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Art of electronics

MicroPET is a high resolution positron emission tomography (PET) scanner designed for imaging small laboratory animals. It consists of a ring of 30 position-sensitive scintillation detectors, each with an 8/spl times/8 array of small lutetium oxyorthosilicate (LSO) crystals coupled via optical fibers to a multi-channel photomultiplier tube. The detectors have an intrinsic resolution averaging 1.68 mm, an energy resolution between 15 and 25% and 2.4 ns timing resolution at 511 keV. The detector ring diameter of microPET is 17.2 cm with an imaging field of view of 112 mm transaxially by 18 mm axially. The scanner has no septa and operates exclusively in 3D mode. Reconstructed image resolution 1 cm from the center of the scanner is 2.0 mm and virtually isotropic, yielding a volume resolution of 8 mm/sup 3/. For comparison, the volume resolution of state-of-the-art clinical PET systems is in the range of 50-75 mm/sup 3/. Initial images of phantoms have been acquired and are reported. A computer controlled bed is under construction and will incorporate a small wobble motion to improve spatial sampling. This is projected to further enhance spatial resolution. MicroPET is the first PET scanner to incorporate the new scintillator LSO and to our knowledge is the highest resolution multi-ring PET scanner currently in existence.

MicroPET: a high resolution PET scanner for imaging small animals

We report the first measurement of the parity-violating asymmetry A(PV) in the elastic scattering of polarized electrons from 208Pb. A(PV) is sensitive to the radius of the neutron distribution (R(n)). The result A(PV)=0.656±0.060(stat)±0.014(syst) ppm corresponds to a difference between the radii of the neutron and proton distributions R(n)-R(p)=0.33(-0.18)(+0.16) fm and provides the first electroweak observation of the neutron skin which is expected in a heavy, neutron-rich nucleus.

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Measurement of the Neutron Radius of 208Pb Through Parity Violation in Electron Scattering

‘Two is better than one’—probes for dual-modality molecular imaging

▪ Abstract The recent rapid increase in interest in tomographic imaging of small animals and of human (and large animal) organ biopsies is driven largely by drug discovery, cancer detection/monitoring, phenotype identification and/or characterization, and development of disease detection methods and monitoring efficacies of drugs in disease treatment. In biomedical applications, micro-computed tomography (CT) scanners can function as scaled-down (i.e., mini) clinical CT scanners that provide a three-dimensional (3-D) image of most, if not the entire, torso of a mouse at image resolution (50–100 μm) scaled proportional to that of a human CT image. Micro-CT scanners, on the other hand, image specimens the size of intact rodent organs at spatial resolutions from cellular (20 μm) down to subcellular dimensions (e.g., 1 μm) and fill the resolution-hiatus between microscope imaging, which resolves individual cells in thin sections of tissue, and mini-CT imaging of intact volumes.

Micro-computed tomography - Current status and developments

In a previous simulation study, we demonstrated the feasibility of using coded apertures together with pixelated detectors tors for small animal SPECT. In this paper; we further explore the potential of this approach with a prototype detector and simulated multipinhole apertures. We also investigated the effect of multiplexing due to overlapped projections on convergence properties, image signal-to-noise ratio (SNR) and spatial resolution. The detector comprises a 48/spl times/44 array of NaI(Tl) crystals, each 1 mm/spl times/1 mm/spl times/5 mm on a 1.25-mm pitch. The crystal array is directly coupled to a Hamamatsu R3941 8 cm position sensitive photomultiplier tube. Multipinhole apertures were simulated by performing repeated SPECT acquisitions of the same object with a single tungsten pinhole translated to different positions in the aperture plane. Image reconstruction is based on a three-dimensional ray driven projector which is an extension of a method described for single pinhole SPECT with a displaced center of rotation. Image estimates are updated using the maximum likelihood expectation maximization (ML-EM) algorithm. The effect of multiplexing was to slow convergence and reduce the achievable SNR by approximately 15% compared with nonmultiplexed data (but the result may be achieved in a fraction of the time). The reconstructed resolution obtained with a resolution phantom was 1.5-mm full width at half maximum and there was no appreciable difference between the resolution of multiplexed and nonmultiplexed data. These results encourage us to develop a prototype coded aperture system for high sensitivity, high resolution small animal SPECT.

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A prototype coded aperture detector for small animal SPECT

The Detector Group at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) and the Biology, Physics, and Applied Sciences Departments at the College of William and Mary are collaborating on the development of a miniature dual modality SPECT-CT system for mouse imaging The detector heads of the SPECT sub-system are designed to be capable of imaging the gamma- and X-ray emissions (28-35 keV) of the radioactive isotope iodine-125 (I-125) Two different sets of I-125 imaging detectors are configured on a gantry that has an open-barrel type design One set of detector heads is based on the 1-in square Hamamatsu R5900-M64 position sensitive photomultiplier tube coupled to crystal scintillator arrays The other detector heads configured on the gantry are two 5-in diameter Hamamatsu R3292-based compact gamma cameras The X-ray radiographic projections are obtained using a LIXI Inc model LF-85-503-OS X-ray imaging system that has an active area of 55 cm in diameter The open-barrel shaped gantry facilitates the positioning of various mini gamma-ray imaging detectors and the X-ray system The data acquisition and gantry control is interfaced through a Macintosh G3 workstation Preliminary SPECT reconstruction results using the R5900 based detector are presented

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SPECT-CT system for small animal imaging

High resolution imaging gamma-ray detectors were developed using Hamamatsu's 5" diameter R3292 position sensitive PMT (PSPMT) and a variety of crystal scintillator arrays. Special readout techniques were used to maximize the active imaging area while reducing the number of readout channels. Spatial resolutions approaching 1 mm were obtained in a broad energy range from 20 to 511 keV. Results are also presented of coupling the scintillator arrays to the PMT via imaging light guides consisting of acrylic optical fibers which enables greater design freedom.

High spatial resolution gamma imaging detector based on a 5" diameter R3292 Hamamatsu PSPMT

Positron emission mammography (PEM) is a new, specialized imaging modality utilizing PET radiopharmaceuticals to detect breast cancer. The capabilities and limitations of PEM in detecting breast tumors were investigated with a series of phantom experiments. The PEM imager was mounted on a standard Lorad biopsy table (separated by 18 cm). In the initial phase of the investigation, basic scanner parameters (resolution, sensitivity, and scatter fraction) were measured. The effects of a number of breast imaging parameters (length of acquisition, breast thickness, and breast density) on detection of breast lesions were then explored utilizing special phantoms. Moderately compressed breasts were simulated with a block of gelatin containing amounts of FDG consistent with 370 MBq injections. Lesions were simulated with four hollow spheres (inner diameters=5 mm, 8 mm, 12 mm, and 15 mm) filled with amounts of FDG representative of uptake in malignant breast tumors (target-to-background concentration ratio=8.5:1). Resolution at the center of the imager was 3.9 mm, sensitivity was 0.059 kcps/kBq/ml and the Compton scatter fraction was approximately 12%. Objects as small as 8 mm in diameter could be detected after 30 s of data acquisition; 5 mm spheres were detectable after 300 s. Object detection capabilities were reduced with increasing breast thickness. In thin compressed breasts (2 cm) even the smallest sphere (5 mm in diameter) could be detected; increasing breast thickness increased the minimum detectable sphere diameter to 8 mm. Increased background activity caused by FDG uptake in metabolically active normal tissue more prevalent in radiodense breasts compared to "fatty" breasts was simulated and shown to reduce the minimum detectable lesion size to 12 mm for the densest breasts. These results demonstrate the potential of PEM for the detection of breast lesions. The addition of the system to a standard biopsy apparatus indicates its potential for use to guide some core biopsies of breast cancers.

The potential role of positron emission mammography for detection of breast cancer. A phantom study

Jefferson Lab., Hampton University, and University of Virginia are collaborating on optimization of dedicated scintimammography mini gamma cameras. Several prototype imagers with a FOV of 4"/spl times/4" and 8"/spl times/6" were tested for an application in a dual modality mode as an adjunct technique to digital mammography imagers, or as stand-alone instruments in a dedicated breast SPECT mode. The goal of this study was to obtain experimental data allowing for selection of the best imaging geometry to detect small lesions labeled with Tc-99m. The design of the small scintimammography gamma camera prototypes used in these studies, with an active FOV from 10 cm/spl times/10 cm and 15 cm/spl times/20 cm, is based on an array of Hamamatsu R7600-00-C8 position sensitive photomultiplier tubes (PSPMTs). Optically coupled to the PSPMT array via specially designed efficient multi-element light guide is a matrix of NaI(Tl) scintillator pixels (made by Bicron Corporation) with each element 3 mm/spl times/3 mm/spl times/6 mm in size and separated by 0.4 mm thick septa. Several designs of this basic structure were used in prototypes used to detect small lesions inserted in gelatin breast phantoms under compression and in a non-compressed SPECT mode. Anthropomorphic torso phantoms were also employed to simulate realistic scatter radiation fields. Two data acquisition systems were used to collect and analyze the data: one based on a Macintosh G3 workstation with FERA ADCs and one based on a PC computer running Windows NT/KmaxNT software which makes use of two sixteen channel ADC PCl cards. The present results show that the preferred imaging geometry is planar imaging with two opposing detector heads and breast under compression, however, further study of the dedicated breast SPECT is warranted.

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R. Wojcik

Papers

A prototype coded aperture detector for small animal SPECT

SPECT-CT system for small animal imaging

High spatial resolution gamma imaging detector based on a 5" diameter R3292 Hamamatsu PSPMT

The potential role of positron emission mammography for detection of breast cancer. A phantom study

Optimization of dedicated scintimammography procedure using detector prototypes and compressible phantoms