Irina Shestakova

Single-photon spatial and energy resolution enhancement of a columnar CsI(Tl) / EMCCD gamma-camera using maximum- likelihood estimation

We examined the spatial resolution of a columnar CsI(Tl), single-photon imaging system using an approach that estimates the interaction position to better than the spread of the light distribution. A columnar scintillator was directly coupled to a 512×512 electron multiplying CCD (EMCCD) camera (16 μm pixels) binned at 2×2 to sample at 32 μm pixels. Optical photons from gamma-ray/scintillator interactions are sampled over multiple pixels. Resultant images show clusters of signal at the original interaction site, clusters from Cs and I K x-rays up to several hundred microns away, and clusters from collimator K x-rays. Also evident are depth-of-interaction effects which result in a broadening of the light distribution. These effects result in a degradation of spatial and energy resolution. Cluster pixel data was processed to better estimate the interaction position within the initial interaction cluster. Anger (centroid) estimation of individual gamma-ray events yielded spatial resolutions better than 100 μm; a result previously achievable only with pixellated semiconductor detector arrays. After proper calibration, depth-of-interaction (DOI) effects are corrected by performing maximum-likelihood 3D position and energy estimation of individual gamma-ray interactions.

A CCD-based detector for SPECT

We are investigating the use of a CCD for high-resolution radionuclide imaging. The use of a CCD has the potential to provide very high spatial resolution on the order of 200 to 400 /spl mu/m, while significantly simplifying the readout electronics. The detector is based on a special CCD with on-chip multiplication gain that allows high-speed operation while maintaining the read noise at a very low level of <1 electron. To achieve high detection efficiency and excellent spatial resolution for incident gamma flux, a specially fabricated thick microcolumnar CsI(Tl) scintillator was optically coupled to the CCD. A prototype SPECT imaging system was assembled by incorporating pinhole/parallel hole collimators in the design. The use of this system for radionuclide imaging has been demonstrated through tomographic imaging of a test phantom filled with /sup 99m/Tc.

Evaluation of a EMCCD detector for emission-transmission computed tomography

A prototype electron multiplying charged coupled device (EMCCD) based gamma camera is evaluated for emission-transmission computed tomography. The detector has an 8.2times8.2 mm2 active sensor area with a 512times512 pixel array, and can be operated with a 3:1 fiber optic taper to expand the effective imaging area to approximately 25times25 mm2 for small animal studies. We demonstrate the possibility of operating the camera in either charge integrating or single photon counting mode for pinhole SPECT with 125I and 99mTc sources. In photon counting mode, we show that energy discrimination can be used to distinguish between these two sources for dual isotope imaging. X-ray CT images of phantoms are taken without a collimator to evaluate the quality of the camera for transmission imaging

High speed X-ray imaging camera for time resolved diffraction studies

We report here on a high-speed X-ray imaging camera, specifically developed for time resolved diffraction studies using synchrotron and laboratory X-ray sources. This camera is capable of acquiring six X-ray images at speeds of up to 2300 frames per second (f/s). The system is based on a modified architecture charge coupled device (CCD) optically coupled to a fiber-optic taper via an image intensifier. The front end of the taper is coupled to a specially designed microstructured CsI(Tl) scintillator screen capable of providing high light output, very high-detection efficiency, and excellent spatial resolution. In addition to the time resolved diffraction studies, this detector will be extremely valuable in applications such as dynamic imaging of small animals, X-ray microtomography, and materials science applications. This paper discusses the design and performance characterization of the imaging system. Additionally, we present some preliminary high-speed X-ray imaging data obtained using laboratory X-ray sources.

Recent advances in columnar CsI(Tl) scintillator screens

Columnar CsI(Tl) screens are now routinely used in indirect digital x ray imaging detectors. The CsI(Tl) scintillator provides high density, high atomic number, and high scintillation efficiency. These properties, coupled with the fact that CsI(Tl) can be grown in columnar form, provide excellent spatial resolution, high x-ray absorption, and low noise resulting in detectors with high overall detective quantum efficiency (DQE(f)). While such screens are now commercially available, developments leading to further improvements in scintillator performance are ongoing at RMD. Here we report on the recent progress in developing very thin (10 μm) to very thick (~3 mm) columnar screens and discuss their application potential in digital radiology and nuclear medicine.

Feasibility of a beta-gamma digital imaging probe for radioguided surgery

We report here on a novel design of a digital, intraoperative imaging probe intended for use in radio-guided surgical procedures in conjunction with radiolabels such as /sup 131/I and /sup 18/F. The probe allows the user to rapidly localize tumors by detecting the highly penetrating gamma rays, and then image the tumor with the short-range beta rays. The system provides a rapid, high-resolution, image of the interrogated area, fulfilling the need for clear delineation of tumors during radio-guided surgical procedures. The beta imaging sensor consists of a microcolumnar CsI(Tl) scintillator screen capable of providing very high detection efficiency, high light output and excellent spatial resolution coupled to a CCD via a flexible, coherent fiberoptic bundle. The gamma sensor is a shielded piece of crystalline CsI(Tl) coupled to a photodiode located behind the image sensitive front end. The feasibility of this design was studied by separately testing the beta imaging and gamma detection components. The operation of the components was characterized with intrinsic performance measurements of count rates, signal-to-noise ratios, spatial resolution, as well as time for acquiring useful images using radionuclide and anthropomorphic phantoms.

Near Simultaneous Combined SPECT/CT Imaging Using EMCCD

At RMD we have developed a detector for near simultaneous, combined, small animal SPECT/CT imaging using a customized electron multiplying CCD (EMCCD) camera. The detector is based on a back-thinned EMCCD coupled to a high resolution CsI(Tl) scintillator via a 3:1 fiberoptic taper, and is capable of operating at frame rates of 30 frames per second in full pixel resolution mode and at 520 frames per second in binning mode. The performance of this system for combined radionuclide and X-ray imaging was evaluated using 99mTc and 125I sources and a mini-focus X-ray generator, respectively. We have demonstrated that this system is capable of providing better than 100 mum intrinsic resolution for both radionuclide and X-ray imaging, making it possible to develop a practical and economical, near simultaneous, combined SPECT/CT system for small animal imaging. Here we discuss the SPECT/CT imaging data acquired by imaging a mouse cardiac phantom filled with a small quantity of 99mTc, and discuss the advantages of this detector for small animal imaging

A new sensor for thermal neutron imaging

Thermal neutrons serve as a useful tool in probing macromolecular structures in protein crystallography and in investigations of new materials. However, neutron techniques are underutilized due to the lack of high performance digital, position sensitive detectors. The primary limiting factor in current detectors is the converter screen which converts the neutron signal into visible light. Here we report on a new type of neutron sensitive screen for use in digital imaging systems. The screen consists of a pixelated, microstructured CsI(Tl) scintillator film sandwiched between two neutron converting layers of GdF/sub 3/. To increase the effective surface area of the GdF/sub 3/ conversion layer and to enhance the contrast resolution of the images, the CsI(Tl) layer is pixelated using micromachining techniques. For testing their imaging performance, the sensors were optically coupled to a CCD system to form an imaging detector. The system was subjected to imaging tests at a thermal neutron port of the University of Massachusetts Lowell Research Reactor. The results of these preliminary imaging experiments are presented here.

Design and performance of an EMCCD based detector for combined SPECT/CT imaging

We have designed and developed a very high sensitivity detector for near-simultaneous SPECT/CT imaging of small animals. The detector is based on a back-thinned electron multiplying charge coupled device (EMCCD) bonded to a fiberoptic window, and optically coupled to a high resolution, high efficiency, very thick microcolumnar CsI(Tl) scintillator via a fiberoptic taper. In addition to the low noise and high spatial resolution inherent to CCDs, the EMCCD provides controllable internal gain, which minimizes read noise even when the device is operated at high frame rates. This allows the detection of incident gamma-ray/X-ray radiation with high spatial resolution and enhanced signal-to-noise ratio (SNR). The use of thick microcolumnar CsI(Tl) offers high detection efficiency for gamma-ray/X-ray radiation while maintaining a high spatial resolution. This combination of the EMCCD and the CsI(Tl) scintillator has resulted in a unique detector that can be employed for near simultaneous functional (SPECT) and anatomical (X-ray CT) imaging at a reduced cost. The design and evaluation of the detector are discussed in this paper

A High Spatial Resolution Sensor For Thermal Neutron Imaging

We are developing a new type of neutron-sensitive sensor with minimal gamma-ray sensitivity for use in digital imaging systems. It consists of a plastic scintillator loaded with boron or gadolinium atoms for enhanced sensitivity to thermal neutrons. To achieve high spatial resolution, the scintillator in a liquid form is introduced into a capillary array (fused glass capillaries with no interstices), and then polymerized. The scintillation light generated by a neutron interaction is confined within a capillary, without lateral spreading through the scintillator. This not only improves spatial resolution, but also enhances image contrast by minimizing the glare fraction. The sensors were optically coupled to a cooled, fiberoptic taper-based CCD system to form an imaging detector. The detector was subjected to characterization and imaging tests at the thermal neutron port of the University of Massachusetts Lowell Research Reactor. Here we present the results that confirm the high resolution, high efficiency, and low gamma-sensitivity of the sensor.