Nuclear Imaging: Scintillation Camera I

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Nuclear Imaging Scintillation Camera I

• Design
• Performance

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Introduction

• Nuclear imaging produces images of the
  distributions of radionuclides in patients
• Uses gamma rays, characteristic x-rays, or
  annihilation photons to form images
• Instruments designed to image gamma ray and x-ray
  emitting radionuclides use collimators
• Vast majority (over 99.95) of emitted photons is
  wasted

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Development

• Earliest successful imaging device was the
  rectilinear scanner
• A single moving radiation detector sampled the
  photon fluence at a small region of the image
  plane at a time
• Improved upon by use of a large-area
  position-sensitive detector (indicating the
  location of each interaction) to sample
  simultaneously the photon fluence over the entire
  image plane
• More expensive permits more rapid image
  acquisition

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Anger scintillation camera

• Developed by Hal Anger at Berkeley from 1952 to
  1958
• Began to significantly replace rectilinear
  scanners in the late 1960s
• Spatial resolution became comparable to that of
  rectilinear scanners
• Tc-99m-labeled radiopharmaceuticals became
  commonly used (replacing I-131 and Hg-203 due to
  short half-life and lower beta emission)

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Anger camera (cont.)

• Permits more rapid acquisition of images and
  enables dynamic studies that depict the
  redistribution of radionuclides to be performed
• Scintillation camera wastes fewer photons
• Images have less quantum mottle (statistical
  noise)
• Can be used with higher resolution collimators,
  producing images of better spatial resolution
• More flexible in its positioning, permitting
  images to be obtained from almost any angle

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Design

• Scintillation camera contains a disk-shaped or
  rectangular NaI(Tl) crystal, typically 0.95 cm
  thick, optically coupled to a large number
  (typically 37 to 91) of 5.1- to 7.6-cm diameter
  PMTs.
• Some designs incorporate a Lucite light-pipe
  between the glass cover of the crystal and the
  PMTs in others, the PMTs are directly coupled to
  the glass cover

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Design (cont.)

• In most cameras, a preamp is connected to the
  output of each PMT
• Between the patient and the crystal is a
  collimator, usually made of lead, that only
  allows x- or gamma rays approaching from certain
  directions to reach the crystal
• A scintillation camera without a collimator does
  not generate meaningful images

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Design (cont.)

• The pattern of photon interactions in the crystal
  forms a 2D projection of the 3D activity
  distribution in the patient
• The PMTs closest to each photon interaction in
  the crystal receive more light than those that
  are more distant, causing them to produce larger
  voltage pulses
• Relative amplitude of the pulses from the PMTs
  contain sufficient information to determine the
  location of the interaction in the plane of the
  crystal

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Analog camera

• Position circuit received pulses from individual
  preamps and, by determining the centroid of these
  pulses, produced an X-position pulse and a
  Y-position pulse
• Summing circuit added the pulses from individual
  preamps to produce an energy (Z) pulse
  proportional to total energy deposited in the
  crystal
• Z pulse sent to a SCA produced a logic pulse
  only if Z pulse within a preset range of energies

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Analog camera (cont.)

• Analog X- and Y-position pulses and the logic
  pulse from each interaction sent to a cathode ray
  tube (CRT)
• CRT produced a momentary dot of light on its
  screen at a position determined by the X and Y
  pulses if a logic pulse from a SCA received
  simultaneously
• Photographic camera aimed at CRT recorded the
  flashes of light, forming an image on film, dot
  by dot

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Hybrid camera

• Analog position and summing circuits
• Analog X, Y, and Z pulses converted to digital
  signals by analog-to-digital converters (ADCs)
• Digital signals sent to digital correction
  circuits
• Corrected digital X, Y, and Z signals converted
  back to analog voltage pulses
• Energy discrimination done in the analog domain
  by SCAs
• Output to CRT as with analog camera or digitized
  again for display on a computer monitor

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Digital camera

• Pulses from individual PMTs are digitized
• Position signals and energy signal formed using
  digital circuitry
• Digital X, Y, and Z signals are corrected, using
  digital correction circuits, and energy
  discrimination applied, in digital domain
• Signals formed into digital images within a
  computer

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Collimators

• Most common collimator is the parallel-hole
  collimator
• Holes may be round, square, or triangular most
  state-of-the-art collimators have hexagonal holes
  and are usually made from lead foil
• Septa must be thick enough to absorb most of the
  photons incident upon them
• Collimators designed for use with radionuclides
  that emit higher-energy photons have thicker septa

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Collimators (cont.)

• Inherent compromise between spatial resolution
  and efficiency (sensitivity) of collimators
• Reducing size of holes or lengthening collimator
  to improve spatial resolution reduces efficiency
• Most scintillation cameras are provided with a
  selection of parallel-hole collimators
• low-energy, high-sensitivity
• low-energy, all-purpose (LEAP)
• low-energy, high-resolution (LEHR)

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Parallel-hole collimator

• Size of the image produced by parallel-hole
  collimator not affected by distance of object
  from collimator
• Spatial resolution degrades rapidly with
  increasing collimator-to-object distance
• In the following diagram
• R is an indicator of spatial resolution

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Pinhole collimator

• Used to produce magnified views of small objects,
  such as thyroid or hip joint
• Consists of small (typically 3- to 5-mm diameter)
  hole in a piece of lead or tungsten mounted at
  apex of a leaded cone
• Produces a magnified image whose orientation is
  reversed magnification decreases as object is
  moved away from pinhole

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Converging collimator

• Many holes, all aimed at a focal point in front
  of the camera
• Magnifies the image magnification increases as
  the object is moved away from the collimator

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Diverging collimator

• Many holes, all aimed at a focal point behind the
  camera
• Produces a minified image amount of minification
  increases as object is moved away from the
  collimator
• May be used to image a large portion of a patient
  on a small (25-cm diameter) or standard (30-cm
  diameter) FOV camera

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Image formation

• Photons from each point in the patient are
  emitted isotropically
• Some photons escape the patient without
  interaction, some scatter within the patient
  before escaping, and some are absorbed in the
  patient
• Many of the escaping photons are not detected
  because they are emitted in directions away from
  the detector
• Collimator absorbs majority of photons which
  reach it

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Image formation (cont.)

• Only a tiny fraction of emitted photons has
  trajectories permitting passage through the
  collimator holes
• Of those reaching the crystal, some are absorbed
  in the crystal, some scatter from the crystal,
  and some pass through the crystal without
  interaction
• Relative probabilities of these events depends on
  the energies of the photons and the thickness of
  the crystal

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Image formation (cont.)

• Of those photons absorbed in the crystal, some
  are absorbed by a single photoelectric
  absorption, others undergo one or more Compton
  scatters before a photoelectric absorption
• It is possible for two photons to simultaneously
  interact with the crystal
• If the energy (Z) signal from the coincident
  interactions is within the energy window of the
  energy discrimination circuit, the result will be
  a single count mispositioned in the image
• Fraction of simultaneous interactions increases
  with the interaction rate of the camera

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Image formation (cont.)

• Spatial resolution and image contrast reduced by
• Interactions in crystal of photons that have been
  scattered in the patient
• Photons that have penetrated the collimator septa
• Photons that undergo one or more scatters in the
  crystal
• Coincident interactions
• Energy discrimination circuits reduce this by
  rejecting photons that scatter in the patient or
  result in coincident interactions

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Measures of performance

• With collimator attached system or extrinsic
• With collimator removed intrinsic
• System measurements give best indication of
  clinical performance
• Intrinsic measurements often more useful for
  comparing performance of different cameras
  isolate camera performance from collimator
  performance

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Uniformity

• Measure of a cameras response to uniform
  irradiation of the detector surface
• Ideal response is a perfectly uniform image
• Intrinsic uniformity measured using a point
  source placed 4 to 5 times the largest dimension
  of the crystal away from the camera
• System uniformity measured using a uniform planar
  source in front of the collimated camera

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Spatial resolution

• Measure of a cameras ability to accurately
  portray spatial variations in activity
  concentration and to distinguish as separate
  radioactive objects in close proximity
• System spatial resolution evaluated by imaging a
  line source, such as a capillary tube filled with
  Tc-99m
• Determined by the collimator resolution and the
  intrinsic resolution of the camera

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Spatial resolution (cont.)

• Intrinsic resolution determined by acquiring an
  image with a sheet of lead containing thin slits
  placed against the uncollimated camera using a
  point source
• In routine practice, intrinsic resolution
  evaluated by imaging a parallel line or
  four-quadrant bar phantom in contact with camera
  face and visually noting the smallest bars that
  are resolved

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Spatial linearity

• Measure of the cameras ability to portray the
  shapes of objects accurately
• Determined from the images of a bar phantom, line
  source, or other phantom by assessing the
  straightness of the lines in the image
• Spatial nonlinearities can significantly degrade
  the uniformity

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Multienergy spatial resolution

• A measure of the cameras ability to maintain the
  same image magnification, regardless of the
  energies deposited in the crystal by the incident
  x- or gamma rays
• Can be tested by imaging several point sources of
  Ga-67, offset from the center of the camera,
  using only one major gamma ray at a time
• Centroid of the count distribution should be at
  the same position in the image for all three
  gamma ray energies

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System efficiency

• The fraction of x- or gamma rays emitted by a
  source that produces counts in the image
• In conjunction with imaging time, determines
  amount of quantum mottle (graininess) in the
  images
• Product of the collimator efficiency, the
  intrinsic efficiency of the crystal, and the
  fraction of interacting photons accepted by the
  energy discrimination circuits

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Collimator efficiency

• Collimator efficiency is the fraction of photons
  emitted by a source that penetrate the collimator
  holes
• A function of the distance between the source and
  the collimator and the design of the collimator
• Intrinsic efficiency is the fraction of photons
  penetrating the collimator that interact with the
  crystal

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Energy resolution

• A measure of its ability to distinguish between
  interactions depositing different energies in its
  crystal
• A camera with superior energy resolution is able
  to reject a larger fraction of photons that have
  scattered in the patient and coincident
  interactions, producing images with better
  contrast
• Calculated from the full-width-at-half-maximum
  (FWHM) of the photopeak divided by the energy of
  the photon, and expressed as a percentage

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Count rate performance

• Usually specified by
• Observed count rate at 20 count loss (typically
  110,000 to 260,000 counts/sec without scatter)
• The maximal count rate (typically 170,000 to
  500,000 counts/sec without scatter)
• Both are reduced when measured with scatter
• High count rates usually achieved by implementing
  a high count-rate mode that degrades spatial and
  energy resolutions