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Ultrasound is an important secure method of scanning especially when a patient is too sensitive
towards radiation e
...
Here we discuss in detail how ultrasound is generated and used in imaging
Principles of Generation of Ultrasound: Ultrasound can be produced using piezoelectric
transducer
...
Two opposite sides of the
crystal are created with the layers of silver to act as electrical contacts as illustrated in following

Two dimensional representation of
quartzcrystal
Thin layersof silver

diagram

Ultrasound Scanning

By: Umair Qureshi

Physics9702/ A2

2

Quartz has a complex structure, made up of a large number of repeating tetrahedral silicate units
as illustrated in following diagram

Negatively
charged oxygen
ions

Positively charged silicon ion

The positions of
the oxygen links
are not rigidly
fixed in these units or lattice and since the oxygen ions are negatively charged, the movement
can be encouraged by applying an electric field
...
e across the layers of silver), then positive
silicon ions are attracted towards the cathode and negative oxygen ions are attracted towards
anode
...
Depending on polarity of the applied voltage,
the crystal structure becomes thinner or thicker as a result of altered charge distribution
...
An alternating voltage applied across
the silver electrodes will setup mechanical vibrations in the crystal
...
When applied frequency is
equal to the natural frequency of vibration of transducer disc, the disc vibrates at resonance and
Ultrasound Scanning

By: Umair Qureshi

Physics9702/ A2

4

creates sound waves at the same frequency as the alternation potential difference in the
surrounding medium
...
An absorber block behind the disc prevents ultrasound waves created at the two surfaces
of the disc cancelling each other out
...
The backing block material is poxy
resin which damps the disc vibrations rapidly at the end of each pulse before next pulse is
produced
...
A simple diagram shows
how the reflected pulses are received
...
The incidence of these two variant zones, give rise to piezoelectricity and
a variable voltage (AC) is generated across the layers of silver
...

A variety of ultrasound is used according to the nature of diagnostics

Important Terminologies:

Ultrasound Scanning

By: Umair Qureshi

Physics9702/ A2

6

In order to be able to explain the principles of the use of ultrasound in diagnosis, it is necessary
to have an understanding of the reflection of ultrasound at boundaries and its absorption in
medium
...
When an ultrasound wave meets the boundary between two different media,
some of the energy (intensity) is reflected and some is transmitted as shown below
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There is an important feature of the medium
which majorly contributes in reflection of ultrasound called acoustic impedance
Specific Acoustic Impedance (Z):
In simple words it is the resistance of a medium towards transmission of ultrasound
...


Ir ( Z2- Z1)2
∝= =
Ii ( Z2+ Z1)2
Coupling Medium:
The intensity reflection coefficient is very large for entering and leaving the human body
(boundary between air and soft tissue)
...
This is achieved by means of a coupling medium such as a gel
that fills any spaces between the transducer and skin
...


Ultrasound Scanning

By: Umair Qureshi

Physics9702/ A2

8

x

I0

IT

Following expression is used to calculate transmitted intensity

IT= Ioe-kx
Where ‘k’is known as absorption coefficient
...

Io

How to solve questions involving multiple reflections and transmission?
It is observed that students get confused when they asked to calculated final reflected intensity
received at probe after multiple reflections and absorptions
...

Consider the following case when incident intensity Iopasses through a medium consisting of
layer 1 and layer 2 of two different acoustic impedance Z1 and Z2 respectively
...

We also know one more thing; that when ultrasound passes through certain medium, some of its
intensity is absorbed and the rest is transmitted to next layer
...
If you use the
values of k and x, you can get the ration of transmitted and incident intensity which is called
intensity absorption coefficient given below
β=

IT
Io

So now we are able to figure out how much intensity is received in the start at boundary B and
how much is reflected back
...
The gel reduces size of impedance
between boundaries at the skin and thus reduces reflection at the skin
...
These pulses are partly reflected and partly transmitted at
boundaries between medium in the body (e
...
The reflected pulses return
to the transducer where they are detected and transferred to the computer as voltage pulses
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Two types of scans are in common use
for the display of ultrasound scan
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A short burst of ultrasound is transmitted to
the body through the coupling medium
...
The reflected pulse is picked up by the
transducer which now acts as a receiver
...
The reflected pulse meets boundaries as it returns to the transducer
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Consequently echoes from deeper in the body tend to be lower intensity
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A
vertical line appears on the screen each time an echo is received
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The distance
between boundaries can be calculated if the speed of ultrasound in the various media is known
...


B-Scan: The B-scan technique basically combines a series of A-scans, taken from a range of
different angles, to reform a 2-dimesional picture
...

The ultrasound probe for a B-scan consists of a series of small crystal, each having a slightly
different orientation
...
Each reflected pulse is shown as a bright spot in the correct orientation of the crystal
on the screen of a CRO
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This image may be photographed or stored in the computer memory
...
The main advantage of the ultrasound scanning is that the health risk factors are
very low compared to X-rays imaging
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With higher frequencies, smaller features within the body can be identified
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Ultrasound Scanning

By: Umair Qureshi

Physics9702/ A2

1

Questions on Ultrasound
1
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The table gives values of the acoustic impedance for air and soft

tissue
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(ii)
Explain why air between an ultrasound probe and the skin (soft tissue) of a person
prevents ultrasound diagnosis taking place
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2
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How ultrasound may be produced
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Ultrasound Scanning

By: Umair Qureshi

Physics9702/ A2

1

Calculate the acoustic impedance of bone and enter your answer in the table above
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(a) Explain the main principles behind the use of ultrasound to obtain diagnostic information about
internal body structures
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(ii) An ultrasound transmitter emits a pulse
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[Nov 13/ Paper 4/ CIE]

Ultrasound Scanning

By: Umair Qureshi

Physics9702/ A2

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Answers

1
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Ultrasound Scanning

By: Umair Qureshi

Physics9702/ A2

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