Bipin Singh

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.

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

Fabricating high-resolution and high-sensitivity scintillator arrays using Laser Induced Optical Barriers

High-resolution positron emission tomography (PET) requires pixelated scintillators-however, if the pixelation process leaves inter-pixel gaps, the loss of material results in loss of sensitivity. PET sensitivity also requires scintillators such as LSO and L YSO to be thicker than 2 cm, due to the high penetrating power of 511 keV gamma rays. Fine pixelation of L YSO is difficult, since it is very hard material and is known to crack under thermal and mechanical stress. We have developed a method to introduce optical barriers within monolithic L YSO crystals to form pix elated arrays with small pixel size and large thickness. Arrays were fabricated using a high-frequency solid-state laser to form optical barriers (interpixel gaps), which can be as thin as 13 μm without affecting the transparency of the crystals. Our method yields near-perfect, extremely high aspect ratio pixels. By controlling parameters such as laser pulse repetition rate and energy density, LYSO crystals can be effectively pixelated with virtually no material loss. We have laser processed L YSO crystals ranging from 5 to 20 mm thick and 0.8×0.8 to 1.5×1.5 mm2 in pixel cross section. When a collimated beam (0.5 mm) of 70 kVp X-rays was incident on one pixel of a 10×10×20 mm3 scintillator array with 0.8×0.8 mm2 pixel size, an average optical crosstalk ratio was measured at 6.5:1, which shows excellent pixel separation. Our technique is ideal for fabricating scintillator arrays for clinical/pre-clinical PET and SPECT systems as well as photon counting CT detectors. Our technique is automated, and is cost-effective.

Development of microcolumnar LaBr3:Ce scintillator

While a wide variety of new scintillators are now available, new cerium-doped lanthanide halide scintillators have shown a strong potential to move beyond their familiar role in conventional gamma ray spectroscopy, toward fulfilling the needs of highly demanding applications such as radioisotope identification at room temperature, homeland security, and quantitative molecular imaging for medical diagnostics, staging and research. Despite their extraordinary advantages, however, issues related to reliable, large volume manufacturing of these high light yield materials in a rapid and economic manner have not been resolved or purposefully addressed. Also, if microcolumnar films of this material could be fabricated, it would find widespread use in a multitude of high-speed imaging/nuclear medicine applications. Here we report on synthesizing LaBr3:Ce scintillators using a thermal evaporation technique, which permits the fabrication of high spatial resolution microcolumnar films and holds a potential to synthesize large volumes of high quality material in a time efficient and cost effective manner. Performance evaluation of the fabricated films and their application for SPECT imaging are also discussed.

A hand-held beta imaging probe for FDG

ObjectivesAdvances in radiopharmaceuticals and clinical understanding have escalated the use of intraoperative gamma probes in surgery. However, most probes on the market are non-imaging gamma probes that suffer from the lack of ancillary information of the surveyed tissue area. We have developed a novel, hand-held digital Imaging Beta Probe™ (IBP™) to be used in surgery in conjunction with beta-emitting radiopharmaceuticals such as 18FDG, 131I and 32P for real-time imaging of a surveyed area with higher spatial resolution and sensitivity and greater convenience than existing instruments.MethodsWe describe the design and validation of a hand-held beta probe intended to be used as a visual mapping device to locate and confirm excision of 18FDG-avid primary tumors and metastases in an animal model.ResultsWe have demonstrated a device which can generate beta images from 18FDG avid lesions in an animal model.ConclusionsIt is feasible to image beta irradiation in animal models of cancer given 18FDG. This technology may be applied to clinical mapping of tumors and/or their metastases in the operating room. Visual image depiction of malignancy may aid the surgeon in localization and excision of lesions of interest.

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

Contrast enhancement in X-ray phase contrast tomography

We demonstrate phase contrast enhancement of X-ray computed tomography derived from propagation based imaging. In this method, the absorption and phase components are assumed to be correlated, allowing for phase retrieval from a single image. Experimental results are shown for liquid samples. Signal-to-noise ratio is greatly enhanced relative to pure attenuation based imaging.

Luminescence Properties and Morphology of ZnSe:Te Films

High-speed imaging applications, such as time-resolved X-ray diffraction, require detectors with high frame rates ranging from hundreds to thousands of frames per second. New high-resolution CCD imagers capable of operating at the required frame rates have been developed; however, the current X-ray to light converters are the performance limiting factor in such applications. Here we report on the development of a structured ZnSe:Te scintillator that promises to provide extraordinarily high scintillation efficiency, emission at 640 nm (ideally suited for CCD sensors), high density of 5.4 g/cm3, and a fast decay time of ~3 mus to ~50 mus with no afterglow, which permits high speed imaging without the problem of ghosting due to persistence. Furthermore, the non-hygroscopic and non-toxic nature of the ZnSe:Te scintillator, along with its stability of response over a wide range of temperatures and extremely high levels of radiation, makes it an ideal material for radiation detection in general and for synchrotron applications in particular. At RMD, we have fabricated microcolumnar ZnSe:Te films measuring ~25 mum to 85 mum in thickness using co-evaporation of ZnSe and ZnTe on suitable substrates. These films show columnar structure with columns ranging from 0.2 mum to 5 mum in diameter. The scintillation light produced by the radiation interaction is channeled within the microcolumns by the mechanism of total internal reflection, thereby providing very high spatial resolution, even when films are made thick to achieve high X-ray absorption.

A high speed functional microCT detector for small animal studies

Dedicated microCT systems for noninvasive screening of small animals are now in routine use. However, speed of operation limits their use for functional studies. At RMD we are addressing this need by developing a digital X-ray detector that can simultaneously provide high speed, high sensitivity, and a large active imaging area. The system consists of a special high speed CCD detector optically coupled to a scintillator. In its current configuration, the system provides 5/spl times/5 cm/sup 2/ active area, spatial resolution of /spl sim/100 /spl mu/m, and can operate at speeds of up to 225 frames per second. The system has been used to acquire volumetric CT data with 360 projections in /spl sim/12 seconds compared to several minutes needed for most commercial systems. This paper presents design of various system components and performance characterization along with the CT reconstruction data on a mouse phantom.