G. De Vincentis

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.

SCINTILLATING ARRAY GAMMA CAMERA FOR CLINICAL USE

Abstract Dedicated gamma cameras for specific clinical application are representing a new trend in Nuclear Medicine. They are based on Position Sensitive Photo Multiplier Tubes (PSPMT). The main intrinsic limitation of large area PSPMT (5″ diameter) is the photocathode glass window. Coupling to a planar scintillation crystal strongly affects the useful active area and the intrinsic spatial resolution. To overcome this limitation at University of Rome “La Sapienza” was developed the first 5″ diameter gamma camera consisting of a Hamamatsu R3292 PSPMT coupled to 50 × 50 YAP: Ce scintillating array. The array pixel size is 2 × 2 mm2 and the overall dimension of multi-crystal is 10 × 10 × 1 cm3. Resistive chains were used to calculate the centroid. The scintillating array produces a focused light spot minimising the spread introduced by PSPMT glass window. The intrinsic spatial resolution varied between 2 and 2.7 mm. The position linearity and useful active area resulted in good agreement with intrinsic one obtained by light spot irradiation. The real limitation was the poor energy resolution of an individual crystal (40%) and the poor uniformity response of PSPMT (within ±15%). A correction matrix was then carried out by which a 57% of total energy resolution was obtained for the whole matrix. The camera is currently operating as Single Photon Emission Mammography (SPEM) and it is producing breast functional images for malignant tumour detection using the same geometry as standard X-ray mammography.

Technetium labeled bombesin-like peptide: Preliminary report on breast cancer uptake in patients

Bombesin-like peptides are neurotransmitters and cancer growth factors. Several tumors, breast cancer among them, show one or more than one of the three known bombesin receptors. We have synthesized and labeled with technetium 99m a new pentadecapeptide, analogue to the leu13 amphibian bombesin (99mTc BN). Labeling yield was 83 +/- 4%. Prone Scintimammography was performed on five patients affected by breast cancers (T categorization: two T1b and three T1c), after injecting 0.7 mg, 185 to 296 MBq (5 to 8 mCi) of the peptide. Total body scan did not show free technetium biodistribution. No adverse reaction was observed. Prone Scintimammography with 99mTc Sestamibi (99mTc SM) was also performed few days later. 99mTc BN detected all 5 cancers, whereas 99mTc SM only four: all the T1c and one T1b cancer. Two of them showed axillary node invasion that was detected by both the radiotracers. A fibroadenoma present on contralateral breast to the one with cancer, was not detected neither by 99mTc SM nor by 99mTc BN. Tumor/breast normal tissue ratio (T/B) was constantly higher with 99mTc BN than with 99mTc SM. Maximal T/B was measured as 1.79 with 99mTc SM and 2.25 with 99mTc BN 5 min after fast i.v. administration. In conclusion our 99mTc BN is taken up by primary breast cancer showing higher T/B than 99mTc SM (p < 0.01). In our limited scale, 99mTc BN appears to be safe and, in our limited scale, even more accurate than 99mTc SM for detecting breast cancer.

First Results from a YAP:Ce Gamma Camera for Small Animal Studies

The YAP (yttrium aluminum perovskite) camera is a novel gamma camera with intrinsic submillimeter spatial resolution and detection efficiency comparable to a standard Anger camera. At the first stage, it is a miniature gamma camera with a field of view of 4/spl times/4 cm/sup 2/ and is currently utilized for radio tracer studies on small animals. The YAP camera consists of a multicrystal array coupled to a position sensitive photomultiplier tube (Hamamatsu R2486) with a parallel hole collimator. The preliminary results are presented and discussed, in particular the intrinsic characteristics of the scintillating array, which are measured, as well as the position linearity and the spatial resolution, with a parallel collimator. Images were obtained from /sup 99m/Tc line source and nude Balb C mice, which have been injected with /sup 99m/Tc MDP, a bone-seeking agent. The observed images show the importance of the small-field YAP camera in radiopharmaceutical research.

Synthesis, Chemical, Radiochemical and Radiobiological Evaluation of a New 99mTc-labelled Bombesin-like Peptide

A new pentadecapeptide bombesin analogue was prepared by Fmoc synthesis, purified by HPLC and identified by electron ionization mass spectrometry. The biological activity of the new peptide was tested on isolated human colonic muscle cells and compared to native bombesin. Labelling of the new biomolecule with Tc-99m yielded a single radioactive species which remained stable at room temperature for eight hours. In a binding assay, the radiolabelled peptide showed high affinity for oat-cell carcinoma (Kd = 9.8 nM) and colorectal adenocarcinoma (Kd = 27.2 nM). Biodistribution studies, performed in normal rodents, indicated uptake by organs that normally express bombesin receptors, such as liver, intestines and kidneys. Scintigraphic studies, performed in nude mice transplanted with small cell lung carcinoma and colon cancer cells, showed significant tumor uptake two hours p.i. The new synthetic pentadecapeptide appears to have promise for several malignancies, including oat-cell lung carcinoma, colorectal cancer and gastroenteropancreatic (GEP) tumors.

Toward a nuclear medicine with sub-millimiter spatial resolution

Abstract The HIRESPET Collaboration is developing a new concept of a gamma camera with sub-millimiter spatial resolution. The first prototype consists of a small field size gamma camera based on a Position Sensitive Photo-Multiplier Tube (PSPMT) coupled to a novel scintillation crystal. The intrinsic spatial resolution of the PSPMT is better than 0.3 mm. The scintillation crystal consists of yttrium aluminium perovskit (YAP:Ce). It has a light efficiency of about 40% relative to NaI, a good gamma radiation absorption ( Z = 39) and a high density (5.37 g/cm 3 ). It is inert and not hygroscopic. To match the PSPMT characteristics, a special crystal assembly has been made consisting of a bundle of YAP pillars, where a single crystal has the transversal dimension of 0.6 × 0.6 mm 2 and a thickness ranging between 1 mm and 28 mm. Each scintillation pillar is optically separated from the other by a reflective layer of 5 μm thick. The preliminary results obtained from the gamma camera prototype (YAP camera) show spatial resolution values ranging between 0.6 mm and 1 mm and an intrinsic detection efficiency comparable with a standard Anger 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.

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.

NEW GENERATION POSITION-SENSITIVE PMT FOR NUCLEAR MEDICINE IMAGING

Recently, Hamamatsu has developed a new dynode structure in a position-sensitive photomultiplier tube (PSPMT) based on metal channel technology, producing a very focused charge distribution. Hamamatsu R5900-C8 is the first metal channel dynode PSPMT with a crossed wire anode. The outline dimension is 28 × 28 × 20 mm3 with an active area of 21 × 22 mm2 and four wire anodes (4X + 4Y). A weighted summing device was realised to calculate the charge distribution centroid where each anode was directly connected to a preamplifier and amplifier. Tests of position linearity, energy response and spatial resolution were carried out coupling the PSPMT to an NaI(Tl) planar crystal and to a YAP:Ce scintillation array. To test the influence of charge spread on position linearity, the photocathode was irradiated by a light spot with 1 mm of aperture size. Position non-linearity resulted when a scintillating array with 1 × 1 mm2 pixel size was coupled to PSPMT. On the contrary, increasing the spread of charge distribution by a planar scintillation crystal, a good linearity response was obtained. Impressive spatial resolution values were obtained from the YAP:Ce scintillating array. They ranged between 0.5 and 0.2 mm irradiating the crystals by 57Co and 137Cs source, respectively.