Topic 9' Image Quality in Nuclear Medicine

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Topic 9. Image Quality in Nuclear Medicine

• Spatial Resolution, Contrast and Noise
• Evaluation of Detection and Observer Performance
• Quality Assurance of Imaging Instruments

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Spatial Resolution

• Spatial resolution refers to the ability of
  imaging instrument to provide the sharpness or
  detail of images.
• Factors affecting spatial resolution include
  collimator resolution (the main factor in nuclear
  medicine). System sensitivity requires certain
  diameter of the collimator holes etc.

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Spatial Resolution

• Factors affecting spatial resolution also include
  intrinsic resolution (due to the statistical
  variation which is photon energy dependent).
• Patient motion in respiratory or cardiac imaging
  causes image blurring
• Image display or recording system can also
  contribute the degrading of spatial resolution.

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Methods for Evaluating Spatial Resolution

• Organ phantom measurement (qualitative) such as
  brain phantom.
• Bar phantom measurement (quantitative). There are
  a number of phantoms four quadrant bar phantom,
  parallel-line phantom, orthogonal hole phantom.
• Point or Line Spread Functions.

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Bar Phantom and Line Spread Function

• The smallest bar space distinguishable can be
  regarded as the system spatial resolution
• The FWHM of a point or line spread function is
  also a parameter for the spatial resolution.
• The FWHM of the spread function is about 1.4-2
  times the resolvable bar pattern

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Frequency Response Curve

• An graph of relative output amplitude versus
  frequency is an audio equipments frequency
  response curve.
• Inexpensive audio systems generally produce the
  mid-range audio frequencies (poor response in
  low and high frequencies)

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Modulation Transfer Function

• An image system responds to the input image
  signal (spatial frequencies) is the modulation
  transfer function (contrast is the amplitude).
  MTF(?)Cout(?)/Cin(?) where Cin(Imax-Imin)/(Imax
  Imin)
• A typical nuclear medicine image system transfers
  lower image spatial frequencies.

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Modulation Transfer Function

• Good low frequency response is needed to outline
  the coarse details of the image and is important
  for the presentation and detection of relatively
  large but low contrast lesion.
• Good high frequency response is necessary to
  portray fine details and sharp edges.

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Modulation Transfer Function

• Systems spatial resolution can be described
  completely by the Modulation Transfer Function
  (MTF)
• MTF is obtained by mathematical analysis of the
  line or point spread functions
• Minimum visible bar patterns (frequency) can be
  related to the basic spatial frequency in MTF
  which drops below about 0.1.

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Modulation Transfer Function

• MTF curves can be obtained for different
  components of an imaging system and the system
  MTF is then given by MTF(?)MTFi(?)xMTFc(?)
• The MTFs can be used for the comparison of
  different systems.

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Contrast

• Image contrast refers to the differences in
  density (or intensity) in parts of the image.
• There are a number of factors that could affect
  the contrast such as the radiopharmaceuticals
  (high lesion-to-background uptake desirable)
• Film contrast (transparency has better contrast
  than Polaroid film) enhance both desire image
  contrast and noise.

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Contrast

• Background count rates can reduce the image
  contrast substantially.
• Scattered radiation and septal penetration have
  the same effect of adding background to the
  image.
• Pulse height analyzer (narrow the window) could
  be used for the scatter rejection but there is a
  trade-off (decrease counts and increase noise)

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Contrast

• Decreased contrast (background, scatter or septal
  penetration) results in poorer visibility of both
  large low contrast objects as well as fine
  details (all structures) in the image.
• Scatters add long tails to the spread function,
  suppress the low frequencies and shift the
  limiting high frequency in MTF.

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Background Removal Example
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Noise

• Image noise may be either random or structured
• Structured noise refers to non-random variation
  in counting rate superimposed on and interfering
  with the perception of the structures of interest
• Structured noise may arise from the radionuclide
  distribution itself or caused by system
  artefacts.
• Random noise is caused by statistical variation
  of count rate and is very important factor in
  nuclear medicine.

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Random Noise

• Random noise is related to the information
  density
• Information density is defined as the counts per
  unit area recorded
• Information density can be increased by increase
  count rate or imaging time
• Information density affects the minimum
  detectable size and contrast of lesions.

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Noise vs Lesion Contrast

• Noise contrast is the percentage standard
  deviation of counts recorded in an area
• A 3-5 times the noise contrast is required for a
  lesion to be detectable
• Lesion contrast requirement increases as lesion
  size decrease
• Random noise may be the detection limiting factor
  for small, low contrast lesions.

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Evaluation of Detection and Observer Performance

• Contrast-Details Studies (C-D)
• Receiver Operating Characteristics (ROC)

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Contrast-Detail Studies

• A phantom with a set of objects of varying sizes
  and contrasts is used to acquire images from
  different systems. An observer is then given the
  images randomly to identify the smallest size
  visible at each level of contrast.
• Useful for comparing the detectability of both
  large low contrast lesions as well as small
  high-contrast lesions.

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Receiver Operating Characteristic Studies

• A set of images is obtained with different image
  systems or techniques to be tested. A observer is
  asked to identify positive or negative with 4
  levels of confidence.
• True positive fraction (TPF) and False positive
  fraction (FPF) are then calculated
• The ROC curves (TPF vs FPF) are then plotted for
  each system for comparison.

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Quality Control Tests

• Operational checks and acceptance testing
• Quality parameters uniformity, spatial
  linearity, spatial resolution, count rate
  response, system sensitivity, multi-window
  spatial registration, energy resolution etc.
• Quality control testing methods common sources
  99mTc and 57Co. Intrinsic and extrinsic

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Acceptance Tests

• All parameters should be measured as the
  reference for the operational checks.
• Acceptance tests are more complicated than the
  routine tests but some of the tests are used for
  the operational checks.

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Quality Control Program

• Physicists or Technologists are responsible.
  Quality control programs depend on every
  individual institution. There are general
  standard guidelines such NEMA standard.
• Current state-of-art computers and software
  programs are available for quality control
  purpose.

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