Nuclear Medicine Image Acquisition Methods

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Nuclear Medicine Image Acquisition Methods

• Chapter 5, New Chapter 7

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Methods of Data Acquisition

• For every photon emitted from the radionuclide
  within the patient and entering the gamma camera,
  three electrical pulses are produced. (X, Y, Z)
• Two of the pulses represent the coordinates on
  the photon interaction site in the plane of the
  NaI crystal. The third pulse represents the
  energy of the incident photon and acts as trigger.

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Methods of Data Acquisition

• Signals produced by the gamma camera are analog
  in nature, they must first be converted to an
  equivalent binary signal using the
  analog-to-digital converter (ADC) before they can
  be understood by the computer.

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Methods of Data Acquisition

• Once the ADC conversion is done, the count is
  entered into the computer memory by one of the
  two following data acquisition methods
• 1. The frame mode
• 2. The list mode

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Frame Mode Acquisition

• Frame mode is the principal method of recording
  counts from the gamma camera to produce a
  radionuclide image.
• The square mosaic commonly (matrix) consists of
  64 x 64, 128 x 128, or 256 x 256 picture elements
  (pixels).
• Thus there are 4K, 16K, and 64K pixels forming
  one radionuclide image.

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Frame Mode Acquisition
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Frame Mode Acquisition

• Each pixel in the image matrix has a one-to one
  correspondence with a given location in the plane
  of the NaI crystal.
• For every scintillation event that produces a
  signal acceptable to the pulse-height analyzer,
  the number of counts in the pixel corresponding
  to the coordinates of the scintillation site is
  incremented by one.

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Frame Mode Acquisition

• If a 64 x 64 matrix is circumscribing a gamma
  camera having a 40-cm diameter field of view,
  then each pixel in the image matrix covers an
  area of 6.25 x 6.25 mm on the face of the camera.
  (400mm/64)
• Spatial resolution of the computer image is
  increased as the number of pixels in the image
  matrix is increased

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Frame Mode Acquisition

• The number of counts should be quadrupled
  whenever the dimensions of an image array are
  doubled
• If the counts are only doubled a higher
  statistical noise is the trade-off for better
  spatial resolution

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Frame Mode Acquisition

• Size of acquisition see text
• Zooming-decreases the background counts and
  increases the resolution through magnification
• The signals for each count are amplified by a
  factor greater than that used in normal mode.

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Frame Mode Acquisition

• The spatial resolution of the computer image is
  ultimately limited by the resolution of the gamma
  camera
• For a 40-cm diameter or a 40 x 50-cm
  field-of-view gamma camera, the maximum camera
  resolution is adequately approximated by a 256 x
  256 matrix

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Byte-Mode vs. Word-Mode Acquisition

• Complete image matrix-includes number of bytes
  used to form a pixel, plus number if rows and
  columns of pixels in matrix

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Byte-Mode vs. Word-Mode Acquisition

• Byte-mode
• Max number of counts in each pixel is 255, extra
  counts causes pixel saturation
• Image contrast and resolution are reduced with
  pixel saturation
• Count density changes in the region over the
  saturated pixels cannot be measured

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Byte-Mode vs. Word-Mode Acquisition

• Word-mode
• Each pixel is 16 bits deep, max number of counts
  is 65,535, no pixel saturation
• Allows more counts to accumulate in each pixel
  that the byte mode
• Twice as much disk and memory needed for each
  image

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Dynamic Frame-Mode Acquisition

• The computer is instructed to acquire a series of
  images, each for a predetermined length of time
• Useful for visualization and quantification of
  flow kinetics of a radionuclide
• Total number of frames computer acquires per unit
  time is the framing rate

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Multiple-gated and List-mode Acquisition

• List-mode
• Short duration acquisition at very high transient
  count rates
• Multiple-gated
• A type of dynamic frame acquisition that uses a
  physiologic signal, i.e. the ECG pulse
• Both detailed later