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REVIEW ARTICLE
Year : 2020  |  Volume : 8  |  Issue : 3  |  Page : 105-109

Physics behind ultrasound; what should I know as a pediatric surgeon?


1 Department of Medical Physics, College of Applied Science, Umm Al-Qura University, Mecca, Saudi Arabia
2 Department of Surgery, Faculty of Medicine, Umm Al-Qura University, Mecca; Department of Surgery, King Faisal Specialist Hospital and Research Centre, Jeddah, Saudi Arabia

Date of Submission01-Apr-2021
Date of Acceptance12-May-2021
Date of Web Publication19-Jul-2021

Correspondence Address:
Dr. Omemh Abdullah Bawazeer
College of Applied Science, Umm Al-Qura University, Mecca
Saudi Arabia
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ssj.ssj_70_21

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  Abstract 

The bedside and intraoperative ultrasound are used frequently in infants and children by the treating surgeon as a part of physical examination. The fundamentals of ultrasound physics are essential for proper image interpretation and understanding common ultrasound artifacts. We aimed to cover the basic physics terminologies, ultrasound components, modes, and tissue echogenicity in this review article.

Keywords: Pediatric surgeon, physics, ultrasound


How to cite this article:
Bawazeer OA, Bawazir O. Physics behind ultrasound; what should I know as a pediatric surgeon?. Saudi Surg J 2020;8:105-9

How to cite this URL:
Bawazeer OA, Bawazir O. Physics behind ultrasound; what should I know as a pediatric surgeon?. Saudi Surg J [serial online] 2020 [cited 2021 Jul 30];8:105-9. Available from: https://www.saudisurgj.org/text.asp?2020/8/3/105/321732


  Introduction Top


Ultrasound is widely used in almost all clinical specialties. It was first used in clinical cardiology, and its use expanded to include other subspecialties. This imaging modality is readily available, easy to use, and has a low cost. In addition, it generates high-resolution images without exposing the patients to radiation risk. Thus, it is essential for acute care physicians to understand the basics of the physics of ultrasound. Physicians using ultrasound should be familiar with the generation of images and understand the limitations of ultrasound and its artifacts.


  Basic Terminologies Top


Sound frequency is the number of ultrasound waves per second. Humans can hear sounds of a frequency ranging from 20 to 20000 Hz. Sounds of higher frequency are called ultrasound.[1] Medical ultrasound devices produce sounds with frequencies ranging from 2 to 15 MHz.[2] Velocity of ultrasound results from the product of wavelength and frequency.[3] The wavelength is defined as the distance between two peak points, and it is inversely proportional to frequency. The cycle is the time required to produce one complete wave, and the number of cycles per second is the frequency, which is measured in Hertz [Figure 1].[3],[4]
Figure 1: Ultrasound basic physics

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High-frequency transducers are used in neonates as they have a thin chest wall that necessitates more resolution than penetration. On the contrary, in older children and adults, a lower frequency transducer is used because penetration is more important in this age group.


  Basic Physics Top


Ultrasound devices emit waves and receive their reflected echoes. Ultra-sonographers frequently use the brightness mode (B-mode), which is the basic ultrasonography mode.[2] The B-mode produces images that are two dimensional (2D), black and white, and have a slice thickness of less than 1 mm. The slices can be transverse, sagittal, oblique, or coronal. The ultrasound waves traverse through different tissues and reflect back to the transducer, producing images.[5] During the propagation of sound waves, they face resistance called acoustic impedance, which varies according to several factors, including the material's density. Ultrasound waves are reflected more from solid materials because of their dense particles.[6] Sound transmission is more in fluid media; consequently, fewer ultrasound waves are reflected from fluids. Therefore, liquids produce black images (echogenic), and bones produce white images; this is attributed to the difference in ultrasound reflection.

In addition, bones are not conductive to sound waves; therefore, a black acoustic shadow will be produced behind them. Similarly, air reflects ultrasound beams strongly and hinders the visualization of the structures behind.[7],[8] Sound travels through different tissues at various speeds, and this variability causes imaging artifacts[9],[10] [Figure 2].
Figure 2: The acoustic impedance with various tissue echogenicity (the higher reflection of ultrasound as material density increases)

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  Generation of Ultrasound Top


The ultrasound transducer has piezoelectric crystals that change electrical signals to mechanical vibrations and emit sound waves.[2],[11] In addition, these crystals produce electric signals when mechanically compressed. Crystals are arranged in arrays adjacent to each other and are electrically connected. Rapid alternating current applied to the crystals generates vibration and emits ultrasound. The transmission period is concise, ranges from 0.5 to 3 μs, and followed by a “receiver phase.” In the receiver phase, the piezoelectric crystals are stimulated by the returning sound waves, which are converted to electric signals [Figure 3]. The duration of this phase is longer than the transmission phase and can reach up to 1 ms. The pulse repetition period is the combined duration of both phases; it is shorter with a shallower depth because of the shortened receiver phase.[1],[12]
Figure 3: The basic concept of piezoelectric transducer function

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  Ultrasound Instrumentation Top


Transducer

Ultrasound wave emission and detection are performed by the transducer, which is the main component of ultrasound devices.[13],[14] The transducer has piezoelectric crystals that convert electrical energy to mechanical energy and vice versa. New transducers have incorporated new pure wave technology, which uses fine ceramic crystals and transfers energy with great precision and efficacy compared to conventional crystals.[15]

Ultrasound gel

The ultrasound gel is an essential component of the ultrasound machine. It prevents energy loss; therefore, it helps to get a clear image.[16]


  Interaction of Ultrasound with Tissues Top


Images are produced through the interaction between ultrasound waves and tissues. There are several types of interactions, and it is essential to be aware of these types so as to better understand image artifacts. Reflection occurs when the ultrasound hits the interface between two mediums, and part of the waves are reflected back to the probe. Several factors affect the number of reflected waves, including the acoustic properties and density of the two mediums. It is difficult to image through the overlaying lung or pneumothorax because of the significant difference in density between the tissue and air. Reflection is also affected by the angle between the tissue and the ultrasound wave (angle of insonation) [Figure 4].[17],[18] When the angle is orthogonal, maximal reflection occurs.
Figure 4: Illustration of angle of insonation with maximum reflection

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An example of this is the imaging of interventricular septum from the four-chamber view. This view could give a false impression of a septal defect because of little reflection from the surrounding structures in-line with the ultrasound wave. Therefore, the interventricular septum should be visualized from the subcostal or parasternal views.

Another interaction that may occur between the waves and tissues is scattering. Scattering occurs when the sound wave hits a smaller structure than the wavelength of the wave. This interaction results in the scattering of the beam in all directions with the loss of most of the signals. However, scattering has a role in generating two-dimensional images, and scatter patterns are characteristics of each organ. Bright areas inside an organ indicate increased scattering. Refraction occurs when the ultrasound waves travel through different mediums with different propagation speeds. The difference in the propagation speed and the angle between the wave and the surface affect the degree of bending. Refraction artifacts may cause objects to appear in altered locations.


  Ultrasound Images Mode Top


B-mode

The B-mode is a 2D image produced by simultaneously scanning an area with a linear array rather than a single one as in A-mode. The returning waves are converted into several dots with varying brightness. The dimensions of the images represent the actual tissue distances, and the intensity of the gray scale is the echo strength. The B-mode is the most common mode used in regional anesthesia and is used to study a specific area's cross-section [Figure 5].
Figure 5: B-mode of the ultrasound

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Doppler mode

This term describes the change in sound's wavelength because of the relative movement between the ultrasound source and the receiver. If the sound waves move toward the receiver, a positive Doppler shift is produced, in which the sound waves are squeezed, and a higher-pitch sound occurs. On the other hand, a negative Doppler shift occurs if the sound waves move away from the receiver, resulting in the stretching of sound waves and low pitch sound. The angle between the emitted sound and the moving structures affects the magnitude of Doppler shift. No Doppler shift occurs with perpendicular angles, and the largest Doppler shift occurs with an angle between 0° and 180°.[19],[20]

Colour Doppler is a method of visually detecting motion or blood flow. Colour Doppler consist of colour mapping of doppler shifts superimposed on B-mode image. The movement of the ultrasound waves toward or away from the receiver produces the color. When the flow is toward the transducer, a red color is produced, and the blue color indicates a flow away from the probe. Therefore, the produced color gives information about the velocity and direction of the blood flow.[21]

There are several applications of color Doppler in clinical practice. Color Doppler can be used in peripheral nerve block under ultrasound guidance to identify the presence and nature of blood vessels. Power Doppler is more sensitive than color Doppler; in addition, it is less affected by the scanning angle. Power Doppler is used to detecting smaller blood vessels, but it does not provide information about the velocity and direction of blood flow.

In pulse-wave Doppler, the transducer emits and receives signals intermittently. Pulse-wave Doppler is site specific and cannot measure the high-velocity flow. In continuous-wave Doppler, the transducer continuously emits and receives the ultrasound waves. Two crystals in the transducer are utilized simultaneously; one is used to emit waves, and the second is used to receive it. Continuous-wave Doppler can determine the high-velocity flow because there are no aliasing phenomena with continuous Doppler. An example of an application of the continuous flow Doppler is the evaluation of ventricular pressure using the tricuspid valve jet.

M-mode

This mode is used to detect the movement structure of signal, producing a picture of waves such as imaging the cardiac valves. M-mode is used in cardiac imaging, but it is not frequently used in local anesthesia.


  Controlling Ultrasound Waves Top


There are several ways to control ultrasound emission from transducers. Sounds can be transmitted in a continuous or interrupted form. An interrupted sound emission produces B-mode, and Doppler mode results from continuous sound emission.

The quality and resolution of the images and the degree of penetration can be controlled by changing the waves' frequency. The resolution increases with higher frequency, but the depth of penetration decreases and vice versa. Increasing the distance and frequencies result in greater attenuation. Therefore, we use low frequencies for the examination of deep structures and obese patients. High-frequency probes are used for the examination of superficial structures.

Gain can be changed, and consequently, the image quality can be changed as well. Increasing the gain amplifies the received ultrasound signal and produces white images. On the other hand, decreasing gain masks the details and yields black images.[8] The gain factor can be changed using time gain compensation, which displays structures with the same brightness if they are equally reflective regardless of their depth.

The transducer emits ultrasound waves perpendicularly. Bending the transducer widens the sonographic field. Flat transducer surface yields parallel waves, and this is called linear array transducer. These linear array transducers have good resolution, high frequency, and less penetration. In addition, linear array transducer produces rectangular images, and convex array transducer images are wider and have more depth. Transducer with a reduced surface with fan-shaped sectors allows the visualization of thoracic structure between the ribs.[22]


  Echogenicity Top


Echogenicity is the ability of tissues to transmit or reflect the sound waves.[23] Based on this character, tissues are classified as hyperechoic such as bones, hypoechoic tissues such as the gray matter, or anechoic tissues such as cerebrospinal fluid. Anechoic structures appear dark or black, hypoechoic is darker than the surrounding structures, and hyperechoic is whiter compared to the adjacent tissues. Isoechoic term refers to a lesion with similar echogenicity to the surrounding tissues [Figure 2].


  Conclusion Top


Understanding the bedside ultrasonography became an essential part of training physicians caring for infants and children. Physicians using ultrasound should be familiar with how images are generated and understand the ultrasound and its artifacts' limitations. This article reviewed the basic physics essential for pediatric surgeons using bedside ultrasound.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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Rahman A, Zhou YQ, Yee Y, Dazai J, Cahill LS, Kingdom J, et al. Ultrasound detection of altered placental vascular morphology based on hemodynamic pulse wave reflection. Am J Physiol Heart Circ Physiol 2017;312:H1021-9.  Back to cited text no. 17
    
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Malaiapan Y, Leung M, White AJ. The role of intravascular ultrasound in percutaneous coronary intervention of complex coronary lesions. Cardiovasc Diagn Ther 2020;10:1371-88.  Back to cited text no. 18
    
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Gill RW, Kossoff MB, Kossoff G, Griffiths KA. New class of pulsed Doppler US ambiguity at short ranges. Radiology 1989;173:272-5.  Back to cited text no. 19
    
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Hamelmann P, Vullings R, Kolen AF, Bergmans JW, van Laar JO, Tortoli P, et al. Doppler ultrasound technology for fetal heart rate monitoring: A review. IEEE Trans Ultrason Ferroelectr Freq Control 2020;67:226-38.  Back to cited text no. 20
    
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Tanoue H, Hagiwara Y, Kobayashi K, Saijo Y. Echogenicity in transrectal ultrasound is determined by sound speed of prostate tissue components. Annu Int Conf IEEE Eng Med Biol Soc 2012;2012:460-3.  Back to cited text no. 23
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]



 

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  In this article
Abstract
Introduction
Basic Terminologies
Basic Physics
Generation of Ul...
Ultrasound Instr...
Interaction of U...
Ultrasound Image...
Controlling Ultr...
Echogenicity
Conclusion
References
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