M.N. Cinti

High spatial and energy resolution gamma imaging based on LaBr3(Ce) continuous crystals

Recently scintillators with very high light yield and photodetectors with high quantum efficiency have been opening a new way to realize gamma cameras with superior performances based on continuous crystals. Pixilated imagers have a spatial resolution limited by pixel size, in contrast with continuous scintillation crystals, where spatial resolution is a statistical function depending on light distribution spread and on generated photoelectrons from scintillation light flash. Continuous LaBr3:Ce crystal, with a light yield almost two times higher than NaI:Tl ones and a lower intrinsic energy resolution, could be the best candidate to carry out a gamma imaging with sub-millimeter spatial resolution and very good energy resolution. Unfortunately standard Anger algorithm produces an intrinsic position non-linearity affecting spatial resolution for small size continuous crystal. In this work we propose a new method to calculate the position mean value by squaring the 2D collected charge distribution on a multi-anodes photomultiplier tube (MA-PMT). In this study we take into account four different detector configurations: three sample of LaBr3:Ce scintillation crystals, 49mm×49mm area, a couple of 4.0 with different surface treatment and a single 10 mm thick, with 3 mm glass window. Moreover a forth one with 5.0mm thickness which was integral assembled with an Hamamatsu H8500. We applied the new position algorithm to simulated data, obtained by Geant4 code and afterwards to the experimental data obtained scanning the different detectors with 0.4 mm Ø collimated Tc99m point source, at 1.5 mm step. The results obtained with the new algorithm show an improvement in position linearity and in spatial resolution of about a factor two. The best values in terms of spatial resolution were 0.9 mm, 1.1 mm and 1.8 mm for integral assembled, 4.0 mm thick and 10 mm thick LaBr3:Ce crystal respectively. These results demonstrate the potential of LaBr crystal for molecular imaging application and more in general for gamma ray imaging

Small animal imaging by single photon emission using pinhole and coded aperture collimation

The design of detectors for radio-imaging of small animals is challenging because of the high spatial resolution required, possibly coupled with high efficiency to allow dynamic studies. Spatial resolution and sensitivity are difficult to attain at the same time with single photon imaging techniques because collimators define and limit performance. In this paper we first describe a simple desktop gamma imager equipped with a pinhole collimator and based on a pixellated NaI(Tl) scintillator array coupled to a Hamamatsu R2486 PSPMT. The limits of such a system as well as the way to overcome them in future systems is shown next. Better light sampling at the anode level would allow better pixel identification for a higher number of pixels, which is one of the parameters defining image quality and improving spatial resolution. The performance of such a design is compared with other designs using other PSPMT types with different light sampling schemes at the anode level. Finally, we show how the substitution of the pinhole collimator with a coded aperture collimator can result in a substantial improvement in system sensitivity while maintaining very good spatial resolution, possibly at a sub-millimeter level. Calculations and simulations of a particular solution show that sensitivity can improve by a factor of nearly 30.

A Geant4 simulation code for simulating optical photons in SPECT scintillation detectors

Geant4 is an object oriented toolkit created for the simulation of High-Energy Physics detectors. Geant4 allows an accurate modeling of radiation sources and detector devices, with easy configuration and friendly interface and at the same time with great accuracy in the simulation of physical processes. While most Monte Carlo codes do not allow the simulation of the transport and boundary characteristics for optical photons transport generated by scintillating crystal, Geant4 allows the simulation of the optical photons. In this paper we present an application of the Geant4 program for simulating optical photons in SPECT cameras. We aim to study the light transport within scintillators, photomultiplier tubes and coupling devices. To this end, we simulated a detector based on a scintillator, coupled to a photomultiplier tube through a glass window. We compared simulated results with experimental data and theoretical models, in order to verify the good matching with our simulations. We simulated a pencil beam of 140 keV photons impinging the crystal at different locations. For each condition, we calculated the value of the Pulse Height Centroid and the spread of the charge distribution, as read out by the anode array of the photomultiplier. Finally, the spatial and the energy resolutions of the camera have been estimated by simulated data. In all cases, we found that simulations agree very well with experimental data.

Optimization of compact gamma cameras for breast imaging

Abstract Mammoscintigraphy is an important tool for secondary breast cancer screening. Good results have been achieved with dedicated gamma cameras for T 1b tumors (size >5xa0mm). We have performed simulations and measurements in order to improve the performance of compact gamma cameras for detecting T 1a tumors (size 3 ) coupled with a Position Sensitive Photomultiplier Tube (PSPMT) having a multianode structure (2×2xa0mm 2 pad size). Measurements allowed to test the validity of the simulation software.

Preliminary evaluation of a monolithic detector module for integrated PET/MRI scanner with high spatial resolution

The proposal of Mindview European Project concerns with the development of a very high resolution and high efficiency brain dedicated PET scanner simultaneously working with a Magnetic Resonance scanner, that expects to visualize neurotransmitter pathways and their disruptions in the quest to better diagnose schizophrenia. On behalf of this project, we propose a low cost PET module for the first prototype, based on monolithic crystals, suitable to be integrated with a head Radio Frequency (RF) coil. The aim of the suggested module is to achieve high performances in terms of efficiency, planar spatial resolution (expected about 1 mm) and discrimination of gamma Depth Of Interaction (DOI) in order to reduce the parallax error. Our preliminary results are very promising: a DOI resolution of about 3 mm, a spatial resolution ranging from about 1 to 1.5 mm and a good position linearity.

Light output spatial distributions of CsI(Tl) scintillation arrays for gamma-rays imaging

Abstract Over the last years Nuclear Medicine and Oncology require the development of new scintigraphic imagers dedicated to particular organs and lesions, with improved performances with respect to presently available devices. In order to enhance the imaging responses, pixellated crystals have to be employed as scintillating devices coupled to position sensitive photomultiplier tubes. Some of major steps forward in this field could be obtained by using the recently produced Hamamatsu photomultiplier tubes. The knowledge of the pixel scintillation light distribution at photocathode allows the imaging performances optimization. To this aim CsI(Tl) arrays pixel light distributions were investigated by using a photographic method and a Hamamatsu multianode 16 channel linear array photomultiplier tube for single-event light output readout. Ten scintillating arrays, with square pixel base area ranging between 1.0×1.0 and 2.0×2.0xa0mm 2 and thickness from 1.0 to 5.0xa0mm, were tested. In this study, the strip scintillation light distribution spread at photocathode was found linearly dependent on the total light guide thickness and crystal-pixel base side.

Position sensitivity of large volume LaBr 3 :Ce detectors

The position sensitivity of two large volume (1” × 1” and 3” × 3”) LaBr<inf>3</inf>:Ce crystals has been measured using collimated beams of 662 keV γ-rays. The crystals have been coupled to a Position Sensitive Photo Multiplier Tube (PSPMT) and/or shielded phototube. The results indicate that, in the case of 662 keV γ -rays, both crystals should allow identification of the the position of the γ-ray first interaction with a 1-2 cm resolution. Similar tests have been done in the past using 140 keV γ-rays and no sensitivity was found. Using GEANT4 simulations we confirmed these results.

A novel parallel hole collimator for high resolution SPET imaging with a compact LaBr3 gamma camera

In this work we propose an analysis of a novel Low Energy (LE) parallel hole collimator for high resolution single photon emission tomography (SPET) applications. This prototype, realized jointly with Nuclear Fields, is a lead parallel hole collimator with 1.0 mm hexagonal hole, 18 mm length, 0.2 mm septa and 10x10 cm2 of useful detection area. It has been planned to match the high spatial resolution performances of a compact gamma camera based on LaBr3:Ce continuous scintillation crystal. The imaging performances of this prototype are compared with others two parallel collimators, for different dimensions and applications, and a tungsten pinhole collimator ones. All the collimators were tested with a compact scintillation gamma camera based on LaBr3:Ce continuous crystal and multi anode photomultipler tube (MA-PMT) Hamamatsu H8500. The high intrinsic spatial resolution of this crystal enhances the response of collimators at short source-to-collimator distance (SCD) overcoming alignment problems with the collimator pattern. From our analysis the collimator prototype seems to be complementary with the use of pinhole one and when coupled to the compact LaBr3:Ce gamma camera can allow a very attractive trade-off between spatial resolution, sensitivity and detection area for radionuclide molecular imaging applications.

Monte Carlo study and experimental measurements of breast tumor detectability with the YAP-PEM prototype

A prototype for positron emission mammography is under development within a collaboration of the Italian Universities of Pisa, Ferrara, Bologna and Roma. The device is composed of two stationary detection heads, each with an active area of 6 cm /spl times/ 6 cm, made of 30/spl times/30 YAP:Ce finger crystals of 2 mm /spl times/ 2 mm /spl times/ 30 mm. The EGSnrc Monte Carlo code has been used to perform a complete simulation of this camera. We have used a fast three-dimensional iterative algorithm (30 s per iteration on a PC-Pentium III 800 MHz processor) for image reconstruction. The performed study indicates that tumors of 5 mm diameter, i.e., 0.065 cm/sup 3/ volume, with 37 kBq/cm/sup 3/ (1 /spl mu/Ci/cm/sup 3/) specific activity embedded in a breast active phantom, are detectable in 10 minutes for a 10:1 tumor/background ratio with an 8.7 Signal-to-Noise Ratio value. Experimental measurements with the small animal tomograph YAP-PET have validated the Monte Carlo predictions.

Position algorithm for monolithic scintillation crystals based on charge projection readout

Nuclear medicine imaging devices commonly use multi-element photo detection systems, composed of an array of N × N elements, each one providing an individual signal. Many strategies have been developed to reduce the number of readout channels, one of the main approaches is the Rows and Columns (R/C) projection logic. In this paper we proposed a modified version of Raised To the Power (RTP) algorithm adapted to R/C logic. In order to validate its efficiency a linear scanning irradiation on two 49× 49 mm2 LaBr3:Ce (0.5%) crystals with different thickness (4 mm and 10 mm) was carried out. Imaging performance analysis was made in terms of position linearity, Field-of-View (FoV) enlargement and spatial resolution. Imaging results from Anger Logic, RTP algorithm based on single element readout and RTP algorithm based on R/C readout were compared. A notable advantage of using RTP algorithms instead of Anger Logic was found: the FoV widens from about 30% to more than 70% of the detector area whereas the spatial resolution is highly improved, especially for off-center interactions, both for 4 mm-thick and 10 mm-thick crystals. Furthermore, imaging performance with the R/C readout is just slightly different from the single element one (FoV reduction less than 7% and SR worsening less than 10%). The R/C adapted RTP algorithm opens doors to high imaging performance with a substantial reduction of complexity and cost in the readout electronics.