Download Examination and optimization of high resolution PET detector modules

Examination and optimization of high resolution
PET detector modules
PhD Theses
CECÍLIA OCSOVAINÉ STEINBACH
Supervisor: Emőke Lőrincz, PhD
Budapest University of Technology and Economics
Department of Atomic Physics
2016.
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Research history
Positron Emission Tomography (PET) is one of the today’s most modern functional
medical imaging techniques. PET applications provide information about the dispersion
of the applied radiotracer within the patient’s body by detecting coincidence of the
annihilation γ -photon pairs. Multicrystal scintillator matrices are used for the detection
that produce UV/visible photons after absorbing the γ-photons. These photons are then
typically detected by photomultiplier tubes (PMT-s) that are coupled to the scintillator
matrix via a light guide layer.
Spatial resolution and image quality is basically defined by the accuracy of determining
the pair of crystal pins that absorbed the γ-photons. Since optical photons play the key
role in this phase of detection, image resolution can be improved by using optical
solutions that influence positively the propagation and detection of photons. To reduce
cost and time requirements of the research, there is an increased demand for reliable
simulation models that are capable to model correctly the optical parameters and
processes of the system.
Department Atomic Physics and Mediso Ltd. conducted joint research over many years
to model and optimize light propagation within the detector module. Present doctoral
thesis is created within the framework of this cooperation.
Objectives
Aim of my PhD work was the development and experimental validation of the optical
model of the PetDetSim simulation tool used for modeling PET detector modules. I looked
for the optical key parameters and their correct incorporation to get a reliable model
within the limits of the given simulation environment. My aim was to measure the
necessary optical parameters, within this to develop a new direct method to measure the
optical scattering length of the scintillator crystals. In the literature the scattering length
is determined indirectly with an error of several orders of magnitude. The quantum
efficiency of PMTs given by the manufacturer needs complex recalculation before its
incorporation into the simulation. I applied a new method considering the reflectance of
the PMT window glass and the photocathode. Important goal of my work was the
experimental validation of the model using a set of easily reproducible measurements that
previously missed from the literature.
The easiest way to identify the crystal pin that absorbed the γ-photon is the so-called
‘Anger-logic’ based on the calculation of the center of gravity of the PMTs’ signals.
Practical goal of my work was to apply the model to work out an optical solution that is
capable to linearize the position of crystal pins (the so-called ‘Anger-position’) in the
Anger-image and thus improve the spatial resolution of the PET.
My results are summarized in 4 thesis statements.
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Theses
1. I improved the optical model of the Detect2000 based PetDetSim simulation tool
used for modeling PET detector modules by applying chamfered crystal geometry that
prevents the photons from closing inside the crystal, and considering angle dependent
reflectance of the PMT’s photocathode, and I experimentally validated the PetDetSim by
a set of measurements consisting of various crystal configurations. [1]-[6]
2. I worked out a new method for the recalculation of the PMTs’ quantum efficiency
given by the manufacturer and for its incorporation into the model considering the
reflectance of the PMT window glass and the photocathode. [1, 7]
3. For the determination of the optical scattering length in scintillator crystals, I worked
out a new direct method that is less sensitive to the accuracy of the measured parameters
and considers the nature of the scattering at the given wavelength. I ascertained that light
scattering is negligible in LYSO crystals at the emission wavelengths of the scintillator.
[8, 9]
4. I designed and built a new structured light guide that is capable to resolve peripheral,
often indistinguishable crystal pins. Applying double prisms matched to the crystals
nearly linearized the previously non-linear Anger positions, and reduced the uncertainty
of calculated positions. [10, 11]
Application of the results
The model I built is used both of my colleagues and the staff of Mediso Ltd. in their
research and development work. Aspects recognized in the modeling have given an
important support for the research running in the Department of Atomic Physics within
the framework of an FP7 project. The approach of modeling the PET detector modules
from the point of view of optics is a niche.
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Publications related to the theses
[1] C. O. Steinbach, Á. Szlávecz, B. Benyó, T. Bükki, and E. Lőrincz, “Validation of
Detect2000 Based PetDetSim by Simulated and Measured Light Output of Scintillator
Crystal Pins for PET Detectors”, IEEE Transactions on Nuclear Science, Vol. 57, No. 5,
pp. 2460–2467, October 2010.
[2] Á. Szlávecz, T. Bükki, C. Steinbach and B. Benyó, “A novel model-based PET detector
block simulation approach”, Biomedical Signal Processing and Control, Vol. 6, No. 1,
pp. 27–33, 2011.
[3] E. Lőrincz, G. Erdei, I. Péczeli, C. Steinbach, F. Ujhelyi, T. Bükki, I. Müller, “Modeling
and Optimization of Scintillator Array for PET Detectors”, IEEE Transactions on
Nuclear Science, Vol. 57, No. 1, pp. 48–54, February 2010.
[4] Á. Szlávecz, B. Benyó, C. Steinbach, T. Bükki, “A novel model and an environment for
PET detector block simulation”, Proceedings of the 7th IFAC Symposium on
Modelling and Control in Biomedical Systems, Aalborg, Denmark, August 12–14, pp.
304–308, 2009.
[5] E. Lőrincz, G. Erdei, I. Péczeli, C. Steinbach, F. Ujhelyi, T. Bükki, I. Müller, “Light Output
Analyzes of Scintillator Crystal Pins and Array for PET Detector Modules”, IEEE
Nuclear Science Symposium and Medical Imaging Conference, pp. 4868–4871,
Dresden, 2008.
[6] Steinbach C., Erdei G., Péczeli I., Ujhelyi F., Lőrincz E., Bükki T., “Szcintillátor
kristálytűk
fényhasznosításának
modellezése
PET
detektormodulhoz”,
Kvantumelektronika, P-5, 2008.
[7] C. O. Steinbach, F. Ujhelyi, E. Lőrincz, “Enhanced model of quantum efficiency for the
optical simulation of photodetectors”, IEEE Nuclear Science Symposium and Medical
Imaging Conference, pp. 2555–2558, Anaheim, 2012.
[8] C. O. Steinbach, F. Ujhelyi, E. Lőrincz, “Measuring the Optical Scattering Length of
Scintillator Crystals”, IEEE Transactions on Nuclear Science, Vol. 61, No. 5, pp. 2456–
2463, 2014.
[9] C. O. Steinbach, F. Ujhelyi and E. Lőrincz, “Optical Scattering Length of LYSO
Scintillator Crystals”, IEEE Nuclear Science Symposium and Medical Imaging
Conference, Conference Record, pp. 2353–2356, 2012.
[10] C. O. Steinbach, G. Erdei, F. Ujhelyi, P. Major, E. Lőrincz, “Optimized Light Sharing
Module for High Resolution PET Block Detectors”, IEEE Transactions on Nuclear
Science, IEEE Transactions on Nuclear Science, Vol. 59, No. 3, pp. 507–512, 2012.
[11] C. O. Steinbach, G. Erdei, I. Péczeli, F. Ujhelyi, T. Bükki and E. Lőrincz “Optimized Light
Sharing Module for PET Block Detectors”, IEEE Nuclear Science Symposium and
Medical Imaging Conference, Conference Record, pp. 2817–2820, 2009.
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