Physics And
Instrumentation of
Ultrasound

WHAT DO YOU
UNDERSTAND ABOUT
ULTRASOUND ?

Bats navigate using ultrasound

Bats make high-pitched chirps which are too high for
humans to hear. This is called ultrasound
Like normal sound, ultrasound echoes off objects
The bat hears the echoes and works out what caused
them
•Dolphins also navigate with ultrasound
•Submarines use a similar method called sonar
•We can also use ultrasound to look inside the body…

• Ultrasound
– Cyclic sound pressure with a
frequency greater than the upper
limit of human hearing.
• Human Ear  Audible
Range Frequency?

The human ear can only
The human ear can
only respond to the
audible frequency range
~ 20Hz - 20kHz to the
audible frequency range ~
20Hz - 20kHz

Medical sonography
(ultrasonography)
 Ultrasound-based diagnostic imaging
technique used to visualize muscles and
internal organs, their size, structures and
possible pathologies or lesions.
 APPLICATIONS?
 ADVANTAGES & DISADVANTAGES?

Diagnostic applications
• Cardiology
• Gynaecology & Obstetrics
• Ophthalmology
• Abdomen
• Urology- to determine, for example, the amount of fluid
retained in a patient's bladder.
• Musculoskeletal - tendons, muscles, and nerves
• Vascular - arteries and veins
• Interventional biopsy - emptying fluids, intrauterine
transfusion

Therapeutic applications
• Therapeutic applications use
ultrasound to bring heat or
agitation into the body.
• Therefore much higher energies
are used than in diagnostic
ultrasound.

ULTRASOUND PHYSICS
Format
 What is sound/ultrasound?
 How is ultrasound produced
 Transducers - properties
 Effect of Frequency
 Image Formation
 Interaction of ultrasound with tissue
 Acoustic impedance
 Image appearance

Sound?
 Sound is a mechanical, longitudinal wave that travels
in a straight line
 Sound requires a medium through which to travel

CATEGORIES OF SOUND
 Infrasound (subsonic) below 20Hz
 Audible sound 20-20,000Hz
 Ultrasound above 20,000Hz
 Nondiagnostic medical applications
<1MHz
 Medical diagnostic ultrasound >1MHz

In 1826 Daniel
Colladon, a Swiss
physicist, and Charles
Sturm, a French
mathematician,
accurately measured its
speed in water. Using a
long tube to listen
underwater (as Leonardo
da Vinci suggested in
1490), they recorded how
fast the sound of a
submerged bell traveled
across Lake Geneva.
Their result--1,435
meters per
second in water of 1.8
degrees Celsius (35
degrees Fahrenheit)--was
only 3 meters per second
off from the speed
accepted today.

Compression wave

Acoustic Variables
• Period
• Wavelength
• Amplitude
• Frequency
• Velocity

Acoustic Variables

Acoustic Variables

Amplitude, A (m)
The maximum displacement that occurs in an
acoustic variable.

Why we use different frequency?

Basic Ultrasound Physics
Amplitude
oscillations/sec = frequency - expressed in
Hertz (Hz)

What is Ultrasound?
 Ultrasound is a mechanical, longitudinal wave with a
frequency exceeding the upper limit of human hearing,
which is 20,000 Hz or 20 kHz.
 Medical Ultrasound 2MHz to 16MHz

ULTRASOUND – How is it produced?
Produced by passing an electrical current through a
piezoelectrical (material that expands and contracts
with current) crystal

Human Hair
Single
Crystal
Microscopic view of scanhead

In ultrasound, the following events
happen:
1. The ultrasound machine transmits high-
frequency (1 to 12 megahertz) sound pulses into
the body using a probe.
2. The sound waves travel into the body and hit a
boundary between tissues (e.g. between fluid
and soft tissue, soft tissue and bone).
3. Some of the sound waves reflect back to the
probe, while some travel on further until they
reach another boundary and then reflect back
to the probe .
4. The reflected waves are detected by the probe
and relayed to the machine.

1. The machine calculates the distance from
the probe to the tissue or organ
(boundaries) using the speed of sound in
tissue (1540 m/s) and the time of the each
echo's return (usually on the order of
millionths of a second).
6. The machine displays the distances and
intensities of the echoes on the screen,
forming a two dimensional image.

Piezoelectric material
 ACapplied to a piezoelectric crystal
causes it to expand and contract –
generating ultrasound, and vice versa
 Naturally occurring - quartz
 Synthetic - Lead zirconate titanate
(PZT)

Ultrasound Production
 Transducer produces ultrasound pulses (transmit 1%
of the time)
 These elements convert electrical energy into a
mechanical ultrasound wave
 Reflectedechoes return to the scanhead which
converts the ultrasound wave into an electrical
signal

Piezoelectric Crystals
 Thethickness of the crystal determines the
frequency of the scanhead
Low Frequency High Frequency
3 MHz 10 MHz

Frequency also affects the QUALITY of the
 The frequency
vs. Resolution
ultrasound image
 The HIGHER the frequency, the BETTER the
resolution
 The LOWER the frequency, the LESS the resolution
 A 12 MHz transducer has very good resolution, but
cannot penetrate very deep into the body
 A 3 MHz transducer can penetrate deep into the
body, but the resolution is not as good as the 12
MHz
Low Frequency High Frequency
3 MHz 12 MHz

Broadband vs. Narrowband
Amplitude
Frequency

Broadband vs. Narrowband
Nerve Visualisation:
 5-10 MHz
 6-13 MHz
 By altering the transmit frequencies one transducer
replaces several transducers
 View a range of superficial to deep structures without
changing transducers

Transducer Design
Size, design and
frequency
depend upon the
examination

Image Formation
Electrical signal produces ‘dots’ on the screen
 Brightness of the dots is proportional to the
strength of the returning echoes
 Location of the dots is determined by travel
time. The velocity in tissue is assumed constant
at 1540m/sec
Distance = Velocity
Time

Image Formation
‘B’ mode

Interactions of Ultrasound with Tissue
 Reflection
 Refraction
 Transmission
 Attenuation

Interactions of Ultrasound with Tissue
 Reflection
 The ultrasound reflects off tissue and returns to
the transducer, the amount of reflection depends on
differences in acoustic impedance
 The ultrasound image is formed from reflected echoes
transducer

Refraction
reflective
refraction
Scattered
echoes
Incident
Angle of incidence = angle of reflection

Interactions of Ultrasound with Tissue
Transmission
 Some of the ultrasound waves continue deeper into
the body
 These waves will reflect from deeper tissue
structures
transducer

Interactions of Ultrasound with Tissue
Attenuation
 Defined - the deeper the wave travels in the
body, the weaker it becomes -3 processes:
reflection, absorption, refraction
 Air (lung)> bone > muscle > soft tissue >blood >
water

Interactions of Ultrasound with
Tissue
• Acoustic impedance (AI) is dependent on the density of
the material in which sound is propagated
- the greater the impedance the denser the material.
• Reflections comes from the interface of different AI’s
• greater ∆ of the AI = more signal reflected
• works both ways (send and receive directions)
Transducer
Medium 1 Medium 2 Medium 3

Interaction of Ultrasound
with Tissue
• Greater the AI, greater the returned signal
• largest difference is solid-gas interface
• we don’t like gas or air
• we don’t like bone for the same reason
GEL!!
• Sound is attenuated as it goes deeper into the body

• Z (Rayls) = Density (kg/m³) x Speed (m/s)

• Incident beam has
normal incidence 90
degree
(perpendicular
incidence) on the
tissue interface, the
magnitude of
reflection can be
calculated (IRC)
• α  Z values

Attenuation & Gain
 Sound is attenuated by tissue
 More tissue to penetrate = more
attenuation of signal
 Compensate by adjusting gain based on
depth
 near field / far field
 AKA: TGC

Ultrasound Gain
 Gain controls
 receiver gain only
 does NOT change power output
 think: stereo volume
 Increasegain = brighter
 Decrease gain = darker

Balanced Gain
 Gain
settings are important to obtaining adequate
images.
bad far field
bad near field
balanced

Reflected Echo’s
 Strong Reflections = White dots
Diaphragm, tendons, bone
‘Hyperechoic’

Reflected Echo’s
Weaker Reflections =
Grey dots
 Most solid organs,
 thick fluid – ‘isoechoic’

Reflected Echo’s
 No Reflections = Black dots
 Fluid within a cyst, urine, blood
‘Hypoechoic’ or echofree

What determines how far ultrasound
waves can travel?
 The FREQUENCY of the transducer
 The HIGHER the frequency, the LESS it can penetrate
 The LOWER the frequency, the DEEPER it can penetrate
 Attenuation is directly related to frequency

Ultrasound Beam Depth
• Need to image at proper depth
• Can’t control depth of beam
• keeps going until attenuated
• You can control the depth of displayed data

Ultrasound Beam Profile
 Beam comes out as a slice
 Beam Profile
 Approx. 1 mm thick
 Depth displayed – user controlled
 Image produced is “2D”
 tomographic slice
 assumes no thickness
 You control the aim
1mm

Goal of an Ultrasound System
 The ultimate goal of any ultrasound
system is to make like tissues look the
same and unlike tissues look different

Accomplishing this goal depends
upon...
 Resolving capability of the system
 axial/lateral resolution
 spatial resolution
 contrast resolution
 temporal resolution
 Processing Power
 ability
to capture, preserve and display the
information

Types of Resolution
 Axial Resolution
 specifies how close together two objects can be along
the axis of the beam, yet still be detected as two
separate objects
 frequency (wavelength) affects axial resolution –
frequency resolution

Types of Resolution
 Lateral Resolution
 the ability to resolve two adjacent objects that are
perpendicular to the beam axis as separate objects
 beamwidth affects lateral resolution

Types of Resolution
 Spatial Resolution
 also called Detail Resolution
 the combination of AXIAL and LATERAL resolution -
how closely two reflectors can be to one another
while they can be identified as different reflectors

Types of Resolution
 Temporal Resolution
 the ability to accurately locate the position of moving
structures at particular instants in time
 also known as frame rate

Types of Resolution
 Contrast Resolution
 the ability to resolve two adjacent objects of similar
intensity/reflective properties as separate objects -
dependant on the dynamic range

Liver metastases

Ultrasound Applications
Visualisation Tool:
Nerves, soft tissue masses
Vessels - assessment of position, size,
patency
Ultrasound Guided Procedures in real
time – dynamic imaging; central venous
access, nerve blocks

Imaging
Know your anatomy – Skin, muscle,
tendons, nerves and vessels
Recognise normal appearances –
compare sides!

Skin, subcutaneous tissue
Epidermis
Loose connective tissue and subcutaneous fat
is hypoechoic
Muscle interface
Muscle fibres interface
Bone

Transverse scan – Internal Jugular Vein and
Common Carotid Artery

Summary
•Imaging tool – Must have the knowledge to
understand how the image is formed
•Dynamic technique
•Acquisition and interpretation dependant
upon the skills of the operator.