Sameer V. Tipnis

Developments in synchrotron x-ray computed microtomography at the National Synchrotron Light Source

Last year, the X27A beamline at the National Synchrotron Light Source (NSLS) became dedicated solely to X-Ray Computed Microtomography (XCMT). This is a third-generation instrument capable of producing tomographic volumes of 1 - 2 micron resolution over a 2 - 3 mm field of view. Recent enhancements will be discussed. These have focused on two issues: the desire for real-time data acquisition and processing and the need for highly monochromatic beam (.1% energy bandpass). The latter will permit k-edge subtraction studies and will provide improved image contrast from below the Cr (6 keV) up to the Cs (36 keV) k-edge. A range of applications that benefit from these improvements will be discussed as well. These two goals are somewhat counterproductive, however; higher monochromaticity yields a lower flux forcing longer data acquisition times. To balance the two, a more efficient scintillator for X-ray conversion is being developed. Some testing of a prototype scintillator has been performed; preliminary results will be presented here. In the meantime, data reconstruction times have been reduced, and the entire tomographic acquisition, reconstruction and volume rendering process streamlined to make efficient use of synchrotron beam time. A Fast Filtered Back Transform (FFBT) reconstruction program recently developed helped to reduce the time to reconstruct a volume of 150 X 150 X 250 pixels3 (over 5 million voxels) from the raw camera data to 1.5 minutes on a dual R10,000 CPU. With these improvements, one can now obtain a quick look of a small tomographic volume (approximately 106 voxels) in just over 15 minutes from the start of data acquisition.

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.

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.

High speed X-ray imaging camera using a structured CsI(Tl) scintillator

New third generation X-ray sources such as the Advanced Photon Source have created a need for a detector that can provide multiple frames of detailed X-ray images on the millisecond time scale. Such detectors will prove invaluable in applications such as time-resolved X-ray diffraction, X-ray microtomography, as well as materials science applications like polymer processing. Currently, detectors capable of acquiring high resolution X-ray images at such high speed do not exist, thus limiting progress in many of these important areas of research. To address these needs we have developed a prototype fast X-ray imaging system, using a structured CsI(Tl) scintillator coupled to a fast-frame 1 K/spl times/1 K CCD. The system has been successfully employed to capture 1024/spl times/64 pixel X-ray images at a rate of 1000 frames per second (fps) with a 12 bit dynamic range. The system exceeds the capabilities of the current high speed X-ray imaging systems which typically operate at the rate of 30 fps. Fabrication of a large area detector is currently underway, using a microstructured CsI(Tl) scintillator coupled to a fast-frame CCD with a 3:1 fiberoptic taper. The camera will operate in a burst mode, acquiring 8 1 K/spl times/1 K images at rates up to 1000 frames per second with 12 bit dynamic range. Higher image capture speeds can be accomplished by reducing the image area. This paper discusses the specific characteristics of the CsI(Tl) screens, experimental details of the prototype and the new design for the large area detector being developed specifically for time-resolved X-ray diffraction experiments in structural biology.

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.

Large area CCD based imaging system for mammography

The authors have recently developed a digital X-ray imaging system based on a high resolution scintillator screen optically coupled to a specially designed large area CCD for use in mammographic procedures. The CCD consists of a 7 K/spl times/4 K array of 12 /spl mu/m pixels, measuring 88.2 mm/spl times/51 mm and operated at room temperature, cooled only by ambient air circulation. With 12 /spl mu/m pixels, the resulting limiting Nyquist frequency is 41.6 lp/mm. The readout electronics and software are designed to capture images with the full CCD resolution as well as with binning. Depending on the requirements of the application, the CCD may be binned in the 2/spl times/2 or the 4/spl times/4 configuration. Since the time required to read 28 million pixels is large, binning provides an option for rapid image acquisition at a reduced resolution. A fiberoptic faceplate is optically bonded to the CCD to simplify coupling of scintillator screens to the sensor. One important feature of the system is the lack of a fiberoptic taper, making the system smaller and easier to handle or move. Structured CsI(Tl) scintillator screens developed at RMD Inc., were used for the system evaluation. This paper discusses the system design, the characterization of the CCD sensor as well as the imaging performance of the system using mammography phantoms.

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.

Comparative study of CsI(Tl) screens for macromolecular crystallography

At RMD we have fabricated structured CsI(Tl) screens tailored for macromolecular x-ray crystallography applications. Diffraction patterns typically consist of several closely spaced Bragg peaks of varying sizes and intensities, and the detection of such features requires screens with high light output, high resolution, and excellent x-ray absorption. Properties of these screens, for example, light output or spatial resolution, were tailored by post deposition treatments to suit the specific needs of the application. Specifically, we have produced up to 45 micrometers thick CsI(Tl) screens with excellent resolution over the spatial frequency range of 0 to 20 lp/mm and very low noise. Imaging characteristics of these screens along with the commercial Gd2O2S (GOS) have been measured using a CCD detector with a fiberoptic taper. Performance of these screens in terms of point spread function (PSF(f)), light output, noise power spectrum (NPS(f)), and the modulation transfer function (MTF(f)) was measured. It is observed that the intrinsic properties of the structured CsI(Tl) screens are heavily influenced by the substrate on which the films are deposited and on the post deposition coatings, thus providing a latitude for modifying the screen properties to match the needs of the application.

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.

Structured LiI scintillator for thermal neutron imaging

We are currently developing, high resolution, high efficiency, micro-columnar LiI films for thermal neutron imaging. The films are produced by the vapor deposition of LiI on a fiberoptic substrate and hermetically sealed in a specially designed aluminum package. Our work has produced up to 1.2 mm thick films with column diameters of approximately 30 /spl mu/m. We have also performed imaging studies by optically coupling some of these films to a fiberoptic taper based CCD. The imaging performance of the system was experimentally evaluated at Radiation Monitoring Devices, Inc., as well as at the thermal neutron port of the University of Massachusetts Lowell Research Reactor. The LiI films exhibited excellent scintillation characteristics with a spatial resolution as high as 4.5 Ip/mm. This paper outlines the film characterization and performance evaluation conducted during the course of the study. The new sensor described is expected to usher in the development of large area, high-resolution digital thermal neutron detectors with improved detection efficiency, dynamic range, and faster readout times than the current sensors.