Ultrasound

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Ultrasound
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Ultrasound Imaging

• Ultrasound - Sound waves (longitudinal) with
  frequencies greater than 20KHz.
• Typical frequencies used in imaging 1 5 MHz
• The transducer produces short pulses and acts as
  a receiver in between pulses.

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Piezoelectric Crystals

• When pressure is exerted on these crystals, a
  voltage is produced between opposite faces of the
  crystal.
• The charges within the crystal structure are
  forced out of balance.

• The process also works in reverse, so an applied
  voltage will result in pressure at the surface of
  the crystal.
• An alternating voltage generates a longitudinal
  oscillation (and therefore a sound wave).
• The frequency of the a.c. is chosen so that it
  matches one of the resonant frequencies of the
  crystal.

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Ultrasound Pulses

• Typical Pulse Data
• f 3 MHz
• v 1500 m/s
• ? - 0.5mm
• Pulse length - 1?s
• Scale of body 5-20cm
• Time until return of pulse 70 ?s

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Building the Image
A-scan. The peaks correspond to reflection from
surfaces inside the body. Taller peak represent
stronger reflections.
B-Scan. The transducer is moved across the body
and the return signals are stored
electronically. The signal strength controls the
brightness/colour of each part of the image
displayed on a screen.
Blood vessels inside the eye
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Resolution

• Diffraction will occur if the sound waves have a
  wavelength that is too long.
• As a general rule, the Ultrasound scanner will
  have a resolution approximately equal to its
  wavelength. Therefore resolution is of the order
  of millimetres (typical ?0.5mm).

Doppler Scans
Movement of the reflecting surface causes a shift
in wavelength of the ultrasound pulse (The
Doppler Effect). e.g. to assess blood vessels and
the heart.
Doppler ultrasound of a kidney