Introduction to QC in Nuclear Medicine

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					Introduction to QC in Nuclear
  Medicine Instrumentation
  ◎ Survey meter
  ◎ Dose calibrator
  ◎ Scintillation well counter & thyroid
      t k
    uptake probe b
  ◎ Gamma camera
  ◎ SPECT
                                               元培放射系 林俊良編製




  Preface to Instrumentation QC and QA

◎ Quality control (QC) refers to the testing
  designed to identify equipment problems.
◎ Quality assurance (QA) refers to a
  system designed to identify problems,
  propose solutions and monitor results to
                               performance.
  achieve the desired level of performance

IAEA:International Atomic Energy Agency
NEMA:National Electrical Manufacturers Association

                                               元培放射系 林俊良編製




                                                             1
         Gas-Filled Detector




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         Gas-Filled Detector (continued)

◎ Characteristics of the Major Voltage Regions Applied
  Across a Gas-Filled Detector




                                                   元培放射系 林俊良編製




                                                                 2
          Gas-Filled Detector (continued)

◎ Recombination and ionization region
  When the voltage between the electrodes is relatively low, the
  field within the gas is weak and many of the ions simply
  recombine, leaving only a small fraction to reach the
  electrodes. (recombination region)

              ionization    At a somewhat higher voltage,
   recomb-      region      referred to as the ionization region,
    ination
                            most of the ions that are formed
                                 h the l t d
                            reach th electrodes.
                            A further, small increase in the
                            voltage does not increase the
                            current once the voltage is sufficient
                            to collect 100% of the ions formed.
                                                          元培放射系 林俊良編製




          Gas-Filled Detector (continued)

◎ Application of ionization region: Dose Calibrator




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                                                                        3
        Gas-Filled Detector (continued)

◎ Geiger region
                         As the voltage is increased into this
                         region,
                         region most of the gas within the
                  Geiger detector is massively involved in the
                  region multiple, successive ionizations.
                         (Avalanche)
                         Once all the gas is involved, no
                         greater gas amplification is possible
                         so that any further increases in
                           lt    have littl effect on th size of
                         voltage h     little ff t    the i    f
                         the pulse of current.
                           A detector operating in this region is
                           called a Geiger counter, or Geiger-
                           Mueller counter.

                                                           元培放射系 林俊良編製




                                                           元培放射系 林俊良編製




                                                                         4
         Gas-Filled Detector (continued)

◎ Principles of Measurement
             g
  a. Measuring Current
   The gas-filled detector has both a positive and a negative
   electrode.
   The positive and negative ions produced in the gas by the
   radiation move in opposite directions, positive ions toward the
   negative cathode and the negative ions toward the anode.
   (Ex:ionization chamber, dose calibrator)
  b. Counting P l
  b C               f Current
         ti Pulses of C     t
   The alternative to measuring the current is counting the
   individual pulses produced as each individual charged particle
   or photon enters the gas.
   (Ex:Geiger counter)

                                                          元培放射系 林俊良編製




        QC of Survey Meter
    ◎ Calibration
     — before initial use and annually
    ◎ Constancy (reference check source)
     — before each day use
    ※ These tests should also be performed following
      adjustment or repair of the instrument.




                                                          元培放射系 林俊良編製




                                                                        5
     Calibration in Survey Meter
◎ The readings of the test are taken from two long-
  lived sources : 57Co & 137Cs
  (The radioactivity of sources should be enough to
  generate readings at approximately one third and
  two third of full scale.)
  — Detect sources from zero to one meter by every
      ten centimeters.
                  10 cm                    1m




                                                元培放射系 林俊良編製




     Calibration in Survey Meter (continued)
— The readings must be within 20% of their expected
  measurement.




                                                元培放射系 林俊良編製




                                                              6
                                                    元培放射系 林俊良編製




     Constancy in Survey Meter

◎ The records of the test are taken by a long-lived
  source (usually 137Cs )

  — Detect the reference source every day in same
   distance.
                       same distance




   — The daily readings must be within 10% of its
     original value.
                                                    元培放射系 林俊良編製




                                                                  7
     QC of Dose Calibrator

◎ Constancy
    daily
 — d il
◎ Linearity
 — at installation and quarterly
◎ Accuracy
 — at installation and annually
◎ Geometry
 — at installation
※ These tests should also be performed following
  adjustment or repair of the instrument.


                                                 元培放射系 林俊良編製




     Constancy in Dose Calibrator

◎ Constancy testing determines the reproducibility of
                                           y
  measurements of a source of known activity from
  day to day.
  — Records are taken by measuring of 137Cs
    standard source.
  — Energy window is setting on same channel.




                                                 元培放射系 林俊良編製




                                                               8
     Constancy in Dose Calibrator                   (continued)


  — The readings should not vary by more than 10%
  from the value recorded at the initial accuracy test.


                     +10%
          Activity
          A




                     -10%
                                standard activity
                                measured activity

                 0          (day)              30

                                                         元培放射系 林俊良編製




     Linearity in Dose Calibrator

◎ A linearity test ascertains how accurately a dose
                                                 g
  calibrator measures activities over a wide range,
  from microcurie to millicurie amounts.
  ※Two methods are available:
        - decay method
        - shielding method (lead sleeves method)
  — (decay method) Record the activity of prepared
    99mTc sample from the highest dose (30 mCi)

    down to 10 μCi by waiting hours.
  — (decay method) Calculate the predicted activity
    for each time the sample was measured.


                                                         元培放射系 林俊良編製




                                                                       9
     Linearity in Dose Calibrator (continued)

◎ Measured activities should be within 10 % of
  calculated activities.




                                                     元培放射系 林俊良編製




     Linearity in Dose Calibrator (continued)

◎ Shielding method
 - A more rapid technique is to measure the dose
     unshielded, then repeat the measurement of the
     same dose shielded within lead sleeves of varying
     thickness.




                                                     元培放射系 林俊良編製




                                                                   10
     Accuracy in Dose Calibrator

◎ Accuracy testing assesses the ability of a dose
                         y                    y
  calibrator to accurately measure the activity of
  radionuclides of different gamma energies.
  — Use at least two different energy sources, such
    as 57Co and 137Cs.
   ※ One of which must have a photon energy
      between 100 and 500 keV
  — Calibrate readings and compare with national
    standard institute.
  — The readings must be within 10% of reference
    standards.

                                                     元培放射系 林俊良編製




     Geometry in Dose Calibrator

◎ Evaluation of geometric variation determines the
               p                    g
  effect of sample volume and configuration on the
  measurement of a sample’s activity.

       1 mCi in o.1 mL   ≠   1 mCi in 1.0 mL

       1 mCi in a vial   ≠ 1 mCi in a syringe




                                                     元培放射系 林俊良編製




                                                                   11
        Geometry in Dose Calibrator (continued)

  — Measure a small amount of activity of sample
    in every probable containers, such as vials, syringes
    and tubes.
  — Increase the volume in the container stepwise by
    adding water, then measure the activity.
  — Calculate the correction factor of each container (or
    each diluted volume.
                            t      ti it
                            true activity
    correction factor =   measured activity

    calibrated activity = measured activity × correction factor


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         Nonimaging Scintillation Detector

◎ Sodium iodide crystal scintillation detector




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                                                                        12
        Nonimaging Scintillation Detector (continued)

◎ Pulse-Height Analyzer




                                                 元培放射系 林俊良編製




        Nonimaging Scintillation Detector (continued)

◎ Thyroid Probe                   ◎ Well Counter




                                                 元培放射系 林俊良編製




                                                               13
       QC of
       Scintillation Counter & Thyroid Uptake Probe

 ◎ Background measurement
  — daily
 ◎ Calibration
  — daily
 ◎ Sensitivity
  — daily
 ◎ Energy resolution
  — quarterly
 ◎ Chi-square test
  — quarterly
 ※ These tests should also be performed following
   adjustment or repair of these instruments.
                                                      元培放射系 林俊良編製




     Calibration in
     Scintillation Counter & Thyroid Uptake Probe

— Use a monoenergetic gamma source of known energy,
   for e a p e, 137Cs o 57Co.
    o example,        or Co
— Adjusting the upper and lower level discriminations (ULD
   and LLD) of the
   pulse-height
   analyzer.
  For 137Cs, the LLD
  and ULD would be
  set at 652 and 672
  keV, for a window
  of 3%.

                                Tc-99m


                                                      元培放射系 林俊良編製




                                                                    14
     Calibration in
     Scintillation Counter & Thyroid Uptake Probe (continued)

— Beginning with the lowest voltage setting, count the
  source for 1 min at each increasing voltage setting,
                                     g     g        g
  increasing the voltage until the maximum count
  rate is observed.
                          Window

     Counts
      per
     minute



                          Voltage

                                                    元培放射系 林俊良編製




     Sensitivity in
     Scintillation Counter & Thyroid Uptake Probe


                                        (Probe)
  Displace a standard source
— Di l         t d d
  against the counting tube
  (probe) with the adequate
  distance, then calculate the
  counting efficiency.                 (source)


         sensitivity          measured count rate
                          =
    (counting efficiency)       true decay rate


                                                    元培放射系 林俊良編製




                                                                  15
    Sensitivity in
    Scintillation Counter & Thyroid Uptake Probe (continued)

◎ Decision of adequate distance in thyroid probe




          (probe)      (graph paper)
                                                point
                                               source




                                                   元培放射系 林俊良編製




    Sensitivity in
    Scintillation Counter & Thyroid Uptake Probe (continued)

◎ Isoresponse dose curve




                                                   ±5%




                                                   元培放射系 林俊良編製




                                                                 16
        Energy resolution in
        Scintillation Counter & Thyroid Uptake Probe

◎ If the conversion of light photons in a
  sodium iodide detector to an electrical
                perfect,
  pulse were perfect the photopeak in a
  gamma spectrum would look like a
  narrow line.
  - A long-lived source (usually 137Cs) is
      placed in the well counter or in front
      of the probe.                             1
  - The energy spectrum of this source
      is plotted by measuring counts in
         p        y          g                          FWHM
      sequential 10-keV window.                0.5
  - Calculate the full width at half
      maximum (FWHM) and the % energy
      resolution.
                                FWHM (keV)           629 662 695
       % energy resolution =                            (keV)
                               photopeak (keV)
                                                                元培放射系 林俊良編製




        Chi Square test in
        Scintillation Counter & Thyroid Uptake Probe

◎ The chi square test is a method to check for random
  errors greater than those that would be predicted.
   Random errors affect the reproducibility (or
   precision) of measurements.
    - Using a long-lived standard (137Cs or 57Co), collect
      a series of measurements (20-50), each for a
      preset time.

                                     Ni: individual measurement
                                      n:number of measurement
                                     N: mean (average) value
                                     X 2: chi square value
                                                                元培放射系 林俊良編製




                                                                              17
         Chi Square test in
         Scintillation Counter & Thyroid Uptake Probe (continued)


(for example:)




                                                     (10005-10016)2




                                                           元培放射系 林俊良編製




 Table
  of
  Chi
Square

                                                       b bilit
                                                    probability




   ※ Acceptable p values are between 0.1 and 0.9.
     Values greater than 0.9 or less than 0.1 indicate that the
     variations in the measurements do not match the expected
     and the instrument should be checked.
                                                           元培放射系 林俊良編製




                                                                         18
          Imaging System (Gamma Camera)




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           Collimators

Collimators are composed of
thousands of precisely
  li  d holes (channels),
aligned h l ( h         l )
which are formed by either
casting hot lead or folding
lead foil.

The collimator conveys
toward the crystal only those
  h t    t    li directly
photons traveling di tl
along the long axis of each
hole.




                                          元培放射系 林俊良編製




                                                        19
         Collimators (continued)


casting hot lead                                  folding lead foil




                                                             元培放射系 林俊良編製




         Collimators        (continued)


  Photons emitted in all other       Without a collimator in front of
  directions are absorbed by         the crystal, the imaging would
  the septa between the holes.
         p                           be indistinct.




                                                             元培放射系 林俊良編製




                                                                           20
For the same bore length, the smaller the diameter
the higher the resolution




                                                     For the same hole diameter, the
                                                     higher the bore the higher the
                                                         l ti
                                                     resolution




                                                                       元培放射系 林俊良編製




 ◎ Angle of
   acceptance




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                                                                                       21
            Parallel-Hole Collimators (continued)

◎ High- and Medium-Energy Collimators

   Low-energy                                         higher energy
   Low energy collimators are not adequate for the higher-energy
   photons of nuclides such as 67Gallium, 131Iodine, 111Indium, nor for
   positron emitters such as 18Fluorine.

   High-energy collimators with thicker septa
   to reduce septal penetration.
   High-energy collimators are useful for
   131
       Iodine and required for 18Fluorine
                                 Fluorine.

   Medium-energy collimators can be used
   to image photons emitted by 67Gallium and
   111
       Indium.


                                                                   元培放射系 林俊良編製




            Parallel-Hole Collimators (continued)

◎ Slant-Hole Collimators
  These are parallel-hole collimators with holes directed at an angle
  to the surface of the collimator.




                                                                   元培放射系 林俊良編製




                                                                                 22
            Parallel-Hole Collimators (continued)

◎ Slant-Hole Collimators (continued)




                                                    元培放射系 林俊良編製




            Nonparallel-Hole Collimators

◎ Converging and Diverging Collimators
   In a converging collimators,
   however, the holes are not
   parallel but are angled inward,
   toward the organ.

   Consequently, the organ
   appears larger at the face of
   the crystal.




                                                    元培放射系 林俊良編製




                                                                  23
           Nonparallel-Hole Collimators (continued)

◎ Converging and Diverging Collimators (continued)
   Diverging collimators is used most often on a camera with a small
   crystal, such as a portable camera.
   Using a diverging collimator, a large organ such as the lung can be
   captured on the face of a smaller crystal.




                                                               元培放射系 林俊良編製




           Nonparallel-Hole Collimators (continued)

◎ Pinhole Collimators
                       hole-the pinhole usually
   These have a single hole the pinhole-usually 2 to 4 mm in
   diameter.
   The image is projected upside down and reversed right to left at
   the crystal.

   A pinhole collimator
   generates magnified images
   of a small organ like the
   thyroid or a joint.




                                                               元培放射系 林俊良編製




                                                                             24
          Nonparallel-Hole Collimators (continued)

◎ Fan Beam Collimators
  These are a cross between a
  converging and a parallel-hole
  collimator.

  They are designed for use on
  cameras with rectangular heads
  when imaging smaller organs
  such as the brain and heart.




                                               元培放射系 林俊良編製




          Nonparallel-Hole Collimators (continued)

◎ Fan Beam Collimators




                                               元培放射系 林俊良編製




                                                             25
             Camera Head



                                     The head housing envelopes and
                                     shields these internal components.




                                      The gantry supports the heavy
                                      camera head




                                                                   元培放射系 林俊良編製




             Camera Head (continued)

The crystal for an imaging camera is a large
slab of thallium-“doped” NaI crystal similar
to that used for the scintillation probes.
                                   p




Thicker crystals have higher sensitivity,
the resolution is lower because gamma
rays may be absorbed farther from the
point at which they entered the crystal.
                                                                   元培放射系 林俊良編製




                                                                                 26
                               Camera Head (continued)




                                                Sixty or more photomultiplier tubes may be
                                                attached to the back surface of the crystal
                                                using light conductive jelly.




                                                                                  元培放射系 林俊良編製




                               Positioning Circuit

The amount of light received by a
photomultiplier tube (PMT) is
related to the proximity of the tube
to the site of interaction of the
gamma ray in the crystal.
interaction of gamma ray




                           closest PMT
                           receive greatest photons
                           — greatest output pulse
          n




                           farthest PMT
                           receive fewest photons
                           — smallest output pulse

                                                                                  元培放射系 林俊良編製




                                                                                                27
          Positioning Circuit (continued)


                                  The positioning circuit uses a
 A positioning circuit improves   “voltage divider” to weigh the
                                        g                 g
 resolution by factoring in the   output of each tube in relation to
 output from adjacent tubes.      its position on the face of the
                                  crystal.




                                                             元培放射系 林俊良編製




          Positioning Circuit (continued)


The output of each of
the preamplifiers
attached to each PMT
is connected to four
directional terminals:
X+, X-, Y+, and Y-.




                                                             元培放射系 林俊良編製




                                                                           28
         Positioning Circuit (continued)




   0.5×0.36




  0.25×0.21



                                           元培放射系 林俊良編製




         Positioning Circuit (continued)




X+ + X- = X = 0.60 - 0.43 = 0.17


       0.17 × 15 cm = 2.55 cm


The output of the Y- and Y+ terminals
are processed in the same way.




                                           元培放射系 林俊良編製




                                                         29
     QC of Gamma Camera (Scintillation Camera)

◎ Field uniformity
 — daily
◎ Spatial resolution
 — weekly
◎ Spatial linearity
 — weekly
◎ Energy resolution
 — quarterly
◎ Sensitivity
 — semiannually
※ These tests should also be performed following
  adjustment or repair of the instrument.
                                                       元培放射系 林俊良編製




     Field Uniformity in Gamma Camera

◎ Field uniformity is the ability of a scintillation camera
     p                        g
  to produce a uniform image when the source
  provides a uniform distribution of photons over the
  detector.
  Clinically, it is the ability of the
  instrument to produce
  accurate images of a
  radionuclide distribution in
                                         nonuniform   uniform
  patients
  ※Two methods are available:
   - intrinsic method (without a collimator)
   - extrinsic method (with a parallel-hole collimator)
                                                       元培放射系 林俊良編製




                                                                     30
      Field Uniformity in Gamma Camera
                                                        (continued)


 ◎ Two different uniformity parameters shall be
   determined:integral uniformity and diff
   d t    i d i t       l if     it             ti l
                                      d differential
   uniformity.

   — Integral uniformity is a measure of the maximum
     detector count deviation in the CFOV or UFOV.
     (maximum deviation)
   — Differential uniformity is a measure of the
     maximum deviation over a limited range designed
     to approximate the size of a photomultiplier tube.
    (maximum rate of change over adjacent detectors)

                                                               元培放射系 林俊良編製




      Field Uniformity in Gamma Camera
                                                        (continued)

◎ Comparison of intrinsic and extrinsic method
                     Intrinsic
                     I ti i                       E ti i
                                                  Extrinsic
Collimator            None                Low-energy, parallel-hole
             99mTc point source           99mTc   liquid flood source
Radiation    99mTc                        57Co
                   liquid flood source           solid planar source
 source      57Co solid planar source


                   so rce
             Point source is placed        Fl d or planar source
                                           Flood      l
 Source      4-5 crystal diameters         is placed on collimator
             away from detector.           face.
 distance
             Flood or planar source
             is placed on crystal face.

                                                               元培放射系 林俊良編製




                                                                             31
        Field Uniformity in Gamma Camera
                                                         (continued)




99mTc   liquid flood phantom       57Co    solid planar source

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        Field Uniformity in Gamma Camera
                                                         (continued)
 ◎ Intrinsic method
                                                 99mTc   point source


                                              Point source is placed
                                              4-5 crystal diameters
      Detector                                away from detector.
 without a collimator

                                   99mTc   liquid flood source or
                                   57Co   solid planar source
                                   Flood or planar source
                                   is placed on crystal face.

                 Detector
            without a collimator
                                                                元培放射系 林俊良編製




                                                                              32
      Field Uniformity in Gamma Camera
                                                      (continued)
◎ Extrinsic method


                                  99mTc   liquid flood source or
                                  57Co   solid planar source
                                  Flood or planar source
                                  is placed on collimator
                                  face.

              Detector
              D t t
          with a collimator



                                                             元培放射系 林俊良編製




     Spatial Resolution and Spatial Linearity
     in Gamma Camera
◎ Spatial resolution is the ability of a scintillation
  camera to reproduce small differences in
    di      lid          t ti in
  radionuclide concentration i
  closely spaced areas.
  Clinically, resolution affects
  the ability to visualize small
  defects.
◎ Spatial linearity is the ability of a scintillation
  camera to produce a uniform image with straight
  lines corresponding to
  straight lines in a phantom.
  Clinically, it is the accurate
  portrayal of true organ shape.

                                                             元培放射系 林俊良編製




                                                                           33
        Spatial Resolution and Spatial Linearity
        in Gamma Camera                  (continued)


 ◎ Common used bar phantoms



 Four-quadrant                                        Hine-Duley
  bar phantom                                          phantom




   Parallel-line                                   Orthogonal
   equal-space                                        hole
     (PLES)                                         phantom
    phantom

                                                           元培放射系 林俊良編製




        Spatial Resolution and Spatial Linearity
        in Gamma Camera                  (continued)




                                              99mTc liquid flood source
Bar phantom                                               or
                                              57Co solid planar source




                                    imaging
                        Detector
                    with a collimator

                                                           元培放射系 林俊良編製




                                                                          34
      Spatial Resolution and Spatial Linearity
      in Gamma Camera                  (continued)


        for spatial resolution          for spatial linearity

  0.40 cm                   0.32 cm


  0.48 cm                   0.24 cm


 for spatial resolution and linearity   for spatial linearity
   0.64 cm
   0.48 cm
   0.40 cm
   0.48 cm
   0.64 cm


                                                                元培放射系 林俊良編製




      Spatial Resolution and Spatial Linearity
      in Gamma Camera                  (continued)



◎ BRH Test phantom
  f spatial resolution and
  for     ti l  l ti     d
  linearity




                                                                元培放射系 林俊良編製




                                                                              35
     Spatial Resolution and Spatial Linearity
     in Gamma Camera                  (continued)



◎ Computerized Dynamic
       p             y
  Line Phantom
  for spatial resolution,
  linearity, and field
  uniformity




                                                         元培放射系 林俊良編製




     Flood Phantoms Used in Gamma Camera
                    Multi-Contrast
 Thyroid Phantom    Liver Phantom         Neck Phantom




  Orthogonal Tri-Hole Phantom    Dynamic Heart Phantom




                                                         元培放射系 林俊良編製




                                                                       36
       Spatial Resolution and Spatial Linearity
       in Gamma Camera                  (continued)


◎ Line source used for spatial resolution check



                                imaging
              Detector
          with a collimator




  ( full-width at half-maximum )          ( full-width at tenth-maximum )
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    uniform            incomplete mixing flood       air bubble in flood




   nonuniform
                              a nonfunctioning photomultiplier tube




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                                                                                37
                                  cracked by impact


  squeezing thumbtack
   between crystal and
         lli t
       collimator



                            severe change in temperature




                                                      元培放射系 林俊良編製




       Sensitivity in Gamma Camera

◎ Sensitivity is the ability to
  detect i i i           t in
  d t t ionizing events i a
  sodium iodide crystal
  expressed in counts per
  second per microcurie
  (cps/μCi).

   sensitivity
       measured count rate (cps)
   =
         true activity (μCi)


                                                      元培放射系 林俊良編製




                                                                    38
        SPECT (Tomographic Camera)




                                                       元培放射系 林俊良編製




        SPECT (Tomographic Camera) (continued)

Single-photon emission computed tomography (SPECT)
           q         p planar views of the radioactivity in an
cameras acquire multiple p                             y
organ.

The data are then processed mathematically to create cross-
sectional views of the organ.

SPECT utilizes the single photons emitted by gamma-emitting
radionuclides such as 99mTc, 67Ga, 111In, and 123I.
                           ,     ,      ,
In contrast to positron emission tomography (PET), which
utilizes the paired 511-keV photons arising from positron
annihilation.


                                                       元培放射系 林俊良編製




                                                                     39
         SPECT (Tomographic Camera) (continued)


 First, the SPECT camera is
          t t d     th t th h d
    constructed so that the head
    can rotate either stepwise or
    continuously about the patient
    to acquire multiple views.

 Second, it is equipped with a
   computer that integrates the
   multiple images to produce the
   cross-sectional views of the
   organ.


                                          元培放射系 林俊良編製




       QC of SPECT (Tomographic Camera)
— Quality control for tomographic
    cameras should first include
    those procedures used to test
    conventional scintillation
    cameras.
◎ Center of Rotation (COR) Correction
 — weekly
◎T        hi Uniformity C
 Tomographic U if            ti
                    it Correction
 — weekly
※ These tests should also be performed
  following adjustment or repair of the
  instrument.
                                          元培放射系 林俊良編製




                                                        40
       COR Correction in SPECT

◎ COR correction is performed to ensure that the axis of
    t ti     the i t b t hi h the detector t t
  rotation, th point about which th d t t rotates,
  falls in the center of the computer matrix for each
  projection acquired.

   For example, in a 64×64
   matrix, the center
                  o ld
   coordinates would be
   located at pixels 32.5 and
   32.5.



                                                    元培放射系 林俊良編製




       COR Correction in SPECT (continued)


   Detector 1
  Point source         90o rotated
   Detector 2
                                        variation<0.5 pixel


 Rotated imaging




                                                    元培放射系 林俊良編製




                                                                  41
         Uniformity Correction in SPECT

  ◎ Tomographic imaging only tolerates a much smaller
    variance in field uniformity.
    Even a small nonuniformity introduce a ring artifact
    (bull’s eye) into the reconstructed images.




                                                       元培放射系 林俊良編製




         Uniformity Correction in SPECT (continued)

— To reduce nonuniformity artifacts in reconstructed
      g
  images, the pplanar
  projections are
  corrected with a
  30-million count flood
  source image
  (for a 64×64 matrix).

   - to yield a standard
         ield
    deviation of only
    1% across the field
    of view


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                                                                     42
       Uniformity Correction in SPECT (continued)

   ( In planar imaging, nonuniformities across the field
     of view are corrected with electronic circuitry. )
◎ For tomographic uniformity correction, after the flood
  correction image (map) is stored in the computer, the
  software calculates an average pixel value for the
  flood matrix.
                                      average flood counts
each pixel correction factor (Fi) =
                                      each pixel counts (Ci)

uncorrected pixel counts (Ci) × Fi = corrected pixel counts



                                                       元培放射系 林俊良編製




       Uniformity Correction in SPECT (continued)




                        correction
                   ×                  =
                           map



                                                       元培放射系 林俊良編製




                                                                     43
Flood Phantoms Used in SPECT




                                    元培放射系 林俊良編製




Flood Phantoms Used in SPECT (continued)




                                    元培放射系 林俊良編製




                                                  44