CERN 11.12.2007

BioCareThe Common PET/CT Detector

CERN December 2007

Antoni Nassalski

A. Nassalski, M. Moszyński, A. Syntfeld-Każuch, Ł. Świderski, T. Szczęśniak, D. Wolski, T. Batsch

The Soltan Institute for Nuclear Studies, PL 05-400 Otwock-Świerk, Poland

• 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

BioCareMolecular Imaging for Biologically Optimized Cancer Therapy

Comparative Study of Scintillators for PET/CT Detectors


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)).


ResultsNumber of photoelectrons and energy resolution for 662 keV ‑rays.

Crystal Size [mm³] NPHE1) E/E2) sc

3)

LSO 5x10x10 (new) 5350±270 9.0±0.4 8.0±0.5

LSO 5x10x10 (old) 5450±270 8.8±0.4 7.8±0.5

LYSO 5x10x10 4910±250 8.9±0.4 7.8±0.5

MLS 5x10x10 5000±250 8.4±0.3 7.3±0.4

GSO 5x10x10 1930±100 8.9±0.4 5.6±0.4

GSO:Ce,Zr 5x10x10 2000±100 8.3±0.3 4.8±0.3

LGSO 4x6x20 3130±150 7.8±0.3 5.6±0.4

LaCl3 5x10x10 5930±300 5.0±0.2 3.1±0.2

LaBr3 5x10x10 12000±400 3.6±0.2 2.2±0.2

BGO 5x10x10 880±40 11.7±0.5 5.8±0.4

CWO 3x10x10 2740±140 7.3±0.3 4.5±0.3

1) Photoelectron yield, phe/MeV , Cs-137 (661.7keV),2 ) Energy resolution, %, Cs-137 (661.7keV),3 ) Intrinsic energy resolution, %, Cs-137 (661.7keV),


Fig. 1. Non-proportionality curves of the studied scintillators.

Fig. 2: Energy resolution of the studied crystals versus -ray energy.

Fig. 3. Intrinsic energy resolution of the tested crystals.


Fig. 4. Light pulse shapes of the selected crystals.

Fig. 5. Time spectra of the selected crystals.


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.


Conclusions

The studies done with classical photomultipliers (PMT) lead to the conclusion that only GSO:Ce 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.


The Road to the Common PET/CT Detector


• 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.

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.



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)


Dead time of about 40 ns, APD gain = 200, Energy threshold at about 30 keV.

Input and output signals of CFD Ortec 935 at the rate of 3 x 106 c/s of 60 keV -rays from a 241Am source.


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.


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

PET mode


Time spectra in the PET mode of the detector.

Note a high time resolution below 1 ns.

PET mode


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.



• 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.

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.


Experimental setupProposed solution: LSO pixels + APD array working in both counting and current modes.

4 LSO pixels (2x2x15mm3) coupled to Hamamatsu S8550 APD array.


Results


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.


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.

The typical doses received in the small animals micro CT studies are 10-100 cGy


IEEE NSS-MIC 2005-2007

Application of Hamamatsu S8550 APD array to the Common PET/CT Detector The Road to the Common PET/CT Detector

Comparative Study of Scintillators for PET/CT Detectors


CERN 11.12.2007

Business

Transcript of CERN 11.12.2007