Physics MJ 42/17 Q4.(a)

 Q. Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures. [6]

Ans: (media = medium plural)

• When a potential difference is applied to a piezoelectric crystal, it produces an ultrasound pulse. (f = 1 to 15 MHz).
• A gel (or coupling medium) is used to fill the air gap between the probe (piezoelectric crystal) and the skin. (Air would reflect back nearly 100% of the ultrasound. The impedance of air is much less than the impedance of skin. z = Acoustic impedance = Density material x speed of sound in material. The greater the difference in z values between 2 media, the less the transmission. All ultrasound gets reflected.)
• The pulse now enters the internal tissues, muscles, etc. It reflects back between the boundaries of media due to the difference in acoustic impedance in media.
• The reflected pulse is received by the ultrasound transmitter. This reflected pulse is displayed and processed.
• The intensity of the reflected pulse gives information about the type of boundary.
• The time delay between sending the pulse and receiving it gives the information on the depth of the boundary.