γ-Camera System Research Articles

Sensitivity and Resolution Study of a Small-Field γ-Camera System on a Tomographic Level

The sensitivity and the spatial resolution of a small-field γ-Camera system based on a Position Sensitive Photomultiplier Tube (PSPMT) on a tomographic level are examined in this study. A cylindrical Gel-Phantom (d = 40 mm, h = 50 mm) with cylindro-conoidal tubes and capillaries (from 64 to 640 mm3 in volume) containing water solution of 99mTc is used as a test phantom in the present work. A total of 24 projections covering the full angle region (0°-360°) are obtained with the γ-Camera system under examination. The planar information is further analyzed to reconstruct the tomographic images taking into account all off-line corrections needed to remove barreloid deformations appearing at the edges of the Field-of- View. The reconstruction procedure is performed with iterative algorithms and for comparison reasons two different techniques (MLEM and accelerated ART) are used. The variety of the 99mTc-volumes in the phantom with the given specific radioactivity and the phantom axial asymmetry, due to the diÆerent radial distances of the tubes in the gel environment, allow a realistic characterization of the system’s performance on a tomographic level. Obtained experimental results for the system sensitivity and spatial resolution are presented and discussed in this work.

Position and Energy Resolution of a γ-Camera based on a Position Sensitive Photomultiplier Tube

Studies of the spatial- and energy-resolution of a small field, high resolution γ-Camera system currently being developed in our laboratory are presented here. The system is based on a cylindrical Position Sensitive Photo-Multiplier Tube (HAMAMATSU R2486) with 32 crossed-wired anodes, arranged in two orthogonal groups. The anode outputs are connected to a resistive current divider network and after pre-amplification are guided to a local digitizing system. A PCI-Analog to Digital Converter card with a maximum sampling rate of 20 MHz and the rest of the Data Acquisition System is controlled by software running on the LabVIEW environment. Position and induced charge of the incident light pulses can be easily reconstructed through a variety of offline algorithms. In the first part of this study the intrinsic response of the PMT and especially its energy and position resolution is presented. The experimental procedure utilizes the controlled pulse light output of a LED guided through fiber glass directly to the PMT’s entrance. The accumulated charge distribution and charge spread as a function of the incident number of photons is studied. In the second part, the response of the integrated γ-Camera system after the application of typical scintillation crystals for 99Tc radioactive phantoms is measured and analyzed.

A Small Field, High Resolution γ-Camera System dedicated to SPECT Imaging

A small field, high resolution γ-Camera system dedicated to radiopharmaceutical research and other clinical SPECT (Single Photon Emission Computed Tomography) applications is currently being developed in our group. The system is equipped with the 3” HAMAMATSU R2486 Position Sensitive PhotoMultiplier Tube (PSPMT) with a 16X+16Y-crossed wire anode and various pixelated and ho- mogeneous scintillation crystals. Planar images are created from the recorded charge signals by applying the resistive chain technique. The main part of this work focuses on the development of new correction methods for the improvement of the spatial resolution and the uniformity of the γ-Camera. The spatial distortion correction technique is based on lookup tables with the coordinates of reference points which are selected during the calibration phase of the system for a given set of collimator and scintillation crystal. The applied algorithm incorporates 2D-interpolation tech- niques and has been developed on a full automated graphics environment making use of the HIGZ (High Level Interface to Graphics and ZEBRA) program libraries from CERN. Both correction methods for the spatial distortion and non-uniformity have been applied to phantom images using several combinations of small capillaries filled with water solution of 99mTc. Comparative studies are shown on planar im- ages for different phantom geometries. The method is also extended to tomographic images and the obtained SPECT improvement in resolution is discussed.

Optical Properties of Continuous and Pixelated Scintillation Crystals

The optical photon distribution produced inside continuous and pixelated scintillation crystals by the absorption of a γ-ray have been studied with the photon transport program DETECT 2000. With this program the charge signals recorded by a multi-wired anode system, like the Position Sensitive PhotoMultiplier Tube (PSPMT) of a γ-Camera, are simulated. Based on the analytical parametrization which is fitted to experimental data, a new position reconstruction method for PSPMTs is proposed in this work. Planar images have been reconstructed with the new method and compared to the traditional charge center of gravity technique. Data are obtained from a small field, high resolution γ-Camera system with a multi- wired crossed anode using the R2486 (HAMAMATSU) PSPMT. Systematic studies for continuous and pixelated inorganic scintillation crystals of CsI(Tl) have been performed for different phantom geometries using small capillaries of 99mTc. The developed method seems to drastically improve the resolution of the reconstructed planar information, even when homogeneous crystals are used.

GEANT4/GATE Simulation Studies in the Emission Tomography

Radiotracer imaging studies for a small field, high resolution ∞-Camera system and a clinical system for Positron Emission Tomography (PET) by means of GATE (GEANT4 Application for Tomographic Emission) simulations are presented in this work. In a validation phase, which preceded the main study, experimentally obtained results for planar images with the existing ∞-Camera system were directly compared to simulated data. A simple phantom structure, consisting of four parallel capillaries filled with 99mTc water solution, was imaged by the γ-Camera system for several phantom-collimator distances and the measured and Monte-Carlo calculated spatial projections were compared. The major objective of this validation study was the optimal description of the most important components, the hexagonal, parallel-hole Pb-collimator and the pixelated CsI scintillation crystal of the γ-imaging system in terms of GATE components. In the main study, a GATE simulation setup for this ∞-Camera detector is used and Monte-Carlo data are accumulated for simple geometrical phantoms with different monophotonic radiotracer energies and relative intensities. In parallel, a commercially available cylindrical shaped PET scanner ring, consisting of 32 sectors with 4 x 6 x 6 LSO scintillation crystals, has been constructed in the GATE environment. Simulation data are obtained for the most usual positron emitters (18F, 11C and 15O) and for several phantom geometries. The spatial resolution of both systems and their overall performance is presented and discussed in this study.

Construction of a High-Resolution Mobile γ-Camera System for Mammography Study

A small-field portable γ-Camera system based on a Position Sensitive Photomultiplier Tube (PSPMT) has been recently developed in our Lab. This γ-Camera system has been characterized on planar and tomographic level and exhibits high efficiency and resolution (0.95 ± 0.5 mm spatial resolution on planar imaging and 0.20μCi sensitivity on a tomographic level). Even though a portable γ-Camera system has many advantages, it shows a strong dependence from the operators hand stability which greatly affects the quality of the acquired image. To overcome these circumstances a need for a motion correction technique arises. In the present work, a simple a low cost solution to overcome this stability problem by means of commercially available equipment is proposed. It is based on the Nintendo’s Wii Remote sensors, a small handheld device which can be easily coupled to the portable camera. It comprises both, a 3D-accelerometer which reports the three detected components of the gravity acceleration and an infrared sensor for a fix-point distance evaluation. The Wii Remote sensors device is wireless and operates with modern communication protocols (Bluetooth). Based on the received accelerometer and infrared-sensor data by a DAQ system developed on the LabVIEW environment, the γ-Cameras DAQ system is capable to store all the primary parameters on an event-by-event basis and to perform the necessary motion correction in real time. Some typical image correction results are presented in the current work.

Slit slat collimator optimization with respect to MTF

Conventional single photon emission computed tomography (SPECT) collimators are not suitable for 511keV imaging. In order to use a SPECT system for single positron emission tomography (SPET), we need to design high-efficiency collimators capable of absorbing 511keV photons. Slit slat (SS) collimators have been proposed for this purpose even though the optimization of such collimators has not yet been performed. In order to investigate the reliability of the system with SS collimator, it was decided to evaluate such a system.A gamma camera system with both parallel hole (PH) and SS collimators was simulated by Monte Carlo method. In these simulation parameters of the simulated system having 38in NaI(Tl) crystal for 140keV γ-rays with PH collimator was compared with practical results and a good correlation was found between the simulated and practical results. In this way, the validity of our simulation code was confirmed. The crystal thickness for simulated γ-camera system with SS collimator for detection of 511keV photons was set to be 58in which resulted in an intrinsic spatial resolution of 4mm. The simulated SS collimators consisted of 50 lead plates of various height, thickness and spacing. The imaging was performed by rotating the SS collimator from 0° to 180° in 73 steps of 2.5° each. The physical parameters such as spatial resolution, efficiency and modulation transfer function (MTF) of this system for different SS collimator designs, (i.e. SS64,3,3, SS80,3,3, SS64,4,5, SS80,4,5, SS85,4,5, where in SSh,s,t is used to indicate the septal height (h), septal separation (s), and septal thickness (t) in millimeters were evaluated. The value of these parameters were compared with the values obtained employing a 511keV PH85,4,2.5 collimator.Based on MTF values, our result showed that the SS85,4,5 collimator has an optimum performance to the other SS collimators. The relative efficiency of the system with SS85,4,5 collimator was about 20 times higher than the efficiency of the system with the PH85,4,2.5 collimator whereas in this case the spatial resolution had deteriorated by almost 8% (i.e. from 14.1 to 15.0mm).The result of this study shows that the SS collimators have great advantages over the conventional PH collimators for 511keV imaging, providing much higher efficiency while maintaining a comparable spatial resolution. Employing this collimator makes it possible to use a SPECT system for metabolic imaging by SPET system.