R. Pellegrini

99Tcm-MIBI uptake in green plants

Uptake of 99mTcm-sestamibi by biological structures depends on delivery and concentration by electrochemical gradients through the biological membranes and can be simply studied using a green plant model in which photosynthesis tightly modulates water and solute regional flow. Photosynthesis creates electrochemical gradients inside chloroplasts and mitochondria. Moreover, it is the driving force for the movement of water and solutes through induction of pore opening which causes capture of CO2 and loss of water vapour. Thus osmotic pressure increases thereby drawing water from the roots. Hypoestes sanguinolenta was used as an experimental model. This plant displays green zones (with several chloroplasts) and red zones (where they are absent). To detect the uptake differences between these zones we used a new, high-resolution gamma camera. Our results show that (a) 99mTcm-sestamibi is actively transported with water and ions by xylem to leaves where it may diffuse at cellular level; (b) activation of photosynthesis by light strongly influences the total uptake and the selective compartmentation in green zones; and (c) the green plants particular physiology tremendously enhances the differences between 99Tcm-sestamibi and 201Tl uptake. We suggest that viable cells, able to create and maintain electrochemical gradients, selectively take up 99Tcm-sestamibi.

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.

Feasibility study for SPECT mammography based on compact imagers rotating around breast vertical axis

The detection limit of invasive carcinoma by standard prone scintimammography appears to be >=1 cm diameter. Since it is desirable to detect lesions at very earliest stages of growth (0.8 cm or less), the development of prototype scintimammographic systems with improved imaging performances is a primary goal. Here, the authors propose a dedicated high resolution breast imaging scanner for SPECT around the Vertical Axis of Rotation (VAOR) with the breast in prone position. The rotating detector module consists of a compact position sensitive photomultiplier tube (PS-PMTHamamatsu R7600-00-C8 coupled to a CsI(Tl) scintillating array. The compactness is the peculiarity of this detector module to allow the lodging in the breast interspace to work close to the chest wall and to minimize the tumor to collimator front distance. A preliminary study was performed by a breast phantom and 5 small FOV gamma camera with an actual rotation radius of 6.5 cm. Images were reconstructured using a filtered back projection implementation. Tumor SNR from planar images shown very low values: on the contrary reconstructed images strongly enhanced the contrast of a tumor 1 cm sized. A further enhancement was obtained by the compact detector module that was able to achieve higher SNR values for tumor less 1 cm sized).

8 inch diameter PSPMT for gamma ray imaging

This work presents preliminary measurements taken with the first prototype Hamamatsu (R6970) 8-inch Position Sensitive Photomultiplier Tube (PSPMT). These measurements are compared with similar ones obtained using a 5 inch PSPMT (Hamamatsu R3292). The new 8 inch PSPMT has 12 dynode stages, 11 with a proximity mesh structure whilst the last one is a back reflector. The entrance window is 7.5 mm thick and has an active area of 180 mm diameter. The crossed-wire anode of the 8 inch tube consists of 36/spl times/36 wires on a 4 mm pitch. These are paired together to give an 18/spl times/18 wire outputs. Both PSPMTs were coupled to a 110 mm diameter, 3 mm thick CsI(Tl) scintillating array in which each pixel has dimensions of 2/spl times/2 mm/sup 2/. Two read-out methods are compared in this paper. The first being the conventional resistive-divider technique. The second method uses a new multi-wire readout technique in which, the charge on each anode wire is individually read out and digitized. Measurements of the spatial resolution, position linearity, energy resolution and intrinsic charge distribution were carried out for both tubes using both read-out systems. Spatial resolution values of approximately 2 mm FWHM were obtained using the 8-inch PSPMT and the multiwire read-out technique. The other measured characteristics were similar to those obtained using the 5-inch PSPMT. These results obtained using the prototype 8-inch PSPMT underline the potential of this detector in the field of imaging in Nuclear Medicine.

Reduced parallel anode readout for 256 ch flat panel PMT

SPECT and PET need a good pixel identification to obtain high quality images. This is why new generations of PSPMT with anode array with smaller step have been developed till the recent flat panel PMT H9500 with 2 square area, 256 anodes array, 3 mm individual pitch. The realization of electronic chains with so high number channels demands dedicated electronics readout with high cost and high management difficulty. In this work we propose a new method of anode number reduction in parallel readout limiting the total chain number to 64. Starting from the evidence that event charge distribution is always contained inside a portion of the FP PMT anodic plane, we assume that only a quarter of the anodes are involved in the detection. Our approach consists on virtually dividing 16times16 anodes in 4 arrays with 64 anodes per each. Once the charge distribution is collected by the 4 anodic planes, each individual anodic charge is projected on one plane. Physically, each i,j-element of one quadrant is associated to the corresponding i,j-element of the other three matrices. In terms of hardware, it is simply realized connecting one to each other the set of four i,j-anodes of the 4 quadrants. In this work an image reconstruction software has been developed and tested by measured charge distribution collected by 256 anodes Hamamatsu FP PMT. The final results, in term of spatial resolution and position linearity, are in agreement with ones collected by the total number of anodes

Preliminary Evaluation of Compact Detectors for Hand-Held Gamma Cameras.

Hamamatsu Photonics has recently developed a new generation of compact Position Sensitive PhotoMultiplier Tubes (pspmt) based on metal channel dynode charge multiplication technology. The R5900 family now has a range of compact tubes that differ in the structure of the anode. The models considered in this paper, the C8 and M16, also differ in the photocathode active area. The C8 has a crossed plate anode configuration consisting of 4X + 4Y strips and an active area of 22 × 22 mm 2 while the M16 has a 4 × 4 anode array with a smaller active area of 18 × 18 mm 2 . In this paper we report our evaluation of the C8 and the M16 tubes for clinical imaging applications such as a hand-held gamma probe, multi-pspmt camera and for tomographic rings. To this aim, measurements of pulse height uniformity, inter-channel gain variation and anode cross talk were performed using a light source coupled to a 1 mm diameter optical fiber. Finally, the pspmts were optically coupled to three CsI(Tl) scintillating arrays with pixel size ranging between 1.5 × 1.5 mm 2 and 4.2 × 4.2 mm 2 to compare the imaging properties.

A study of intrinsic crystal-pixel light-output spread for discrete scintillation gamma-ray imagers modeling

This paper is focused on discrete scintillation imaging devices, made by using crystal arrays and metal-channel dynode Hamamatsu 1 and 2 square position sensitive photomultiplier tubes (PSPMT). These devices are particularly suitable for nuclear medicine based high-resolution Single Photon Emission Computed Tomography (SPECT) applications. A more adequate analytical model describing the light-output spread from a single crystal-pixel, was studied. Previously experimental data published by us were reviewed using this model. The parameter which describes the intrinsic pixel light-output spread was obtained using a 1D model-version, that adequately fits the measured single-event charge-strip integrals. Furthermore the intrinsic spread was found linearly dependent, in the examined experiments, on a crystal-pixel shape factor (defined as the ratio between the pixel-blind-surface-area to the pixel-volume). Finally, a simulation tool was developed on these basis, to predict and to optimize the imager response by evaluating the impact of each device component on the response.

Scintillator and photodetectors comparison for compact gamma cameras

The design of high resolution compact gamma cameras requires careful choice of scintillation crystal and photo-detector as well as their degree of pixellation. In this paper we evaluate the imaging properties of photo detectors for clinical imaging application. We compare the performances of two kinds of scintillators (CsI(Tl) and NaI(Tl)) coupled to PSPMTs of the Hamamatsu R5900 family (C8, C12, M16 and M64). To compare the imaging properties, the PSPMTs were optically coupled to CsI(Tl) and NaI (TI) scintillating arrays with pixel size of 2 /spl times/ 2 mm/sup 2/. Spatial resolution as well as energy resolution have been measured.

CsI(Tl) micropixel scintillation array for ultrahigh resolution gamma-ray imaging

The aim of this paper is to investigate on the spatial resolution limit by coupling a CsI(Tl) micropixel scintillation array to PSPMTs for ultrahigh resolution gamma-ray imaging. To perform this study an array with 0.2 mm pixel size, 0.4 mm pitch, and 1 mm thickness were realized by Spectra Physics (Hilger). The present scintillation arrays technology is suitable to produce large crystal areas. In this paper we present spatial resolution and position results obtained by coupling the micropixel scintillation array to Hamamatsu square PSPMTs: 1 R8520-C12, 1 R5900-L16 and 2 H8500 Flat panel PMT. Preliminary measurements demonstrate that micropixel array shows better performance coupled to H8500 model, in term of the uniformity response. This setup carries out an intrinsic spatial resolution lower limit of about 0.6 mm FWHM at 50% FWHM energy resolution, defining it as minimum scintillation array pitch detectable at 122 keV. Furthermore, from the results obtained by R5900-L16, the position response will be improved reducing the anode size (3 mm size) for a better sampling of the scintillation light.