast enhanced ultrasound
A pelvic ultrasound is a diagnostic tool used to image pelvic pathology and to image the uterus and ovaries or urinary bladder. Ultrasound is commonly used during pregnancy to check on the development of the fetus. 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 transvaginally (in a woman) or transrectally (in a man). See:-
- Gynecologic ultrasonography
- Obstetric ultrasonography
Upper Abdominal Ultrasound
An "abdominal ultrasound" is a diagnostic tool to image the solid abdominal organs such as the pancreas, aorta, inferior vena cava, liver, gall bladder, bile ducts, kidneys, and spleen. The sound waves are scattered by gas in the bowel limited giagnostic capabilities in this area. The appendix can sometimes be seen when inflammed eg: appendicitis
- Treating benign and malignant tumors and other disorders, via a process known as Focused Ultrasound Surgery (FUS) or HIFU, High Intensity Focused Ultrasound. These procedures generally use lower frequencies than medical diagnostic ultrasound (from 250 kHz to 2000 kHz), but significantly higher time-averaged intensities. The treatment is often guided by MRI, as in Magnetic Resonance guided Focused Ultrasound.
- More powerful ultrasound sources may be used to clean teeth in dental hygiene or generate local heating in biological tissue, e.g. in occupational therapy, physical therapy and cancer treatment.
- Focused ultrasound sources may be used to break up kidney stones or 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 (usually a type of ceramic). 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; well above the capabilities of the human ear. Any frequency above the capabilities of the human ear is referred to as 'ultrasound'. 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 (a form of impedance matching), 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 return is 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.) Which transducer elements received the echo (often there are multiple elements on a transducer). 2.) How strong was the echo. 3.) How long did it take 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 brightness. 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 (A,B,C,etc.). The time that it took to receive the echo determines the 'row' (1,2,3,etc.), and the strength of the echo determines the brightness that the cell should change to (white for a strong echo, black for a weak echo, and varying shades of grey for everything in between.)
Ultrasonography (sonography) 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 acoustical impedance, part of the sound wave is reflected, which the probe detects 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. The difference between gases and solids is so great that most of the acoustic energy is reflected, and so imaging of objects beyond that region is not possible.
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. Although part of the acoustic energy is lost every time an echo is formed, this effect is small compared to the attenuation of sound due to absorption.
To generate a 2D-image, the ultrasound beam is swept, either mechanically, or electronically using a phased array of acoustic transducers. The received data are processed and used to construct the image.
The frequencies used for medical imaging are generally in the range of 1 to 13 MHz. Higher frequencies have a correspondingly shorter wavelength, and so images can have a greater resolution. 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.
Most sonographic machines can also produce colour images, of sorts. This done simply by assigning a range of colours to encode the amplitudes of the received echoes. In addition, 3D images can be generated by acquiring a series of 2D images, often using a specialised probe.
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.
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 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 vasculature 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 colour 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, pulsing sound.
Strictly speaking, most modern ultrasound machines do not use the Doppler effect to measure velocity, as they rely on pulsed wave Doppler (PW). 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 (since frequency is the rate of change of phase). 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 (CW) 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 (this is the major advantage of PW systems - the time between the transmitted and received pulses can be converted into a distance with knowledge of the speed of sound).
In the ultrasound community (although not in the signal processing 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.
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 (e.g. computed X-ray tomography, DEXA or magnetic resonance imaging).
Weaknesses of ultrasound imaging
- Large body habitus, obese patients limit image quaility as the overlying adipose tissue (fat) scatters the sound and geater 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 grater 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 (plueral effusion) 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 scan head 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 (apart from demarcating pleural effusions).
- 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?". There are some reports of low birth weight babies being born to mothers who had more than the recommended ultrasound examination.
There may be these side-effects:-
- Heat development: Local tissue absorb the ultrasound energy and increases the temperature of those tissues
- Bubble formation: dissolved gases come out of the solution due to local heat increases
However, there are no substantiated side-effects documented in studies.
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.
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 MalmÃ¶. On December 16 the same year, the method was used to generate an echo-encephalogram (ultrasonic probe of the brain). Edler and Hertz published their findings in 1954.
Parallel developments in Glasgow, Scotland (coincidentally also a major shipbuilding centre) by Professor Ian Donald and colleagues at the Glasgow Royal Maternity Hospital (GRMH) 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 marine 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.