Rui Ferreira Marques

Whole-Body Single-Bed Time-of-Flight RPC-PET: Simulation of Axial and Planar Sensitivities With NEMA and Anthropomorphic Phantoms

A single-bed, whole-body positron emission tomograph based on resistive plate chambers has been proposed (RPC-PET). An RPC-PET system with an axial field-of-view (AFOV) of 2.4 m has been shown in simulation to have higher system sensitivity using the NEMA NU2-1994 protocol than commercial PET scanners. However, that protocol does not correlate directly with lesion detectability. The latter is better correlated with the planar (slice) sensitivity, obtained with a NEMA NU2-2001 line-source phantom. After validation with published data for the GE Advance, Siemens TruePoint and TrueV, we study by simulation their axial sensitivity profiles, comparing results with RPC-PET. Planar sensitivities indicate that RPC-PET is expected to outperform 16-cm (22-cm) AFOV scanners by a factor 5.8 (3.0) for 70-cm-long scans. For 1.5-m scans (head to mid-legs), the sensitivity gain increases to 11.7 (6.7). Yet, PET systems with large AFOV provide larger coverage but also larger attenuation in the object. We studied these competing effects with both spherical- and line-sources immersed in a 27-cm-diameter water cylinder. For 1.5-m-long scans, the planar sensitivity drops one order of magnitude in all scanners, with RPC-PET outperforming 16-cm (22-cm) AFOV scanners by a factor 9.2 (5.3) without considering the TOF benefit. A gain in the effective sensitivity is expected with TOF iterative reconstruction. Finally, object scatter in an anthropomorphic phantom is similar for RPC-PET and modern, scintillator-based scanners, although RPC-PET benefits further if its TOF information is utilized to exclude scatter events occurring outside the anthropomorphic phantom.

Low-temperature performance of a large area avalanche photodiode

Abstract A Large Area Avalanche Photodiode was studied, aiming to access its performance as light detector at low temperatures, down to −80°C. The excess noise factor, F , was measured and found to be approximately independent of the temperature. A linear dependence of F on the APD gain with a slope of 0.00239±0.00008 was observed for gains >100. The detection of low intensity light pulses, producing only a few primary electron–hole pairs in the photodiode, is reported.

TOFtracker: gaseous detector with bidimensional tracking and time-of-flight capabilities

Particle identification by time-of-flight requires the simultaneous measurement of the passing time and the trajectory of particles. It may be usefull that each tracking station measures both quantities, providing better timing accuracy by redundancy and independence from an external initial time counter. In here we demonstrate a 5-gap timing RPC equipped with patterned electrodes coupled to both charge-sensitive and timing circuits, yielding a time accuracy of 77 ps along with a position accuracy of 38 mm.

A direct time-of-flight reconstruction for whole-body single-bed RPC-PET: Results from lesion and anthropomorphic simulated data

A single-bed, whole-body positron emission tomograph based on resistive plate chamber detectors has been proposed (RPC-PET). It has been shown by simulation that RPC-PET with an axial field-of-view (AFOV) of 2.4m is feasible and yields an absolute sensitivity enhancement of c. one order of magnitude superior to that of typical cylindrical, crystal-based PET scanners, with 16cm AFOV. In addition to its time-of-flight (TOF) advantage, RPC-PET offers potential very-high spatial resolution at the detector level. This must be properly handled by a fully-3D reconstruction algorithm capable of processing the very inclined lines-of-response (LOR) from large AFOV systems such as RPC-PET. For RPC-PET, these are acquired within a single-bed examination. Consequently, one or several segmented images representing the full human body during reconstruction are necessary. In this paper, we show that a direct-TOF implementation of the ordered subset expectation maximization (OSEM) algorithm allows for all events acquired with RPC-PET to be directly processed without rebinning and directly inserted inside the object image by means of a TOF-kernel. Such kernel (1) avoids typical (and slow) voxel-wise image navigation, (2) respects the time uncertainty dictated by the imaging RPC detectors, and (3) permits handling list-mode data iteratively with CT-based attenuation correction and OSEM computed both on-the-fly. We present reconstructed result from blind simulations of a single-bed, whole body PET system, corresponding to (1) six simulated spherical sources immersed in a homogeneous activity background, (2) an anthropomorphic phantom including a realistic grey-to-white matter uptake ratio of four, (3) and an additional anthropomorphic phantom with oncological lesions. We show that overall specificity for lesion detection may be increased (false positives decreased) by multiple similar, but independent reconstructions that eliminate false candidates arising from image statistical noise.

Preliminary characterization of the external proton beam from a PET cyclotron for use in neutron and proton radiobiology and other dosimetric studies

Different cyclotron models capable of accelerating protons up to 20MeV have been worldwide installed. Although their purpose is mainly positron emission tomography (PET) radioisotope production, they are equipped with several beam lines suitable for scientific research. Each beam line may typically deliver proton currents up to 150 μA (1×1015 particles/s). Radiobiological and dosimetric studies can be performed using these beam lines, which may contribute to further improve ion therapy and material radiation hardness results, among other applications. We report experimental results aiming at characterizing the proton beam achievable outside a PET cyclotron. In addition, we simulate this experimental setup by using Geant4. We show that simulation is consistent with previous published experimental data and with our first-measured results. These point to a beam angular spreading which may be utilized to establish an irradiation setup within the bunker. We estimate that the dose achievable with such setup may span 4-orders-of-magnitude, useful in radiobiology, ranging from 10mGy to 100 Gy. Finally, we show by simulation that neutron and γ-ray dose on a realistic, in-bunker target is negligible down to at most the 1% level. Further quantification for lower levels is ongoing.

Pulse shape analysis in the liquid xenon multiwire ionisation chamber for PET

A method of computation of the pulse shape in an ionisation chamber with wire collectors has been developed and verified experimentally with a test chamber. Good agreement between the calculated waveforms and the acquired signals is obtained. Having proved its reliability, the same method was applied to study the dependence of the signal pulse shape and amplitude on the location of primary ionisation due to 511 keV /spl gamma/-rays in the liquid xenon multiwire ionisation chamber for PET. A method of real-time amplitude correction for this effect is suggested, resulting in significant improvement of energy resolution. The simulated resolution for 511 keV /spl gamma/-rays is 15%.

Radiobiology with cyclotron proton beams: A viability study

Cyclotrons capable of accelerating protons up to about 20 MeV have been worldwide installed. Although their purpose is mainly positron emission tomography (PET) radioisotope production, they are equipped with several beam lines suitable for scientific research. Each beam line may typically deliver proton currents up to 150 μA (1015 particles/s). Radiobiological and radiophysiological studies using of these beam lines may contribute to further improve proton therapy results, namely by giving input to pertinent scientific questions, with some of these questions analyzed here. Once the rationale for radiobiological and radiophysiological studies is substantiated, we investigate the viability of building an experimental setup adjusted to one beam line of a proton cyclotron recently installed at the University of Coimbra (a Cyclone® 18/9 HC, from IBA — Ion Beam Applications, S.A.). First GEANT4-based Monte-Carlo simulations are compared with well-estabished ion simulation codes. The precision of these results, as well as that of other published experimental data, points to experimental challenges to be addressed in order to thrive towards a high-precision dosimetric setup. Such high-precision is required by law for radioherapeutical applications, which we would like to mimic experimentally so that potential future results have impact on the medical physics community. We finally show a setup scheme, adjustable to one beamline of the cyclotron, capable of equally providing very low (1 cGy) and high dosage (100 Gy).

Towards a high-dynamic dose-range irradiation setup for radiobioloy and radiophysiology

The number of cyclotrons capable of accelerating protons to about 20 MeV is increasing throughout the world. In Portugal, an IBA (Ion Beam Applications, Belgium) model Cyclone 18/9 cyclotron was installed at ICNAS (Instituto de Ciencias Nucleares Aplicadas a Saude) for positron emission tomography (PET) in 2010. Such facility is equipped with eight beam lines suitable for scientific research. Each beam line may deliver proton currents up to 150 μA (1×1015 particles/s). Radiobiological and dosimetric studies, among others, can be performed using these beam lines. In this work, we report on experimental results and Geant4 and SRIM/TRIM simulations, which aim at investigating the possible use of the 18 -MeV proton beam from the PET cyclotron to irradiate a selected region of a target with 1mm diameter. We prove by simulation and experiment that we are indeed able to irradiate a selected region of a target with 1mm diameter (i.e., the beam spot is very sharp). In addition, simulations indicate that neutron and γ-ray dose on a realistic target is negligible down to at most the 1 % level when compared with the proton dose. Moreover, the experimental and simulation results show good beam uniformity in the target-selected region. In addition to the previously-mentioned results, we measured the Bragg peak of the protons from ICNAS cyclotron by using a stacked target consisting of aluminum foils interleaved with polyethylene sheets, readout by custom-made electronics. Finally, calculations show that the proposed irradiation setup may span 4 orders of magnitude, ranging from 1 cGy to 100 Gy. Such a setup could satisfy user requirements in several fields, including ion therapy radiobiological and animal studies and biological science.

Whole-body single-bed time-of-flight RPC-PET: Simulation of axial and planar sensitivities with NEMA and anthropomorphic phantoms

A single-bed, whole-body positron emission tomograph based on resistive plate chamber detectors has been proposed (RPC-PET). RPC-PET with an axial field-of-view (AFOV) of 2.4 m is feasible and yields an absolute NEMA NU 2-1994 sensitivity enhancement of 20 (4.5) in respect to 16-cm AFOV PET systems with (without) time-of-flight (TOF). These results, however, do not correlate directly with lesion detectability. It is the planar (slice) sensitivity that dictates the exposure time necessary to obtain enough statistics to detect a lesion. This planar sensitivity is currently obtained with a NEMA NU 2-2001 line-source phantom. After validating our simulations with measurements published for existing scanners (GE Advance, Siemens TruePoint and TrueV), we study here by simulation the axial sensitivity profiles of state-of-the-art BGO- and LSO-based PET scanners, and compare the results with RPC-PET. Planar sensitivity results indicate that RPC-PET is expected to outperform 16-cm AFOV scanners by a factor 6 for a 70-cm long scan. If the axial extent is elongated to 1.5 m (approximately head to mid-legs), the sensitivity gain increases to 12. Yet, PET systems with larger AFOV do provide a larger solid angle coverage, at the expense of larger attenuation in the object. In order to quantitate these competing effects, we have studied both point- and line-sources immersed in a water cylinder. For 1.5-m-long scans, the planar sensitivity drops over one order of magnitude in all scanners simulated. In this scenario, RPC-PET outperforms 16-cm AFOV scanners by a factor of 55 (9) with (without) considering the TOF benefit. Regarding the TOF benefit, RPC-PET has a sensitivity gain of 18 when compared to modern, 16-cm AFOV scanners with 600 ps FWHM TOF resolution. We finally show that object scatter in an anthropomorphic phantom is similar for a whole-body, single-bed RPC-PET in respect to a modern, scintillator-based scanner.

Development of a PET cyclotron based irradiation setup for proton radiobiology

An out-of-yoke irradiation setup using the proton beam from a cyclotron that ordinary produces radioisotopes for positron emission tomography (PET) has been developed, characterized, calibrated and validated. The current from a 20 μm thick aluminum transmission foil is readout by home-made transimpedance electronics, providing online dose information. The main monitoring variables, delivered in real-time, include beam current, integrated charge and dose rate. Hence the dose and integrated current delivered at a given instant to an experimental setup can be computer-controlled with a shutter. In this work, we report on experimental results and Geant4 simulations of a setup which exploits for the first time the 18 MeV proton beam from a PET cyclotron to irradiate a selected region of a target using the developed irradiation system. By using this system, we are able to deliver a homogeneous beam on targets with 18 mm diameter, allowing to achieve the controlled irradiation of cell cultures located in biological multi-well dishes of 16 mm diameter. We found that the magnetic field applied inside the cyclotron plays a major role for achieving the referred to homogeneity. The quasi-Gaussian curve obtained by scanning the magnet current and measuring the corresponding dose rate must be measured before any irradiation procedure, with the shutter closed. At the optimum magnet current, which corresponds to the center of the Gaussian, a homogenous dose is observed over the whole target area. Making use of a rotating disk with a slit of 0.5 mm at a radius of 150 mm, we could measure dose rates on target ranging from 500 mGy/s down to 5 mGy/s. For validating the developed irradiation setup, several Gafchromic® EBT2 films were exposed to different values of dose. The absolute dose in the irradiated films were assessed in the 2D film dosimetry system of the Department of Radiotherapy of Coimbra University Hospital Center with a precision better than 2%. In the future, we plan to irradiate small animals, cell cultures, or other materials or samples.