Nicola Belcari

Multi-ray-based system matrix generation for 3D PET reconstruction.

Iterative image reconstruction algorithms for positron emission tomography (PET) require a sophisticated system matrix (model) of the scanner. Our aim is to set up such a model offline for the YAP-(S)PET II small animal imaging tomograph in order to use it subsequently with standard ML-EM (maximum-likelihood expectation maximization) and OSEM (ordered subset expectation maximization) for fully three-dimensional image reconstruction. In general, the system model can be obtained analytically, via measurements or via Monte Carlo simulations. In this paper, we present the multi-ray method, which can be considered as a hybrid method to set up the system model offline. It incorporates accurate analytical (geometric) considerations as well as crystal depth and crystal scatter effects. At the same time, it has the potential to model seamlessly other physical aspects such as the positron range. The proposed method is based on multiple rays which are traced from/to the detector crystals through the image volume. Such a ray-tracing approach itself is not new; however, we derive a novel mathematical formulation of the approach and investigate the positioning of the integration (ray-end) points. First, we study single system matrix entries and show that the positioning and weighting of the ray-end points according to Gaussian integration give better results compared to equally spaced integration points (trapezoidal integration), especially if only a small number of integration points (rays) are used. Additionally, we show that, for a given variance of the single matrix entries, the number of rays (events) required to calculate the whole matrix is a factor of 20 larger when using a pure Monte-Carlo-based method. Finally, we analyse the quality of the model by reconstructing phantom data from the YAP-(S)PET II scanner.

An optimization-based method for geometrical calibration in cone-beam CT without dedicated phantoms

In this paper we present a new method for the determination of geometrical misalignments in cone-beam CT scanners, from the analysis of the projection data of a generic object. No a priori knowledge of the object shape and positioning is required. We show that a cost function, which depends on the misalignment parameters, can be defined using the projection data and that such a cost function has a local minimum in correspondence to the actual parameters of the system. Hence, the calibration of the scanner can be carried out by minimizing the cost function using standard optimization techniques. The method is developed for a particular class of 3D object functions, for which the redundancy of the fan beam sinogram in the transaxial midplane can be extended to cone-beam projection data, even at wide cone angles. The method has an approximated validity for objects which do not belong to that class; in that case, a suitable subset of the projection data can be selected in order to compute the cost function. We show by numerical simulations that our method is capable to determine with high accuracy the most critical misalignment parameters of the scanner, i.e., the transversal shift and the skew of the detector. Additionally, the detector slant can be determined. Other parameters such as the detector tilt, the longitudinal shift and the error in the source-detector distance cannot be determined with our method, as the proposed cost function has a very weak dependence on them. However, due to the negligible influence of these latter parameters in the reconstructed image quality, they can be kept fixed at estimated values in both calibration and reconstruction processes without compromising the final result. A trade-off between computational cost and calibration accuracy must be considered when choosing the data subset used for the computation of the cost function. Results on real data of a mouse femur as obtained with a small animal micro-CT are shown as well, proving the capability of the proposed calibration method. In principle, the method can be adapted to other cone-beam imaging modalities (e.g., single photon emission computed tomography).

Energy, Timing and Position Resolution Studies With 16-Pixel Silicon Photomultiplier Matrices for Small Animal PET

A high resolution small animal PET scanner that employs Silicon Photomultiplier (SiPM) matrices as photodetectors is under development at the University of Pisa and INFN Pisa. The first SiPM matrices fabricated by the Center for Scientific and Technological Research, FBK-irst (Trento, Italy), are being evaluated for this purpose. The devices are composed of 16 (4 times4) pixel elements of 1 mmtimes1 mm in a common substrate. The first tests have been carried out employing the ASIC MAROC2 for the readout. Energy and timing resolution, and position determination tests have been performed coupling both pixellated and continuous LYSO scintillator crystals to the matrix, and the results have been compared with the ones obtained for single SiPMs. The first tests on position determination with continuous crystals and SiPM matrices have been performed. An intrinsic spatial resolution of 0.61 mm FWHM has been obtained.

Analysis of image sharpness reproducibility on a novel engineered micro-CT scanner with variable geometry and embedded recalibration software.

This study investigates the reproducibility of the reconstructed image sharpness, after modifications of the geometry setup, for a variable magnification micro-CT (μCT) scanner. All the measurements were performed on a novel engineered μCT scanner for in vivo imaging of small animals (Xalt), which has been recently built at the Institute of Clinical Physiology of the National Research Council (IFC-CNR, Pisa, Italy), in partnership with the University of Pisa. The Xalt scanner is equipped with an integrated software for on-line geometric recalibration, which will be used throughout the experiments. In order to evaluate the losses of image quality due to modifications of the geometry setup, we have made 22 consecutive acquisitions by changing alternatively the system geometry between two different setups (Large FoV - LF, and High Resolution - HR). For each acquisition, the tomographic images have been reconstructed before and after the on-line geometric recalibration. For each reconstruction, the image sharpness was evaluated using two different figures of merit: (i) the percentage contrast on a small bar pattern of fixed frequency (f = 5.5 lp/mm for the LF setup and f = 10 lp/mm for the HR setup) and (ii) the image entropy. We have found that, due to the small-scale mechanical uncertainty (in the order of the voxel size), a recalibration is necessary for each geometric setup after repositioning of the systems components; the resolution losses due to the lack of recalibration are worse for the HR setup (voxel size = 18.4 μm). The integrated on-line recalibration algorithm of the Xalt scanner allowed to perform the recalibration quickly, by restoring the spatial resolution of the system to the reference resolution obtained after the initial (off-line) calibration.

Reprogrammable Acquisition Architecture for Dedicated Positron Emission Tomography

We have developed a flexible, cost-efficient PET architecture adaptPositron Emission Tomographyable to different applications and system geometries, such as positron emission mammography (PEM) and in-beam PET for dose delivery monitoring (ibPET). The acquisition system has been used to implement modularized dual planar detectors with very low front-end dead time, as required in PEM or in ibPET. The flexibility is obtained thanks to the FPGA-based, reprogrammable, TDC-less coincidence processor. The final goal is to propose an effective acquisition methodology and the construction of a compact, low-cost instrument able to provide early diagnosis and to improve the effectiveness of follow-up studies for smaller tumours with respect to those studied with present clinical equipment (e.g., whole-body PET, SPECT, or scintigraphy).

Experimental validation of the filtering approach for dose monitoring in proton therapy at low energy

The higher physical selectivity of proton therapy demands higher accuracy in monitoring of the delivered dose, especially when the target volume is located next to critical organs and a fractionated therapy is applied. A method to verify a treatment plan and to ensure the high quality of the hadrontherapy is to use Positron Emission Tomography (PET), which takes advantage of the nuclear reactions between protons and nuclei in the tissue during irradiation producing beta(+)-emitting isotopes. Unfortunately, the PET image is not directly proportional to the delivered radiation dose distribution; this is the reason why, at the present time, the verification of depth dose profiles with PET techniques is limited to a comparison between the measured activity and the one predicted for the planned treatment by a Monte Carlo model. In this paper we test the feasibility of a different scheme, which permits to reconstruct the expected PET signal from the planned radiation dose distribution along beam direction in a simpler and more direct way. The considered filter model, based on the description of the PET image as a convolution of the dose distribution with a filter function, has already demonstrated its potential applicability to beam energies above 70 MeV. Our experimental investigation provides support to the possibility of extending the same approach to the lower energy range ([40, 70] MeV), in the perspective of its clinical application in eye proton therapy.

Silicon photomultipliers and SiPM matrices as photodetectors in nuclear medicine

A high performance detector head with matrices of silicon photomultipliers (SiPMs) is under development at the University of Pisa. The detector head will be employed in the construction of a high spatial resolution, MR compatible small animal PET scanner. Silicon photomultipliers from FBK- irst (Trento, Italy) are being evaluated for this purpose. SiPM elements of 1 mm x 1 mm size and SiPM matrices composed of four (2 x 2) pixel elements have been tested. An intrinsic timing resolution of 60 ps sigma has been measured. The results with LSO crystals show an energy resolution of 20% FWHM at 511 keV, and a coincidence timing resolution of 600 ps rms. New devices with improved characteristics and active area, as well as SiPM matrices with 16 (4 x 4) SiPM elements have been produced, and will be evaluated.

A PET Prototype for “In-Beam” Monitoring of Proton Therapy

The in-beam PET is a novel PET application to image the beta+ activity induced in biological tissues by hadronic therapeutic beams. Thanks to the correlation existing between beam-delivered dose profiles and beam-induced activity profiles, in vivo information about the effective ion paths can be extracted from the in-beam pet image. in situ measurements, immediately after patient irradiation, are recommended in order to exploit the maximum statistics, by also detecting the contribution provided by the very short lived isotopes, e.g. 15O. A compact, dedicated tomograph should then be developed for such an application, so as to be used in the treatment room. We developed a small PET prototype in order to demonstrate the feasibility of such a technique for the monitoring of proton therapy of ocular tumors at the CATANA facility (Catania, Italy). The prototype consists of two planar heads with an active area of about 5 cm times 5 cm. Each head is made up of a square position sensitive photomultiplier (Hamamatsu H8500) coupled to a matrix of the same size of LYSO scintillating crystals (2 mm times 2 mm times 18 mm pixel dimensions). Dedicated, compact electronic boards are used for the signal multiplexing, amplification and digitization. The distance between the pair can be varied from 10 cm up to a maximum of about 20 cm. The validation of the prototype was performed on plastic phantoms using 62 MeV protons at the CATANA beam line. Different dose distributions were delivered and a good correlation between the distal fall-off of the activity profiles and of the dose profiles was found, i.e., better than 2 mm along the beam direction.

Proton range monitoring with in-beam PET: Monte Carlo activity predictions and comparison with cyclotron data

GOAL Proton treatment monitoring with Positron-Emission-Tomography (PET) is based on comparing measured and Monte Carlo (MC) predicted β(+) activity distributions. Here we present PET β(+) activity data and MC predictions both during and after proton irradiation of homogeneous PMMA targets, where protons were extracted from a cyclotron. METHODS AND MATERIALS PMMA phantoms were irradiated with 62 MeV protons extracted from the CATANA cyclotron. PET activity data were acquired with a 10 × 10 cm(2) planar PET system and compared with predictions from the FLUKA MC generator. We investigated which isotopes are produced and decay during irradiation, and compared them to the situation after irradiation. For various irradiation conditions we compared one-dimensional activity distributions of MC and data, focussing on Δw50%, i.e., the distance between the 50% rise and 50% fall-off position. RESULTS The PET system is able to acquire data during and after cyclotron irradiation. For PMMA phantoms the difference between the FLUKA MC prediction and our data in Δw50% is less than 1 mm. The ratio of PET activity events during and after irradiation is about 1 in both data and FLUKA, when equal time-frames are considered. Some differences are observed in profile shape. CONCLUSION We found a good agreement in Δw50% and in the ratio between beam-on and beam-off activity between the PET data and the FLUKA MC predictions in all irradiation conditions.

A Time Efficient Optical Model for GATE Simulation of a LYSO Scintillation Matrix Used in PET Applications

A time efficient optical model is proposed for GATE simulation of a LYSO scintillation matrix coupled to a photomultiplier. The purpose is to avoid the excessively long computation time when activating the optical processes in GATE. The usefulness of the model is demonstrated by comparing the simulated and experimental energy spectra obtained with the dual planar head equipment for dosimetry with a positron emission tomograph (DoPET). The procedure to apply the model is divided in two steps. Firstly, a simplified simulation of a single crystal element of DoPET is used to fit an analytic function that models the optical attenuation inside the crystal. In a second step, the model is employed to calculate the influence of this attenuation in the energy registered by the tomograph. The use of the proposed optical model is around three orders of magnitude faster than a GATE simulation with optical processes enabled. A good agreement was found between the experimental and simulated data using the optical model. The results indicate that optical interactions inside the crystal elements play an important role on the energy resolution and induce a considerable degradation of the spectra information acquired by DoPET. Finally, the same approach employed by the proposed optical model could be useful to simulate a scintillation matrix coupled to a photomultiplier using single or dual readout scheme.