Chapter 13 RealTime Imaging

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Chapter 13Real-Time Imaging
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Temporal Resolution

• Frame Rate
• Number of frames per second
• Determined by two factors
• Sounds speed in the medium
• Depth of imaging
• Speed of sound in soft tissue is considered to be
  constant at 1.54 km/s, therefore, the maximum
  imaging depth determines the frame rate
• Units
• Hertz

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Temporal Resolution

• Pertains to accuracy in time
• Describes the ability to precisely position
  moving structures from instant to instant
• The greater the frame rate, the better the
  temporal resolution
• Temporal resolution is determined by the frame
  rate
• Units - Hertz

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Frame Rate

• Tframe time for one frame
• Relationship of frame rate Tframe
• Inversely related
• Reciprocals
• Tframe x frame rate 1
• Frame rate 10 Hz
• Tframe 1/10 Hz 0.1 second
• Frame rate 50 Hz
• Tframe 1/50 Hz 0.02 second

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Frame Rate - Determinants

• Image depth
• of pulses in each image
• Sector size
• of scan lines per image (line density)
• of focal points
• One pulse per scan line per of focal points

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Imaging Depth
Higher Frame Rate

• Tframe pulse x PRP
• of pulses 100
• PRP 1/1000th second
• 100 x 1/1000 1/10th second
• Frame rate 10 Hz

Lower Frame Rate
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Imaging Depth vs. Frame Rate

• Tframe pulse x PRP
• Depth x
• of pulses 100
• PRP 0.001 second
• 100 x 0.001 0.1 second
• Frame rate 1/0.1 s 10 Hz
• Depth (x) is decreased by ½
• Go-return time is cut in half
• of pulses 100
• PRP 0.0005 second
• 100 x 0.0005 0.05 second
• Frame rate 1/.05 s 20 Hz

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Imaging Depth Frame Rate Inversely Related
Shallow image Higher frame rate
Deep image Lower frame rate
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Frame Rate - Calculation

• Depth 5 cm
• Scan lines 100 per image
• What is the frame rate?

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Number of Pulses per Image

• Relationship of pulses per frame vs. frame rate
• Inversely related
• Determinate of number of pulses needed to create
  each image
• Three factors
• Number of pulses per scan line
• Multi- vs. single focus
• Sector size
• Lines per angle of sector
• Line density

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Multi-focus
Single focus one pulse per scan line higher
frame rate
Multi-focus three pulses per scan line lower
frame rate
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Multi-focus

• Example
• Single focus with image made up of 100 scan lines
• One pulse per scan line 100 pulses per image
• Four focal zones with image made up of 100 scan
  lines
• Four pulses per scan line 4 x 100 400 pulses
  per image
• Main advantage of multiple focal zones
• Improves accuracy of individual images
• Each scan line has superior lateral resolution
  due to being narrow over a wide range of depths
• What do you want?
• Better temporal resolution, or
• Better lateral resolution

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Single vs. Multi-focused Image
Single Focus Higher Frame Rate
Multi-focus Lower Frame Rate
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Sector Size
Narrower Image Higher frame rates

• Sector Size Field of View
• Sector size vs. Frame rate
• Inversely related

Wider Image Lower frame rates
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Sector Size vs. Frame Rate

• 30 degree sector
• 1 transmit line for each degree of sector
• 30 sound pulses per image
• 90 degree sector
• 1 transmit line for each degree of sector
• 90 sound pulses per image
• Frame rate decreases temporal resolution is
  reduced

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Line Density

• Line density the spacing between sound beams
• Line density vs. frame rate
• Inversely proportional
• Decreased line density
• Decreased pulses per image
• Increased frame rate
• Higher temporal resolution
• Increased line density
• Increased pulses per image
• Decreased frame rate
• Lower temporal resolution

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Line Density

• Example
• 90 degree sector
• One line for every 2 degrees
• 45 pulses to make an image
• 90 degree sector
• One line for every 1 degree
• 90 pulses to make an image
• Frame time decreases
• Decreased temporal resolution

• Main advantage of high line density
• Improves the accuracy of individual images
• Aka, improved spatial resolution

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Line Density
Line Density High
Line Density Low
Frame Rate 18 Hz
Frame Rate 60 Hz
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Relationship of Temporal Resolution Image
Quality

• Increased temporal resolution
• Each image contains much less information
• Produced by
• Fewer focal points
• Narrower field of view
• Low line density
• Makes a good movie
• Needed with dynamic, fast moving structures, e.g.
  fetal heart
• Decreased temporal resolution
• Each image contains more information
• Produced by
• More focal points
• Wider field of view
• High line density
• Makes a good picture
• Non-moving structures, e.g. gallbladder

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Summary Temporal Resolution