Journal of Clinical Neonatology

: 2020  |  Volume : 9  |  Issue : 3  |  Page : 157--161

Physics behind ultrasound – What should i know as a neonatologist?

Iyer Harohalli Venkatesh 
 Consultant Neonatologist, Department of Neonatology, Manipal Hospital, Bengaluru, Karnataka, India

Correspondence Address:
Dr. Iyer Harohalli Venkatesh
Manipal Hospital, Bengaluru, Karnataka


The bedside ultrasound is used in the care of sick neonates very often by the treating neonatologist as an extended physical examination. The basic physics involved in the ultrasound technology is very essential to understand the functioning of all the knobs used to derive both the structure and the physiology of the images. This review article covers the essentials of physics including sound, transducers, echogenicity, Doppler, and their interplay to acquire quality images.

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Venkatesh IH. Physics behind ultrasound – What should i know as a neonatologist?.J Clin Neonatol 2020;9:157-161

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Venkatesh IH. Physics behind ultrasound – What should i know as a neonatologist?. J Clin Neonatol [serial online] 2020 [cited 2020 Sep 30 ];9:157-161
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Sound is produced as a vibration when a wave passes through the medium – air, liquid, or solid. The human ear can hear the sound in the range of 20–20,000 Hz. The sound wave with a frequency more than the maximum human audible frequency is called ultrasound.[1] The sound propagates in a wavy fashion. The particles of sound come together to create compression and disperse farther away to produce rarefaction. The important properties of an ultrasound sound wave more often used in the interpretation of the images are the velocity, the frequency, and the wavelength [Figure 1]. The wavelength and the frequency are inversely proportional to each other. The neonates have a thin chest wall that requires more resolution than the penetration of the sound wave, and hence, the high-frequency probes are used. In pediatric and adult patients, the penetration of ultrasound waves is important, and hence, the lower-frequency probe is used [Figure 2] and [Figure 3]. The velocity is the product of the frequency and the wavelength. The wavelength is the distance between two peak points (point of compression) and is denoted by lambda. The time required to produce one complete wave is called a cycle. The number of wave cycles produced in 1 s is called the frequency and is measured in hertz. The velocity of sound is the speed at which the wave traverses and is the product of the frequency and the wavelength.{Figure 1}{Figure 2}{Figure 3}


Transducer is the main component in the ultrasound machine responsible for the formation and detection of images. The lead zirconate titanate is the Piezoelectric element in the ultrasound, which helps in the conversion of one energy to another electrical energy to mechanical energy and mechanical energy to electrical energy, helping the images to get captured on the cathode tube.[2],[3] The pure wave technology is another newer concept incorporated in the transducer. Here, the fine ceramic crystals are pure and arranged uniformly. They transfer energy with great precision and efficacy compared to conventional piezoelectric crystals. The footprint, the curvilinear probes are used commonly in neonates [Figure 4] and [Figure 5].{Figure 4}{Figure 5}

 Ultrasound Gel

The gel is made up of glycerine, propylene glycol, perfume dyes, and de-ionized water [Figure 6]. The bond between the gel over the skin and the transducer prevents the loss of energy and thereby helps in getting a clear image.{Figure 6}


It is the ability of the tissue to either transmit or reflect the ultrasound waves in the context of surrounding tissue.[4],[5],[6],[7] Depending on the echogenicity, the structure is either hyperechoic (bone), hypoechoic (grey matter), or anechoic (cerebrospinal fluid) [Figure 7], [Figure 8], [Figure 9].{Figure 7}{Figure 8}{Figure 9}

 Doppler Ultrasound

The utility of sound waves produced by ultrasound in detecting the movement of blood in the blood vessel is the basis for the Doppler ultrasound. The principle used is called the Doppler shift wherein there is a relative frequency shift of the received echoes of moving red blood cells [Figure 10]. When the transmitted frequency is less than the receiver frequency, the shift is called positive, and when the transmitted frequency is more than the receiver frequency, the shift is called negative. The difference between the two frequencies will determine the frequency shift.[8] The analogy is the moving ambulance and the standing person. The moving ambulance vehicle is comparable to the movement of blood, and the standing person is comparable to the transducer. Movement of the vehicle toward the standing person produces more frequency of sound and away from the person produces less frequency of sound.{Figure 10}

The Doppler ultrasound helps in determining the direction and velocity of the blood flow. The color Doppler, the pulse-wave (PW) Doppler, and the continuous-wave Doppler are the three types.

 Angle of Insonation

The angle that is produced between the tissue of interest with the ultrasound beam. The strong echo is produced when the angle of incidence approaches the angle of reflection [Figure 11]. The ultrasound beam should be parallel to the blood flow as much as possible.[9]{Figure 11}

Color Doppler

The sound waves get converted into different colors to determine the velocity and direction of blood flow. The red blood cell in the blood vessel is the moving object, and the probe placed on the surface is the stationary. The flow moving toward the probe will have more frequency than the flow that happens away from the probe. It is comparable to the moving ambulance toward and away from the stationary person. The principle called Blue Away Red Toward is often used to understand the direction of blood flow. The blood flow toward the transducer is Red and the flow away from the transducer is Blue. The maximum frequency that is required to reconstruct the signal is given by the Nyquist limit [Figure 12].[10]{Figure 12}

Pulse-wave Doppler

The same crystal in the transducer receives and transmits the signal intermittently. The ultrasound comes in pulses. Unlike the continuous-wave Doppler, this is site-specific and can not measure the high-velocity flow (>2 m/s) in view of the phenomena of aliasing. In the determination of the E/A ratio as a part of hemodynamically significant patent ductus arteriosus the PW Doppler at the mitral valve is performed [Figure 13].{Figure 13}

Continuous-wave Doppler

The piezoelectric crystals within the transducer work continuously emitting and transmitting the ultrasound wave. Here, two piezoelectric crystals are utilized. One is to receive the signal and the other one to send. The high-velocity flow can be determined as there are no phenomena of aliasing with continuous Doppler [Figure 14]. The site of origin of velocity cannot be determined at is not specific. This is used in the evaluation of right ventricular pressure using the tricuspid jet.{Figure 14}


The bedside ultrasound has become an integral part of the care of sick neonates. The understanding of physics behind the ultrasound is the foundation key before learning the anatomy and functions of the essential structures in the body, including heart, brain, lung, and kidneys, using ultrasound. This article encompasses the basic idea that one should master before getting into the depth of ultrasound knowledge.

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