In: Physics
Why is ultrasound energy impenetrable in air when it is propagating in an adjacent soft tissue medium? What are some harmful and beneficial functions of air in an ultrasound imaging exam.
Ultrasound waves are sound waves with frequencies over 20 Khz. The speed of sound in air is 330m/s wheras in body soft tissues it is 1540 m/s. The difference in the intensities of ultrasound occur at the interfernces of two medium due to the property called Accoustic Impedence denoted by Z.
The speeed and accoustic impedences are given by
Where, B = Bulk modulus
= density of medium
Z = accoustic impedence
The reflection coefficient is given by
where Z1 and Z2 are the accoustic impedence of the two mediums
A lesser value of means less intensity is reflected whereas larger value means large part of the intensity is reflected and less is transmitted. For example, if the value of is 0.04, it means 4% of the intensity is reflected and other is transmitted.
For air, Z1 = 429 kg/m2s
for body soft tissues, Z2 = 1.5X106 kg/m2s
Thus 99.89% of the intensity is reflected back in air wheras only 0.11% of the intensity is transmitted, Thus we can say that ultrasound are impentrable in air when it is propagating at the interference of the body tissue and air .
Ultrasound waves perform very poorly whenever there is air or gas between the transducer and the interested organ. As the ultrasound waves are reflected back in air, we cannot get the image of the organ of interest.