D.G. Marks

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.

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.

A 48/spl times/48 CdZnTe array with multiplexer readout

We report results of gamma-ray imaging and energy-resolution tests of a 48/spl times/48 CdZnTe array. Our detectors have 125 /spl mu/m square pixel electrodes produced by photolithography and are indium-bump-bonded to a multiplexer readout circuit. Using a collimated beam of 140 keV gamma rays of 120 /spl mu/m diameter centered on one pixel, we found that the majority of events produced significant charge deposition in nearby pixels. Charge and energy are transported out of the pixel by charge diffusion, photoelectron range, Compton scattering, and escape of K X-rays. These effects also distort single-pixel spectra, although photopeaks are still discernible at 140 keV. When signals from neighboring pixels are summed together to correct for this charge spreading, an energy resolution of 10 keV is obtained at 140 keV. Corrections will be simpler and energy resolution should be better for the 380 /spl mu/m pixels of the 64/spl times/64 CdZnTe arrays we are constructing for an ultra-high-resolution brain imager.

A 48×48 CdZnTe array with multiplexer readout

We report results of gamma-ray imaging and energy-resolution tests of a 48/spl times/48 CdZnTe array. Our detectors have 125 /spl mu/m square pixel electrodes produced by photolithography and are indium-bump-bonded to multiplexer readout circuit. Using a collimated beam of 140 keV gamma rays of 120 /spl mu/m diameter centered on one pixel, we found that the majority of events produced significant charge deposition in nearby pixels. Charge and energy are transported out of the pixel by charge diffusion, photoelectron range, Compton scattering, and escape of K X-rays. These effects also distort single-pixel spectra, although photopeaks are still discernible at 140 keV. When signals from neighboring pixels are summed together to correct for this charge spreading, an energy resolution of 10 keV is obtained at 140 keV. Corrections will be simpler and energy resolution should be better for the 380 /spl mu/m pixels of the 64/spl times/64 CdZnTe arrays we are constructing for an ultra-high-resolution brain imager.

SEMICONDUCTOR PIXEL DETECTORS FOR GAMMA-RAY IMAGING IN NUCLEAR MEDICINE

Abstract Semiconductor pixel detectors hold great promise for replacing scintillation cameras in nuclear medicine; improvements in spatial resolution, energy resolution and sensitivity should result. The current status of this subject is briefly reviewed. The fabrication of hybrid, 48 × 48 CdZnTe pixel arrays for use in gamma-ray imaging is described. Each detector array is indium-bump bonded to a Hughes 48 × 48 multiplexer chip; the design is similar to that of an infrared focal-plane array. The 48 × 48 CdZnTe array is shown to perform well as a gamma-ray imaging system with 125 μm spatial resolution (at 60 keV), equivalent to the pixel spacing. A correction technique for charge spreading between pixels is demonstrated. The implications of macroscopic crystal defects on array performance are briefly discussed.

Maximum-likelihood estimation for semiconductor detector arrays

We propose a rigorous statistical treatment of data from semiconductor detector arrays using maximum-likelihood estimation. For pixellated and cross-striped electrodes, a single gamma-ray interaction can produce signals in multiple electrodes. The set of signals generated from a single gamma-ray interaction are random variables with a probability law dependent on the position and energy of the gamma ray. If the signals are fixed then the probability law becomes the likelihood function. The maximum-likelihood estimate is then the position and energy that maximize the likelihood. We applied this estimator to a 48/spl times/48 pixellated CdZnTe array with 125 /spl mu/m pixel spacing. The likelihood law was evaluated with Monte-Carlo integration using a model of all physical processes affecting signal generation. By simplifying the physical model such that each gamma ray can undergo only one scattering process, computation time is greatly reduced with no measurable effect on energy resolution. The energy estimates formed histograms with 7.5 keV full-width-half-maximum (FWHM) at 140 keV, and 6 keV FWHM at 60 keV. Currently used methods show at best 11 keV FWHM at 140 keV and 9 keV FWHM at 60 keV. Comparison with simulations shows that these devices behave close to our model.

Use of CdZnTe pixel arrays with multiplexer readout to map detector crystal properties

We demonstrate the use of pixel arrays to map CdZnTe detector materials properties in situ. Our readout technique uses a multiplexer containing a gated integrator and is sensitive to DC currents. We have made extensive measurements of leakage current in one Cd/sub 0.9/Zn/sub 0.1/Te hybrid detector array that has size 25/spl times/25/spl times/1.5 mm/sup 3/ and 380 /spl mu/m pixel pitch. The pixel I-V curves are essentially ohmic and the leakage pattern is qualitatively similar at different temperatures. Histograms of pixel resistivity are relatively flat in the range 5-60 G/spl Omega/ cm. Pixel Arrhenius plots have slopes that differ with pixel resistivity.

Modular 64×64 CdZnTe Arrays With Multiplexer Readout for High-Resolution Nuclear Medicine Imaging

We are developing modular arrays of CdZnTe radiation detectors for high-resolution nuclear medicine imaging. Each detector is delineated into a 64×64 array of pixels; the pixel pitch is 380 jim. Each pixel is connected to a corresponding pad on a multiplexer readout circuit. The imaging system is controlled by a personal computer. We obtained images of standard nuclear medicine phantoms in which the spatial resolution of approximately 1.5 mm was limited by the collimator that was used. Significant improvements in spatial resolution should be possible with different collimator designs. These results are promising for high-resolution nuclear medicine imaging.

Improving spatial resolution in semiconductor gamma-ray detectors using a list-mode EM algorithm for fluence estimation

We present a method for applying the expectation-maximization (EM) algorithm to semiconductor gamma-ray imaging detectors to reconstruct images from sets of electrode data. The EM algorithm iteratively seeks an image described by a distribution of fluence, or gamma rays per unit area, that has the greatest probability of producing the observed set of data. The algorithm acts without estimating interaction positions for individual gamma rays. When applying the algorithm to data from a Cd/sub 0.8/Zn/sub 0.2/Te detector with square pixel electrodes spaced 125 /spl mu/m we get a spatial resolution of 40 /spl mu/m for 60 keV gamma rays, compared with 104 /spl mu/m resolution when individual interaction positions are estimated.