Ultrasound in Medicine

by Sampad Sengupta

Over the last 50 years, ultrasound has grown to become a major component of medical imaging.

Ultrasound’s history can be traced back to Leonardo da Vinci, who first recorded experiments listening to sound transmitted through water by placing a tube into the sea to evaluate what he could hear. This may be considered to be the beginning of sonar research which was a precursor to twentieth century medical ultrasound. In 1794, an Italian biologist named Lazzaro Spallanzani analysed the basic mechanisms of spatial orientation of bats, proposing other mechanism of spatial orientation than the visual – ophthalmic system, thus showing the echolocation of bats. In the 1950s, Ian Donald, a Scottish physician first explored the use of ultrasound after seeing it used in Glasgow shipyards to look for flaws in metallurgy. His article ‘Investigation of Abdominal Masses by Pulsed Ultrasound’, published 7 June 1958 in the medical journal ‘The Lancet’, was one of the defining publications in the field. Introduction of real-time ultrasonography in recent years with the development of extraordinary equipment has revolutionised the diagnostic imaging world.

What is ultrasound?

Sound is a mechanical vibration distinguished by frequency and loudness. The velocity of sound waves is constant at 1500 ms^-1 and is determined by the product of wavelength and frequency. Velocity being constant, higher frequencies means shorter wavelengths.

Ultrasound is an oscillating sound pressure wave with a frequency greater than the upper limit of the human hearing range. It is not different from normal audible sound in terms of physical properties, but only by the fact that humans cannot hear it. An average human has audible range of up to 20000 Hz (20 kHz) whereas ultrasound devices operate with frequencies from 20 kHz up to several gigahertz. An ultrasonic transducer is a device that converts energy into ultrasound, or sound waves above the normal range of human hearing. Transducers of ultrasound machines have different frequencies and are in the range of 2-10 MHz.  Higher frequency probes (transducers) give better resolution which means they are more able to distinguish two targets close together. However, they have decreased penetration. On the other hand, lower frequency probes have much lower resolution but higher penetration. In essence, higher frequency probes are used for superficial structures and lower frequency probes are used for deeper structures.

How is the image produced?

The image is produced in three steps _ producing the sound, receiving echoes, and then interpreting this echo. Arc-shaped sound waves are produced by a transducer, which travel into the body and come into focus at a desired depth. Newer piezoelectric transducers (usually made of ceramic) use techniques which allows the user to change the direction and depth of focus. As air is not the best conductor of sound waves, a water-based gel is placed between the patient’s skin and the face of the probe to enable the efficient transmission of the waves into the body. The sound wave is partially reflected from the layers between different tissues. Specifically, sound is reflected anywhere there are changes in density in the body, e.g. blood cells in blood plasma, small structures in organs, etc. Some of the reflections return to the transducer which causes vibrations, which are then turned into electrical pulses that travel to the ultrasound scanner where they are processed and transformed into a digital image, taking into account several factors such as the intensity of the echo, the time taken for the echo to be received and so on.

Where is it used?

Ultrasonography has now been accepted as a cost effective, non-ionising (therefore relatively harmless) imaging technique in most fields of medicine. It has a vast role in pregnancy related conditions where the fluid around the baby acts as an acoustic window to visualise the different structures of the baby and its development. Among the various field in medicine, ultrasonography has been harnessed in cardiology in the form of echocardiography as a non-invasive useful test or investigation. Gall bladder pathologies or kidney-related problems are mostly detected by this method. Although sensitivity and specificity of this imaging technique is not as high as computerised tomography (CT scan) or magnetic resonance imaging (MRI) yet this technique is well accepted as preliminary step for cancer screening.

Although relatively harmless, injudicious use of ultrasound may cause damage to the tissues by microvacuolation, heat generation, cavitation, microstreaming, etc. Therefore, the tests or imaging should be performed only where there is a genuine need for it and not indiscriminately. The investigation and interpretation is highly operator dependent. Therefore, advances in the technology and sophistication of the equipment must be accompanied by rigorous training for individuals in sonography. 3-D imaging and endoscopic ultrasonography are recent developments in this field. Further research and development in the field of medical physics and interpretation of the imaging in ultrasonography in the context of health problems will help improve the quality of care for patients.