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Medical ultrasonography

Medical ultrasonography is an ultrasound-based diagnostic imaging technique used to visualize internal organs, their size, structure and their pathological lesions.

Uses

Ultrasonography is widely utilized in medicine. It is possible to perform diagnosis or therapeutic procedures with the guidance of ultrasonography (for instance biopsies or drainage of fluid collections). Typically uses a hand-held probe (often called a scan head) that is placed directly on and moved over the patient: a water-based gel ensures good coupling between the patient and scan head.

Medical ultrasonography is used in, for example:

Technology

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 acoustical impedence, 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 impedences, 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 impedence of the material. Part of the acoustic energy is lost every time an echo is formed.

Unlike regular sound, ultrasound can be directed into a single direction. The echoes received by a stationary probe will result in a single dimensional signal showing peaks for every major material change.

To generate a 2D-image, the probe is swivelled, either mechanically or through a phased array of ultrasound transducers. The data is analysed by computer and used to construct the image. In a similar way, 3D images can be generated by computer using a specialised probe.

Some ultrasonography machines can produce colour images, of sorts. From the amount of energy in each echo, the difference in acoustic impedence can be calculated and a colour is then assigned accordingly.

The frequencies used for medical imaging are generally in the range of 1 to 10 MHz. Higher frequencies have a correspondingly lower 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 better penetration of deeper tissues, a lower frequency (3-5MHz) may be used.

Doppler ultrasonography

Ultrasonography 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. 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 particularily useful in cardiovascular studies (ultrasonography of the vasculature 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 or power Doppler. It is often presented audibly using stereo speakers: this produces a very distinctive, although synthetic, sound.

Strengths and weaknesses

Strengths of ultrasound imaging

  • 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 as well as some aspects of the function 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. DEXA, computed X-ray tomography or magnetic resonance imaging).

Weaknesses of ultrasound imaging

  • Classical ultrasound devices have trouble penetrating bone but current research on ultrasound bone imaging will make it possible with dedicated devices in the future.
  • 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 impedence. 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 that are far removed from the body surface, 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.

History

Sweden

Medical ultrasonography was invented in 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 for 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 lent from the ship construction company Kockums in Malmö. 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.

Scotland

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.

References

  • Donald I, MacVicar J, Brown TG. Investigation of abdominal masses by pulsed ultrasound. Lancet 1958;1(7032):1188-95. PMID 13550965
  • Edler I, Hertz CH. The use of ultrasonic reflectoscope for the continuous recording of movements of heart walls. Kungl Fzsiogr Sallsk i Lund Forhandl. 1954;24:5. Reproduced in Clin Physiol Funct Imaging 2004;24:118-36. PMID 15165281.
  • S. A. Kana (2003). Introduction to physics in modern medicine. Tsylor & Francis. ISBN 0-415-30171-8.

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