Richard C. Lanza

Coded aperture imaging

We have simulated the performance of various apertures used in coded aperture imaging—optically. The annulus, twin annulus, Fresnel zone plate and the uniformly redundant array have been decoded using a non-coherent correlation process. Ways of reducing the ‘d.c.’ background of the various apertures are discussed. Results of imaging extended and continuous-tone planar objects are presented.

Development and validation of a Monte Carlo simulation of photon transport in an Anger camera

The geometric component of the point spread function (PSF) of a gamma camera collimator can be determined analytically, and the penetration component can be calculated readily by numerical ray-tracing. A Monte Carlo simulation of photon transport which includes collimator scatter is developed. The simulation was implemented with an array processor which propagates up to 1024 photons in parallel, allowing accurate estimates of the total radial PSF in less than a day. The simulation was tested by imaging monoenergetic point sources of Tc-99m, Cr-51, and Sr-85 (140, 320, and 514 keV, respectively) on a General Electric Star Cam with low-energy, general-purpose, and medium-energy collimators. Comparisons of measured and simulated PSFs demonstrate the validity of the model and the significance of collimator scatter in the degradation of image quality.

Optimal coded aperture patterns for improved SNR in nuclear medicine imaging

During a Nuclear Medicine project that called for the optimal design of a coded aperture we found that lowthroughput masks do not always provide a Signal-to-Noise Ratio (SNR) advantage. In this paper, we present the simulations of the performance of some coded aperture patterns chosen from different families and compare the results with theoretical predictions. A general expression for the SNR and its particular form for different patterns are provided. The choice of the optimal pattern family is discussed with reference to the characteristics of the object to be imaged and in light of the effect of near-field artifacts. No-Two-Holes-Touching (NTHT) arrays based on Modified Uniformly Redundant Arrays (MURAs) proved to offer the best compromise between SNR performance and practical fabrication constraints. r 2001 Elsevier Science B.V. All rights reserved.

Near-field artifact reduction in planar coded aperture imaging

Coded apertures for imaging problems are typically based on arrays having perfect cross-correlation properties. These arrays, however, guarantee a perfect point-spread function in far-field applications only. When these arrays are used in the near-field, artifacts arise. We present a mathematical derivation capable of predicting the shape of such artifacts. The theory shows that methods used in the past to compensate for the effects of background nonuniformities in far-field problems are also effective in reducing near-field artifacts. The case study of a nuclear medicine problem is presented to show good agreement of simulation and experimental results with mathematical predictions.

Fast neutron resonance radiography for elemental imaging: theory and applications

Fast neutron resonance radiography (NRR) has been devised as an elemental imaging method with applications such as contraband detection and mineral analysis. In NRR, a two-dimensional (2-D) elemental mapping of hydrogen, carbon, nitrogen, oxygen, and the sum of other elements is obtained from fast neutron radiographic images, taken at different neutron energies and chosen to cover the resonance cross-section features of one or more elements. Images are formed using a lens-coupled plastic scintillator charged coupled device (CCD) combination. In preliminary experiments, we have produced NRR images of various simulants using a variable energy beam based on the Li(p,n)Be reaction and a variable energy proton beam. As an alternative to this method, we have studied NRR imaging using the D-D reaction, d(d,He)n, at fixed incident D energy and scanning through various neutron energies by using the angular variation in neutron energy. The object and detector rotate together around the neutron source; different energy (2-6 MeV) neutrons can be obtained at different angles from the target. The radiographic transmission image provides a 2-D mapping of the sum of elemental contents (weighted by the attenuation coefficients). Transmission measurements taken at different neutron energies (angles) then form a set of linear equations, which can then be solved to map individual elemental contents.

Large area imaging detector for long-range, passive detection of fissile material

Recent events highlight the increased risk of a terrorist attack using either a nuclear or a radiological weapon. One of the key needs to counter such a threat is long-range detection of nuclear material. Theoretically, gamma-ray emissions from such material should allow passive detection to distances greater than 100 m. However, detection at this range has long been thought impractical due to fluctuating levels of natural background radiation. These fluctuations are the major source of uncertainty in detection and mean that sensitivity cannot be increased simply by increasing detector size. Recent work has shown that this problem can be overcome through the use of imaging techniques. In this paper we describe the background problems, the advantages of imaging and the construction of a prototype, large-area (0.57 m/sup 2/) gamma-ray imager to detect nuclear materials at distances of /spl sim/100 m.

First dark matter search results from a surface run of the 10-L DMTPC directional dark matter detector

Abstract The Dark Matter Time Projection Chamber (DMTPC) is a low pressure (75 Torr CF4) 10 liter detector capable of measuring the vector direction of nuclear recoils with the goal of directional dark matter detection. In this Letter we present the first dark matter limit from DMTPC from a surface run at MIT. In an analysis window of 80–200 keV recoil energy, based on a 35.7 g-day exposure, we set a 90% C.L. upper limit on the spin-dependent WIMP-proton cross section of 2.0 × 10 − 33 cm 2 for 115 GeV/c2 dark matter particle mass.

A Single arm spectrometer detector system for high-energy heavy ion experiments

The recent availability of 14.6 GeV/c per nucleon 16O and 28Si ions from the Brookhaven National Laboratory Tandem-AGS accelerator facility has prompted the design, construction and operation of a large-solid-angle (25 msr) magnetic spectrometer with particle identification from ∼0.5 to ∼4.7 GeV/c. A small-solid-angle Cherenkov counter complex views the target through the magnet and extends the particle identification up to ∼15 GeV/c. This experiment (E-802) employs event characterization detectors, a charged-particle multiplicity array, a highly segmented lead-glass detector, and a zero degree calorimeter. The facility measures momentum spectra of identified heavy-ion-produced hadrons with high resolution (Δp/p≤0.005) as a function of collision centrality given by triggers from the event characterization detectors. Construction and performance details of the spectrometer components and auxiliary detectors are described.

Source-search sensitivity of a large-area, coded-aperture, gamma-ray imager

We have recently completed a large-area, coded-aperture, gamma-ray imager for use in searching for radiation sources. The instrument was constructed to verify that weak point sources can be detected at considerable distances if one uses imaging to overcome fluctuations in the natural background. The instrument uses a rank-19, one-dimensional coded aperture to cast shadow patterns onto a 0.57 m/sup 2/ NaI(Tl) detector composed of 57 individual cubes each 10 cm on a side. These are arranged in a 19/spl times/3 array. The mask is composed of four-cm thick, one-meter high, 10-cm wide lead blocks. The instrument is mounted in the back of a small truck from which images are obtained as one drives through a region. Results of first measurements obtained with the system are presented.

Small animal imaging by single photon emission using pinhole and coded aperture collimation

The design of detectors for radio-imaging of small animals is challenging because of the high spatial resolution required, possibly coupled with high efficiency to allow dynamic studies. Spatial resolution and sensitivity are difficult to attain at the same time with single photon imaging techniques because collimators define and limit performance. In this paper we first describe a simple desktop gamma imager equipped with a pinhole collimator and based on a pixellated NaI(Tl) scintillator array coupled to a Hamamatsu R2486 PSPMT. The limits of such a system as well as the way to overcome them in future systems is shown next. Better light sampling at the anode level would allow better pixel identification for a higher number of pixels, which is one of the parameters defining image quality and improving spatial resolution. The performance of such a design is compared with other designs using other PSPMT types with different light sampling schemes at the anode level. Finally, we show how the substitution of the pinhole collimator with a coded aperture collimator can result in a substantial improvement in system sensitivity while maintaining very good spatial resolution, possibly at a sub-millimeter level. Calculations and simulations of a particular solution show that sensitivity can improve by a factor of nearly 30.