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Ultrasound image production

image production:

  • as each pulse is emitted, its line of sight is known accurately & therefore the resulting echoes from sound/tissue interactions along this line are represented on the monitor in accurate spatial position.
  • multiple lines of sight produced at known angles from the transducer then construct an image of the underlying tissue, composed of thousands of dots. This image is known as a frame.
  • multiple frames produced in rapid succession form the moving or real-time image seen on the monitor.
  • frame rates in modern systems are typically 10-16 frames per second & a flicker free image is achieved using electronic smoothing techniques.

image modes:

amplitude mode (A-mode):
  • 1 dimensional representation of amplitude of returning echoes within a single beam as a amplitude vs distance graph such as on a simple cathode ray oscillator (CRO)
brightness mode (B-mode):
  • standard 2D mode where dot brightness of a sweep of beam represents echo intensity over a sector
motion mode (M-mode):
  • detects movement such as foetal heart beating
Doppler spectral:
  • detects Doppler change in returning frequency and represents on a 1D Fast Fourier Transform graph as velocity of moving interface vs time which is used for determining flow velocities with heart and blood vessels
  • requires much more beam power than B-mode and thus NEVER used to assess fetal heart in 1st TM
  • when combined with B-mode, this is called a duplex scan
colour flow Doppler:
  • creates a colour map representation of the returning Doppler change in frequencies superimposed on a B-mode image providing visual representation of both speed and direction of the moving interface (eg. RBC's within a blood vessel)
  • enables confirmation on B-mode image that a structure indeed is a blood vessel and depending on the angle of incidence of the beam displays the direction of flow within it.
  • uses more power than B-mode but not as much as in Doppler spectral
  • need to only apply it to area required and ensure that area is relatively free of B-mode artefacts as well as select appropriate Doppler settings such as speed range, gain, etc else will have much Doppler artefact displayed.
  • when combined with a duplex scan, is called a colour duplex scan
amplitude Doppler or power Doppler:
  • similar to colour flow Doppler

pre-processing:

  • manipulation of echo data before it is stored in the scan converter memory
  • eg. TGC, FOV, frame averaging, dynamic range, & digitisation

post-processing:

  • eg. read zoom

image optimisation:

transmission power:

  • regulates the amount of energy exciting the transducer crystal & thus the strength of the beam
  • keep to a minimum required for depth penetration to minimise exposure
  • increased power results in:
    • better penetration into tissues
    • increased heat production in tissues
    • reduction in axial resolution
    • increased reverberation artefacts

overall gain:

  • amplification of the received signal
  • does not increase exposure
  • affects the whole image equally
  • undue increase in gain may obscure subtle changes in texture or produce artifactual echoes in fluid-filled structures (eg. GB)

time gain compensation (TGC):

  • as the sound beam passes through tissue its strength is attenuated, thus the echoes returning from deeper structures are weaker than those from superficial structures resulting in the monitor image being darker as it goes deeper in a non-linear fashion
  • to compensate for this, the TGC control allows the gain to be boosted according to how long it took the echo to come back
  • when appropriate TGC is applied, the monitor density should be uniform irrespective of depth of tissue
  • the TGC curve can be adjusted for different tissues:
    • linear for abdomen
    • hyperbolic for pelvic where one does not want to enhance the superficial bladder 
  • machines usually offer near gain & far gain controls

focus:

  • modern transducers use excellent focussing technology to achieve good resolution through the depth of field
  • US systems usually have user-controlled focal zones, giving best definition at the designated depth of focus
  • the firing of crystals changes to cone the beam to its narrowest point at the zone you choose
  • on some systems, it is possible to expand the focal zone by adding more than one focal point, this will allow focussing over a larger portion of the depth of field but has the trade off of slowing frame rate
  • thus:
    • position focal point at area of interest
    • practice using more than one focal zone to watch effect on frame rate

depth:

  • the depth control alters the depth of the field of view
  • the number of pixels in the image area is fixed so setting the field of view also sets the number of pixels used to represent each square centimetre of the patient
  • thus:
    • use smallest depth of field necessary to image the structure of interest
    • small depth of field - superficial structures
    • large depth of field - deep structures

dynamic range:

  • refers to the range of echoes from strong to weak, available to be displayed on the monitor at a particular time
  • thus:
    • decreasing it gives fewer grays & increases contrast
      • eg. help distinguish endometrium from myometrium
    • increasing it gives more grays & decreases contrast

zoom:

  • used to magnify structure of interest
  • real-time or write-zoom mode:
    • increases line density & pixels/unit area but decreases field of view
  • read-zoom mode:
    • a magnification of a particular part of the field of view and does not affect image resolution

frame averaging:

  • sometimes called persistence or smoothing
  • controls the accumulation (or averaging) of echo information over two or more frames
  • increasing it can enhance subtle texture differences but can cause blurring of the image & will result in a reduction in effective frame rate

B-mode colour:

  • many US systems allow use of colour mapping instead of pure gray scale display of B-mode images
  • this may increase the apparent contrast as the eye can detect differences in colour more easily than differences in shades of gray

cine memory:

  • storage of a number of previous frames on system freeze
  • the saved sequence of frames can be reviewed at will, manipulated & imaged
  • very useful in scanning the young or elderly who may be uncooperative

Image resolution:

  • the ability to distinguish echoes in terms of space, time or strength
  • good resolution is critical to the production of high quality images

contrast resolution:

  • the ability to demonstrate differentiation between tissues having different characteristics eg. liver vs spleen

temporal resolution:

  • the ability to accurately show changes in the underlying anatomy over time eg. in echocardiography

spatial resolution:

  • ability to detect and display structures that are close together

axial resolution:

  • objects in same axial path
  • usually 0.5-2mm (better than lateral resolution)
  • dependent on:
    • 0.5 x spatial pulse length (SPL) - the shorter the better
    • determined by:
      • the number of cycles in one pulse
        • damping levels - transducer-dependent
        • transmit power - the greater the voltage, the longer the crystal rings & thus the longer the SPL, leading to a slight decrease in axial resolution
      • the wavelength
        • thus higher frequencies exhibit better axial resolution
    • received gain settings effect the length of the voltage signals generated by the returning echoes, the higher the gain, the poorer the axial resolution
    • field of view (FOV) settings effect the display of pixels per unit area of the patient - the smaller FOV setting makes best use of the available scan converter memory

lateral resolution:

  • objects in different axial paths
  • usually 1-3mm
  • dependent on:
    • width of the ultrasound beam - the wider the poorer the lateral resolution
      • ultrasound machines assume all received echoes are from the central axis of the beam, so if two targets are within the beam at the same point of time, the echoes are assumed to have come from the same target & only one structure will be registered on the image
      • if the beam is narrower than the distance between the two targets so that only one target is within the beam at any one time, and as the transducer is swept across the body, both targets will be interrogated separately & both will be registered on the image.
      • thus best where beam is narrowest ie. has tightest focus.
      • correct positioning of the focal zones is critical in gaining the best lateral resolution for a given transducer

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us_image.txt · Last modified: 2008/11/19 23:37 (external edit)