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Medical Ultrasound for A-Level Physics
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Medical Ultrasound for A-Level Physics
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Sound waves are mechanical longitudinal waves. Ultrasound is very high frequencysound, with frequencies above 20000 Hz, that is, beyond the range of human
audibility, which is roughly in the range 20 Hz – 20000 Hz.
Sonar Principle
Whenever a sound wave passes from one medium to another in which the speed of
sound is different, refraction and reflection will take place. The reflected wave
bounces back, while the refracted wave passes into the second medium. The extent
through which a wave reflects back rather than passing through is determined by the
change in wave speed, which depends on the acoustic impedance (Z) of the second
medium.
The acoustic impedance of a medium depends on the density of the medium and on
the speed of sound in it.
Z c f
Sound is reflected at the boundaries between materials that have different acoustic
impedances. The ratio of the intensity of the reflected sound to the intensity of the
incident sound is the intensity reflection coefficient ().
2 2
Z Z1 I Z Z2
2 r 1
Z2 Z1 Ii Z1 Z2
If Z1=Z2, the media are impedance matched and no sound wave is reflected. When
Z1 and Z2 are very different, most of the incident energy is reflected.
, Reflected energy
Material Z (kg m-2 s-1) Interface as a % of
incident energy
Air 4.39×102 Air/Soft tissue 0.99 99
Bone 6.0×106 Air/Bone 1 100
Soft tissue 1.63×106
Fat 1.38×106 Fat/Muscle 0.01 1
Muscle 1.7×106
Brain 1.58×106 Blood/Brain 9.9×10-6 9.9×10-4
Blood 1.59×106 Blood/Bone 1 100
Skull bone 7.78×106
Water 1.5×106
Ultrasound for Diagnostics
Ultrasound is used for exploring the body – to monitor the growth, development and
general health of a baby in its mother’s womb and also to examine the health of soft
tissue organs such as the bladder.
Ultrasound for Therapy
Ultrasound is used to treat certain medical conditions. It can help some types of sore
tissue to heal rapidly and can also destroy gallstones and kidney stones.
Principle of Ultrasound Scanning
Medical ultrasonics uses a probe to send out pulses of ultrasound and receives the
echoes from each boundary within the body. A short pulse with the frequency of a
few MHz is applied to a transducer in the probe. This transmits a pulse of sound
waves and receives the reflections. The time interval between the emission of the
pulse and its returning to the probe indicates the distance of the interface from
where it got reflected.
2d
t
v where d is the distance between the probe and the interface, v is the speed
of sound in the medium and t is the time taken for the wave to return to the probe.
This is the sonar principle. By using a computer processor and other analysing
devices, the reflected pulses are used to produce a two dimensional image of the
different layers.
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