F. Calvino

Quantification of dopaminergic neurotransmission SPECT studies with 123I-labelled radioligands. A comparison between different imaging systems and data acquisition protocols using Monte Carlo simulation

Purpose123I-labelled radioligands are commonly used for single-photon emission computed tomography (SPECT) imaging of the dopaminergic system to study the dopamine transporter binding. The aim of this work was to compare the quantitative capabilities of two different SPECT systems through Monte Carlo (MC) simulation.MethodsThe SimSET MC code was employed to generate simulated projections of a numerical phantom for two gamma cameras equipped with a parallel and a fan-beam collimator, respectively. A fully 3D iterative reconstruction algorithm was used to compensate for attenuation, the spatially variant point spread function (PSF) and scatter. A post-reconstruction partial volume effect (PVE) compensation was also developed.ResultsFor both systems, the correction for all degradations and PVE compensation resulted in recovery factors of the theoretical specific uptake ratio (SUR) close to 100%. For a SUR value of 4, the recovered SUR for the parallel imaging system was 33% for a reconstruction without corrections (OSEM), 45% for a reconstruction with attenuation correction (OSEM-A), 56% for a 3D reconstruction with attenuation and PSF corrections (OSEM-AP), 68% for OSEM-AP with scatter correction (OSEM-APS) and 97% for OSEM-APS plus PVE compensation (OSEM-APSV). For the fan-beam imaging system, the recovered SUR was 41% without corrections, 55% for OSEM-A, 65% for OSEM-AP, 75% for OSEM-APS and 102% for OSEM-APSV.ConclusionOur findings indicate that the correction for degradations increases the quantification accuracy, with PVE compensation playing a major role in the SUR quantification. The proposed methodology allows us to reach similar SUR values for different SPECT systems, thereby allowing a reliable standardisation in multicentric studies.

Study of the point spread function (PSF) for 123I SPECT imaging using Monte Carlo simulation

The iterative reconstruction algorithms employed in brain single-photon emission computed tomography (SPECT) allow some quantitative parameters of the image to be improved. These algorithms require accurate modelling of the so-called point spread function (PSF). Nowadays, most in vivo neurotransmitter SPECT studies employ pharmaceuticals radiolabelled with 123I. In addition to an intense line at 159 keV, the decay scheme of this radioisotope includes some higher energy gammas which may have a non-negligible contribution to the PSF. The aim of this work is to study this contribution for two low-energy high-resolution collimator configurations, namely, the parallel and the fan beam. The transport of radiation through the material system is simulated with the Monte Carlo code PENELOPE. We have developed a main program that deals with the intricacies associated with tracking photon trajectories through the geometry of the collimator and detection systems. The simulated PSFs are partly validated with a set of experimental measurements that use the 511 keV annihilation photons emitted by a 18F source. Sensitivity and spatial resolution have been studied, showing that a significant fraction of the detection events in the energy window centred at 159 keV (up to approximately 49% for the parallel collimator) are originated by higher energy gamma rays, which contribute to the spatial profile of the PSF mostly outside the geometrical region dominated by the low-energy photons. Therefore, these high-energy counts are to be considered as noise, a fact that should be taken into account when modelling PSFs for reconstruction algorithms. We also show that the fan beam collimator gives higher signal-to-noise ratios than the parallel collimator for all the source positions analysed.

Evaluation of the geometric, scatter and septal penetration components in fan beam collimators using Monte Carlo simulation

The quantitative analysis of SPECT data requires an accurate determination of the collimator point spread function (PSF). The aim of this work is to characterize the PSFs of fan beam and parallel collimators by using Monte Carlo simulation. Given a particular collimator configuration, a detailed hexagonal hole array is generated and information describing its geometry is stored in a look-up table. When a photon crosses the collimator front plane, a forty-hole array is placed around its impact position using this table. Each photon is then tracked up to the detector surface by using the Monte Carlo code PENELOPE and its associated geometry handling routines. Particle counters are defined that score the probability of impact on the detector as a function of the final photon position. Four sets of counters are employed so as to differentiate contributions to the geometric, septal penetration, coherent (Rayleigh) and incoherent (Compton) scatter components. Furthermore, sensitivity quantification and pulse-height energy spectra are calculated for different source locations. Monte Carlo results have been compared with sensitivity values obtained experimentally and good agreement was found. The authors results show that for /sup 99m/Tc imaging, the geometric component represents about 95% of the fan beam PSF, whereas the incoherent scattering component is negligible.

Study of charm photoproduction mechanisms

AbstractThis paper presents results on charm photoproduction in the energy interval 40 to 160 GeV, obtained from the high-statistics charm samples of the NA 14/2 experiment at CERN. We measure the charm cross-section, the distributions inxF andp2T and various production ratios and charge asymmetries. The total non-diffractive open-charm cross-section per nucleon is measured to ben

Modelling of high-energy contamination in SPECT imaging using Monte Carlo simulation

sigma _{(gamma N to cbar cX)}

Measurement of Ds± and Cabibbo-suppressed D± decays

n at 〈Eγ〉 =100 GeV. We discuss the photoproduction of charm in terms of theoretical and phenomenological models. We compare the measuredp2T andxF distributions with first-order QCD calculations of photon-gluon fusion and obtain a value for the charm-quark mass ofmc=1.5+0.2−0.1GeV/c2.

Modeling of high-energy contamination in SPECT imaging using Monte Carlo simulation

Abstract A small cylindrical track detector was built as an array of single-wire drift cells with aluminized mylar cathode tubes. Point measurement resolution of ∼ 90 μm was achieved with a drift gas of 50% argon-50% ethane at atmospheric pressure. The chamber construction, electronics, and calibration are discussed. Performance results from PEP colliding-beam data are presented.

Branching ratios and properties ofD-meson decays

123I is a commonly used radioisotope employed in neurotransmitter SPECT studies. In addition to an intense line at 159 keV, the decay scheme of this radioisotope includes a low yield (~3%) of higher energy photons which have a non-negligible contribution to the final image when low-energy high-resolution (LEHR) collimators are used. This contribution of high-energy photons may achieve ~28% of the total counts in the projections. The aim of this work is to model each energy component of the high-energy Point Spread Function (hPSF) for fan-beam LEHR collimators in order to develop faster Monte Carlo (MC) simulations of high-energy ray contamination. The modelling of hPSF was based on the results of simulating photons through the collimator-detector system using the MC code PENELOPE. Since low-energy PSFs models for fan-beam collimators must tend to a Gaussian distribution, we use the same function for the hPSF modelling for high-energy photons. The parameters of these Gaussian functions were obtained by minimizing the root mean square (RMS) error between each simulated hPSF and the function g(x,y) using the efficiency of the simulated hPSFs as a constraint The RMS attained with fit of g(x,y) to the simulated hPSFs was always smaller than ~2% of the mean efficiency per pixel of the image. A very strong dependence of the efficiency on the type and thickness of the backscatter material behind the crystal was found. The hPSFs were parameterized for a wide range of energies, ranging from 350 keV to 538 keV. Our results indicate that Gaussian distributions approximate in a suitable way the hPSF responses for fan-beam collimators. This model will be an important tool to accelerate MC simulations of radiolabelled compounds which emit medium- or high-energy rays