H. Bradford Barber

SemiSPECT: A small-animal single-photon emission computed tomography (SPECT) imager based on eight cadmium zinc telluride (CZT) detector arrays

The first full single-photon emission computed tomography (SPECT) imager to exploit eight compact high-intrinsic-resolution cadmium zinc telluride (CZT) detectors, called SemiSPECT, has been completed. Each detector consists of a CZT crystal and a customized application-specific integrated circuit (ASIC). The CZT crystal is a 2.7 cm x 2.7 cm x -0.2 cm slab with a continuous top electrode and a bottom electrode patterned into a 64 x 64 pixel array by photolithography. The ASIC is attached to the bottom of the CZT crystal by indium-bump bonding. A bias voltage of -180 V is applied to the continuous electrode. The eight detectors are arranged in an octagonal lead-shielded ring. Each pinhole in the eight-pinhole aperture placed at the center of the ring is matched to each individual detector array. An object is imaged onto each detector through a pinhole, and each detector is operated independently with list-mode acquisition. The imaging subject can be rotated about a vertical axis to obtain additional angular projections. The performance of SemiSPECT was characterized using 99mTc. When a 0.5 mm diameter pinhole is used, the spatial resolution on each axis is about 1.4 mm as estimated by the Fourier crosstalk matrix, which provides an algorithm-independent average resolution over the field of view. The energy resolution achieved by summing neighboring pixel signals in a 3 x 3 window is about 10% full-width-at-half-maximum of the photopeak. The overall system sensitivity is about 0.5 x 10(-4) with the energy window of +/-10% from the photopeak. Line-phantom images are presented to visualize the spatial resolution provided by SemiSPECT, and images of bone, myocardium, and human tumor xenografts in mice demonstrate the feasibility of preclinical small-animal studies with SemiSPECT.

Applications of semiconductor detectors to nuclear medicine

Abstract Progress in the development of semiconductor detectors is being applied to improving the resolution and imaging performance of nuclear medicine cameras. Nuclear medicine is briefly described. Efforts to develop semiconductor cameras for both planar and tomographic imaging are reviewed.

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.

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.

Application of II-VI materials to nuclear medicine

Semiconductor gamma-ray detector arrays made of II-VI materials such as CdTe or CdZnTe hold great promise for improving the spatial resolution and energy resolution of nuclear medicine imaging systems. This field has benefited greatly from technologies developed in infrared imaging. This report surveys the state of the art for producing high-resolution semiconductor arrays with emphasis on II-VI materials and considers the prospects for producing a semiconductor detector gamma camera. A number of practical designs are reviewed that make use of single-charge-carrier dominance effects to improve useful photopeak fraction and thus efficiency.

Progress in BazookaSPECT: High-resolution, dynamic scintigraphy with large-area imagers

We present recent progress in BazookaSPECT, a high-resolution, photon-counting gamma-ray detector. It is a new class of scintillation detector that combines columnar scintillators, image intensifiers, and CCD (charge- coupled device) or CMOS (complementary metal-oxide semiconductors) sensors for high-resolution imaging. A key feature of the BazookaSPECT paradigm is the capability to easily design custom detectors in terms of the desired intrinsic detector resolution and event detection rate. This capability is possible because scintillation light is optically amplfied by the image intensifier prior to being imaging onto the CCD/CMOS sensor, thereby allowing practically any consumer-grade CCD/CMOS sensor to be used for gamma-ray imaging. Recent efforts have been made to increase the detector area by incorporating fiber-optic tapers between the scintillator and image intensi_er, resulting in a 16x increase in detector area. These large-area BazookaSPECT detectors can be used for full-body imaging and we present preliminary results of their use as dynamic scintigraphy imagers for mice and rats. Also, we discuss ongoing and future developments in BazookaSPECT and the improved event- detection rate capability that is achieved using Graphics Processing Units (GPUs), multi-core processors, and new high-speed, USB 3.0 CMOS cameras.

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.

Variations in pulse-height spectrum and pulse timing in CdZnTe pixel array detectors

The signal generated in pixel array detectors for gamma-ray imaging can be strikingly different from the signal seen in single-element detectors. When the pixel size is small compared with the detector thickness, the signal induced in the readout circuit becomes dominated by one charge carrier. If the small pixel is biased positive with respect to the continuous electrode the sensed signal will be due mostly to electron motion while the degrading effects of hole trapping and variation in interaction depth become far less important. To confirm experimentally the predictions of charge transport theory we fabricated CdZnTe test pixels of various sizes. Using a source of alpha radiation we measured the pulse timing properties for electron and hole transport as a function of pixel size. Pulse-height spectra were taken with gamma radiation from 99mTc. The results are in good agreement with the model for signal induction.

Progress of BazookaSPECT

Recent progress on a high-resolution, photon-counting gamma-ray and x-ray imager called BazookaSPECT is presented. BazookaSPECT is an example of a new class of scintillation detectors based on integrating detectors such as CCD(charge-coupled device) or CMOS(complementary metal-oxide semiconductor) sensors. BazookaSPECT is unique in that it makes use of a scintillator in close proximity to a microchannel plate-based image intensifier for up-front optical amplification of scintillation light. We discuss progress made in bringing about compact BazookaSPECT modules and in real-time processing of event data using graphics processing units (GPUs). These advances are being implemented in the design of a high-resolution rodent brain imager called FastSPECT III. A key benefit of up-front optical gain is that any CCD/CMOS sensor can now be utilized for photon counting. We discuss the benefits and feasibility of using CMOS sensors as photon-counting detectors for digital radiography, with application in mammography and computed tomography (CT). We present as an appendix a formal method for comparing various photon-counting integrating detectors using objective statistical criteria.

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.