Nuclear Imaging: Emission Tomography I

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Nuclear ImagingEmission Tomography I

• Single Photon Emission Computed Tomography (SPECT)

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SPECT

• Single photon emission computed tomography
  (SPECT) generates transverse images depicting the
  distribution of x- or gamma ray emitting nuclides
  in patients
• Standard planar projection images are acquired
  from an arc of 180 degrees (most cardiac SPECT)
  or 360 degrees (most noncardiac SPECT) about the
  patient

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SPECT (cont.)

• Most SPECT systems use one or more scintillation
  camera heads that revolve about the patient
• Transverse images are reconstructed using either
  filtered backprojection (as in CT) or iterative
  reconstruction methods

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Image acquisition

• Camera head or heads revolve about the patient,
  acquiring projection images from evenly spaced
  angles
• May acquire images while moving (continuous
  acquisition) or may stop at predefined angles to
  acquire images (step and shoot acquisition)
• Each projection image is acquired in frame mode

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Image acquisition (cont.)

• If camera heads produced ideal projection images
  (i.e., no attenuation by patient and no
  degradation of spatial resolution with distance
  from camera), projection images from opposite
  sides of patient would be mirror images
• 180-degree arc would be sufficient for transverse
  image reconstruction
• Attenuation greatly reduces number of photons
  from activity in the half of patient opposite
  camera head this information is blurred by
  distance

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Image acquisition (cont.)

• SPECT projection images usually acquired in
  either a 64 x 64 (60 or 64 projections) or a 128
  x 128 (120 or 128 projections) pixel format
• Using too small a pixel format reduces spatial
  resolution of the projection images and of the
  resultant reconstructed transverse images
• Using too few projections creates radial streak
  artifacts in the reconstructed transverse images

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Cardiac image acquisition

• Most cardiac SPECT studies acquired with a
  180-degree arc from the 45-degree right anterior
  oblique (RAO) view to the 45-degree left
  posterior oblique (LPO) view
• Projection images of the heart from the opposite
  180 degrees have poor spatial resolution and
  contrast due to greater distance and attenuation

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Orbits

• Camera heads on older SPECT systems used circular
  orbits around the patient while acquiring images
• Satisfactory for imaging of the brain
• Loss of spatial resolution in body imaging
  because of distance from surface
• Newer systems provide noncircular orbits that
  keep camera heads in close proximity to surface
  of body throughout the orbit

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Transverse image reconstruction

• After projection images are acquired, they are
  usually corrected for axis-of-rotation
  misalignments and for nonuniformities
• Following these corrections, transverse image
  reconstruction is performed using either filtered
  backprojection or iterative methods

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Filtered backprojection

• Similar technique as for CT imaging
• Choice of filter kernel for a particular type of
  study is determined by the amount of statistical
  noise in the projection images
• Mainly determined by injected activity,
  collimator, and acquisition time per image
• and their spatial resolution
• Determined by collimator and the typical
  distances from the camera head(s) from the organ
  being imaged

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Filter kernels

• Projection images of better spatial resolution
  and less quantum mottle require a filter with
  higher spatial frequency cutoff to avoid loss of
  spatial resolution in the reconstruction
  transverse images
• Projection images of poorer spatial resolution
  and greater quantum mottle require a filter with
  lower spatial frequency cutoff to avoid excessive
  quantum mottle in the reconstructed transverse
  images

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Iterative reconstruction

• An initial activity distribution in the patient
  is assumed
• Projection images are calculated from the assumed
  activity distribution, using the known
  characteristics of the scintillation camera
• Calculated projection images are compared with
  actual projection images and, based on this
  comparison, the assumed activity distribution is
  adjusted
• Process repeated several times until calculated
  projection images approximate the actual ones

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Iterative reconstruction (cont.)

• Calculation of projection images takes into
  account the decreasing spatial resolution with
  distance from the camera face
• If a map of the attenuation characteristics of
  the patient is available, the calculation of the
  projection images can include the effects of
  attenuation
• Iterative methods can partially compensate for
  effects of decreasing spatial resolution with
  distance, photon scattering in the patient, and
  attenuation in the patient

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Attenuation correction

• X- or gamma rays that must traverse long paths
  through the patient produce fewer counts, due to
  attenuation, than those from activity closer to
  the near surface of the patient
• Transverse image slices of a phantom with a
  uniform activity distribution will show a gradual
  decrease in activity toward the center
• Attenuation effects are more severe in body SPECT
  than in brain SPECT

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Attenuation correction (cont.)

• A common correction method assumes a constant
  attenuation coefficient throughout the patient
• Some SPECT cameras have radioactive sources to
  measure the attenuation through the patient
• After acquisition, the transmission projection
  data are reconstructed to provide maps of tissue
  attenuation characteristics across transverse
  sections of the patient, similar to x-ray CT
  images
• These attenuation maps are used during SPECT
  image reconstruction to provide
  attenuation-corrected SPECT images

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Attenuation correction (cont.)

• Transmission data usually acquired simultaneously
  with the acquisition of the emission projection
  data
• Performing the two separately poses significant
  problems in the spatial alignment of the two data
  sets
• Radionuclide used for transmission measurements
  is chosen to have primary gamma-ray emissions
  that differ significantly in energy from those of
  the radiopharmaceutical

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SPECT collimators

• Most commonly used is the high-resolution
  parallel-hole collimator
• Fan-beam collimators mainly used for brain SPECT
• FOV decreases with distance from collimator
• If used for body SPECT, portions of the body are
  excluded from the FOV, creating artifacts in the
  reconstructed images

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Multihead SPECT cameras

• Two or three scintillation camera heads reduce
  limitations imposed by collimation and limited
  time per view
• Permits use of higher resolution collimators for
  a given level of quantum mottle
• Requirement for electrical and mechanical
  stability of the camera heads
• Y-offsets and X- and Y-magnification factors of
  all heads must be precisely matched throughout
  rotation

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