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Basics of ultrasound

What is ultrasound?

  • ultrasound is sound whose frequency is above the range of human hearing:
    • human hearing range 20-20,000Hz
    • diagnostic ultrasound 1-20MHz:
      • standards exist up to 15MHz
      • 3MHz commonly used for adult abdominal scans (wavelength = 0.51mm)
      • 5MHz commonly used for transvaginal scans or paediatric scans (wavelength = 0.31mm)
  • diagnostic ultrasound is the use of ultrasound waves to evaluate a patient's internal organs by transmitting ultrasound waves into the body and, because various internal structures reflect and scatter sound differently, the returning echoes can be collected and converted to electrical impulses which can then be displayed as visual images.
  • propagation of sound is the transfer of energy through a medium, with some of the energy being imparted to the medium.
  • as with any sound, it will not travel through a vacuum and is not electromagnetic.
  • see sound for basic wave terminology such as period (T), frequency (f), wavelength (l), velocity ©, power and intensity as well as characteristics of sound waves such as attenuation, reflection, scatter, refraction, absorption.

Medical ultrasound machine:

  • an ultrasound machine consists of:
    • an ultrasound transducer which emits the ultrasound beam of a given frequency and detects the reflected echoes which are transduced into electrical signals and sent to the machine
    • the machine which:
      • controls some aspects of the emission of the ultrasound characteristics
      • interprets the returning electrical signals and displays them as a visual image in an image “frame”

Ultrasound transducers:

a basic transducer consists of a tube containing:

  • an array of acoustically & electrically isolated piezo-electric crystals that both:
    • emit ultrasound at a certain frequency in response to an applied voltage
      • fundamental frequency dependent on the thickness of the crystal
      • amplitude dependent on the voltage applied
      • crystal expands or contracts depending on the polarity of the applied voltage
      • continues emitting a wave whilst the voltage is applied and then the resonance decays after voltage ceased
      • the array of wave fronts from each element combine to form a single wave front - Huygen's principle
    • detects reflected ultrasound and creates an electric signal in proportion to the wave intensity
  • a damper to shorten the crystals resonating decay duration after the applied voltage is discontinued
    • a very short pulse is required for diagnostic US images to be produced
    • the damper material is similar to placing a hand on a ringing bell to stop it ringing and thus shorten the “ring-down” time
  • wires to each crystal

pulse wave ultrasound:

  • diagnostic ultrasound machines generate short pulses of ultrasound waves lasting only ~1% of the duration of a pulse interval while the remaining 99% of the interval, the transducer is interpreting the returning echoes
  • the rate at which pulses are emitted is called the Pulse Repetition Frequency (PRF) which is usually about 1-5kHz
    • if the PRF is too fast, there will not be enough time to allow reading of echoes returning from deep tissues, thus:
      • maximum PRF = speed of sound in medium / (2 x maximum readable depth)
  • echo amplitude assessment:
    • the returning echoes have an intensity in the range of 1010 to 1015 below the intensity of the original beam sent
    • this is much to wide a range to display, so a logarithmic display is used, but as this still leaves the data at a range of 100-150dB, further compressions are done on the data to end up with a final gray scale of 8-25 shades of gray
    • dynamic range:
      • the actual degree of compression can usually be set by the operator by changing the “dynamic range” setting with the resulting effect on the contrast of the displayed image
      • high dynamic range (low contrast) settings are good for assessing organ parenchymal patterns
      • low dynamic range (high contrast) settings are good for assessing vascular structures
  • harmonic mode optimisation:
    • some newer machines are capable of using detection of the 2nd harmonic frequency of the original beam frequency which allows reduction in some artefacts such as diffuse reverberation but requires a more intense initial beam

beam profile:

  • the shape of the ultrasound beam is important to the quality of the image it produces
  • a beam profile is made up of 3 parts:
    • near field (Fresnel zone):
      • region of generally parallel laser-like waves 
      • the part of the beam useful for imaging purposes as it is here that the resolution is best
      • length is often termed the transition distance
      • narrow beams are created by various focussing techniques:
        • curved lens in front of the crystal
        • electronic means - used in modern transducers
    • transition point:
      • the point at which near field ends and divergence begins
    • far field (Frauhofer zone)
      • region in which wave divergence occurs
  • diffraction:
    • in addition to the central “primary” beam, other off-axis beams are generated which may cause artefacts and are thus minimised by manufacturers:
      • side -lobes
      • grating lobes as a result of the array of crystals 

transducer types:

  • sector:
    • narrow near field, wide far field - for deep structures eg. heart with narrow intercostal window
    • mechanical sector
    • electronic sector (phased array)
  • linear array:
    • parallel beams 90deg to transducer face ⇒ rectangular field of view
    • elements pulsed sequentially in groups in order to produce a wider aperture & resultant improved beam shape
    • usually high frequency, superficial probes
  • curved array:
    • similar to linear, but transducer face is convex creating a divergent field of view
    • provide wide near & far fields
  • annular array
  • specialised (intracavity, intraoperative)

transducer frequencies:

  • 2.5-3.5MHz - heart
  • 3.5-5MHz - abdomen
  • 5MHz - pancreas, GB, transvaginal
  • 7.5-10MHz - foreign bodies, etc

examination safety assessment:

  • modern machines also document indexes to give an idea of intensity of sound used for the scan image such as:
    • MI index
    • TI index
  • see also: biologic effects of sound
us_basics.txt · Last modified: 2008/11/19 23:23 (external edit)