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Unit 3
Questions 13 - 17
Ultrasound is now one of the key methods
used in medicine for exploring the internal structure of the body. Medical
ultrasonics works by sending pulses of ultrasound into the body and then
monitoring the returning echoes from different boundaries within the body.
This principle relies on a particular property of the different tissues
in the body called the specific acoustic impedance. This can be calculated for
a material by multiplying the speed of sound in the material by the density of
the material. When ultrasound reaches a boundary between tissues with two
different specific acoustic impedances a small proportion of the ultrasound is
reflected. This reflection is then analysed to produce an image. The following
table gives some specific acoustic impedances
| Tissue or Medium |
Specific acoustic impedance |
| Air |
0.00044 x 106 |
| Fat |
1.38 x 106 |
| Water |
1.5 x 106 |
| Brain |
1.58 x 106 |
| Blood |
1.59 x 106 |
| Muscle |
1.6 x 106 |
| Bone |
6 x 106 |
Question 13
What are the units
of specific acoustic impedance
A kgm3s-1
B kgm-2s-1
C
kgm-3s-1
D kgm2s-1
Question 14
The strength of the reflection at a
boundary is primarily determined by the difference in acoustic impedances, the
greater the difference the stronger the signal. For which of the following
boundaries would the strongest reflection be seen
A brain-blood
B water-brain
C blood-bone
D fat-blood
In fact the ratio of the sound intensity reflected Ir to the sound
intensity initially falling on the boundary Ii is termed the intensity
reflection coefficient a. This value depends on the two specific acoustic
impedances of the materials Z1 and Z2. It is given as
a = Ir / Ii =
(Z2-Z1)2 /
(Z2+Z1)2
Question 15
For the boundary between blood and muscle a would have a value of roughly
A 1.1 x 103
B 11
C 1.1 x
10-3
D 1.1 x 10-6
Question 16
And a will have a value closest to 1 for which of the following
boundaries
A water-brain
B blood-muscle
C
water-muscle
D blood-bone
Question 17
The
speed of ultrasound in the tissues mentioned is generally around
1500ms-1. The frequency used is of the order of 2MHz. What would be
the wavelength of these sound waves
A 7.5 mm
B 4.5 x
10-2mm
C 4.5 x 10-6mm
D 7.5 x
10-8mm
SECTION
ANSWERS
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