Topic 8' Gamma Camera II

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Topic 8. Gamma Camera (II)

• Basic Performance Characteristics
• Detector Limitations
• Design and Performance Characteristics of
  Parallel-Hole Collimators
• Performance Characteristics of Converging,
  Diverging, and Pinhole Collimators
• Measurements of Gamma Camera Performance.

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Intrinsic Spatial Resolution Limits

• Intrinsic spatial resolution refers to the limit
  of spatial resolution achievable by the detector
  and the electronics
• Intrinsic spatial resolution is limited primarily
  by two factors multiple scattering and
  statistical fluctuation in the distribution of
  light photons (the later is the main factor).
• Intrinsic resolution depends on detector crystal
  thickness and ? ray energy. A thinner crystal is
  used for Anger camera to achieve better spatial
  resolution.
• Larger number of PM tubes and improvement of
  light collection efficiency result in better
  resolution.

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Intrinsic Resolution vs Photon Energy
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Intrinsic Resolution vs Crystal Thickness
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Detection Efficiency

• The crystal thickness of NaI(Tl) in Anger camera
  is smaller (6-12mm) than that of a well counter
  (2-5cm).
• Anger camera is designed for optimal detection of
  ? ray energies of 100-200keV.

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Energy Resolution
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Problems at High Counting Rates

• Pulse pile-up is the problem at high count rate
  and it results in count loss and image
  distortion.
• Count loss depends on the whole energy spectrum
  but the apparent dead-time depends on the window
  fraction. Scatter causes narrower of the window
  fraction therefore longer dead-time.

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Problems at High Counting Rates

• Pulse pile-up causes image distortion. Two
  scattered event may be added to form a photo-peak
  which produces a location between the two
  scattered events.

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Problems at High Counting Rates

• The general effect of the pulse pile-up is to
  cause a loss of image contrast and details

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Problems at High Counting Rates

• Pulse pile-up rejection circuit can be used to
  improve the pulse pile-up but it will reduce the
  maximum achievable count rate
• Deadtime can be improved by shortening the
  effective charge integration time for the output
  signal from the PM tube but it will reduce the
  amount of light collected by the PM tube,
  therefore, degrade the intrinsic resolution.

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Problems at High Counting Rates

• Count rate performance should be one of the
  important factors for Anger cameras

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Pulse Pile-up Correction
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Image Non-Linearity

• Non-linearity refers to that a straight line
  object appear as curved line image
• Non-linearity occurs when the X and Y positional
  signals do not change linearly with the
  displacement distance.
• Pincushion distortion is an inward bowing
  image and Barrel distortion is an outward
  bowing image
• A PM tube may have high light collection
  efficiency in the centre which may result in a
  pincushion in the centre and barrel between
  PM tubes. This kind of images could results in a
  wavy line pattern.

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Nonlinearity Example
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Image Non-Uniformity

• Non-Uniformity refers to the intensity variation
  from a uniform flood source
• The causes could be the non-uniform detection
  efficiency (small differences in pulse height
  spectrum) and non-linearity of PM tubes (more
  server) or instrument malfunctions.
• Edge packing refers to the bright ring around
  the edge of the image. It is caused by the
  internal light reflection of the detector
  crystal. Edge packing is usually masked and the
  useful field of view is smaller than the actual
  detector size

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Non-Uniformity Correction Techniques

• Cosmetic approach
• Adjust individual PM tube gains (compensate for
  the detection efficiency difference)
• Correction factors matrix. Test image is
  normalised and the correction factor for each
  matrix element is used to add or subtract from
  the image.
• Advanced approach
• A set of microprocessors are used to store
  correction matrices for regional differences in
  pulse height spectra and for position distortion.
  
• Images are corrected on event by event basis (on
  fly).

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Non-uniformity Correction Example
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Uniformity Profiles
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Gamma Camera Tuning
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Design and Performance Characteristics of
Parallel Hole Collimators

• Basic limitations in collimator performance
• Septal thickness
• Geometry of collimator holes
• System resolution

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Basic Limitation in Collimator Performance

• Collimator is a limiting factor in camera system
  performance.
• Collimator resolution refers to the sharpness or
  details of the ? ray image projected onto the
  detector --- worse than the intrinsic resolution.
• Collimator efficiency refers to the fraction of ?
  rays that pass through it -- a few percent or
  less.

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Septal Thickness

• Septal thickness is designed to prevent ? rays
  from penetrating from one hole to another (allow
  less than 5 to pass through).
• Septal thickness should be as small as possible
  in order to gain maximum efficiency
• High atomic number and density material should be
  used (lead is the choice)

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Septal Thickness

• The required thickness t2dw/(l-w) where
  (t/w)(2dt)/l if ? is small
• If 5 penetration is allowed,
  tgt6d/µ/l-(3/ µ) because e-µwlt0.05 therefore
  µwgt3

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General Comments

• The septal thickness depends on the ? ray
  energies to be used because µ depends on the ?
  ray energy.
• Energy ranges in nuclear medicine are often
  classified as low energy lt150keV medium energy
  lt400keV and high energy lt1Mev.
• Low energy collimators are generally fragile
  because they are only a few tenth of mm.
• Low energy collimators are used whenever possible
  to obtain maximum collimator efficiency (foggy
  background may be superimposed on the image for
  high energy).

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Geometry of Collimator Holes

• The performance of a collimator is affected by
  its shape, length and diameter (round or
  hexagonal are the best)
• Spatial resolution and detection efficiency are
  the two important performance parameters
• Collimator spatial resolution is defined as the
  FWHM of the radiation profile from a point or
  line source of radiation projected by the
  collimator onto the detector.

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Collimator Resolution

• A parallel holes collimator resolution is given
  by Rcd(leb)/le where le l-2µ-1 is the
  effective collimator length (taking into account
  of the ? ray penetration)

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Collimator Resolution (2)

• Spatial resolution of a parallel collimator
  increases (worse) as the distance between the
  collimator and the source increased

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Collimator Efficiency

• The detection efficiency is given by
  gK2(d/le)2d2/(dt)2
  where K is a constant that depends on the holes
  shape. The first part is the geometric factor
  (solid angle subtended by a collimator hole) and
  the second part is the fraction that is not
  covered by the septa (the area ratio of the holes
  and total detector).

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Collimator Efficiency

• Collimator efficiency for a source in air is
  independent of source-to-collimator distance b.

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Collimator Efficiency

• Invariance of collimator efficiency with source
  to collimator distance applies to point sources,
  line sources and uniform sheet source in air with
  parallel hole collimators.

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Resolution and Efficiency

• The relationship between the resolution and
  efficiency is gRc2
• For a given septal thickness, collimator
  resolution is improved only at the expense of
  decreased collimator efficiency and vice versa.

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System Resolution

• System resolution depends on a number of factors
  such as, scattering, septal penetration,
  intrinsic resolution and collimator resolution
  with the collimator and intrinsic resolutions are
  the main factors.
• System resolution (consider intrinsic and
  collimator only) can be expressed as Rs2Ri2Rc2
• System resolution is determined primarily by
  collimator resolution.

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Performance Characteristics of Converging,
Diverging and Pinhole Collimators
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Performance Characteristics of Converging
Collimator
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Performance Characteristics of Diverging
Collimator
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Performance Characteristics of Pinhole Collimator
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Performance Characteristics of Converging,
Diverging and Pinhole Collimators
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Some Comments

• Resolution always is best with the source as
  close as possible to the collimator
• For point source in air, the efficiency increases
  as the source to collimator distance increase
  with converging collimator (maximum at focus
  point) and decreases for diverging and pinhole
  collimators. No change for flood source as long
  as the source cover the entire detector.
• Diverging, converging and pinhole collimators may
  be useful for the change of field of view but the
  image distortion caused by the magnification with
  depth may be a problem.

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Measurements of Gamma Camera Performance

• Intrinsic Resolution
• System Resolution
• Spatial Linearity
• Uniformity
• Counting Rate Performance
• Energy Resolution
• System Sensitivity