R. Scafè

Multi-PSPMT scintillating camera

Gamma ray imaging is usually accomplished by the use of a relatively large scintillating crystal coupled to either a number of photomultipliers (PMTs) (Anger Camera) or to a single large Position Sensitive PMT (PSPMT). Recently the development of new diagnostic techniques, such as scintimammography and radio-guided surgery, have highlighted a number of significant limitations of the Anger camera in such imaging procedures. In this paper a dedicated gamma camera is proposed for clinical applications with the aim of improving image quality by utilizing detectors with an appropriate size and shape for the part of the body under examination. This novel scintillation camera is based upon an array of PSPMTs (Hamamatsu R5900-C8). The basic concept of this camera is identical to the Anger Camera with the exception of the substitution of PSPMTs for the PMTs. In this configuration it is possible to use the high resolution of the PSPMTs and still correctly position events lying between PSPMTs. In this work the test configuration is a 2 by 2 array of PSPMTs. Some advantages of this camera are: spatial resolution less than 2 mm FWHM, good linearity, thickness less than 3 cm, light weight, lower cost than equivalent area PSPMT, large detection area when coupled to scintillating arrays, small dead boundary zone (<3 mm) and flexibility in the shape of the camera.

Tumor SNR analysis in scintimammography by dedicated high contrast imager

The introduction of a new gamma camera fully dedicated to scintimammography (Single Photon Emission Mammography-SPEM), and more recently with a full breast FoV, allowed to make clinical examination in cranio-caudal projection like in RX-mammography, with breast mildly compressed. Such cameras are based on pixellated scintillation array and position sensitive photomultiplier (PSPMT). Reducing the collimator-tumor distance, the geometric spatial resolution and contrast was enhanced. Unfortunately, due to the scintimammographic low counting, poor contrast images are still obtained, in particular for small tumor. The aim of this paper is to evaluate how a camera based on pixellated detector can improve the SNR values for small tumor by an effective correction of the spatial response. The procedure is based on good pixel identification. A Small Gamma Camera (SGC) was arranged using metal channel dynode PSPMT photomultiplier (Hamamatsu R7600-C8) coupled to different CsI (Tl) scintillator array, with field of view (FoV) with an all purpose collimator. This PSPMT kind drastically reduces the charge spread improving the intrinsic characteristics of the imager. The dimensions of the CsI (Tl) arrays were the same of PSPMT active area (22/spl times/22 mm/sup 2/). Considering the very high intrinsic spatial resolution, a look up table was realized to accurately correct the gain and spatial non-uniformities. We used a breast and torso phantom to characterize the SNR as a function of scintillation pixel size, thickness of the breast, tumor size and depth. The data showed that the SNR depends principally on the match between the tumor and pixel size. In particular, for a 6 mm diameter tumor, the best SNR results were obtained by a 2/spl times/2 mm/sup 2/ pixelled array. For larger tumors, up to 10 mm diameter, a greater pixel size, like 30 mm/sup 2/ or 4/spl times/4 mm/sup 2/, optimizes the SNR value. We compared the results of this camera with the analogous ones obtained by a SPEM gamma camera and by a standard Anger Camera.

Detection of sentinel node in breast cancer: pilot study with the imaging probe.

The commonly used gamma probes are easy to use but also give rough information when employed in radioisotope-guided surgery. When images are required for exact localization, a gamma camera as well as a probe have to be used. Position-sensitive photomultipliers have contemporaneously allowed high-resolution scintigraphy and miniaturization of gamma cameras. We have assembled a miniature gamma camera with a 1-square-inch field of view and an intrinsic resolution of about 1 mm. When the minicamera is collimated with a large-holed, highly sensitive collimator, it acquires a spatial resolution of 3 mm. This prototype has been tested in the detection of difficult-to-image breast cancer sentinel nodes. Five nodes that had not been found with the usual technique of an Anger camera plus conventional probe were checked with the miniature camera that we named imaging probe: it actually is small enough to be used as a probe and large enough to give an image. One of the five nodes was found and imaged. It was small, disease-free, close to the tumor and probably hidden by the Compton halo around the peritumoral injection site. Our pilot study shows that the imaging probe, although still a prototype, has certain advantages over conventional methods when lymph node localization is required during surgery.

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

Evaluation of flat panel PMT for gamma ray imaging

Abstract The first position sensitive PMT, Hamamatsu R2486, developed in 1985, represented a strong technological advance for gamma-ray imaging. Hamamatsu H8500 Flat Panel PMT is the last generation position sensitive PMT: extremely compact with 2 in. active area. Its main features are: minimum peripheral dead zone (1 mm ) and height of 12 mm . It was designed to be assembled in array to cover large detection area. It can represent a technical revolution for many applications in the field of gamma-ray imaging as for example nuclear medicine. This tube is based on metal channel dynode for charge multiplication and 8×8 anodes for charge collection and position calculation. In this paper we present a preliminary evaluation of the imaging performances addressed to nuclear medicine application. To this aim we have taken into account two different electronic readouts: resistive chain with Anger Camera principle and multianode readout. Flat panel PMT was coupled to CsI(Tl) and NaI(Tl) scintillation arrays. The results were also compared with the first generation PSPMT.

Scintillator and photodetector array optimization for functional breast imaging

Nuclear Medicine methods have been proposed as a means of imaging primary breast lesions and regional metastatic involvement based on tumor physiology. Recently, the positive predictive value of scintimammography using 99 Tc labelled SestaMIBI has been reported to be as high as 81%, with an associated negative predictive value of 97%. Visualization of small (o1 cm) lesions using scintimammography may be complicated, however, by the effects of overlying and underlying background uptake of MIBI in the breast soft tissue and the deterioration of lesion contrast with distance from the gamma camera. For these reasons dedicated compact gamma cameras have been proposed and successfully used. Nevertheless, the detection of very small tumors (o5–10 mm) is still very difficult. Many parameters affect the breast small tumors detection. The degree of pixellation of both the scintillator and photodetector is critical for the intrinsic position resolution of the detector and for the overall imaging performance. In this paper, we examine the basic imaging properties of systems using arrays of scintillators and pixellated photodetectors. The influence of readout systems is also taken into account. Simulations as well as preliminary experimental results are presented. r 2002 Published by Elsevier Science B.V.

Radioguided biopsy of osteoid osteoma: usefulness of imaging probe.

Aims and Background When removal of osteoid osteoma is performed with open biopsy, the surgeon can be guided by radioactivity of 99mTc-MDP (methylene D- phosphonate) acquired by a probe. Material and methods We compared the performance of a commercially available ZnCdTe probe (Neoprobe 2000) and a one-square-inch-field-of-view imaging probe (IP) on two patients undergoing open biopsy for osteoid osteoma. Triphasic bone scintigraphy was performed before operation and Neoprobe as well as IP were used in the operating room by two nuclear physicians. When the surgeon asked for guidance, each nuclear physician had to indicate a precise direction. Results The surgeon asked for guidance once in the first operation, on a patient with osteoid osteoma of the femur, and four times in the second operation, for osteoid osteoma of the acetabulum. The indications provided by IP were correct 5/5 times, whereas the commercial probe was correct 3/5 times. Both devices were able to assess the surgical radicality. After biopsy, bone samples were divided into high-count and low-count samples. Pathological examination confirmed the presence of osteoid osteoma in high-count samples. Conclusions IP has already been used to guide biopsy, but only in breast disease. The present work confirms its good performance also in orthopedics as a portable mini gamma camera that can be used in the operating room.

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.

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.

CsI(Tl) Micro-Pixel Scintillation Array for Ultra-high Resolution Gamma-ray Imaging

The aim of this paper is to investigate the intrinsic spatial resolution limit by coupling a CsI(Tl) micro-pixel scintillation array to position sensitive photomultipliers (PSPMTs) for ultra-high resolution gamma-ray imaging. On this purpose, 1 mm thick array with 0.2 mm pixel side, 0.4 mm pitch has been realized by Spectra Physics (Hilger). The present scintillation arrays technology is suitable to produce larger crystal areas. In this paper we present spatial resolution and positioning results obtained by coupling the micro-pixel scintillation array to Hamamatsu square PSPMTs: 1rdquo R8520-C12, 1rdquo R5900-L16 and 2rdquo H8500 Flat panel PMT. Preliminary measurements demonstrate better performance in term of uniformity response when micro-pixel array is coupled to a H8500 PSPMT model. This setup carries out an intrinsic spatial resolution lower limit of about 0.6 mm FWHM at 50% FWHM energy resolution, defining it as the minimum scintillation array pitch detectable at 122 keV. The results obtained by R5900-L16 with a better sampling of the scintillation light has shown an improvement of the position linearity in spite of a worse spatial resolution due to the poor light output of scintillation array.