P. Bennati

Design of compact pinhole SPECT system based on flat panel PMT

The present development of new gamma imagers has allowed to realize detectors with ultra high spatial resolution and very compact size for PET as well as for SPET application. In this paper we analyze and discuss the possible design of new pinhole SPECT scanners based on heads which consist of flat panel PSPMT and different design of scintillation arrays like NaI(Tl), 1 mm pixel size, and CsI(Tl) multi layers array, mounted in off centered configuration to improve the intrinsic spatial resolution of the imagers. The results show that an array configuration 2/spl times/2 Hamamatsu flat panel PSPMTs coupled to NaI(Tl) scintillation array with 1 mm pixel size, represents the best trade off between compactness and spatial resolution of pinhole SPET scanner. The use of off centered CsI(Tl) scintillation array coupled to a single flat panel PSPMT allows to arrange a high sensitivity and very compact pinhole SPET scanner at very low cost only worsening of 50% spatial resolution than an Anger gamma camera pinhole SPECT.

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

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.

Breast MRI: Are T2 IR sequences useful in the evaluation of breast lesions?

AIM To evaluate the potential role of signal intensities calculated in T2 images as an adjunctive parameter in the analysis of mass-like enhancements classified as BIRADS (Breast Imaging Reporting and Data System) assessment categories 2, 3, 4 or 5 with the standard T1 criteria. MATERIALS AND METHODS After a retrospective review of 338-breast Magnetic Resonance Imaging (MRI) performed for the evaluation of a suspicious lesion we selected a group of 65 mass-like enhancements ranging from 5 to 20mm, classified as BIRADS assessment categories 2, 3, 4 or 5, histologically proved. In all cases we calculated the ratio between the signal intensity (SI) of the nodule and the pectoralis major muscle (LMSIR, lesion to muscle signal intensity ratio) with a multiROIs (region of interest) analysis on T2 images. A ROC analysis was performed to test the ability of the two diagnostic parameters separately considered (BIRADS and LMSIR) and combined in a new mono-dimensional variable obtained by a computerized discriminant function. RESULTS Histological examination assessed 34 malignant lesions (52.3%) and 31 benign lesions (47.7%). The evaluation of ROC curves gave the following results: BIRADS area under the curve (AUC) 0.913, S.E. 0.0368, LMSIR AUC 0.854, S.E. 0.0487, combined BIRADS-LMSIR AUC 0.965, S.E. 0.0191 with a definitive increase in the AUC between the overall ROC area and those of the two diagnostic modalities separately considered. DISCUSSION T2-weighted SI assessment with LMSIR measurement improves the diagnostic information content of standard breast MRI and can be considered a promising potential tool in the differential diagnosis of mass-like enhancements judged as borderline lesions (BIRADS 3 and 4).

New devices for imaging in nuclear medicine.

Pinhole gamma camera imaging offers the ability to obtain high resolution images from single gamma ray emitting radiotracers playing a reasonable tradeoff between very small field of view (FoV) and sensitivity. On the other hand the total spatial resolution of a pinhole imaging device is predominantly affected by the detector intrinsic spatial resolution for reduced magnification factors. To design very compact pinhole SPET scanners with very high intrinsic spatial resolution, authors investigated a miniature gamma camera based on the newly developed Hamamatsu H8500 flat panel photomultiplier. The PSPMT was coupled to the following scintillation arrays: CsI(Tl) array with 0.2-mm, 1-mm, 1.4-mm pixel size and NaI (Tl) with 1-mm pixel size. The imaging performances were evaluated by 57Co spot and flood irradiations. NaI(Tl) array shows a better pixel identification for 1 mm pixel size, proving to be a good candidate to make a large area photodetector based on multi PSPMTs closely packed. Although CsI(Tl) array had the smallest pixel size, the low light output limited the best intrinsic spatial resolution to about 0.5 mm.

Dual isotope imaging with LaBr3:Ce crystal and H8500 PSPMT

The introduction of Lanthanum Bromide crystal, characterized by a fast and high light emission, offers the possibility to improve both imaging spatial resolution and energy resolution in SPECT in the 80–300 kev energy range, without compromising detection efficiency. The expected performances may be limited if the crystal is used with a multi-anode PMT with Position Sensitive response due to the wide differences in the segmented anode pad gains, ranging from 2:1 to 10:1, . These characteristics may strongly deteriorate the overall energy resolution and, consequently, impair the detector imaging capability. For this reason, a 256 independent channel electronic read out was developed, based on an FPGA control, to individually read the charge on each anode. The electronics features a very low electronic noise ( < 1%) and a wide dynamic range. The readout electronics was used to build a gamma camera based on a single 100 mm × 100 mm continuous LaBr3:Ce crystal coupled to a 2 × 2 array SBA photocatode (38{%} QE) Hamamatsu H8500. An offline calibration procedure is also shown in order to compensate the anode gain variation and to exploit the LaBr3:Ce capabilities, obtaining on the whole detection area an 8.0–8.5{%} energy resolution at 140 kev. The high energy resolution performances of this gamma camera permitted to discriminate emissions from two different isotope ( Tc99m and Co57) with very close photon energy (140 and 122 kev respectively). This capability can be used to provide the gamma image with references coming from Co57 point sources (marker) fixed at known positions. These results confirm the LaBr3:Ce crystals as one of the most interesting for all single photon emission applications.

Factors affecting flat panel PMT calibration for gamma ray imaging

Hamamatsu H8500 Flat Panel PMT represents the last technological advancement in gamma ray imaging. Compact size makes it attractive for medical imaging application. To study and compare image performance two Flat Panel PMTs were coupled to CsI(Tl) and NaI(Tl) scintillation arrays with 3 mm and 1.8 mm pixel size respectively and they were connected to multi-anode electronic readout (64 channel). Furthermore a pulsed blue LED coupled to an optical fiber was utilized to scan the tube with different light distribution spreading. The study took into account how PMT anode gain uniformity response, light distribution and intensity, influence spatial resolution, position linearity and image noise. Gain calibration was firstly studied because of PMT gain anode non uniformity response, which range between 27:100 and between 45:100 respectively. Furthermore each crystal pixel produces different charge distribution and this depends on the matching between anode and scintillation array lattice. The amount of anode charge can change more than a factor five for narrow light distributions. Tube gain setting results critical, in fact because of energy resolution of each anode spectra, only a factor five pulse height variation can be adequately converted by ADC. In addition there is a further gain anode variation due to PMT non uniformity response of a factor 3. This mentioned two elements, do not allow to convert all pulses in the useful pulse height ADC range. As a consequence image position distortion and background are produced. Flat Panel shows good image performance. However, because of the big anode size and PMT gain non uniformity response, the gain setting can be critical to obtain the best image performance for scintillation light distribution comparable with anode size.

Continuous DoI determination by gaussian modelling of linear and non-linear scintillation light distributions

The Depth of Interaction (DoI) detection is crucial in many medical imaging applications such as small ring PET and high resolution SPECT. In this work we investigate the possibility to discriminate the DoI using continuous crystals. A LaBr3(Ce) crystal has been used in the detection system for its intrinsic high light yield, that especially at low energies (e.g. 140 keV) reduces considerably the statistical uncertainties increasing the DoI discrimination power. The innovative suggestion of this work is the use of spectrometric observables to discriminate events on top and bottom of the crystal, under the hypothesis that scintillation light distributions can be parameterized by a gaussian model. The spread of the light cone (σ) is proportional to the DoI simply by geometrical considerations, but under the gaussian hypothesis relations between the spectrometric variables (maximum high I and integral of the distribution N) and the DoI become a straightforward consequence. Two methods are proposed and discussed: a linear treatment of the light distribution and a non linear (quadratic) manipulation of it. The expected correlations between the spectrometric variables (N and I), according to the gaussian model, are checked using a specific Monte Carlo simulation of the experimental apparatus. Those are then compared with experimental data obtained irradiating the LaBr3:Ce crystal with a Tc99m collimated source. A close agreement between experimental data and MC is verified. Finally, a preliminary test on experimental data has been performed irradiating the crystal with a Co57 source, in order to investigate the strong dependence of the non linear manipulation of the light distribution to the DoI.

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.

A gamma camera with the useful field of view coincident with the crystal area

In recent years a great deal of attention was given to the Hamamatsu Position Sensitive PMTs, due to their suitability for building compact and high resolution gamma cameras. In this framework a great number of solution was proposed for the readout, that vary from the analog readout made with a resistive chain, in some cases including a hardware threshold, to fully digital readouts that read each of the PSPMT anodes individually. In order to obtain the best performance from the PSPMT, a readout that reads each channel individually must be chosen, but a great care must be also taken in designing and tuning the calculation of the point in which the gamma photon has actually interacted with the crystal. This paper describes a 5 cm by 5 cm, gamma camera, based on the Hamamatsu H8500 PSPMT and a continuous La Br3: Ce crystal, that makes the readout of each channel individually, up to a sampling frequency of 250 K Sample/sec, and uses a new algorithm, based on mean square estimation, to reconstruct the parameters of the light distribution produced by the interaction with the gamma photon. The system has many advantages: it does not suffer from reduction of the useful field of view of the detector, that is observed in the center of gravity based approach due to the truncation of the light distribution, determined by the limited dimension of the scintillating crystal. This characteristic allows the full exploiting of the detector surface (at the price of a slight lowering of the resolution a the very borders of the crystal); moreover it is easy to compensate for the gain variations in the PSPMT anodes and in the analog front-ends, avoiding spatial distortion of the gamma image; finally, for very high rate sources, the algorithm is inherently able to discriminate for double hits.