Topic 7. Gamma Camera (I)

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Topic 7. Gamma Camera (I)

• General Comments
• Basic Principles of the Anger Camera
• Types of Gamma Cameras

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General Comments

• Why ? rays? (penetrating through the body, easily
  stopped by lead, ß emission or Auger electrons
  can not get out of body)
• Why NaI(Tl) detector (reasonable compromise
  between efficiency and cost etc. )
• Historically, ? ray imaging started from matrix
  detectors of late 40s to rectilinear scanner, and
  to the Anger scintillation camera of late 50s
  which is the most used today.

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Basic Principles of the Anger Camera

• System Components
• Detector System and Electronics
• Collimators
• Event Detection in Gamma Cameras

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System Components

• Collimator
• NaI(Tl) crystal
• Light Guide (optical coupling)
• PM-Tube array
• Pre-amplifier
• Position logic circuits (differentialaddition
  etc.)
• Amplifier (gain control etc)
• Pulse height analyzer
• Display (Cathode Ray Tube etc).

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NaI(Tl) Crystal Assembly
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Detector System and Electronics(1)

• Typical detector in Anger camera NaI(Tl) crystal
  with 1.25cm thick x 30-50 cm in diameter (thinner
  for low energies, 6mm)
• Thinner crystal is preferred for Anger camera in
  order to get better intrinsic resolution
  therefore better image (sacrifice intrinsic
  efficiency)

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Detector System and Electronics(2)

• Optical coupling materials (silicon fluid,
  grease, or lucite light pipes) are placed between
  the NaI(Tl) crystal and the array of
  photomultiplier tubes --called light guide or
  pipe
• Array of PM tubes (37,61,75 or 91, round,
  hexagonal or square shapes) arranged in hexagonal
  pattern

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Detector System and Electronics(3)

• Part of the signal processing circuitry
  (preamplifier, pulse height analyzers, amplifier,
  pulse pile-up rejection etc.) is attached to each
  PM tube and sealed in a light-tight protective
  housing

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Position Circuitry(1)

• The photomultiplier tubes are divided into
  horizontal halves to obtain X and X- signals and
  vertical halves to obtain Y and Y- signals.
• Four summing matrix circuits are used to sum up
  for x,x-,y and y- signals from each TM tubes
  where each of these signals is the product of
  signal amplitude and position factor.
• A separate summing circuit is used to sum up a
  total signal Z from all PM tubes (signal
  amplitude only, no position factor)

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Light Sharing Between PM Tubes
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Position Circuitry(2)

• The radiation position is then determined by
  Xk(X-X-)/Z and Yk(Y-Y-)/Z where k is a scale
  constant, Z is the total signal amplitude and
  proportional to the incoming radiation energy.
• The positional signals X and Y must be normalised
  by total signal Z because X and Y themselves
  depend on the both signal and positional factors
  (different radiation energy gives different
  signal amplitude at the same position)

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PM Tubes and Signal Positions
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PMT Energy Window Correction
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Pulse Height Displays

• Pulse height analyzer is used to analyse the Z
  signal and if accepted, signal will be displayed
  on the monitor (CRT etc.) at the position
  determined by X and Y.
• Two kind of display modes can be used to display
  the energy spectrum, namely, Z pulse display and
  multi-channel analyzer display.

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Collimators

• Absorptive collimation is used for most ? ray
  image formation (inefficient method for
  utilisation of radiation)
• Four basic collimator types are used with Anger
  camera and similar camera-type imaging device
  pinhole, parallel hole, diverging and converging
  collimators.

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Parallel-Hole Collimator
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Pinhole Collimator(1)

• A cone shape lead with a small pinhole of a few
  millimeter in diameter, about 20-25 cm from the
  pinhole aperture to the detector.
• The image is inverted and could be magnified or
  minified depend on the object position I/Of/b
  where I and O are image and object sizes
  respectively, f is the distance between the
  pinhole and the detector, b is the distance
  between the object and the pinhole.

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Pinhole Collimator(2)

• The size of the imaged area changes with the
  distance between the object and the collimator b
  DD/(I/O) where D is the diameter of the
  detector and D is the images area.
• Image size changes with the distance between the
  object and the collimator b, therefore, image is
  distorted in 3 dimension.

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Parallel Hole Collimator

• Parallel holes are drilled or cast in lead
• Sept is the walls between holes and its thickness
  is chosen to prevent ? rays from crossing from
  one hole to the next.
• Image is the same size as the source distribution
  to the detector.
• Slant-hole collimator is a titled parallel holes
  collimator.

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Diverging Collimator

• Diverging from the collimator face towards the
  object.
• The converging point is about 40-50cm behind from
  the collimator.
• Image is minified I/O(f-t)/(fb) where f is the
  distance between the front of the collimator and
  the converging point, t is the thickness of the
  collimator and b is the distance between the
  object and the front of the collimator.
• Useful image area becomes larger as the image
  becomes more minified. Image size depends on the
  object distance b (image has distortion).
• Useful for small detector to image large organ.

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Converging Collimator

• Holes converge to a point in front of the
  collimator (about 40-50 cm from the collimator)
• Images are magnified non-inverted if the
  objects are placed between the converging point
  and the collimator surface I/O(ft)/(ft-b).
  (image distortion due to b dependence)
• Images are inverted magnified if the objects
  are placed between the converging point and twice
  the convergence length and an inverted minified
  image beyond that distance (not often used).
• Useful for using large detector to image small
  organs

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Parallel vs Converging Collimators
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Gamma Camera Detection Events
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Image Display and Recording Systems(1)

• Persistence CRT displays light spots that do not
  fade immediately
• Non-persistence CRT display images for film
  decording
• Polaroid film is convenient in use but expensive.
  It is a positive type and has limited range of
  optical densities (limit both contrasts and
  latitude, the useful exposure range)
• Transparency film is a negative film type (darker
  for greater exposure) and has better contrasts
  and latitude than Polaroid film

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Image Display and Recording Systems(2)

• Polaroid cameras often have three separate
  lenses, with different lens aperture opening, to
  provide three deferent densities on a single film
  simultaneously.
• Laser film printers are now replacing the old
  film making practice in nuclear medicine.

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Stationary or Mobile

• Anger camera can be sued for static or dynamic
  imaging
• Stationary cameras are designed to be at a fixed
  location while mobile camera has wheels.

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Types of Gamma Camera
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Types of Gamma Camera
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Types of Gamma Camera
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Scanning Camera

• Scanning Anger cameras are used for whole body
  imaging
• Either detector or patient support table may be
  set to move
• Diverging collimator may be used to cover entire
  width of the patients body.
• Whole body images can be printed on a single film.

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Whole Body Bone Scan
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Dynamic Sequence of Planar Images
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