Alexander Stolin

Investigation of efficiency and spatial resolution using pinholes with small pinhole angle

Pinhole collimators are well suited for small animal SPECT, in which high spatial resolution is required over a small field of view (FOV). Typical pinhole collimators have pinhole angles of 90/spl deg/ or more. If their full FOV is used, the trajectories of imaged gamma-rays can be up to 45/spl deg/ away from the pinhole axis, and form angles of incidence at the surface of a planar detector as large as 45/spl deg/. The large incident angle produces a depth-of-interaction (DOI) blur thus reducing spatial resolution for these off-axis locations. This loss of the detector resolution is masked by the accompanying improvement of the pinhole point spread function (PSF) due to the reduced effective aperture size. The magnitude of these effects as a function of the angle between the gamma-ray trajectory and the pinhole axis depends on detector absorption efficiency, and on the physical parameters of the pinhole collimator, including pinhole type, composition, and pinhole angle. In addition, the reduced effective pinhole size results in reduced efficiency for off-axis source points. In this paper we investigate the angular dependence of these effects for pinhole collimators with different compositions and pinhole angles. Results are compared to theoretical expressions that take into account gamma-ray penetration of the pinhole material.

Investigation of spatial resolution and efficiency using pinholes with small pinhole angle

Pinhole collimators are well suited for small animal SPECT, in which high spatial resolution is required over a small field of view (FOV). Typical pinhole collimators have pinhole angles of 90/spl deg/ or more. If their full FOV is used, the trajectories of imaged gamma rays can be more than 45/spl deg/ away from the pinhole axis, and form angles of incidence at the surface of a planar detector greater than 45/spl deg/. The large incident angle produces a depth-of-interaction (DOI) blur thus reducing spatial resolution for these off-axis locations. This loss of resolution is masked by the accompanying improvement of the pinhole point spread function (PSF) due to the reduced effective aperture size. The magnitude of these effects as a function of the angle between the gamma ray trajectory and the pinhole axis depends on detector absorption efficiency, and on the physical parameters of the pinhole collimator, including pinhole type, composition, and size pinhole angle. In addition, the reduced effective pinhole results in reduced efficiency for off-axis source points. In this paper we investigate the angular dependence of these effects for pinhole collimators with different compositions and pinhole angles. Results are compared to theoretical expressions that take into account gamma ray penetration of the pinhole material.

Tri-modality small animal imaging system

Our group is developing a scanner that combines x-ray, single gamma, and optical imaging on the same rotating gantry. Two functional modalities (SPECT and optical) are included because they have different strengths and weaknesses in terms of spatial and temporal decay lengths in the context of in vivo imaging, and because of the recent advent of multiple reporter gene constructs. The effect of attenuation by biological tissue on the detected intensity of the emitted signal was measured for both gamma and optical imaging. Attenuation by biological tissue was quantified for both the bioluminescent emission of luciferace and for the emission light of the near infrared fluorophore cyanine 5.5, using a fixed excitation light intensity. Experiments were performed to test the feasibility of using either single gamma or x-ray imaging to make depth-dependent corrections to the measured optical signal. Our results suggest that significant improvements in quantitation of optical emission are possible using straightforward correction techniques based on information from other modalities. Development of an integrated scanner in which data from each modality are obtained with the animal in a common configuration will greatly simplify this process.

Characterization and comparison of X-ray detectors for use in small animal imaging

In this paper we describe the results of our characterization of four X-ray detectors in order to determine their relative strengths and weaknesses for use in the CT component of a dedicated CT-SPECT scanner that we are developing for small animal imaging. Two of the four detectors are custom-built charge coupled device (CCD)-based detectors and the other two are commercially available detectors based on complementary metal oxide (CMOS) technology. This paper describes the design of each detector and presents the results of measurements of imaging characteristics such as the detector dark noise, sensitivity, the modulation transfer function (MTF), the noise power spectrum (NPS), and the detective quantum efficiency (DQE)- Compared to the CMOS detectors, the CCD based detectors exhibit superior DQE over most of the frequency range of interest.

Image reconstruction for small animal SPECT with two opposing half cones

Pinhole imaging is a promising approach for high spatial resolution single gamma emission imaging in situations when the required field of view (FOV) is small, as is the case for small animal imaging. However, all pinhole collimators exhibit steep decrease in sensitivity with increasing angle of incidence from the pinhole axis. This in turn degrades the reconstruction images, and requires higher dose of radiotracer. We developed a novel pinhole SPECT system for small animal imaging which uses two opposing and offset small cone-angle square pinholes, each looking at half of the FOV. This design allows the pinholes to be placed closer to the object and greatly increases detection efficiency and spatial resolution, while not requiring larger size detectors. Iterative image reconstruction algorithms for this system have been developed. Preliminary experimental data have demonstrated marked improvement in contrast and spatial resolution.

Dual-modality scanner for small animal imaging

We describe the design and performance of a dual-modality system for small animal imaging combining a high resolution SPECT component with an X-ray CT component. All components are mounted on a barrel type gantry which is able to rotate in 0.001 degrees increments about the animal. The SPECT subsystem consists of a four custom-built gamma cameras. The cameras can be equipped with either parallel hole or pinhole collimators depending on the study in question. The cameras can be operated in pairs to implement a unique half cone geometry setup in order to decrease the overall acquisition time as well as maximize spatial resolution. The average intrinsic spatial and energy resolutions of the gamma detectors is 1.9 mm and 17% respectively. The X-ray CT subsystem combines various interchangeable microfocus X-ray sources with either a high resolution CCD based detector or a flat panel Hamamatsu C7942 CMOS array. The CCD detector employs a 7k by 4k Phillips CCD chip with dimensions of 5 cm times 9 cm, large enough to require no minification between the Gd2O2S phosphor and the chip. The CT component has been installed and successfully used in a variety of small animal studies.

Characterization of imaging gamma detectors for use in small animal SPECT

In this paper we investigate the basic properties of an imaging gamma ray detector built using a square 2/spl times/2 array of Hamamatsu H8500 position sensitive photomultiplier tubes (PSPMTs) coupled to a pixelated NaI(Tl) crystal array. The measured imaging properties are compared with those of a detector containing an identical crystal array, but based on a single Hamamatsu R3292 PSPMT resulting in a round field of view. For both detectors, the crystal array contains 1.4 mm /spl times/ 1.4 mm /spl times/ 6 mm crystals with 1.6 mm center-to-center spacing. Specifically, we present the results of investigations of the basic properties, including intrinsic spatial resolution, detection efficiency, energy resolution, and sensitivity uniformity. Intrinsic spatial resolution was measured at three different locations on the detector surface giving an average of 1.8 mm FWHM for both detectors. Energy resolution was the same for both detectors and was 15% FWHM at 140 keV and 31% at 31 keV. The measured detection efficiency at 140 keV was /spl sim/35% and /spl sim/33% for the square and round detectors respectively. The detector response non-uniformity, defined as the ratio of the standard deviation of pixel values to the average pixel value in the central 75% of the field of view, was 8% for the square detector and 23% for the round one. These measurements indicate that the performance of the two detectors is similar, with the exception of sensitivity uniformity, in which the square detector performance is superior. The results of initial testing of a detector with a 3/spl times/4 H8500 PSPMT array coupled to 1.2 mm /spl times/ 1.2 mm /spl times/ 6 mm NaI crystal array are also presented. The intrinsic spatial resolution of the 3/spl times/4 detector was measured to be about 2.1 mm FWHM. The energy resolution was 22.7% at 140 keV. The measured detection efficiency at 140 keV was /spl sim/27% The detector response nonuniformity was measured to be about 40%. Although the sensitivity non-uniformity of the 3/spl times/4 detector was not as good as that of the other two detectors, the ability to more readily scale the field of view of the square PSPMT-based detector, makes this design a promising one for use in small animal SPECT.

Design and characteristics of a small animal multi-modality scanner

In a collaborative effort between the University of Virginia, Jefferson Lab, and Johns Hopkins University, we have developed a CT-SPECT scanner for small animal imaging. The scanner forms the basis of what is to be a trimodality scanner after the addition of an optical imaging component. The SPECT component of this system consists of a custom built gamma camera containing a 2 times 2 array of Hamamatsu H8500 position sensitive photomultiplier tubes coupled to a pixelated array of NaI(Tl) crystals. The detector is equipped with a pinhole collimator. The CT component consists of an X-ray source with a focal spot size of 50 microns and a Hamamatsu 7940DP-03 X-ray detector utilizing complementary metal oxide technology with a detector element size of 50 microns. Both subsystems are attached to a rotating, barrel-type gantry. Basic performance tests, including measurements of intrinsic resolution, intrinsic detector efficiency, energy resolution, and detector response uniformity for the gamma camera and measurements of the RMS dark noise, sensitivity, dynamic range, and imaging performance functions (MTF, NPS, DQE) for the X-ray camera were conducted. A CT-SPECT scan of a MicroSPECT phantom resulted in reconstructed images where the smallest details of the phantom, hot rods 1.2 mm in diameter with a center-to-center spacing of 2.4 mm, are clearly resolved. Dual modality scans of live mice were performed in order to detect the localization of PECAM-antibody tagged with Tc-99m. The scan time for CT was 6 minutes; the SPECT scan time varied from 30 minutes to 2 hours, depending on the amount of residual radioactivity in the animal

Half-cone SPECT system for small animal imaging

Molecular imaging studies using small animals such as mice require the development of high-sensitivity, high-resolution pinhole SPECT imaging systems. It is well known that the photon detection efficiency drops remarkably as the pinhole angle increases, and spatial resolution decreases as the magnification factor decreases. To obtain high efficiency and high resolution, the pinhole should be put as close as possible to the object while the pinhole angle remains as small as possible. To address these conflicting requirements, we proposed a half-cone imaging geometry in which the detector looks at only half of the FOV in each view, while the FOV being reconstructed remain the same. Our preliminary results demonstrated that SPECT system using this geometry could achieve higher sensitivity and better spatial resolution