Chapter 15 Displays and Image Storage

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Chapter 15Displays and Image Storage
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Displays

• Cathode Ray Tube (CRT)
• Funnel shaped, glass vacuum tube
• Electrons, which contain video information are
  emitted from an electron gun in the narrow end of
  the tube
  
• Electron beam travels through magnetic fields
  which focus sweep the beam across the wide end
  of the tube
  
• The interior surface of the screen is coated with
  phosphors, which glow when struck by the
  electrons

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Fields Frames

• Television Image
• 525 closely spaced horizontal lines
• The odd numbered lines (odd field) are written
  first
  
• Occurs in 1/60th of a second
• The even numbered lines (even field) are written
  second
  
• Occurs in 1/60th of a second
• Frame the combination of one odd one even
  field
  
• Time required for one frame 1/30th of a second
• Therefore, frame rate 30 Hz
• Known as interlaced display
• Interlaced displays eliminate flicker
• Humans can detect flicker in non-interlaced
  displays with frame rates of 30 Hz, but not with
  an interlaced display with field rates of 60 Hz

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Fields Frames
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Bistable vs. Grayscale Displays

• Bistable images are composed of two shades
• Black white
• Grayscale displays have multiple levels of
  brightness
  
• Different shades of gray are assigned to
  different echo amplitudes to facilitate
  differentiation the various biologic tissues

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Display Controls
Contrast determines the range of brilliancies
within the displayed image. Bistable images are
high contrast.
Low Contrast
High Contrast
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Display Controls
Brightness determines the brilliance of the
displayed image
Bright
Not Bright
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Scan Converters

• Scan converter
• Stores and later displays information
• Storage in the scan converter is called
  writing
  
• Data is read from the scan converter and is
  displayed on the CRT
  
• Data can be altered between the writing and
  reading processes
  
• Modern scan converters use computer technology
  and are called digital scan converters
  
• Old technology analog

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Scan Converters

• Change the format of the information, translating
  from a spoke format into the video format
  

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Analog vs. Digital Numbers

• Analog numbers real world numbers, found in
  everyday life, e.g. weight, length, etc.
  
• Can have an unlimited continuous range of
  values, e.g. 164 pounds, 163.998673 pounds,
  164.004432 pounds
  
• Digital numbers associated with computer
  devices
  
• Have discrete values, e.g. 163.12 pounds 163.48
  pounds would be reported as 163 pounds
  

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Analog Scan Converter

• First form of scan converter
• Funnel shaped tube containing a silicon wafer or
  dielectric matrix where electrons are stored
  
• Limitations have made analog scan converters
  obsolete
  
• Image fade
• Stored charges on the silicon wafer dissipate
  over time
  
• Image flicker
• Caused by switching between read write modes
• Instability
• Picture quality depends on many factors
• Length of use
• Room temperature
• Humidity
• Deterioration
• Image degrades as the device ages

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Digital Scan Converter

• Uses computer technology to convert images into
  numbers
  
• Known as digitizing
• Image is stored in computer memory as a series of
  0s 1s
  
• Numbers are processed and re-translated into an
  image prior to display on the monitor
  
• Advantages
• Uniformity Consistent grayscale quality
  throughout the image
  
• Stability Does not fade or drift
• Durability not affected by age or heavy use
• Speed nearly-instant processing
• Accuracy error free
• Two important elements
• Pixel
• Bit

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Pixel

• The smallest element of a digital picture
• Pixel Density is the number of picture elements
  per inch

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Pixel

• High pixel density ? improved spatial resolution
• High pixel density creates an image with greater
  detail
  

Large pixels, low pixel density, low spatial
resolution
Small pixels, high pixel density, improved
spatial resolution
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Bit

• The smallest amount of computer memory
• Bistable, with values of 0 or 1
• Decimal numbers
• Based on ten choices, 0 9 everyday numbers
• Binary numbers
• Computer based or digital numbers
• A group of bits in a series of zeroes and ones
• Byte
• A group of eight bits of computer memory
• 10011111
• Word
• Made up of two bytes, or 16 bits

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Bit

• A pixels shade of gray is determined by the
  cluster of bits assigned to it
  
• When more shades of gray appear on image there
  are more bits per pixel
  
• More shades of gray have superior contrast
  resolution
  

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Bits
Many bits assigned to each pixel, producing many
shades of gray and improved contrast resolution
Few bits assigned to each pixel, producing fewer
shades of gray
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Calculating the Number of Gray Shades

• When determining the number of gray shades a
  system can display, multiply the number 2 by
  itself the same number of times as there are
  bits.
• Example an 8 bit memory
• 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 256
• An 8 bit memory can display 256 different shades
  of gray
  

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Summaries
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Analog Digital Image Data

• Reception
• Transducer produces very low voltage analog
  electrical signals
  
• Weak signals are susceptible to contamination by
  electrical noise
  
• Conversion from analog to digital form makes
  these weak signals much less susceptible to noise
  contamination
  
• Digital information must be converted back to
  analog form for display on a CRT
  

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Analog Digital Image Information

• Image information translation is a 5-step
  process
  
• Electrical signals crated by the transducer are
  converted from analog to digital form by the
  analog-to-digital converter
  
• Digital information is stored in the scan
  converters computer memory
  
• Any processing occurring before storage is
  preprocessing
  
• Stored digital information continues to be
  processed by the ultrasound systems computer
  
• Any processing occurring after storage in the
  digital scan converter is postprocessing
  
• Digital signals are translated back into analog
  form by a digital-to-analog converter for display
  
  
• Presentation of the analog information on the
  video display
  

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Analog Digital Image Information

Preprocessing Occurs
Scan Converters Computer Postprocessing Occurs
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Pre- Post- Processing

• Preprocessing
• The manipulation of image data before storage in
  the scan converter
  
• Sonographer controlled
• Permanently alters image data cannot be undone
• Postprocessing
• The manipulation of image data after storage in
  the scan converter
  
• Sonographer controlled
• Postprocessing changes can be reversed
• Alteration to frozen images must be
  postprocessing
  

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Pre- Post- Processing
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Magnification

• Aka zoom
• Improves anatomic detail by enlarging a chosen
  portion of an image (region of interest) to fill
  the entire screen
  
• Two forms
• Read magnification
• Write magnification

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Read Magnification

• Occurs after image storage in the scan converter
• Three steps
• Ultrasound system scans the anatomy
• Image is converted from analog to digital form
  stored in the scan converter
  
• ROI is chosen
• The system reads displays only the original
  data from the chosen ROI
  
• The ROI is NOT rescanned

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Read Magnification

• Characterized by
• Number of pixels or scan lines in the magnified
  image is the same as in the original image.
  
• Spatial resolution does not change because the
  number of pixels in the ROI is unchanged.
  
• Pixels are larger in the zoomed image
• Postprocessing only
• Performed on frozen images

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Write Magnification

• Applied during data acquisition
• Four steps
• Ultrasound system scans the anatomy creates an
  image
  
• Image is converted from analog to digital form
  stored in the scan converter
  
• ROI is chosen the system discards all old data
  in the scan converter
  
• Ultrasound system rescans on the ROI and writes
  new data into the scan converter

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Write Magnification

• Characterized by
• After the ROI is identified, all new image
  information is acquired
  
• Number of pixels or scan lines in the ROI is
  greater than that in the ROIs portion of
  original image
  
• Increase number of pixels in the ROI improves
  spatial resolution
  
• Preprocessing only
• Cannot be used once the image is frozen

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Read Write Magnification
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Recording Archiving Techniques

• Many different forms, technologies
• Paper media
• Magnetic media
• Chemically mediated photographs
• Optical media
• PACS
• (P)icture (A)rchiving (C)ommunications
  (S)ystem
  
• Three major advantages
• Virtually instant access to archived studies
• No degradation of data
• Ability to electronically transmit stored images
  reports to remote sites, e.g.
  interpreting physicians home