H.B. Barber

Signals induced in semiconductor gamma-ray imaging detectors

The signal induced in a readout circuit connected to a pixel electrode in a semiconductor gamma-ray imaging array is calculated by solving the Laplace equation. Two approaches are presented that use Green functions in solving the boundary value problem: decomposition into basis functions, and construction of an infinite series of image charges. Another approach is developed based on the Ramo–Shockley theorem, which makes use of weighting potentials. These potentials may be readily calculated in three dimensions using a Fourier-transform propagation technique. An analytic solution is found for the special two-dimensional case of a strip detector. Experiments on CdZnTe square-pixel test structures using alpha radiation confirm the expected trends in pulse shape as a function of pixel size. Signals observed simultaneously on adjacent pixels also follow the predicted division of currents. Trends with pixel size are also confirmed in the shape of pulse-height spectra taken using a 99mTc source.

Ultra-high-resolution brain SPECT imaging: simulation results

The spatial resolution in a reconstructed single photon emission computed tomography (SPECT) image is influenced by the intrinsic resolution of the detector, and the photon-counting efficiency of SPECT systems is also determined by the intrinsic resolution. The authors demonstrate that improvements in detector resolution can lead to both improved spatial resolution in the image and improved counting efficiency compared to conventional systems. This paradoxical conclusion results from optimizing the geometry of a multiple-pinhole coded-aperture system when detectors of very high resolution are available. Simulation studies that demonstrate the image quality that is attainable with such detectors are reported. Reconstructions are performed using an iterative search algorithm on a custom-designed parallel computer. The imaging system is described by a calculated system matrix relating all voxels in the object space to all pixels on the detector. A resolution close to 2 mm is found on the reconstructed images obtained from these computer simulations with clinically reasonable exposure times. This resolution may be even further improved by optimization of the multiple-pinhole aperture. >

Compact CT/SPECT small-animal imaging system

We have developed a dual-modality CT/SPECT imaging system for small-animal imaging applications. The X-ray system comprises a commercially available micro-focus X-ray tube and a CCD-based X-ray camera. X-ray transmission measurements are performed based on cone-beam geometry. Individual projections are acquired by rotating the animal about a vertical axis in front of the CCD detector. A high-resolution CT image is obtained after reconstruction using an ordered subsets-expectation maximization (OS-EM) reconstruction algorithm. The SPECT system utilizes a compact semiconductor camera module previously developed in our group. The module is mounted perpendicular to the X-ray tube/CCD combination. It consists of a 64 /spl times/ 64 pixellated CdZnTe detector and a parallel-hole tungsten collimator. The field of view is 1 square inch. Planar projections for SPECT reconstruction are obtained by rotating the animal in front of the detector. Gamma-ray and X-ray images are presented of phantoms and mice. Procedures for merging the anatomical and functional images are discussed.

Pixellated CdZnTe detector arrays

Abstract This paper reports the first high spatial resolution room-temperature semiconductor gamma detector arrays. Results of wafer-scale fabrication of 48 by 48 element CdZnTe (CZT) arrays with 125 μm element pitch are presented. Performance characteristics are presented, including pulse height spectra using charge-sensitive preamplifier readout from witness test pixels, which show excellent energy resolution. High bulk and surface resistivities have been verified for these arrays, thus good energy resolution with multiplexer readout is expected.

Tomographic small-animal imaging using a high-resolution semiconductor camera

We have developed a high-resolution, compact semiconductor camera for nuclear medicine applications. The modular unit has been used to obtain tomographic images of phantoms and mice. The system consists of a 64 x 64 CdZnTe detector array and a parallel-hole tungsten collimator mounted inside a 17 cm x 5.3 cm x 3.7 cm tungsten-aluminum housing. The detector is a 2.5 cm x 2.5 cm x 0.15 cm slab of CdZnTe connected to a 64 x 64 multiplexer readout via indium-bump bonding. The collimator is 7 mm thick, with a 0.38 mm pitch that matches the detector pixel pitch. We obtained a series of projections by rotating the object in front of the camera. The axis of rotation was vertical and about 1.5 cm away from the collimator face. Mouse holders were made out of acrylic plastic tubing to facilitate rotation and the administration of gas anesthetic. Acquisition times were varied from 60 sec to 90 sec per image for a total of 60 projections at an equal spacing of 6 degrees between projections. We present tomographic images of a line phantom and mouse bone scan and assess the properties of the system. The reconstructed images demonstrate spatial resolution on the order of 1-2 mm.

Gamma-ray imaging using a CdZnTe pixel array and a high-resolution, parallel-hole collimator

The poor performance of current parallel-hole collimators is an impediment to planar high-resolution gamma-ray imaging, even when high-resolution semiconductor detector arrays are available. High-resolution parallel-hole collimators are possible but have not been fabricated because conventional collimator construction techniques severely limit achievable bore size and septal thickness. We describe development and testing of a high-resolution collimator with 4096 260-/spl mu/m square bores and 380 /spl mu/-m pitch, matched to our existing 2.5 cm/spl times/2.5 cm hybrid 64/spl times/64 CdZnTe arrays with multiplexer readout. The collimator is a laminar composite of about 100 layers of W sheets produced by photolithography and has an efficiency of 5/spl times/10/sup -5/. We have demonstrated sub-millimeter spatial resolution at 140 keV in both phantom and animal imaging using this system. We present images of resolved individual vertebrae in the spine of a mouse and lymphatic channels and nodes in a rat. The collimator and semiconductor array could form a compact module for use in a wide variety of gamma-ray imaging systems.

Progress in the development of large-area modular 64/spl times/64 CdZnTe imaging arrays for nuclear medicine

Previous efforts by our group have demonstrated the potential of hybrid semiconductor detector arrays for use in gamma-ray imaging applications. In this paper, we describe progress in the development of a prototype imaging system consisting of a 64/spl times/64-pixel CdZnTe detector array mated to a multiplexer readout circuit that was custom designed for our nuclear medicine application. The detector array consists of a 0.15 cm thick slab of CdZnTe which has a 64/spl times/64 array of 380 /spl mu/m square pixel electrodes on one side produced by photolithography; the other side has a continuous electrode biased at -150 V. Electrical connections between the detector electrodes and corresponding multiplexer bump pads are made with indium bump bonds. Although the CdZnTe detector arrays characterized in this paper are room-temperature devices, a slight amount of cooling is necessary to reduce thermally generated dark current in the detectors. Initial tests show that this prototype imager functions well with more than 90% of its pixels operating. The device is an excellent imager; phantom images have a spatial resolution of 1.5 mm, limited by the collimator bore.

A Low-Cost Approach to High-Resolution, Single-Photon Imaging Using Columnar Scintillators and Image Intensifiers

Results are presented for a low-cost, ultra-high resolution gamma camera for small-animal SPECT and molecular imaging. The detector, known as Bazooka SPECT, employs a second-generation image intensifier which is directly coupled to a columnar CsI(Tl) scintillator. Operating in photon-counting mode, individual gamma-ray interactions are seen as clusters of signal, and significant improvement in spatial resolution is obtained by estimating the interaction position via Anger (centroid) estimation for individual clusters. Amplification of scintillator light prior to the optical path is advantageous compared to similar, low-light CCD gamma-cameras which solely apply gain in the CCD. With amplification from the image intensifier, the system is no longer limited by light loss from the optical path. This allows for a customizable optical system via lenses in a macro photography configuration and for the use of a low-cost, highspeed CCD. Experimental results show that Bazooka SPECT has an intrinsic resolution of approximately 150mum when the entire 25mm intensifier output screen is imaged onto the CCD. Better than 100mum resolution is attainable with less demagnification from the optical system. Initial indications are that this detector will serve as an attractive, inexpensive modular camera for high-resolution, multiple-pinhole, small-animal SPECT and molecular imaging.

Comparison of in vivo scintillation probes and gamma cameras for detection of small, deep tumours

An in vivo probe was compared with a gamma camera for the task of detecting radiolabelled tumour models in a water phantom. The probe, which contained a 1 cm diameter NaI(T1) detector, was designed and used by us for surgical staging studies of gynaecological patients. Tumours were spherical sources of different sizes and activities per unit volume of cobalt-57. The phantom was a tank of water (451) containing dissolved radioactivity to simulate background activity. Detector-to-source separations and tank depths were also varied. Camera images of 10 min duration were compared with probe counts of 15 s. For each configuration a large number of source and background runs were analysed using an ideal-observer ROC technique. Area under the ROC curve was used as the figure of merit. Results show that for approximately uniform background the probe should perform substantially better than the gamma camera in detecting small, deep tumours provided that the probe can be manoeuvred to within a few centimetres of the tumour. Mathematical modelling of our results indicates that this conclusion is not dependent on radiopharmaceutical dose, tumour uptake or camera type.

Progress in developing focal-plane-multiplexer readout for large CdZnTe arrays for nuclear medicine applications

We report on the construction and initial testing of a 48 × 48 CdZnTe array with 125 μm pixel spacing and multiplexer readout. Large portions of the array function well but there was a loss of pixels near one corner of the array due to non-interconnecting indium bumps. This problem is readily correctable. Excellent single-pixel spectra were obtained with a 99mTc source using an adjustment technique that accounts for energy deposited in neighboring pixels. A point-spread function (PSF) taken at 140 keV yielded a spatial resolution of 230 μm, much better than required for nuclear medicine applications. No problems were found that are not readily correctable or of much less significance for CdZnTe arrays having larger pixel spacing. We are now constructing 64 × 64 CdZnTe arrays with 380 μm pixel spacing for use in an ultra-high resolution brain SPECT imaging system.