Question

 

I. Explain the term acoustic impedance and state the tissue properties which determine its value.

II. A medium attenuates ultrasound at a rate of 0.7 dB cm −1 MHz −1 . A target at a depth of 5 cm below the transducer is imaged with a 3 MHz ultrasound pulse. By how many dB will the echo from the target be attenuated compared to a similar target at a depth of 1 cm?

III. Explain why bone and gas limit the areas of clinical application of ultrasound.

Answer 1

Accoustic imbedance can be defined as , The opposition made by the system to the accoustic flow resulting from an applied accoustic pressure.

Simply saying it is the resistance an ultrasound encounters when it passes through a tissue

Accoustic impedance depends on

1. Density of the tissue d
2. Speed of the sound wave c

If density of the tissue increases accoustic impedance increases

PART B) Attenuation coefficient

the equation relating attenuation constant and the intensity of sound waves is

where is the intensity at a distance x

is the incidenting intensity at the surface of the tissue

is the attenuation constant

intensity of the wave at 5 cm depth is

The echo due to reflection from a particular interface is expressed as the ratio of reflected wave amplitude to incident wave amplitude

P is the pressure amplitudes and Z is the acoustic impedance

Now after reflection only 0.03 times the initial intensity is remaining if it is reflecting from that surface it agian get attenuated

where d is the length and f is the frequency

here Io was 3MHz now after reflection Io reduced to 0.03Io SO

If the target wasat 1cm depth

so the reflecting intensity is 0.49 Io

attenuation

PART C)

Due to the accoustic impedance provided by bone tissues penetrating through bones is limited.

Due to the abrubt change in the impedance provided by air between organ of interest. For example tissues of gastrointestinal tract , the gas inside the tract and the tissues will provide different accoustic impedance