BioCare The Common PETCT Detector

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BioCareThe Common PET/CT Detector
CERN December 2007

Antoni Nassalski
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BioCare
Molecular Imaging for Biologically Optimized
Cancer Therapy

• To avoid the need for successive PET and CT
  scans, dual modality positron emission tomography
  / X-rays tomography (PET/CT) scanners are
  required,
• PET detector LSO fast and bright crystal,
• CT detector CWO low afterglow,
• There are no scintillators showing both
  advantages,
• Classical CT scanner uses a current mode of the
  detector work with a very large dynamic range of
  detected signals up to 216 c/s,
• PET scanner in the current mode, is sensitive to
  the afterglow.

Antoni Nassalski BioCare The Common PET/CT
Detector
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Comparative Study of Scintillators for PET/CT
Detectors
Antoni Nassalski BioCare The Common PET/CT
Detector
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Our contribution to the project (WP1)

• The first step (2004-2005) was to select the
  best scintillator, which could be used in PET/CT
  scanners,
• Such a detector should be characterized by
• fast light pulse,
• high light output,
• high atomic number and density,
• low afterglow.

What we have measured

• Light output,
• Non-proportionality of the light yield versus
  energy of ?-rays,
• Energy resolution and intrinsic energy resolution
  of scintillators,
• Decay time constant of the light pulse,
• Time resolution for 511 keV annihilation quanta,
• Afterglow of the crystals caused by X-ray
  irradiation (induced by 60 keV ?-rays from a
  strong 241Am source (13.9 GBq)).

Antoni Nassalski BioCare The Common PET/CT
Detector
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Results
Number of photoelectrons and energy resolution
for 662 keV ?-rays.
1) Photoelectron yield, phe/MeV , Cs-137
(661.7keV), 2 ) Energy resolution, , Cs-137
(661.7keV), 3 ) Intrinsic energy resolution, ,
Cs-137 (661.7keV),
Antoni Nassalski BioCare The Common PET/CT
Detector
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Fig. 2 Energy resolution of the studied crystals
versus ?-ray energy.
Fig. 3. Intrinsic energy resolution of the
tested crystals.
Fig. 1. Non-proportionality curves of the studied
scintillators.
Antoni Nassalski BioCare The Common PET/CT
Detector
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Fig. 4. Light pulse shapes of the selected
crystals.
Fig. 5. Time spectra of the selected crystals.
Antoni Nassalski BioCare The Common PET/CT
Detector
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Fig. 6 Experimental set-up for the afterglow
measurements (not to scale)
Fig. 7. Afterglow of the selected crystals
measured in the second range after illumination
of the crystal by a strong source 241Am ?-rays
for 5 minutes.
Antoni Nassalski BioCare The Common PET/CT
Detector
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Conclusions
The studies done with classical photomultipliers
(PMT) lead to the conclusion that only GSOCe and
most likely LaBr3 might be considered for the
common PET/CT detector, GSO has a low afterglow,
only 3 times higher than that of BGO, LaBr3 has a
very high light output and an acceptable
afterglow.
Antoni Nassalski BioCare The Common PET/CT
Detector
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The Road to the Common PET/CT Detector
Antoni Nassalski BioCare The Common PET/CT
Detector
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What we have measured
The next step (2006) was to study a detector
consisting of LaBr3, LSO or LYSO pixel crystals
coupled to an avalanche photodiode (APD). The
measurements covered tests of the detectors in
PET and CT modes, respectively.

• Light output,
• Non-proportionality of the light yield versus the
  energy of ?-rays,
• Energy resolution and intrinsic energy resolution
  of scintillators,
• Decay time constant of the light pulse,
• Time resolution for 511 keV annihilation quanta,
• Afterglow of the crystals caused by X-ray
  irradiation (induced by 60 keV ?-rays from a
  strong 241Am source (13.9 GBq)).
• Characterization of the PET detector, and
  determining the relation between counting rate
  and mean current of the APD in the X-ray
  detection.

Antoni Nassalski BioCare The Common PET/CT
Detector
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Experimental setup

• Proposed new solution LSO pixel APD working
  in both counting and current modes.

LSO, LYSO and LaBr3 pixels (4x4x20mm3) coupled to
Hamamatsu S8664-55 APD (5x5 mm2)
Antoni Nassalski BioCare The Common PET/CT
Detector
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Input and output signals of CFD Ortec 935 at the
rate of 3 x 106 c/s of 60 keV g-rays from a
241Am source.
Dead time of about 40 ns, APD gain 200, Energy
threshold at about 30 keV.
Antoni Nassalski BioCare The Common PET/CT
Detector
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CT mode
The figure presents the relation between the
counting rate and the mean current of the
APD, It shows that the detectors work
effectively in the counting rate mode up to 4x106
c/s for LaBr3, For higher counting rates, dead
time limits the proportionality between counting
rate and APD current, representing, in fact, the
recorded dose of X-rays.
Antoni Nassalski BioCare The Common PET/CT
Detector
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PET mode
Energy spectra of 22Na measured with the detector
working as the common PET/CT detector. Note a
good energy resolution of 12 measured with LSO
and 7.8 with LaBr3
Antoni Nassalski BioCare The Common PET/CT
Detector
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PET mode
Time spectra in the PET mode of the
detector. Note a high time resolution below 1
ns.
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Detector
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Conclusions

• The studies done with photomultipliers (PMT) lead
  to the conclusion that LaBr3 and GSO might be
  considered for the common PET/CT detector,
• The studies done with avalanche photodiode (APD)
  lead to the conclusion that LaBr3 and LSO might
  be considered for the common PET/CT detector,
• We are able to count and discriminate the X-ray
  dose in both counting and current mode,
• It allows bypassing an influence of afterglow at
  low doses by the counting mode and increasing the
  dynamic range at high doses by the current mode
  of the detector work,
• Counting the individual X-ray photons in the CT
  mode has the potential to reduce the dose
  received by the patient, even up to 4 orders of
  magnitude (in the case of animal PET),
• To achieve an acceptable position resolution in
  the CT mode a much smaller pixel crystals, 1x1
  mm2 or 2x2 mm2, coupled to the APD array will be
  required,
• It leads to new problems a lower light output of
  a smaller pixel crystal, the APD gain
  stabilization, etc.

Antoni Nassalski BioCare The Common PET/CT
Detector
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What we have measured
The final step (2007) was to study a detector
consisting of 4 LSO 2x2x15 mm3 pixel crystals
coupled to a Hamamatsu S8550 APD array. The
measurements covered tests of the detectors in
PET and CT modes, respectively.

• Light output,
• Non-proportionality of the light yield versus the
  energy of ?-rays,
• Energy resolution and intrinsic energy resolution
  of scintillators,
• Decay time constant of the light pulse,
• Time resolution for 511 keV annihilation quanta,
• Afterglow of the crystals caused by X-ray
  irradiation (induced by 60 keV ?-rays from a
  strong 241Am source (13.9 GBq)).
• Characterization of the PET detector, and
  determining the relation between counting rate
  and mean current of the APD in the X-ray
  detection.

Antoni Nassalski BioCare The Common PET/CT
Detector
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Experimental setup

• Proposed solution LSO pixels APD array
  working in both counting and current modes.

4 LSO pixels (2x2x15mm3) coupled to Hamamatsu
S8550 APD array.
Antoni Nassalski BioCare The Common PET/CT
Detector
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Results
Antoni Nassalski BioCare The Common PET/CT
Detector
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Conclusions

• These studies showed that the detector based on
  LSO crystal coupled to the Hamamatsu S8550 APD
  array is applicable in nuclear medicine, in
  particular for positron emission tomography
  combined with X-rays tomography,
• The proposed setup might be used particularly in
  applications where compact size, high quantum
  efficiency, good time and energy resolutions and
  insensibility to magnetic fields are required,
• We are able to count and discriminate the X-ray
  dose in both counting and current mode,
• Counting the individual X-ray photons in the CT
  mode has the potential to reduce the dose
  received by the patient, even up to 4 orders of
  magnitude.

Antoni Nassalski BioCare The Common PET/CT
Detector
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The typical doses received in the small animals
micro CT studies are 10-100 cGy
Schematic diagram of the phantom used in the CT
simulator.
CT counting images obtained by exposing the
phantom to the 241Am source for 5, 10, 20 and 30
s/X-ray sum.
Antoni Nassalski BioCare The Common PET/CT
Detector
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IEEE NSS-MIC 2005-2007
Comparative Study of Scintillators for PET/CT
Detectors
The Road to the Common PET/CT Detector
Application of Hamamatsu S8550 APD array to the
Common PET/CT Detector
Antoni Nassalski BioCare The Common PET/CT
Detector