Scintillation, Anger, Gamma Camera & Quality Control

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Scintillation, Anger, Gamma Camera Quality
Control
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The performance of a scintillation camera must be
assessed everyday it is used to assure the
acquisition of diagnostically reliable images.
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Scintillation, Anger, and Gamma cameras

• Cameras come in a variety of configurations,
  single, double, and triple head models
• Some allow for whole-body imaging, portable use
  and tomographic imaging ( SPECT single photon
  emission computed tomography)
• Planar imaging is appropriate for many studies,
  SPECT imaging allows for greater sensitivity and
  resolution of imaging deep tissues.

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Gamma Camera
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Gamma Camera
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Almost anything can affect proper function and
performance of a scintillation camera.

• Changes or failure of individual system
  components or subsystems

• Environmental conditions
• electrical power supply fluctuations
• physical shock
• temperature changes
• humidity
• dirt
• background radiation

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Crystals

• Crystals used in anger (gamma) cameras are
  extremely sensitive to moisture and are sealed in
  a aluminum housing.
• They are also sensitive to temperature (rapid
  changes in temp.) can produce fractures of the
  crystal.

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Crystals

• Vary in size from 7 to 25
• Either circular or rectangular
• Crystals are ¼ to ½ thick with 3/8 being the
  most common. Thicker the crystal the greater the
  probability the incoming photon will interact,
  deposit its energy, and be detected.

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Crystals

• The thicker the crystal the better the
  sensitivity will behowever
• The thicker the crystal the poorer the spatial
  resolution will be..because of the complex
  interaction b/w crystal, photomultiplier tube,
  and light pipe that is used to optically couple
  the two.

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Positioning logic

• Anger cameras have an array of PMT
  (photomultiplier tubes) attached to the back of
  the scintillation crystal
• of tubes is determined by size shape of both
  the crystal and individual PMT
• Common for cameras to have either 37, 55, or 61
  PMT

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Positioning logic

• When a scintillation event occurs each PMT
  produces an output pulse
• Amplitude of pulse from a PMT is directly
  proportional to the amount of light
  (scintillations) the photocathode has received.
• PMTs closest to scintillation event produce
  largest output.

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Positioning Logic

• Older PMTround cross section
• Current PMT-hexagonal cross section to cover more
  of crystal area which allows for more efficient
  detection of scintillation photons.
• The more the number of PMTs, the better the
  spatial resolution and linearity.

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Positioning Logic

• If only PMT with the largest pulse were used for
  x,y positioning, the spatial resolution would be
  equal to the cross-sectional size of each tube.
• Combining PMT pulses allows better resolution
  with x,y coordinates based on Centroid (center of
  mass) approach.

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Positioning Logic

• Application of Centroid to Analog Cameras
• Analog network creates four signals (x, x-, y,
  y-).
• PMT coordinate directions are inversely
  proportional to the square of the distance from
  respective coordinate and is controlled in a
  predetermined, fixed way by the resistor network.

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Positioning Logic

• The sum of these 4 signals z pulse
• The x coordinate of the interaction is
• x (x - x-) / z
• The y coordinate of the interaction is
• y (y - y-) / z

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Energy discrimination

• Pulse Height Analyzer is used in Anger (gamma)
  cameras to select a centerline and window
• Photopeak energy is used as the centerline and
  size of window is determined by percentage of the
  centerline energy ex10 of 140Kev to a range
  126Kev 154Kev /- 10

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Energy discrimination

• Wider the window the greater the sensitivity.
• However if the window is to wide it will allow
  compton scatter and resolution will decrease.
• Window width ie 10-20 etc is selected based on
  resolution and sensitivity requirements of study.
  

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Energy Discrimination

• If photon is scattered in the patient, direction
  of flight will not intersect the site of origin,
  only the site of the Compton interaction.
• A large of photons striking the crystal have
  been scattered in the patient.
• Z pulse in PHA used to discriminate against
  scattered photons.

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Energy Discrimination

• Window must be properly peaked or field of view
  will not be uniform.
• Windows off set on high side of peak result in
  appearance of hot PM tubes on the flood.
• Windows off set on low side of peak result in
  appearance of cold PM tubes on the flood.

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Image formation

• Image can be formed in two ways analog and
  digital
• Virtually all cameras (analog digital) form
  images via digital acquisition
• Virtually all cameras are interfaced with a
  computer
• Counts from the radioactivity that are collected
  are stored in the computers memory

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Image formation

• Camera crystals are electronically divided into a
  matrix consisting of many small areas called
  pixels (picture elements)
• Each pixel is assigned a separate storage
  location on the computers memory.
• At the beginning each storage locations are set
  at zero.

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Image formation

• Once a count (radioactivity) is detected on the
  crystal the storage location responds by
  assigning it a 1
• At the conclusion of an acquisition, each storage
  location contains a number of counts
  (radioactivity) that have been registered at that
  location on the crystal.

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Image formation

• Matrix size determines amount of pixels used.
• If a 64 x 64 matrix is used than 4096 pixels are
  used in acquisitions
• Matrix size influences spatial resolution of an
  image. The larger the matrix size the better the
  spatial resolution

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Image formation

• Acquisitions can be acquired in either frame mode
  (most common) or list mode.
• Framing rate is the amount of each image is
  acquired
• Frame mode all counts are collected into a
  storage matrix in a given time period. Matrix
  size and framing rate are preselected.

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Image formation

• One frame of data is acquired by either number of
  counts acquired or a fixed length o time
• Dynamic frame mode is the framing rate in
  sec/frame for a determined amount of time ex 20
  sec per frame for 30mins.
• Matrix and framing rate cannot be changed after
  acquisition.

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Image formation

• Two factors are considered when selecting dynamic
  framing study
• Purpose of study and counting rate.
• Quantitative study usually requires a shorter
  framing rate to provide more data ex renal scan.
  
• Qualitative study requires more counts per frame
  to provide better resoultion

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Image formation

• Higher the counting rate, the shorter the framing
  rate. Ex 0.2-0.5sec/frame more frames per time
  than 2-5sec/frame
• Most common matrix sizes are 64x64, 128x128,
  256x256, larger that matrix better the spatial
  resolution.

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Image formation

• Zoom mode may be used when a small organ occupies
  only a portion of the field of view.
• Zoom mode increases resolution prior to
  acquisition, postacquisition zoom makes image
  larger yet decreases resoultion.

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Image formation

• Information density number of counts per square
  centimeter, directly affects resolution
• Must obtain adequate counts in a frame resolve
  activities b/w adjacent pixels contrast.
• Increasing counts decreases signal-to-noise ratio
  and improves quality of image.

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The most useful tests to help determine the
proper function of a camera are reflected in the
parameters of field uniformity, spatial
resolution, linearity and sensitivity.

• These are all measured at the time of
  installation
• confirm specifications
• provide standard for all subsequent performance
  evaluations/test
• Initial measurements are part of acceptance
  testing upon receipt of a new camera as well as
  after a camera has been serviced.

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UNIFORMITY

• The most basic measurement of camera performance
  is a flood-field uniformity -
• this is the ability of the camera to
  depict a uniform distribution of activity as
  uniform (full-field activity should cause a
  scintillation event across the entire camera
  head, the source should cover the entire camera
  head)
• It is assessed by flooding the field of the
  camera with a uniform field of radiation and then
  assessing the uniformity of the image that it
  produces.
• Flood fields are examined daily for homogeneity.

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Uniformity
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Inhomogeneous flood fields can result from

• Inadequate mixing of Technetium flood phantom
• Computer correction turned off
• Loss of coupling between crystal and PMTs
• Bad PMT voltage adjustment
• Camera pulse height analyzer off-peak for the
  radionuclide

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• Most of the nonuniformity in a camera detector
  occurs as a result of spatial distortion (the
  mispositioning of events)
• To correct distortion, reference images are
  acquired and digital correction maps are
  generated and stored.
• Each map contains values that represent x,y
  coordinate shifts.
• Microprocessor circuitry repositions each count
  in real time during acquisition of the study
  using these shifts.
• With many new cameras it is best to create
  correction maps with the same radionuclide that
  is used for patient imaging.
• In some cameras several correction maps are
  maintained for all the possible radionuclides
  that might be used and it is up to the
  technologist to select the appropriate one.

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• Variation in the position of a pulse from
  different areas of the camera within the pulse
  height window may also produce nonuniformities.
• This spatially dependent energy variation can
  also be corrected by microprocessor circuitry.
• The combination of energy variation and spatial
  distortion is responsible for the loss of spatial
  resolution and imperfect linearity and
  uniformity.
• In more advanced cameras all of these things are
  accounted for through really long boring math
  that no one wants to know.
• Some systems may even account for
  collimator-specific uniformity corrections.

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Uniformity

• Field uniformity may be done either as intrinsic
  or extrinsic
• Intrinsiccollimators off. Monitors the condition
  of sodium iodide crystals and electronics.
• Extrinsiccollimators on. Monitors the camera as
  it is used clinically.

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Uniformity

• Uniformity images must be inspected daily for
  nonuniformity and compared to previous flood
  (uniformity) images.

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SPATIAL RESOLUTION

• Can be defined in terms of the amount by which a
  system smears out the image of a very small point
  source or a very thin line of radioactivity.
• It can be thought of as the distance by which two
  small point sources must be separated to be
  distinguished as a separate in the reconstructed
  image.

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• It is usually measured through the use of a
  transmission phantom (bar phantom)
• It consists of alternating patterns in lead to
  produce closely spaced areas of differing
  activity levels, which will allow for the
  analysis of resolution.
• The better the spatial resolution, the better the
  ability to detect small abnormalities which will
  present themselves as different radionuclide
  concentrations in clinical images.

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• Resolution patterns should be used without a
  collimator to measure intrinsic performance.
• This can be difficult or impossible along with
  time consuming with some multihead cameras.
• It can also be useful to assess the resolution
  with a point or line source.
• The spread of the point or line is an indication
  of the degree of blurring, or the loss of the
  resolution in the camera.
• Resolution should be checked weekly.

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• Four quadrant resolution studies are usually
  required weekly.
• To perform four quadrant studies you must rotate
  the barphantom 90 degrees so that all quadrants
  of the detector are tested.

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LINEARITY

• The ability to reproduce a linear activity source
  as linear in the image.
• A phantom with either an arrangement of bars or
  holes is usually used.
• The image produced should look exactly like the
  phantom that was used.
• For example straight lines should be reproduced
  as straight lines and holes should be reproduced
  as holes

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• Linearity should be checked weekly
• Linear defects in a flood field are usually the
  result of a cracked crystal and less likely the
  result of a collimator defect.
• Linearity is checked along with spatial
  resolution with the bar phantoms

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Florida Administrative Code 64E-5.613Quality
Control of Diagnostic Instrumentation

• Each licensee shall establish written quality
  control procedures for all equipment used to
  obtain images or information from radionuclide
  studies. The procedures shall be recommended by
  equipment manufacturers or be approved by the
  department. The Licensee shall perform quality
  control as specified in written procedures and
  retain a copy of the quality control results for
  3 years.

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NEMA standards

• National Electrical Manufactures Association
  (NEMA) is a trade association for the
  manufactures of electrical products.
• Nuclear section on diagnostic imaging and therapy
  systems division developed the NEMA standards
  publication for performance measurements of
  scintillation cameras

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NEMA standards

• Document defines standards by which scintillation
  cameras may be measured.
• Published standards is to define methods by which
  scintillation cameras performance can be measured
  
• A comparison can be made between performance
  claims by different manufactors.

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NEMA standards

• American Association of Physicists in Medicine
  has published information on scintillation camera
  quality control
• Published information can provide additional
  guidance in designing quality control procedures.

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NEMA standards

• American Society of Nuclear Cardiology has also
  published guidelines for Instrumentation quality
  assurance, performance, and quality control
  procedures for transmission emission tomographic
  systems (SPECT)

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Question The most basic measurement of camera
performance is A) Linearity B) Spatial
Resolution C) Flood-Field Uniformity D)
Sensitivity
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Question The most basic measurement of camera
performance is A) Linearity B) Spatial
Resolution C) Flood-Field Uniformity D)
Sensitivity
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• Question
• How often should spatial resolution be checked?
• Daily
• Weekly
• Quarterly
• Annually

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• Question
• How often should spatial resolution be checked?
• Daily
• Weekly
• Quarterly
• Annually

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Are you ready..

• To explode!!!!!!! Turn on the lights!!!!!
• ANY QUESTIONS