Medical Equipment
MASH 203
Ultrasound Equipment (US)
The Physics of Ultrasound
The Principles of Ultrasound Diagnostic Equipment
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
The Physics of Ultrasound
The term Ultrasound refers to acoustical waves above
the range of human hearing (frequencies higher than 20 kHz).
Ultrasound waves
are vibrations or disturbances
consisting of alternating zones of compression and
rarefaction in physical medium such as gas, liquid, or
solid matter.
The following will be studied:
- Frequency
- Velocity
- Generation
- Reflection
- Beam pattern
- Resolution
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Ultrasound Frequency
Humans can hear only a limited rang of frequencies (Audible spectrum).
The frequency range of audible sound is approximately 20 Hz to 20 kHz.
Frequencies lower than 20 Hz are Infrasound and frequencies higher than 20
kHz are called Ultrasound.
Diagnostic ultrasound equipments used ultrasound frequencies in the range of
approximately 1 MHz to 15 MHz.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Velocity of Ultrasound Waves
Velocity of ultrasound waves to each material of human body
Material
Velocity m/sec
Air
330
Blood
1570
Brain
1540
Fat
1450
Soft tissue
1540
Kidney
1560
Liver
1550
Muscle
1580
Skull bone
4080
Water
1480
The average velocity in human body is about 1530 m/sec, this
average velocity is significant in the study of equipment principles.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Generation of Ultrasound Waves
Phenomena of Piezo-Electric Effect

If we apply voltage in pulses the transducer will expand and
contract.

By applying this voltage in very quick pulses we can vibrate the
transducer and produce the ultrasound wave.

If any external forces is applied to the transducer a voltage is
generated.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Voltage-Ultrasound Conversion
In the practice, high voltage pulses of about 100 V are applied
on the transducer.
The transducer converts the voltage pulses into the ultrasound
waves, which may be transmitted into the human body.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Ultrasound-Voltage Conversion
When the ultrasound waves reach an object they are reflected
back to the transducer.
The reflection echoes are then converted by the transducer
into voltage to be used as reception signals.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Voltage-Ultrasound Conversion
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Ultrasound Reflection

Ultrasound waves transmitted in the human body.

At the border of different structures some of the transmitted waves
are reflected back according to the acoustic impedance different
between two structures.

Acoustic impedance:
Z=ρ*C
o Z: Acoustic impedance
o ρ: Density
o C: Velocity
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Acoustic Impedance
Acoustic impedance of each material in the human body
Material
Acoustic impedance
Air
0.00043
Blood
1.62
Brain
1.6
Fat
1.38
Soft tissue
1.6
Kidney
1.62
Liver
1.66
Muscle
1.7
Skull bone
7.8
Water
1.52
The average impedance in human body is about 1.6.
Acoustic impedance of each material is different from each other
and that is why US waves are reflected from the different
structures.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Ultrasound Beam Pattern

As it shown above, if there is no acoustic lens on the transducer
surface, the ultrasound beam is transmitted in a straight line in the
near field then begins divert from a certain point in the far field as
shown above. This is called an unfocused beam.

If there is an acoustic lens on the transducer surface, the ultrasound
beam will be focused and convert as a certain point. This is called
the focal point.

Generally, the focused beam can give a clearer image.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Resolution

There are two major parameters, which influence the diagnostic
image quality these are axial and lateral resolution.

Resolution can be defined as the ability of the equipment to
distinguish two separate objects.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Axial Resolution

Axial resolutions describe resolution in the axial direction.

Axial resolution is determined by the US wave length and
frequency.

If the US beam uses a short wave length as is shown above we can
distinguish two separate objects which are based closed together in
line in axial direction. This is because wave length of the US beam
is shorter than the distance between the two objects.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Axial Resolution

If the US beam uses a longer wave length as shown below, we
cannot distinguish two separate objects, this is because the wave
length is longer than the distance between the two objects.

Because a higher frequency means a shorter wave length, we can
say generally that: the higher the US frequency, the higher the
axial resolution.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Lateral Resolution

Lateral resolution describes resolution in the lateral direction.

If the US beam is wide, as shown in the top figure, the displayed
object size may be bigger than the actual object size, so lateral
resolution is poor.

If the US beam is narrow the displayed object size will be a closer
to the actual size, so lateral resolution is better.

Generally, the narrow the US beam, the higher the lateral
resolution.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Relationship between Frequency,
Resolution and Penetration

The table below shows the relationship between frequency,
resolution and penetration.

In general, the higher the US frequency the higher the resolution
and the lower the penetration.

Conversely, the lower the U.S frequency the lower the resolution
but the higher the penetration.

To determine a wide range of diagnostic requirements we need a
wide range of US frequencies.

That is why there is a wide selection of probe frequency for studies
of different organs.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
The Principles of Ultrasound
Diagnostic Equipment
Ultrasound Equipment
US diagnostic equipment uses a pulse
reflection system
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
The Principles of Ultrasound
Diagnostic Equipment

Pulse generator: A pulse circuit generates a high voltage pulses,
then the T-delay line circuit applies appropriate delay time for each
transmitted pulse for electronic focusing of the US beam.

When the US probe touches the patient skin surface, US waves are
transmitted to the patient.

The delayed high voltage pulses are applied to the piezo-electric
transducer elements.

They produce the US waves, which will entire the patient, and are
reflected back by the intern structures.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
The Principles of Ultrasound
Diagnostic Equipment

The reflected echo signals are received by the same probe.

Then they are input into the R-delay line circuit to compensate for
the transmission delay factor of the pulses and mix the echo signals.

Then the receiver circuit amplifies the mixed echo signal from the
R-delay line.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
The Principles of Ultrasound
Diagnostic Equipment

In new equipment the echo signals are produced using a Digital
Scan Converter D.S.C.

So that the image data is thought enter the frame menu and read
sequentially out in a format suitable for display on TV-monitor. The
result is real time imaging.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Absorption
The absorption of MHz US waves in each material
Material
Absorption at 1 MHz
dB/cm
Air
12
Blood
0.2
Brain
0.9
Fat
0.6
Soft tissue
0.8
Kidney
1
Liver
0.9
Muscle
2.3
Skull bone
13
Water
0.002
Absorption is measured in decibels per centimeters (dB/cm).
The absorption in air and skull bone is very high.
If absorption is very high, the US waves cannot pass through.
As a general rule US diagnosis is not restructures containing gas or
structures hidden behind bon such as lung, stomach and brain.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Relationship between Depth and
Time

The average velocity of US in the human body is about
1530 m/sec.

At this velocity an US wave will take about 6.5 µsec to travel from
the probe transducer to a target 1 cm below the skin as shown.

An additional 6.5 µsec is required for the US wave to return to the
probe transducer.

As a result the total time taken for the US wave to be transmitted
and received to and from a target, which is 1 cm from the
transducer, is about 13 µsec as shown.
1sec :1530 m  X :1cm 
X
Dr. Mohammed Fayez Al Rez
1
 6.5  sec
1530 100 cm
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Ultrasound Equipment - Unit 3
Relationship between Depth and
Time

The US transmission timing and reception period are determined by
rate pulse.

The rate pulse allows a reception period of up to 260 µsec. This 260
µsec translate to distance of about 20 cm depth in the human body.

Generally, the maximum scanning depth is limited to this 20 cm.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Display Modes for US Imaging
The most common display modes for US imaging
are:

A-Mode.

B-Mode.

M-Mode.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Principles of A-Mode

A-Mode display gives information about the distance between
tissue boundaries e.g. to measure organ thickness.

If we observe an iconoscopes display, let the vertical direction be
the time or depth axis and let the horizontal direction be the
amplitude of the received signal wave.

This display method is called Amplitude Mode or just
A-Mode.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Principles of B-Mode

The B of B-Mode stands for brightness.

Shifting the position of US transmission and reception results in
scanning.

Echo signals received and then go to intensity modulation so that
across sectional image can be displayed on CRT.

Using this display method we can see internal structure in a patient.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Principles of M-Mode

The M of M-Mode stands for motion.

This display mode is useful for studying moving targets such as
inertial mitral valve.

The transducer position should remain fixed when using this mode,
while the display mode used customarily screened in a movement
code scrolling.

Scrolling allows movement of the valve to be displayed as an
image.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Principles of M-Mode
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
A, B and M-Mode
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Principles of Electronic Focus

In actual practice a block of transmission elements are driven for
transmission each US wave.

That the transducer elements are not driven at the same time, the
first US pulse will be driven from transducers 1 and n then
transducers 2 and n-1 and so on.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Principles of Electronic Focus

This sequential driving causes the US beam to convert at a focal
point and that is called Electronic Focusing.

The distance of the focal point from the probe is determined by the
delay time between transmission pulses.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
T & R - Delay Line

For transmission a pulses input to the transmission delay line.

The transmission delay line generates a delay time between pulses
to achieve a desired focusing distance.

For reception the echo signals are input to the reception delay line,
where they are a phase corrected and mixed together.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
B-Mode Scanning Method
The two most common scanning modes are:

Linear scanning

Sector scanning
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Principles of Linear Scanning

In practice many tiny transducers called transducer elements
arrayed in straight line.

The US scanning beam is produced by simultaneously driving a
block of several transducer elements.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Principles of Linear Scanning

The block is new to across the links of the elements array by
electronic switching.

This effect moving the scanning beam across the links of the
transducer.

One scanning line of the US image is produced from the
information gathered between each switching interval of the US
beam.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Linear Scanning

On the left hand side is the probe and the shape of the beam.

On the right hand side is the display format of linear scanning.

The advantages of linear scanning are:
o A large field of view at shallow regions of the body, and
o Easier identification of skin orientation.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Principles of Sector Scanning
For sector scanning all the transducer elements are used for
transmission and reception.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Principles of Sector Scanning
The angle of the US beam is changed by the delay time between
transmission pulses as shown.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Sector Scanning

On the left hand side is the probe and the shape of the beam.

On the right hand side is the display format of sector scanning.

The advantages of sector scanning are:
o A large field of view at deep region of the body, and
o A small skin contact area.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Sector Scanning
Therefore the sector probe can transmit US waves to a very small
acoustic window such as between the ribs to exam the heart as
shown.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Sensitivity Time Control (STC)

The transmitted US waves attenuated gradually through absorption as
they penetrate the human body.

This means that echoes reflected from near the skin surface will be
stronger than echoes reflected from deep regions of the human body.

The STC or sensitivity time control can be used to compensate for this
difference in echo signals strength from shallow to deep regions as
shown.

As echo strength decreases over distance, gain of the signal for different
depth can be increased independently, so that the echoes all have the
same strength.
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Sensitivity Time Control (STC)
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Types of Ultrasound Probes

Linear Array:
o Frequency range: 5 MHz to 13 MHz
o Length: 30 to 60 mm
o Applications: Orthopedics, Breast, Musculoskeletal

Convex array:
o Frequency range: 3.5 MHz to 9 MHz
o Length: 20 to 60 mm
o Applications: Abdomen, GYN, OB, Urology

Micro-convex:
o Application: Pediatrics
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Types of Ultrasound Probes

Phased array:
o Frequency range: 2 MHz to 8 MHz
o Angle: 60 to 90 degree
o Applications: cardiology (heart)

Multi-Plane TEE-Transducer:
o Frequency range: 4.0 to 8.0 MHz
o Image plane rotation: 0 and 180 deg
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3
Types of Ultrasound Probes

Fingertip / Intraoperative:
o Frequency range: 5 MHz to 10 MHz

Endocavity
o Frequency range: 5 MHz to 10 MHz
o Application: Endovaginal, Endorectal

Endoscopic / Mini-probes
o Frequency range: 5 MHz to 20 MHz
Dr. Mohammed Fayez Al Rez
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Ultrasound Equipment - Unit 3