Using Ultrasound for Medical Imaging

In medical imaging, ultrasound is used mainly to detect the shape of a foetus. Ultrasound is also used for imaging the internal of a body. Since the wavelength of an ultrasound is in the order of about 1 cm, it cannot be used to detect structures that are small. Ultrasound is most effective with structures like the bone, or internal organs and foetus.

Principle of Using Ultrasound for Medical Imaging

The principle used in medical imaging is called echo sounding. Ultrasound is directed into the body. Since different materials have different acoustic impedance, when an ultrasound is incident on a boundary of two materials, there would be a reflected component of the ultrasound. This reflected component would then be received by the detector and used to compose the image of the reflected surfaces in the body.

Acoustic Impedance

Acoustic impedance is defined as the product of the density of medium and the speed of sound in the medium.

Mathematically,

 Z= \rho \times c

The unit of Z would be \text{kg m}^{-2}\text{ s}^{-1}

The meaning of acoustic impedance is how good the sound wave travels in that medium. Large value means the sound wave travels better. The acoustic impedance of some common materials are listed below:

Acoustic impedance of some common materials in the body
Acoustic impedance of some common materials in the body

There will be significant reflection of the ultrasound waves at boundaries of two materials that have widely different acoustic impedance. For example, significant reflection occurs between blood and done boundaries. Hence, we can use ultrasound to investigate the shape of the bone. This would not be a good way to investigate the structure of muscle because muscle and blood have similar acoustic impedance.

Use of Gel in Ultrasonic Scanning

When a pregnant woman performs an ultrasound scan, the doctor would always smear a layer of gel onto her abdomen before performing the scan. The boundary between air and the skin causes significant reflection. The acoustic impedance of air is 0.0004 but skin has an acoustic impedance  of about 1.6. Most of the incident ultrasonic waves would be reflected, leaving very little waves to penetrate the body to be reflected by the other internal structures of the body. Using gel removes the air medium such that the boundary would be between the gel and the skin, which have very similar acoustic impedance. Smearing of gel onto the skin is known as impedance matching.

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