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13 August, 07:41

A beam of 1.0 MHz ultrasound begins with an intensity of 1000 W/m². After traveling 12 cm through tissue with no significant reflection, the intensity is about

A. 750 W/m²

B. 500 W/m²

C. 250 W/m²

D. 125 W/m²

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Answers (1)
  1. 13 August, 08:05
    0
    Option C

    Explanation:

    Given:

    - Depth of tissue d = 12 cm

    - frequency of ultrasound f = 1 MHz

    - Input Intensity I_i = 1000 W/m^2

    - attenuation coefficient soft tissue a = 0.54

    Find:

    - Out-put intensity at the required depth

    Solution

    - The amount of attenuation in (dB) with the progression of depth is given by:

    Attenuation = a*f*d

    Attenuation = 0.54*12*1

    Attenuation = 6.48 dB

    - The relation with attenuation and ratio of input and output intensity is given by:

    Attenuation = 10*log_10 (I_i / I_o)

    6.48 dB = 10*log_10 (I_i / I_o)

    I_i / I_o = 10^ (0.648)

    I_o = 1000 / 10^ (0.648)

    I_o = 225 W/m^2

    - Hence the answer is option C: I_o = 250 W/m^2
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