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Medical imaging: ultrasound

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Alison Coutts, MSc, RGN

Lecturer in applied biological sciences, St Bartholomew school of nursing and Midwifery, CityUniversity, London.

What is it?

What is it?

Ultrasound uses high frequency sound waves and their echoes to create an image. Similar technology is used by SONAR in submarines and in geology, and in the echolocation techniques used by bats, whales and dolphins. In healthcare ultrasound has been used since the late 1950s. Modern scanners provide a ‘real time’ image and can show continuous movement in the structures.

How does it work?

In order to obtain an ultrasound image, a transducer generates a high-frequency sound wave between 3.5-7 Mhertz (3.5-7 million cycles a second). These are generated using one or more piezoelectric crystals, which vibrate when an electric current is applied. These vibrations produce sound waves that travel outwards and can be directed into a patient’s body.

Each time the sound waves cross an interface, for example between fluid and tissue or between soft tissue and bone, some are reflected back to the transducer. The reflected sound waves strike the same piezoelectric crystals, which in turn emit an electric current that is relayed to the central processing unit (CPU). The CPU calculates the distance between the boundary and the probe from the time it has taken to be emitted and reflected back. In healthcare the waves are usually reflected back within millionths of a second.

From this the CPU generates an image, which is displayed on a computer monitor. The image can also be printed, or stored on a disk.

Recently ultrasound machines have been developed that can provide three-dimensional images by moving the transducer, and the CPU combining the resulting hundreds of two dimensional images. These detailed images can be used to visualise tiny structures and tumours.

Doppler ultrasound exploits the Doppler effect, whereby a moving object will change the frequency of the reflected sound waves, making them higher if the object is moving away from the transducer, and lower if moving towards the transducer. From this the CPU can calculate the speed the object is moving. In the case of blood vessels, the CPU can use the diameter of the vessel to calculate the rate of blood flow, in millilitres per minute.

In future microbubbles may be used to enhance the quality of the image: tiny bubbles (typically 3mm in diameter) can be introduced and will resonate rapidly in the presence of high-frequency waves (Blomley et al, 2001).

What is it used for?

Ultrasound has many uses and is relatively cheap, non-invasive and does not use ionising radiation.

Obstetric ultrasound was one of the first uses of this technology. It is particularly useful in the following situations:

  • Diagnosis and assessment of early pregnancy;
  • Assessment of fetal viability following threatened miscarriage;
  • Determination of gestational age and monitoring of fetal growth, and presentation prior to delivery;
  • Placental localisation;
  • Detecting alterations in the volume of amniotic fluid;
  • Diagnosing fetal malformations - some of these can threaten the pregnancy while others are trivial in themselves, but may suggest the presence of chromosomal or other problems;
  • Monitoring the fetus during procedures, for example during amniocentesis: ultrasound almost eliminates the risk of the fetus being struck by the needle.

Ultrasound scans are so useful that pregnant mothers can be routinely scanned two or even three times in each pregnancy. It must be remembered that, while many prospective parents enjoy these early glimpses of their baby, some find them intrusive: For example, some do not wish to know the gender of the fetus. Furthermore, not all parents want to know everything about possible fetal malformations - some prefer to allow nature to take its course, so healthcare professionals should check how much information is wanted before undertaking a scan (McFayden, 1998).

Gynaecology uses ultrasound in diagnosing and monitoring tumours of the breast and ovary.

Cardiology exploits the ability of ultrasound to provide moving images. An echocardiogram provides several minutes of video showing the flow of blood through the chambers of the heart, and the movement of the valves. Ultrasound can also provide accurate estimates of the flow of blood through major blood vessels, including in obstetrics.

Urology again ultrasound’s ability to measure movement fluid is useful in monitoring blood flow through the kidney, diagnosing kidney stones and providing an early diagnosis of prostate cancer.

How is it done?

The transducer is placed directly on the patient’s skin. As sound waves of this frequency will not travel through air a jelly is used to ensure good contact between the transducer and the skin. The image is obtained immediately. As no ionising radiation is used the ultrasonographer and a nurse or relative may stay with the patient without risk to their health. The room is often darkened to make the screen easier to see. In obstetric ultrasound the mother may be asked to have a full bladder - this is in order to move abdominal contents so that they do not come between the transducer and the uterus and its contents.

The quality of the image deteriorates in the presence of air, for instance in the bowel, and in very obese patients.

Safety considerations

Ultrasound is generally considered completely safe, but some concerns have been expressed, mainly about using it repeatedly in early pregnancy. The concerns surround the release of high energy, causing an increase in local heat, or the formation of bubbles (cavitation) as gases come out of solution due to higher temperatures. There have been suggestions that such re[eated screenings result in smaller fetuses, babies with speech or hearing difficulties, and an increased risk of childhood leukaemia. However, larger studies have failed to confirm these anxieties (Naumberg et al, 2000). However, 3D ultrasound and other recent developments use more sound waves and we the possibility of some damage should not be ignored (Wiseman and Kiehl, 2007).

Nursing care

There is no specific care required for these procedures. A nurse may stay with the patient, and indeed the ultrasonographer is almost always present. Patients do not require sedating, and the procedure is not painful. Patients are not usually required to remain still, but they may require help with positioning.

References

Blomley, M.J.K. et al (2001) Microbubble contrast agents: a new area in ultrasound. British Medical Journal; 322: .

McFadyen, A. et al (1998) First trimester ultrasound screening. British Medical Journal; 317: 694-695.

Naumberg, E. et al (2000) Prenatal ultrasound examinations and risk of childhood leukaemia: case control study. British Medical Journal; 320: 282-283.

Wiseman, C., Kiehl, E. (2007) Picture perfect: benefits and risks of 3D ultrasound. Maternal and Child Nursing; 32: 2, 102-111.

Further reading

Banerjee, A.K. (2006) Radiology Made Easy. Cambridge: CambridgeUniversity Press.

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