Haris Kudrolli

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.

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.

High-Performance and Cost-Effective Detector Using Microcolumnar CsI:Tl and SiPM

We are developing a technique to fabricate high spatial resolution and cost-effective photon counting detectors using silicon photomultipliers (SiPMs) and microcolumnar structured scintillator. Photon counting detectors using SiPMs are of much interest to the gamma- and X-ray detector community, but they have limitations at low energy due to their dark noise. In this paper, we report on vapor deposition of CsI:Tl directly onto a SiPM, a technique that improves optical coupling and allows for detection of low energy gamma- and X-rays. It simultaneously addresses related issues of light loss and light spread in the scintillator, thereby improving the performance of the detector. Devices made by this technique may be used for both photon counting and gamma- and X-ray imaging.

A Method for Fabricating High Spatial Resolution Scintillator Arrays

Scintillators like thallium doped cesium iodide (CsI:Tl) can be fabricated in microcolumnar form using physical vapor deposition (PVD). The microcolumns channel the scintillation light to the photodetector which results in an improved spatial resolution. This has lead to widespread use of microcolumnar CsI:Tl in digital X-ray radiography. We present here a PVD-based method to aggregate microcolumns into structures called macrocolumns to form scintillator arrays suitable for use in nuclear imaging. In this novel approach, patterned substrates with shallow grooves 20 μm wide, 50 μm deep, with pitch ranging 100 - 500 μm were fabricated and adopted. CsI:Tl scintillator was vapor deposited onto these substrates. The optimal deposition parameters resulted in microcolumnar CsI:Tl, which displayed a macrocolumnar structure dictated by the underlying pattern of the substrate. Scanning electron micrographs (SEM) show that the microcolumns within the macrocolumns are highly oriented and perpendicular to the surface of the substrates. Energy resolution approaching that of a single crystal CsI:Tl was achieved. Since the microcolumns are densely packed with minimal gap, they behave as a macrocolumn or a single pixel. Our technique for fabricating scintillator arrays is a cost-effective alternative to mechanical pixelation of scintillators. This technique results in a high fill factor scintillation detector with minimized inter-macrocolumn gap, and high-yield detector arrays without issues related to material loss in mechanical pixelation. Coupling these structured scintillators to silicon photomultipliers (SiPMs) and applying Anger logic, we resolved scintillator pixels that were almost 1/10th the size of the SiPM macro-pixels. Combining this structured CsI:Tl scintillator with SiPMs results in a compact detector that is ideal for X-ray, gamma-ray, and charged particle detection, such as beta and gamma imaging probes and hand held cameras.

Continuous Phoswich™ detector for molecular imaging

We report on the development of a novel scintillator in which the decay time of its light emission varies continuously with the depth of an interaction in the crystal. The depth-of-interaction (DOI) information is thus encoded in the signal timing, which can be used to localize the position of the gamma interaction within the scintillator with high accuracy. This concept relies on the fact that decay times in certain scintillators vary considerably with the amount of dopant concentration. We are exploiting this property to create scintillators in which dopant concentration varies continuously and monotonically with depth in the crystals. Synthesis of such structures is accomplished using a specialized vapor deposition technique, which provides us with the control to vary the dopant concentration in the crystal during growth. Our technique also provides a reliable and cost-effective means to synthesize this seemingly complex structure in the large physical volumes required to provide the high absorption efficiency and large sensor areas required for PET and SPECT imaging, respectively. To date we have produced Continuous Phoswich™ scintillator (CPS™) structures measuring up to 7 cm in diameter and approaching 1 cm in thickness using cerium-doped lanthanum chloride (LaCl3:Ce). Controlled vapor deposition is used to create a Ce3+ concentration gradient of 1% to 30% over the specimen thickness. This paper discusses the fabrication and characterization of CPS LaCl3:Ce scintillators and Continuous Phoswich detectors (CPD™), and illustrates the continuous DOI capability of the CPS LaCl3:Ce/PMT detector.

Ultrafast multipinhole single photon emission computed tomography iterative reconstruction using CUDA

We have developed an ultrafast statistical iterative reconstruction method for multipinhole single photon emission computed tomography using high performance graphics processing unit computing and have demonstrated a significant performance improvement in reconstruction using computer-generated and experimental sinogram data.

Depth-of-Interaction Compensation Using a Focused-Cut Scintillator for a Pinhole Gamma Camera

Preclinical SPECT can potentially be a powerful platform to study fundamental biological processes and drug interactions in small animals. Gamma cameras for such SPECT systems require high spatial resolutions in order to adequately map the uptake of radioisotopes in small animals. Pinhole collimators offer one of the best technically feasible ways to achieve a high resolution. However, pinhole geometry introduces parallax errors, particularly toward the edge of the field of view, limiting the system spatial resolution. The parallax errors arise from the variable depth of interaction (DOI) of gammaray/scintillator events, especially when gamma rays enter a scintillator at steep angles. There have been several efforts to address parallax errors in pinhole SPECT, including algorithm-based DOI modeling and correction and the use of a curved fiber bundle to collimate light from a curved scintillator [1, 2]. Another way to overcome parallax errors in a pinhole gamma camera is to use a focused-cut scintillator, which is pixellated so that the pixels are focused towards the pinhole of a collimator [3]. Thus, the path of a primary ray that has passed through the pinhole only intersects with a single pixel in the scintillator. Here, we experimentally test a pinhole gamma camera with a focused-cut (FC) scintillator. We measure the resolution across a continuous scintillator and across a straight-cut (SC) pixellated scintillator and show that the thicker FC scintillator has comparable parallax error in comparison with a thinner SC scintillator (3 mm vs. 1 mm). Thus, FC scintillators are shown to offer both the high resolution of thin pixellated scintillators and the high sensitivity of thicker scintillators.

Laser pixelation of thick scintillators for medical imaging applications: x-ray studies

To achieve high spatial resolution required in nuclear imaging, scintillation light spread has to be controlled. This has been traditionally achieved by introducing structures in the bulk of scintillation materials; typically by mechanical pixelation of scintillators and fill the resultant inter-pixel gaps by reflecting materials. Mechanical pixelation however, is accompanied by various cost and complexity issues especially for hard, brittle and hygroscopic materials. For example LSO and LYSO, hard and brittle scintillators of interest to medical imaging community, are known to crack under thermal and mechanical stress; the material yield drops quickly with large arrays with high aspect ratio pixels and therefore the pixelation process cost increases. We are utilizing a novel technique named Laser Induced Optical Barriers (LIOB) for pixelation of scintillators that overcomes the issues associated with mechanical pixelation. In this technique, we can introduce optical barriers within the bulk of scintillator crystals to form pixelated arrays with small pixel size and large thickness. We applied LIOB to LYSO using a high-frequency solid-state laser. Arrays with different crystal thickness (5 to 20 mm thick), and pixel size (0.8×0.8 to 1.5×1.5 mm2) were fabricated and tested. The width of the optical barriers were controlled by fine-tuning key parameters such as lens focal spot size and laser energy density. Here we report on LIOB process, its optimization, and the optical crosstalk measurements using X-rays. There are many applications that can potentially benefit from LIOB including but not limited to clinical/pre-clinical PET and SPECT systems, and photon counting CT detectors.

Application of an EMCCD camera for calibration of hard x-ray telescopes

Recent technological innovations now make it feasible to construct hard x-ray telescopes for space-based astronomical missions. Focusing optics are capable of improving the sensitivity in the energy range above 10 keV by orders of magnitude compared to previously used instruments. The last decade has seen focusing optics developed for balloon experiments [1] and they will soon be implemented in approved space missions such as the Nuclear Spectroscopic Telescope Array (NuSTAR) [2] and ASTRO-H [3]. The full characterization of x-ray optics for astrophysical and solar imaging missions, including measurement of the point spread function (PSF) as well as scattering and reflectivity properties of substrate coatings, requires a very high spatial resolution, high sensitivity, photon counting and energy discriminating, large area detector. Novel back-thinned Electron Multiplying Charge-Coupled Devices (EMCCDs) [4] are highly suitable detectors for ground-based calibrations. Their chip can be optically coupled to a microcolumnar CsI(Tl) scintillator [5] via a fiberoptic taper. Not only does this device exhibit low noise and high spatial resolution inherent to CCDs, but the EMCCD is also able to handle high frame rates due to its controllable internal gain. Additionally, thick CsI(Tl) yields high detection efficiency for x-rays [6]. This type of detector has already proven to be a unique device very suitable for calibrations in astrophysics: such a camera was used to support the characterization of the performance for all NuSTAR optics [7]–[9]. Further optimization will enable similar cameras to be improved and used to calibrate x-ray telescopes for future space missions. In this paper, we discuss the advantages of using an EMCCD to calibrate hard x-ray optics. We will illustrate the promising features of this detector solution using examples of data obtained during the ground calibration of the NuSTAR telescopes performed at Columbia University during 2010/2011. Finally, we give an outlook on ongoing development and optimizations, such as the use of single photon counting mode to enhance spectral resolution.

Direct and indirect detectors for X-ray photon counting systems

Most currently available X-ray or gamma ray imaging detectors are based on energy integration over a certain period of time. We have been developing X-ray and gamma ray detectors based on the photon counting (with energy determination) concept using both direct and indirect radiation conversion, together with associated application-specific integrated circuits (ASICs). As an alternative to our ASIC design approach, we are also exploiting the potential of state-of-the-art silicon photomultipliers (SiPMs) and discrete electronics. In this paper we discuss the advantages and disadvantages of these two approaches and we report our latest results on our ASIC design efforts and our achievements on SiPM/CsI:Tl detector configurations. We will also discuss the potential uses and advantages that each offers to applications in medicine, imaging, homeland security and industry.