Scintillation detectors

1
Instrumentation

• Scintillation detectors
• Band theory of solids
• Outer electrons have energy levels that lie in a
  valence band
• Above valence band is conduction band
• Region between valence and conduction bands is
  the forbidden gap and represents electron
  energies that do not exist in a pure crystal. In
  a ground state, the valence band is completely
  filled with electrons the conduction band is
  empty. When energy is applied to the crystal,
  valence electrons are lifted to the conduction
  band. As electrons fall back to valence band,
  resultant energy is 350-550 nanometers-visible
  light.

2
Instrumentation

• NaI crystal commonly used in NM
• Hermetically sealed in aluminum (Al absorbs
  Alphas Betas)
• Added Thallium impurities create luminesence
  centers in the forbidden zone.
• 20-30 light photon produced per 1 keV of energy
  absorbed

3
Instrumentation

• The light photons are converted to electrical
  signals in the PMTs

4
Instrumentation

• Spectometry
• Pulse height analysis
• Refers to the use of a scintillation counting
  system to obtain an energy spectrum from a
  radioactive source
• Simply a histogram of the pulse height which is
  proportional to the energy deposited in the
  crystal.
• Spectrum has 2 major components Compton plateau
  and photopeak

5
Instrumentation

• Compton plateau
• broad range of energies produced by Compton
  scatter interactions in the crystal
• Right side limit of plateau is the Compton
  edge-Compton interactions in which incoming x or
  gamma rayis backscatterd 180 degrees
• Photopeak
• Highest pulse height

6
Instrumentation

• Resolution
• Ability of a system to accurately depict two
  separate events in space, time or energy as
  separate events.
• The amount by which the system smears out a
  single event space, time or energy.
• The worse the energy resolution of a PHA, the
  broader the photopeak

7
Instrumentation

• FWHM
• Energy resolution can be quantified as the full
  width at half maximum of the photopeak.
• Find the of counts at top of photopeak and then
  locating the points on either side of the peak
  where the counts are half of the peak counts. The
  width (at half max) is then divided by the pulse
  height (energy) at the apex of the photopeak and
  multiplied by 100 to produce an energy resolution
  measurement in percent

8
Instrumentation

• FWHM
• energy resolution(FWHM/photopeak center) x 100
• The smaller the , the better the energy
  resolution
• Typical values
• Cs-137 7 - 9
• Tc-99m 8- 12

9
Instrumentation

• Spectometry systems are generally used to
  determine which radionuclides (and quantities)
  are present in a mixed sample.
• A well counter assays radioactive samples in test
  tubes.
• Lead-shielded NaI detector 1-3 inches in diameter

10
Instrumentation

• Probe systems count radioactivity in people
• Uses a flat field collimator-provides relatively
  uniform detection sensitivity across the region
  of the thyroid.

11
Instrumentation

• Liquid Scintillation counting
• Used to assess the activity of small sources of
  beta emitters (tritium-H-3 or C-14)
• Radioactive samples dissolved in a liquid that
  scintillates
• 3 components organic solvent (99), primary
  fluor, secondary fluor (wave length shifter)
• Quenching refers to any undesirable reduction in
  light from the scintillation cocktail.

12
Instrumentation

• Factors affecting count rate
• Time
• Efficiency
• Geometry
• Attenuation
• Random decay

13
Instrumentation

• Collimators
• .5 in to 2 in thick lead
• Lead between each hole is called the septum
• Interface between patient and crystal
• Discriminate based on direction of flight

14
Instrumentation

• Parallel hole collimator
• Array of parallel holes perpendicular to the
  crystal face
• Presents a real-size image to the detector
• Resolution is best at the collimator surface
• Sensitivity independent of distance of source
  from collimator in most applications
• Resolution degrades with increasing distance

15
Instrumentation

• Converging collimators
• Array of tapered holes that aim at a point some
  distance in front of the collimator (focal point)
• Image presented to crystal is magnified
• Best resolution at the surface of the collimator
• Sensitivity increases as the source is moved from
  the camera face back to the focal plane and then
  decreases as it passes the focal plane

16
Instrumentation

• Diverging collimators
• upside down converging collimators
• Array of tapered holes that diverge from a
  hypothetical focal point behind the crystal.
• Image is minified
• Useful for large organs

17
Instrumentation

• Pinhole collimator
• Thick, conical collimators with a single 2 to 5
  mm hole in the bottom center
• Image gets smaller as the object is moved away
  from the pinhole collimator
• Camera image is magnified from the collimator
  face to a distance equal to the length of the
  collimator

18
Instrumentation

• Spatial Resolution
• Reflects the systems ability to distinguish 2
  separate events
• Quantified by FWHM
• In practice, FWHM in mm will be nearly identical
  to the minimum distance by which 2 point or line
  sources must be separated to be distinguished as
  separate events.
• Resolution can be increased by using many more
  smaller holes or making the collimator longer.

19
Instrumentation

• Sensitivity
• Is the overall ability of the system to detect
  the radioactive emissions from a source. The
  higher the sensitivity, the greater fraction of
  emissions are detected
• Sensitivity can be increased by increasing the
  size or shortening the length of the holes.

20
Instrumentation

• There is always a trade off between resolution
  and sensitivity.
• Resolution Diameter (length distance/length)
• Sensitivity (Diameter/length)2
  (diameter/diameter thickness)2
• Geometric spatial resolution is one of several
  factors that influence actual spatial resolution
  in an image. Other factors include intrinsic
  camera resolution, collimator resolution,
  scatter, patient resolution effect.

21
Instrumentation

• Crystals
• The most common crystal used in Nuclear Medicine
  is Thallium activated NaI.
• Thicker crystals increase the chance of a photon
  interaction (higher sensitivity) at the expense
  of a loss of resolution
• 1/4 crystals have 1mm better intrinsic
  resolution than ½ crystals.
• When counting Tc-99m, ¼ crystals have 15 less
  sensitivity than ½ crystals.
• Need 3/8-1/2 crystals to effectively count
  gamma gt 200keV