P. Arce

GAMOS: A Geant4-based easy and flexible framework for nuclear medicine applications

The use of Monte Carlo has proved to be an essential tool in nuclear medicine, to assist in the design of new medical devices, in the development of new image reconstruction algorithms and correction techniques in PET and SPECT, for the precise determination of the dose in radiotherapy, etc. Among the several general-purpose codes, Geant4 is widely used for its modern technology, its flexibility and its wide range of applications. Nevertheless the use of Geant4 in nuclear medicine requires often a long learning-curve that implies a good knowledge of C++ and the Geant4 codes itself to write the code needed to obtain the required results. GAMOS, the Geant4-based Architecture for Medicine-Oriented Simulations, facilitates the use of Geant4 by providing a simple script language that covers almost all the needs of a nuclear medicine simulation. Its modular and flexible design, based on the use of the plug-in technology, as well as a clear documentation and detailed examples, makes easy to extend the framework to cover any extra need an expert user may have. We describe in this paper the basic components of GAMOS as well as provide a few examples of its use in PET and Radiotherapy simulations.

Modeling and simulation of PET scanner based on pixelated solid-state detector

Novel conceptual design of Pixel-PET scanner, based on pixelated solid-state detector, is presented. Pixel-PET simply solves most of the intrinsic limitations that are inherited in the current PET scanners, and in particular those that are based on scintillating crystals, such as low detection efficiency, low energy resolution, low spatial resolution, parallax effect, image noise from scattered photons, and non-compatibility with strong magnetic field. The simulation results followed by image reconstruction show, when compared with state-of-the-art PET scanner for head, based on scintillating crystals (LSO/LYSO), the detection efficiency increases by a factor of 2.3 and the scattered photons are reduced from 98% to 3%, thus allowing to resolve the 1.2mm rod in Derenzo-like phantom with peak-to-valley ratio of 3 to 1.

Simulation of the Expected Performance of a Seamless Scanner for Brain PET Based on Highly Pixelated CdTe Detectors

The aim of this work is the evaluation of the design for a nonconventional PET scanner, the voxel imaging PET (VIP), based on pixelated room-temperature CdTe detectors yielding a true 3-D impact point with a density of 450 channels/cm3, for a total 6 336 000 channels in a seamless ring shaped volume. The system is simulated and evaluated following the prescriptions of the NEMA NU 2-2001 and the NEMA NU 4-2008 standards. Results show that the excellent energy resolution of the CdTe detectors (1.6% for 511 keV photons), together with the small voxel pitch (1 × 1 × 2 mm3), and the crack-free ring geometry, give the design the potential to overcome the current limitations of PET scanners and to approach the intrinsic image resolution limits set by physics. The VIP is expected to reach a competitive sensitivity and a superior signal purity with respect to values commonly quoted for state-of-the-art scintillating crystal PETs. The system can provide 14 cps/kBq with a scatter fraction of 3.95% and 21 cps/kBq with a scatter fraction of 0.73% according to NEMA NU 2-2001 and NEMA NU 4-2008, respectively. The calculated NEC curve has a peak value of 122 kcps at 5.3 kBq/mL for NEMA NU 2-2001 and 908 kcps at 1.6 MBq/mL for NEMA NU 4-2008. The proposed scanner can achieve an image resolution of ~ 1 mm full-width at half-maximum in all directions. The virtually noise-free data sample leads to direct positive impact on the quality of the reconstructed images. As a consequence, high-quality high-resolution images can be obtained with significantly lower number of events compared to conventional scanners. Overall, simulation results suggest the VIP scanner can be operated either at normal dose for fast scanning and high patient throughput, or at low dose to decrease the patient radioactivity exposure. The design evaluation presented in this work is driving the development and the optimization of a fully operative prototype to prove the feasibility of the VIP concept.

GAMOS: An easy and flexible way to use GEANT4

The wide range of physics models available in GEANT4, as well as its outstanding geometry and visualization tools, has made it gain widespread use in several fields of physics, like high energy, medical, space, etc. Nevertheless the use of GEANT4 often requires a long learning-curve, which implies a good knowledge of C++ and the GEANT4 code itself. GAMOS facilitates the use of GEANT4 by avoiding the need to use C++, providing instead a set of user commands. One of the novelties of GAMOS with respect to similar simulation codes lies in its flexibility, which makes it appropriate for simulation in many physics fields. This flexibility is sustained by the wide range of geometrical configurations, primary generators and physics lists supported and by the comprehensive set of tools that help the user in extracting detailed information from the simulation through user commands. The use of the plug-in technology contributes to this flexibility, as it facilitates the extension of the framework to include extra functionality not foreseen by the framework authors. GAMOS counts already with several hundreds registered users in the five continents; while it is more frequently used in the medical physics field, its use has also been extended to other fields, like high energy physics, space physics, neutron shielding, etc.

Pixelated CdTe detectors to overcome intrinsic limitations of crystal based positron emission mammographs.

A positron emission mammograph (PEM) is an organ dedicated positron emission tomography (PET) scanner for breast cancer detection. State-of-the-art PEMs employing scintillating crystals as detection medium can provide metabolic images of the breast with significantly higher sensitivity and specificity with respect to standard whole body PET scanners. Over the past few years, crystal PEMs have dramatically increased their importance in the diagnosis and treatment of early stage breast cancer. Nevertheless, designs based on scintillators are characterized by an intrinsic deficiency of the depth of interaction (DOI) information from relatively thick crystals constraining the size of the smallest detectable tumor. This work shows how to overcome such intrinsic limitation by substituting scintillating crystals with pixelated CdTe detectors. The proposed novel design is developed within the Voxel Imaging PET (VIP) Pathfinder project and evaluated via Monte Carlo simulation. The volumetric spatial resolution of the VIP-PEM is expected to be up to 6 times better than standard commercial devices with a point spread function of 1 mm full width at half maximum (FWHM) in all directions. Pixelated CdTe detectors can also provide an energy resolution as low as 1.5% FWHM at 511 keV for a virtually pure signal with negligible contribution from scattered events.

Characterization of CdTe detector for use in PET

CdTe diode detectors with Schottky contact have been characterized in terms of energy resolution and time of response. A resolution of 0.98% at 511keV has been achieved with a 4 mm × 4 mm × 2 mm detector at 900 V/mm and −7 °C. At the same bias and temperature conditions, two identical CdTe detectors show a coincidence time FWHM of 25 ns. Additionally, the effect of a strong magnetic field on the charge sharing has been studied for a 9-pixel array detector of 55 µm × 55 µm × 800 µm pixel size connected to MediPix2 front end electronics. No effect on the charge sharing distribution has been observed up to 4 T. These results show that CdTe Schottky diodes are excellent candidates for the development of next generation nuclear medical imaging devices such as PET, Compton gamma camera, and especially PET-MRI when used in a magnetic field immune configuration.

Modeling, simulation, and evaluation of a compton camera based on a pixelated solid-state detector

A novel Compton camera design based on pixelated solid-state detectors is proposed and evaluated via Monte Carlo simulation, using the Geant4-based Architecture for Medicine-Oriented Simulations GAMOS. For the image reconstruction, the Stochastic Origin Ensemble (SOE) method has been used. The efficiency of the reconstruction of Compton prompt events is constant up to activities of 107 Bq. The signal-to-noise ratio (SNR), i.e., the ratio between real coincidences and mis-reconstructed ones, was above 85% for photon energies ranging from 141 to 511 keV. For a 18F isotope source, a sensitivity of 12 cps/kBq has been obtained. For a 99mTc isotope source, a sensitivity of 15 cps/kBq has been obtained. Using the NEMA NU-4 2008 standard for the PSF estimation, values for the FWHM of 1.80 mm for the spatial resolution with a 18F radioactive source and 3.82 mm with a 99mTc source were obtained.

A technique for optimised navigation in regular geometries

The simulation of a DICOM file describing the geometry of a patient through a 3-dimensional grid of several million voxels represents a big challenge in terms of time and memory consumption. We have developed a fast technique to navigate in these regular voxelised geometries in the GEANT4 framework. It takes advantage of the regular structure of the geometry to optimise the location of voxels at tracking time. An option to skip on the fly the voxel boundaries when two contiguous voxels share the same material reduces the number of steps and therefore the simulation time. When the number of materials in the phantom is of the order a few dozens, enough to reach the required precision in medical physics applications, this technique is several times faster than the optimised navigation algorithms implemented in GEANT4, while keeping optimal memory and initialisation time.

Simulation of pseudo-clinical conditions and image quality evaluation of PET scanner based on pixelated CdTe detector

A novel conceptual design of PET scanner, known as a Voxel Imaging PET (VIP) [1], based on pixelated solid state detector, has been proposed to overcome the intrinsic limitations of state-of-art PET devices based on scintillating crystals. For this study, the VIP scanner has been simulated in different pseudo-clinical conditions in order to assess its image quality performance. The evaluation of the PET image quality follows the NEMA prescriptions. Results are also presented for both analytic and iterative image reconstruction methods. The preliminary results from simulations show that the image reconstruction performance of the VIP scanner under realistic clinical conditions provides good images with low dose. The VIP scanner is able to detect down to 1 mm in diameter hot regions in small phantoms without background activity and down to 5 mm in diameter hot spheres in big phantoms in the presence of background activity. Moreover, good quality and high contrast images can be obtained with a considerably lower number of coincidences with respect to the usual crystal PETs.

Neutron spectra around a tandem linear accelerator in the generation of 18F with a bonner sphere spectrometer

A Bonner sphere spectrometer was used to measure the neutron spectra produced at the collision of protons with an H2(18)O target at different angles. A unique H2(18)O target to produce (18)F was designed and placed in a Tandem linear particle accelerator which produces 8.5MeV protons. The neutron count rates measured with the Bonner spheres were unfolded with the MAXED code. With the GEANT4 Monte Carlo code the neutron spectrum induced in the (p, n) reaction was estimated, this spectrum was used as initial guess during unfolding. Although the cross section of the reaction (18)O(p,n)(18)F is well known, the neutron energy spectra is not correctly defined and it is necessary to verify the simulation with measurements. For this reason, the sensitivity of the unfolding method to the initial spectrum was analyzed applying small variation to the fast neutron peak.