Vivek V. Nagarkar

Structured CsI(Tl) scintillators for X-ray imaging applications

We are developing large-area, thick, structured CsI(Tl) imaging sensors for a wide variety of X-ray imaging applications. Recently we have fabricated structured CsI(Tl) scintillators ranging from 30 /spl mu/m (16 mg/cm/sup 2/) to 2000 /spl mu/m (900 mg/cm/sup 2/) in thickness and up to 15/spl times/15 cm/sup 2/ in area. Even 2000-/spl mu/m-thick film showed well-controlled columnar growth throughout the film. Material characterization confirmed that the film is crystalline in nature and that the stoichiometry is preserved. To improve the spatial resolution of thick films, post-deposition treatments were performed. The effect of these treatments on film characteristics was quantitatively evaluated by measuring signal output, modulation transfer function [MTF(f)], noise power spectrum [NPS(f)], and detective quantum efficiency [DQE(f)]. The data show that by proper film treatments, the film DQE(f) can be improved.

High resolution X-ray sensor for non destructive evaluation

Nondestructive evaluation (NDE) using X-rays is becoming indispensable for detecting microdefects in new materials currently used in aerospace and other engineering disciplines. Existing X-ray sensors pose limitations on the speed of operation due to persistence of the sensor and a problematic tradeoff between the sensor thickness and spatial resolution. To address these limitations we are developing a large area structured CsI(Tl) imaging sensor for NDE using CCD based radiographic and computed tomographic systems. The sensor is formed by vapor deposition of CsI(Tl) onto a specially designed fiber optic substrate. Our work has produced X-ray sensors with a factor of 4.5 greater light output, at least three orders of magnitude faster decay time response, and greater spatial resolution (16% modulation transfer function, MTF(f), at 14 line pairs per millimeter (1p/mm)) compared to the currently used high density Tb/sub 2/O/sub 3/ doped fiber optic glass scintillators. These performance advances will address the limitations of existing detector technology by producing high quality images and fast scan times required for real-time NDE inspection. Performance measurements for prototype CsI(Tl) scintillator converters are presented. With these new sensors the development of larger area fiber optic taper based CCD detectors with millisecond data acquisition capabilities and high spatial resolution suitable for NDE applications will be possible.

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 comparison of x-ray detectors for mouse CT imaging

There is significant interest in using computed tomography (CT) for in vivo imaging applications in mouse models of disease. Most commercially available mouse x-ray CT scanners utilize a charge-coupled device (CCD) detector coupled via fibre optic taper to a phosphor screen. However, there has been little research to determine if this is the optimum detector for the specific task of in vivo mouse imaging. To investigate this issue, we have evaluated four detectors, including an amorphous selenium (a-Se) detector, an amorphous silicon (a-Si) detector with a gadolinium oxysulphide (GOS) screen, a CCD with a 3:1 fibre taper and a GOS screen, and a CCD with a 2:1 fibre taper and both GOS and thallium-doped caesium iodide (CsI:Tl) screens. The detectors were evaluated by measuring the modulation transfer function (MTF), noise power spectrum (NPS), detective quantum efficiency (DQE), stability over multiple exposures, and noise in reconstructed CT images. The a-Se detector had the best MTF and the highest DQE (0.6 at 0 lp mm(-1)) but had the worst stability (45% reduction after 2000 exposure frames). The a-Si detector and the CCD with the 3:1 fibre, both of which used the GOS screen, had very similar performance with a DQE of approximately 0.30 at 0 lp mm(-1). For the CCD with the 2:1 fibre, the CsI:Tl screen resulted in a nearly two-fold improvement in DQE over the GOS screen (0.4 versus 0.24 at 0 lp mm(-1)). The CCDs both had the best stability, with less than a 1% change in pixel values over multiple exposures. The pixel values of the a-Si detector increased 5% over multiple exposures due to the effects of image lag. Despite the higher DQE of the a-Se detector, the reconstructed CT images acquired with the a-Si detector had lower noise levels, likely due to the blurring effects from the phosphor screen.

CdTe quantum dots and polymer nanocomposites for x-ray scintillation and imaging.

Investigations are reported on the x-ray scintillation and imaging application of CdTe quantum dots (QDs) and their polymer nanocomposites. Aqueous CdTe QDs with emissions ranging between 510 and 680 nm were prepared and incorporated into polyvinyl alcohol or polymethyl methacrylate polymer matrices. The x-ray luminescent properties were evaluated and a resolution of 5 lines∕mm was obtained from the nanocomposite films. Additionally, the fast decay time, nonafterglow, and superior spectral match to conventional charge coupled devices, show that CdTe QD nanocomposites have high promise for x-ray imaging applications.

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.

Quantum Dot-Organic Polymer Composite Materials for Radiation Detection and Imaging

Colloidal semiconductor nanocrystals exhibit physical properties that are characteristic of intermediate size scales between molecular states and solid state materials, and are often called quantum dots. Solid state semiconductor materials have been used extensively as scintillation detectors for ionizing radiation. We describe the use of semiconductor quantum dot-organic polymer composites for use as scintillation detectors and report the use of quantum dot-polymer composite thin films for X-ray imaging.

Compressive x-ray phase tomography based on the transport of intensity equation

We develop and implement a compressive reconstruction method for tomographic recovery of refractive index distribution for weakly attenuating objects in a microfocus x-ray system. This is achieved through the development of a discretized operator modeling both the transport of intensity equation and the x-ray transform that is suitable for iterative reconstruction techniques.

System integration of FastSPECT III, a dedicated SPECT rodent-brain imager based on BazookaSPECT detector technology

FastSPECT III is a stationary, single-photon emission computed tomography (SPECT) imager designed specifically for imaging and studying neurological pathologies in rodent brain, including Alzheimers and Parkinsonss disease. Twenty independent BazookaSPECT [1] gamma-ray detectors acquire projections of a spherical field of view with pinholes selected for desired resolution and sensitivity. Each BazookaSPECT detector comprises a columnar CsI(Tl) scintillator, image-intensifier, optical lens, and fast-frame-rate CCD camera. Data stream back to processing computers via firewire interfaces, and heavy use of graphics processing units (GPUs) ensures that each frame of data is processed in real time to extract the images of individual gamma-ray events. Details of the system design, imaging aperture fabrication methods, and preliminary projection images are presented.

New design of a structured CsI(Tl) screen for digital mammography

Columnar CsI(Tl) screens are now routinely used for digital x-ray imaging in a wide variety of applications such as mammography, dental radiography, and non-destructive testing. While commercially available CsI(Tl) screens exhibit excellent properties, it is possible to significantly improve their performance. Here, we report on a new design of a columnar CsI(Tl) screen. Specifically, columnar CsI(Tl) screens were subjected to mechanical pixelation for minimizing the long range spread of scintillation light within the film, thus enhancing spatial and contrast resolution, and increasing the detective quantum efficiency (DQE(f)) of the digital imaging detector. To date we have fabricated up to 200 μm thick pixelated CsI(Tl) screens for mammography, and characterized their performance using a CCD camera. This paper presents a comparison of the new pixelated CsI(Tl) screens, conventional columnar CsI(Tl) screens, and Gd2O2S(Tb) screens. The data show that pixelated screens substantially improve the DQE(f) of the digital imaging system.