125 research outputs found
Initial Measurements with the PETsys TOFPET2 ASIC Evaluation Kit and a Characterization of the ASIC TDC
For a first characterization, we used the two KETEK-PM3325-WB SiPMs each
equipped with a 3x3x5 mm LYSO scintillation crystal provided with the
PETsys TOFPET2 ASIC Evaluation Kit. We changed the lower of two discriminator
thresholds (D_T1) in the timing branch from vth_t1 = 5 - 30. The overvoltage
was varied in a range of 1.25 V - 7.25 V. The ambient temperature was kept at
16{\deg}C. For all measurements, we performed an energy calibration including a
correction for saturation. We evaluated the energy resolution, the coincidence
resolving time (CRT) and the coincidence rate. At an overvoltage of 6 V, we
obtained an energy resolution of about 10% FWHM, a CRT of approximately 210 ps
FWHM and 400 ps FWTM, the coincidence rate showed only small variations of
about 5%. To investigate the influence of the ambient temperature, it was
varied between 12{\deg}C - 20{\deg}C. At 12{\deg}C and an overvoltage of 6.5 V,
a CRT of approx. 195 ps FWHM and an energy resolution of about 9.5% FWHM could
be measured. Observed satellite peaks in the time difference spectra were
investigated in more detail. We could show that the location of the satellite
peaks is correlated with a programmable delay element in the trigger circuit.Comment: This paper is under review with IEEE TRPMS. It has been presented in
a talk at the PSMR 2018. This version of the manuscript was submitted as
revision 2 to TRPMS after incrporating the comments of the reviewers. Only
minor textchanges were made. Results, values and figures did not chang
Development and validation of a measurement-driven inter crystal scatter recovery algorithm with in-system calibration
In PET a high percentage of gamma photons being detected undergo Compton
scattering in the scintillator. Scintillator blocks are often built from
optically isolated crystals. Depending on the angle of incidence and the
scintillator geometry this might lead to inter crystal scatter (ICS) events,
where energy is deposited in two or more crystals in the detector, which common
positioning and reconstruction algorithms cannot resolve. Therefore, ICS events
worsen the spatial resolution and the signal-to-noise ratio in the
reconstructed image. We want to address this challenge by recovering individual
crystals from ICS events with their corresponding energy deposits. This
information could ultimately be fed into an image reconstruction framework. In
this work, we established an algorithm based on a detector that couples a
readout channel to each crystal (one-to-one coupling), which combines a
measurement-driven calibration and a fitting routine to achieve the recovery of
crystal interactions from measured light patterns. Using Geant4 simulations, we
validated and optimized this approach by comparing the recovered events to the
simulation ground truth. We showed that, with the best performing algorithm
versions, all correct crystals could be identified for 95-97% of the simulated
events and the crystal energies as well as the event energy sum could be
recovered adequately. For the event energy sum a deviation of less than 5%
could be achieved for 96% of all events. Overall, the developed ICS recovery
algorithm was successfully validated for one-to-one coupled detector. Future
application for other detector configurations should be possible and will be
investigated. Additionally, using the new crystal interaction information to
determine the most likely first interaction crystal is being examined to
improve efficiency and signal-to-noise ratio in the PET reconstruction.Comment: 18 pages, 10 figure
Improving the Timing Resolution of Positron Emission Tomography Detectors Using Boosted Learning -- A Residual Physics Approach
Artificial intelligence (AI) is entering medical imaging, mainly enhancing
image reconstruction. Nevertheless, improvements throughout the entire
processing, from signal detection to computation, potentially offer significant
benefits. This work presents a novel and versatile approach to detector
optimization using machine learning (ML) and residual physics. We apply the
concept to positron emission tomography (PET), intending to improve the
coincidence time resolution (CTR). PET visualizes metabolic processes in the
body by detecting photons with scintillation detectors. Improved CTR
performance offers the advantage of reducing radioactive dose exposure for
patients. Modern PET detectors with sophisticated concepts and read-out
topologies represent complex physical and electronic systems requiring
dedicated calibration techniques. Traditional methods primarily depend on
analytical formulations successfully describing the main detector
characteristics. However, when accounting for higher-order effects, additional
complexities arise matching theoretical models to experimental reality. Our
work addresses this challenge by combining traditional calibration with AI and
residual physics, presenting a highly promising approach. We present a residual
physics-based strategy using gradient tree boosting and physics-guided data
generation. The explainable AI framework SHapley Additive exPlanations (SHAP)
was used to identify known physical effects with learned patterns. In addition,
the models were tested against basic physical laws. We were able to improve the
CTR significantly (more than 20%) for clinically relevant detectors of 19 mm
height, reaching CTRs of 185 ps (450-550 keV)
Classification of Chemicals According to UN-GHS and EU-CLP: A Review of Physical Hazard Classes and Their Intricate Interfaces to Transport and Former EU Legislation
The Globally Harmonized System of Classification and Labelling of Chemicals (UN-GHS) is being implemented in more and more countries all over the world; the EU has done so with the CLP-Regulation (EU-CLP). Compared to the undeniably important questions on health and environmental hazards, the classification of physical hazards of chemicals often has not been in the focus, although their implementation can be challenging and there are traps and pitfalls to be avoided. The following overview of the classification systematics for physical hazards aims at a principle understanding without detailing all criteria or test methods. Similarities and differences between the classification systems of the UN-GHS and EU-CLP, the transport of dangerous goods and the former EU system are reviewed with regard to the physical hazard classes. Available physical hazard classifications for the transport of dangerous goods and according to the former EU system can be used as available information when classifying according to the GHS. However, the interfaces of these classification systems and their limitations have to be understood well when concluding on GHS/CLP classifications. This applies not only to industry when applying CLP but especially to legislators when adapting legislation that in one way or another refers to the classification of chemicals
A Finely Segmented Semi-Monolithic Detector tailored for High Resolution PET
Preclinical research and organ-dedicated applications require high-resolution
positron emission tomography (PET) detectors to visualize small structures and
understand biological processes at a finer level of detail. Current commercial
systems often employ finely pixelated or monolithic scintillators, each with
its limitations. We present a semi-monolithic detector, tailored for
high-resolution PET applications, and merging concepts of monolithic and
pixelated crystals. The detector features slabs measuring (24 x 10 x 1) sq. mm,
coupled to a 12 x 12 readout channel photosensor with 4 mm pitch. The slabs are
grouped in two arrays of 44 slabs each to achieve a higher optical photon
density. We employ a fan beam collimator for fast calibration to train
machine-learning-based positioning models for all three dimensions, including
slab identification and depth-of-interaction (DOI), utilizing gradient tree
boosting (GTB). Energy calculation was based on a position-dependent energy
calibration. Using an analytical timing calibration, time skews were corrected
for coincidence timing resolution (CTR) estimation. Leveraging
machine-learning-based calibration in all three dimensions, we achieved high
detector spatial resolution: down to 1.18 mm full width at half maximum (FWHM)
detector spatial resolution and 0.75 mm mean absolute error (MAE) in the
planar-monolithic direction along the slabs, and 2.14 mm FWHM and 1.03 mm MAE
for depth-of-interaction (DOI) at an energy window of (435-585) keV. Correct
slab interaction identification exceeded 80%, alongside an energy resolution of
13.8% and a CTR of 450 ps FWHM. Therewith, the introduced finely segmented,
high-resolution slab detector demonstrates an appealing performance suitable
for high-resolution PET applications. The current benchtop-based detector
calibration routine allows these detectors to be used in PET systems.Comment: 14 pages, 11 figures, IEEE NSS MIC RTSD 202
Initial PET Performance Evaluation of a Preclinical Insert for PET/MRI with Digital SiPM Technology
Hyperion-IID is a positron emission tomography (PET) insert which allows
simultaneous operation in a clinical magnetic resonance imaging (MRI) scanner.
To read out the scintillation light of the employed LYSO crystal arrays with a
pitch of 1 mm pitch and 12 mm in height, digital silicon photomultipliers (DPC
3200-22, Philips Digital Photon Counting) (DPC) are used. The basic PET
performance in terms of energy resolution, coincidence resolution time (CRT)
and sensitivity as a function of operating parameters, such as the operating
temperature, the applied overvoltage, activity and configuration parameters of
the DPCs, were evaluated on system level. The measured energy resolution did
not show a large dependency on the selected parameters and is in the range of
12.4-12.9% for low activities and degrades to ~13.6% at activities of ~100 MBq.
The CRT strongly depends on the selected trigger scheme (trig) of the DPCs. We
measured approximately 260 ps, 440 ps, 540 ps and 1300 ps for trig 1-4,
respectively. The trues sensitivity for a NEMA NU 4 mouse-sized scatter phantom
with a 70-mm-long tube of activity was dependent on the operating parameters
and was determined to be 0.4-1.4% at low activities. The random fraction stayed
below 5% at activities up to 100 MBq and the scatter fraction was evaluated as
~6% for an energy window of 411-561 keV and ~16% for 250-625 keV. Furthermore,
we performed imaging experiments using a mouse-sized hot-rod phantom and a
large rabbit-sized phantom. In 2D slices of the reconstructed mouse-sized
hot-rod phantom ({\O} = 28 mm), the rods were distinguishable from each other
down to a rod size of 0.8 mm. There was no benefit of the better CRT of trig 1
over trig 3, where in the larger rabbit-sized phantom ({\O} = 114 mm), we could
show a clear improvement of image quality using the time-of-flight information.Comment: Final journal version including the supplemntal data. The images in
the supplement were compressed to meet the arXiv file size limi
- …