Ultrasound

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Ultrasound

• By Saja Abdo

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Objectives

• History
• Physics of ultrasound
• 1. basic principles of sound
• 2. principles of ultrasound
• 3. how ultrasound wave impact by tissue
• Uses of ultrasound

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History

• 1 Jan 1915 Hydrophones covert acoustic energy
  into electrical energy, and is used for
  underwater navigation for submarines and
  iceburgs.
• Use of Ultrasound in Therapy 1 Jan 1920Used as a
  physical therapy treatment on a soccer team in
  Europe.
• 1 Jan 1942Neurologist Karl Dussik was the first
  to use ultrasound for medical diagnosis.

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• 1 Jan 1953, First echocardiogram completed
  through an echo test conducted from a Siemens
  shipyard.
• 1 Jan 1970, Imaging blood flow through the
  chambers and different depths of the heart.
• 1 Apr 1986, 3-D ultrasound was developed and 3-D
  images were captured of a fetus. A 3D ultrasound
  is acquired by emitting high-frequency sound
  waves.

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• 2 D
  3D
• 1 Jan 2000, 4-D ultrasound (real time) was
  developed .
• 2D / 3D / 4D Ultrasound also Video in 9 Apr
  2013.

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Physics of ultrasound

• Basic principles of sound
• Sound is a mechanical, longitudinal wave that
  travels in a straight line
• Sound requires a medium through which to travel
• The wave travels by compressing and rarefacting
  matter.
• rarefaction can by easily observed by
  compressing a spring and releasing it.
• Depending on the matter encountered, the wave
  will travel at different velocities.

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Cycle

• 1 Cycle 1 repetitive periodic oscillation

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Frequency

• Number of cycles per second
• Measured in Hertz (Hz)
• -Human Hearing 20 - 20,000 Hz
• -Ultrasound gt 20,000 Hz
• -Diagnostic Ultrasound 2.5 to 10 MHz

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Principles of 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.
• Diagnostic Ultrasound 2.5 to 10 MHz.
• Produced by passing an
• Produced by passing an electrical current through
  a piezoelectric crystal.
• MHz to 16MHz

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Piezoelectric material

• AC applied to a piezoelectric crystal causes it
  to expand and contract generating ultrasound,
  and vice versa.
• Naturally occurring - quartz.
• Synthetic - Lead zirconate titanate (PZT).

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Ultrasound Production

• Transducer contains piezoelectric(elements/
  crystals)which produce the ultrasound pulses
  (transmit 1 of the time).
• These elements convert electrical energy into a
  mechanical ultrasound wave

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The Returning Echo

• Reflected echoes return to the scan head where
  the piezoelectric elements convert the ultrasound
  wave back into an electrical signal.
• The electrical signal is then processed by the
  ultrasound system

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Piezoelectric Crystals

• The thickness of the crystal determines the
  frequency of the scan head

Low Frequency 3 MHz
High Frequency 10 MHz
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Frequency vs. Resolution

• The frequency also affects the quality of the
  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.

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Image Formation

• Probe emits a sound wave pulse
• measures the time from emission to return of the
  echo .
• Wave travels by displacing matter, expanding and
  compressing adjacent tissues.
• It generates an ultrasonic wave that is
  propagated, impeded, reflected, refracted, or
  attenuated by the tissues it encounters .

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• 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

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Producing an image

• Important concepts in production of an U/S image
• Propagation velocity
• Acoustic impedance
• Reflection
• Refraction
• Attenuation

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Propagation Velocity

• Sound is energy transmitted through a medium.
• Each medium has a constant velocity of sound (c).
• Tissues resistance to compression density or
  stiffness.
• Product of frequency (f) and wavelength (?)
• c f ?
• Frequency and Wavelength therefore are directly
  proportional if the frequency increases the
  wavelength must decrease.

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• Propagation velocity
• Increased by increasing stiffness
• Reduced by increasing density
• For example
• Bone 4,080 m/sec
• Air 330 m/sec
• Soft Tissue Average 1,540 m/sec

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Acoustic Impedance

• Acoustic impedance (z) of a material is the
  product of its density and propagation velocity.
  
  Z pc
• Homogeneous mediums reflect no sound.
• acoustic interfaces create visual boundaries
  between different tissues.
• Bone/tissue or air/tissue interfaces with large
  ?z values reflect almost all the sound.
• Muscle/fat interfaces with smaller ?z values
  reflect only part of the energy.

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Interactions of Ultrasound with Tissue

• Reflection
• Refraction
• Transmission
• Attenuation

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Refraction

• A change in direction of the sound wave as it
  passes from one tissue to a tissue of higher or
  lower sound velocity.
• U/S scanners assume that an echo returns along a
  straight path.
• Distorts depth reading by the probe.
• Minimize refraction by scanning perpendicular to
  the interface that is causing the refraction.

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Reflection

• The production of echoes at reflecting interfaces
  between tissues of differing physical properties.
  
• There are 2 types
• Specular - large smooth surfaces
• Diffuse - small interfaces or nooks and crannies

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Specular Reflection

• Large smooth interfaces (e.g. diaphragm, bladder
  wall) reflect sound like a mirror.
• Only the echoes returning to the machine are
  displayed.
• Specular reflectors will return echoes to the
  machine only if the sound beam is perpendicular
  to the interface.

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Diffuse Reflector

• Most echoes that are imaged arise from small
  interfaces within solid organs.
• These interfaces may be smaller than the
  wavelength of the sound.
• The echoes produced scatter in all directions.
• These echoes form the characteristic pattern of
  solid organs and other tissues.

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Transmission

• Some of the ultrasound waves continue deeper into
  the body.
• These waves will reflect from deeper tissue
  structures.

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• Attenuation
• The intensity of sound waves diminish as they
  travel through a medium
• In ideal systems sound pressure (amplitude) is
  only reduced by the spreading of waves
• In real systems some waves are scattered and
  others are absorbed, or reflected
• This decrease in intensity (loss of amplitude) is
  called attenuation.
• the deeper the wave travels in the body, the
  weaker it become-3 processes reflection,
  absorption, refraction
• Air (lung)gt bone gt muscle gt soft tissue gtblood gt
  water

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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
• Increase gain brighter
• Decrease gain darker
• Gain settings are important to obtaining adequate
  images.

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bad far field
bad near field
balanced
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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

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Uses of ultrasound

• An ultrasound scan can be used in several
  different ways, such as monitoring an unborn
  baby, diagnosing a condition or guiding a surgeon
  during certain procedures.
• Pregnancy
• Ultrasound scans are routine procedure
  for pregnant women. They produce images of the
  unborn baby inside the womb, and display them on
  a monitor.
• Most women are offered at least two
  ultrasound scans during pregnancy
• the first scan (at around eight to 14 weeks) can
  help to determine when the baby is due.

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• the second scan (usually between 18 and 20 weeks)
  checks for structural abnormalities, particularly
  in the baby's head or spine.
• Diagnosing conditions
• Ultrasound scans can help diagnose problems in
  many parts of your body, including your
• 1. liver (cirrhosis) 2.
  gallbladder (gallstones)
• 3. lymph nodes 4.
  ovaries
• 5. Testes
  6. breasts
• 7. blood vessels (aneurysm) 8. joints,
  ligaments and tendons
• 9. Skin
  10. eyes

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• Echocardiogram (ECG)
• An ultrasound scan can be used to examine
  the size, shape and movement of your heart. For
  example, it can check that the structures of your
  heart, such as the valves and heart chambers, are
  working properly and your blood is flowing
  normally. This type of ultrasound scan is called
  an echocardiogram (ECG).
• Biopsy
• Ultrasound can be used to guide doctors
  during certain procedures, such as a biopsy
  (where a tissue sample is taken for analysis).
  This is to make sure the surgeon is working in
  the right area and is often used when
  diagnosing breast cancer.

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References

• Novelline, Robert (1997). Squire's Fundamentals
  of Radiology (5th ed.). Harvard University Press.
  pp. 3435.
•  Bruno Pollet Power Ultrasound in
  Electrochemistry From Versatile Laboratory Tool
  to Engineering Solution John Wiley Sons,(2012) 
  
•  Corso, J. F. ,"Bone-conduction thresholds for
  sonic and ultrasonic frequencies". Journal of the
  Acoustical Society of America 35 (11) 17381743.
  (1963).
•   Takeda, S. Morioka, I. Miyashita, K.
  Okumura, A. Yoshida, Y. Matsumoto, K. "Age
  variation in the upper limit of
  hearing, European Journal of Applied, (1992).
•   Physiology 65 (5) 403408. Retrieved 2008
• Hearing by Bats (Springer Handbook of Auditory
  Research, 5. Art Popper and Richard R. Fay
  (Editors). Springer, (1995).

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