Sonography and Impendence

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Impendence in sonography refers to the resistance that ultrasound encounters while operating on tissues. Z is an objective characteristic of tissue. Impedance is affected by a number of variables, including tissue density (kg/m3) and sound wave speed (c) (in m/s) (Hedrick, 2012). Equation relates these elements:

Z = d x c

The impendence and tissue density are inversely correlated. The impendence is similarly affected by sound speed, however the effect is less obvious. When a scan is to be performed at the interface between several types of tissues, impedance effects in sonography are apparent. The ability of the ultrasound wave to penetrate from one type of tissue to another is highly dependent on the difference in their impedance. When the difference is large, then the sound is reflected. The reflection is grasped at a macroscopic level intuitively (Kremkau, 2011). The reflection can be compared to an echo when one has to yell into a canyon. The sound wave is reflected when it comes across a dense medium and is expected to reverberates back to the source. As sound, do not just pass through the rock due to a large difference in the impendence of both media. A similar situation occurs in sonography when ultrasound beam passes through muscle tissues then encounter a bone, it will reflects off from the bone as the difference in density between the bone and the tissues is high (Kremkau, 2011). The amount of reflection that occurs in a perpendicular direction is expressed by

Where Z1 and Z2 represent the impedance in tissue 1 and tissue 2, respectively

Examples of impedance for bodily tissues (in kg/m2s)

Bodily tissue

Impendence (kg/m2s) (106)



















Sonography depends heavily on the principle of echo such as in ultrasound. The reflected wave is used to produce the image that is then applied for medical purposes. For instance, if the aim is to detect any form of internal fracture, the image produces will be produce as different response waveforms (Narouze, 2011). The area with an internal fracture will be seen as an interface between two tissues. They are further used to identify abnormal growths inside the body such as in the treatment of cancer. The abnormal growths are seen as changes in the wave penetration into tissues. The growth and the normal tissue have different impendence leading to reflection of wave if the growths have a higher density as compared to the tissue.

For a homogeneous tissue without either fracture, growth or presence of a foreign object, the distance will be uniform as the tissue is assumed to have same density. The detector, which in the case of sonography is the imaging tool, will give a uniform response as the echo is produced at the same distance (Narouze, 2011). In case of discontinuities such as the presence of growth, echo will be discontinuous and the imaging device will show an existence of interfacing tissues. Impendence is also applied to enhance study of body internal structure. Since body is constituted of parts with different impedance due to difference in their densities, it is easy to study the structure of certain part (Narouze, 2011). For instance, if the study were directed towards the internal parts of the skull or skull fracture, it would be easy to use the echo application. The wave is passed over the skull and the receiver will display its continuity.

Most of the current technological treatments have been made possible through application of the impendence in sonography. In the above figure, it is the representation of the sonographer machine. The display is connected to the imaging unit and prints the behavior of the echo as received from the tissue. The transducer produces the required wave through the specimen. The transducer then receives the reflected echo from the specimen (Kremkau, 2011). Often, the echo is produced when the transfer media is changing or there is a disjoint in the internal part of the specimen. This echo is converted into a necessary energy form such as waveform, electrical pulse, or current. The display may present this information as an image when connected to the imaging unit of as a waveform when connected to an electronic device (Hedrick, 2012). In this case, the impendence of the specimen and air is different. Since the wave will not necessarily penetrate through the second medium, some of the energy is reflected back. This forms the largest part of impendence application.

The difference in the impedance between air and skin it is important to use a better separating medium between the probe and the skin. Such a medium is oil smeared on the skin and the probe operated with a continuous contact with the skin. As much as possible, air bubbles should not be allowed to come between the probe and the skin. Since the body tissues have slightly different impendence, there will be slight fluctuations (Hedrick, 2012). Therefore, it will be possible to reproduce the image of the external and internal issues.


Hedrick, Wayne R. 2012. Technology for Diagnostic Sonography. London: Elsevier Health Sciences.

Kremkau, Frederick W., Flemming Forsberg, and Frederick W. Kremkau. 2011. Sonography principles and instruments. St. Louis, Mo: Elsevier/Saunders.

Samer N. Narouze. 2011. Atlas of Ultrasound-Guided Procedures in Interventional Pain Management. Springer Science+Business Media, LLC.

April 13, 2023




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Sound Density Theory

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