Ultrasound

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Ultrasound

• Deniz Nevsehirli

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• Ultrasound is a medical imaging technique that
  uses high frequency sound waves and their
  reflections.

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• A basic ultrasound machine consists of the
  following parts
• Transducer Probe - the part that sends and
  receives the sound pulses.
• Central Processing Unit (CPU) - computer that
  does all of the calculations and contains the
  electrical power supplies for itself and the
  transducer probe.
• Transducer Pulse Controls - changes the
  amplitude, frequency and duration of the pulses
  emitted from the transducer probe.
• Monitor - displays the image from the ultrasound
  data processed by the CPU.

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How it works?

• The machine transmits high-frequency sound pulses
  ranging from 1 to 5 Megahertz into the body using
  a probe.
• As the sound waves travel into the body they
  encounter a boundary between tissues (e.g. soft
  tissue and bone).
• During the transition between two materials with
  different physical properties, some of the waves
  get reflected back to the probe, while some get
  refracted and travel further until they reach
  another boundary and get reflected.

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Behavior of an ultrasound beam within tissue

• In part A we see the exponential attenuation of
  ultrasound beam intensity within an homogenous
  material
• In Part B we see reflection and refraction
  occurring at interface between two materials with
  different physical properties.

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How it works?

• The probe detects the reflected waves and
  transfers to the machine.
• The distance from the probe to the tissue or
  boundaries is calculated by the machine using the
  speed of sound in tissue (1,540 m/s) and the time
  elapsed until the detection each echo (normally
  on the order millions per second).
• The machine displays the distances and
  intensities of the echoes on the screen.

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An Example of an Ultrasound Image
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Major Uses of Ultrasound

• Obstetrics and Gynecology
• Urology
• Cardiology
• To observe structures or functions of the hearth
  to identify abnormalities.
• To measure blood flow through the heart and major
  blood vessels.
• Lungs filled with air and ribs limits the
  application.

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A-Mode Applications

• Excellent resolution for short penetration
  distances using high frequencies. (5-15 MHz)
• 1 dimensional images are obtained.
• Small size transducer is an advantage.

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B-Mode Applications

• Light intensity versus time is used to generate
  images.
• Stronger reflections cause brighter line.
• Mechanical Scanning
• Transducer is moved and placed on different
  locations on the patient. From each position an
  image is obtained and combined to form the
  display.
• Electronic Scanning
• Linear array of transducers are used. (64 or
  more)
• Faster operation. Several hundreds of images per
  second.

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C-Mode Applications

• Through transmission imaging is used.
• Separate transducers are used.
• By moving transducers, 2d images are obtained.
• Two tissue properties are derived
• Total attenuation is calculated using absorption,
  scattering and reflection losses.
• Index of refraction along the path using the time
  delay which is inversely proportional to the
  velocity.

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M-Mode Applications

• Quantitative and qualitative analysis of heart
  valve motion are obtained.
• Transducer is stationary. Positions of heart
  valve leaflets are displayed at varying times
  using echoes.
• Vertical deflection is scanned at a rate of 2-3
  cardiac cycles to form a single display. And the
  vertical axis is given in units of mm/sec.

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Doppler Ultrasound

• Doppler ultrasound is based upon the Doppler
  Effect.
• When the object reflecting the ultrasound waves
  is moving, it changes the frequency of the
  echoes.
• If the object is moving toward the probe, this
  will result in an increase in the frequency.
• If the object is moving away from the probe, this
  will result in a decrease in the frequency.
• The change in frequency depends on how fast the
  object is moving. Doppler ultrasound measures the
  change in frequency of the echoes to calculate
  how fast an object is moving.
• Reflected by moving red blood cells, doppler
  ultrasound has been used mostly to measure the
  rate of blood flow through the heart and major
  arteries.

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Doppler Ultrasound

• Pulsed Wave Doppler Imaging
• The time interval between transmission and
  reception is used to calculate distance. So it is
  depth selective.
• Gives precise information about the location of
  target area and the flow.
• Minimum range problem.
• Aliasing problem
• Continuous Wave Doppler Imaging
• Receives information about all moving reflectors
  along the path of the beam.
• Is not depth selective because there is no basis
  for the measurement of the time delay.
• Maximum velocity is calculated.
• Color Doppler Imaging
• Pulsed Doppler Imaging is used to obtain a static
  image of blood flow velocity waveforms.
• Different flow directions and rates are coded
  with different colors.

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Future of Ultrasound

• Biomedical engineers at Duke University's Pratt
  School of Engineering have created a new
  three-dimensional ultrasound cardiac imaging
  probe.
• Inserted inside the esophagus, the probe creates
  a picture of the whole heart in the time it takes
  for current ultrasound technology to image a
  single heart cross section.
• Transesophageal echocardiography, (TEE) is one
  form of ultrasound cardiac imaging. In this
  technique a probe is inserted down the patient's
  throat and behind the heart to capture ultrasound
  heart images.

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Future of Ultrasound

• Current TEE systems can quickly generate only
  two-dimensional cross-sectional images. This
  limitation makes it impractical for use in
  guiding therapeutic treatment devices such as
  ablation probes. 2-D probe must repeatedly
  repositioned during treatments so, instead,
  fluoroscopy (X-ray) is used.
• But the use of X-ray imaging results in radiation
  exposure for patients and requires lead-shielding
  for clinicians. In addition, such procedures take
  up to seven hours to complete.
• Since 3-D imaging requires significantly more
  sensors than 2-D imaging. The new Duke 3-D probe
  has the size of normal TE probes but contains an
  array of 504 individual ultrasound sensors. (8
  times the usual number 64)
• The probe generates ultrasound at 5 million
  vibrations per second. Having 504 sensors, it
  provides great sensitivity and a sharp image. And
  because the image is large enough to map the
  whole volume of the heart, fewer images need to
  be taken, reducing the required time.

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Dangers of Ultrasound

• Since ultrasound is energy, the question is what
  is this energy doing to the tissues.
• There are two major possibilities of problem with
  ultrasound
• Tissues or water absorb the ultrasound energy and
  that increases local temperature.
• Solubility of gases decrease with increasing
  temperature. Dissolved gases can form bubbles due
  to local heat caused by ultrasound.
• Although, there are no records of bad effects of
  ultrasound in studies in either humans or
  animals, ultrasound should still be used only
  when necessary.

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References

• Lectures on Ultrasound by Prof. Yekta Ülgen,
  Institute of Biomedical Engineering BOGAZIÇI
  UNIVERSITY
• http//www.online-medical-dictionary.org
• http//electronics.howstuffworks.com/ultrasound.ht
  m
• http//ocw.mit.edu/OcwWeb/index.htm
• http//www.cardiovascularultrasound.com/articles/b
  rowse.asp
• http//www.physorg.com/news4277.html
• http//www.mgdinc.com/pdfs/Comparison20of20Conti
  nuous20Wave20Doppler20vs20Pulse20Doppler20Pr
  ofiling20Technology.pdf
• http//www.centrus.com.br/DiplomaFMF/SeriesFMF/dop
  pler/capitulos-html/chapter_01.htm