arXiv subjects

Christoph Lerche

Publications and source records attributed to Christoph Lerche.

4 recordsLinked to original sources

arXiv2026

PET Attenuation Map Generation for Brain Imaging from Virtual Non-Contrast and Virtual Non-Iodine Images Obtained with Photon Counting CT Scanners

This study presents a transformation scheme for converting measured Hounsfield units (HUs) into linear attenuation coefficients (LACs) for 511 keV gamma photons in iodine contrast-enhanced angiographic CT images. The method is based on spectral CT data acquired from tissue-equivalent inserts of the Sun Nuclear electron density phantom. The resulting transformation will be used to develop an attenuation map template for our in-house developed BrainPET 7T system. Three approaches were used to obtain the attenuation maps. The first approach is based on angiographic spectral CT scans with iodinated contrast agent administration, and the computation of virtual non contrast (VNC) and virtual non-iodine (VNI) images. In this case, two images are combined: bone regions are segmented from the VNI images, and the corresponding voxel values in the VNC images are replaced, yielding a single quantitative monochromatic CT image. The second approach relies exclusively on VNC images, whereas the third approach uses only VNI images. The transformation schemes from HUs to LACs were established for different tube voltages (120 kV and 140 kV) and virtual monochromatic energies (64 keV, 67 keV, 70 keV, 76 keV, 80 keV, and 100 keV), showing the HUs dependency on the energy setting of the reconstructed virtual monochromatic energies. The accuracy and precision of the proposed transformation scheme were further evaluated using previously acquired angiographic CT datasets. Spectral CT can be expected to minimize the systematic error when converting HUs to LACs for 511 keV gamma rays using virtual monochromatic images (VMIs).

physics.med-ph↗

arXiv2026

Roadmap: Image reconstruction in emission tomography for the era of fast Monte Carlo simulation

Quantitative image reconstruction in positron emission tomography and single photon emission computed tomography depends on how accurately the forward model describes photon emission, transport and detection. Monte Carlo simulation is the most complete model available, yet it is mostly used as an offline reference rather than inside the reconstruction. This roadmap asks what it would take to change that. The main obstacle is not only simulation time. Iterative reconstruction requires the expectation of the measured data and the gradient of the data fidelity, whereas Monte Carlo simulation produces noisy realisations of a complete acquisition. GPU implementations, variance reduction and learned surrogate models have made simulations much faster. At the same time, modern scanners acquire ever larger data sets with larger system models, and further gains in quantitative accuracy require more detailed physics. We discuss how fast Monte Carlo simulation can improve the modelling of the system response, scatter, positron range and the joint estimation of activity and attenuation, how noisy simulations can enter reconstruction algorithms, and which computing hardware makes this feasible. Monte Carlo simulation is most valuable where analytic models fail, and most practical solutions will combine both. The most immediate benefit of fast simulators is the validation and calibration of faster analytic models, together with more accurate, absolutely quantitative scatter estimates. Key open problems are the use of noisy simulations as expectations, in particular for sparse data, the computation of gradients and adjoints, the allocation of a limited simulation budget, and the use of scattered events as a source of information rather than a nuisance. Faster simulators are necessary but not sufficient, and progress requires simulators and reconstruction methods to be developed together.

physics.med-ph↗

arXiv2016

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 (Ø = 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 (Ø = 114 mm), we could show a clear improvement of image quality using the time-of-flight information.

physics.ins-det↗

arXiv2002

On the Infrared Exponent for Gluon and Ghost Propagation in Landau Gauge QCD

In the covariant description of confinement, one expects the ghost correlations to be infrared enhanced. Assuming ghost dominance, the long-range behavior of gluon and ghost correlations in Landau gauge QCD is determined by one exponent kappa. The gluon propagator is infrared finite (vanishing) for kappa =1/2 (kappa > 1/2) which is still under debate. Here, we study critical exponent and coupling for the infrared conformal behavior from the asymptotic form of the solutions to the Dyson-Schwinger equations in an ultraviolet finite expansion scheme. The value for kappa is directly related to the ghost-gluon vertex. Assuming that it is regular in the infrared, one obtains kappa = 0.595. This value maximizes the critical coupling alpha_c(kappa), yielding alpha_c^max = (4 Pi/Nc) 0.709 approx. 2.97 for Nc=3. For larger kappa the vertex acquires an infrared singularity in the gluon momentum, smaller ones imply infrared singular ghost legs. Variations in alpha_c remain within 5% from kappa = 0.5 to 0.7. Above this range, alpha_c decreases more rapidly with alpha_c -> 0 as kappa -> 1 which sets the upper bound on kappa.

hep-ph↗