Enrique W. Izaguirre

Simulation and Validation of Point Spread Functions in Pinhole SPECT Imaging

Modeling and assessment of point spread functions (PSFs) of pinhole collimators is essential for the design of small animal single photon emission computed tomography (SPECT) imaging systems, and are gaining increasing importance with the advent of multipinhole imaging techniques. PSFs also can be used in resolution recovery methods implemented in reconstruction algorithms. Therefore, we have developed and validated a ray-tracing approach to calculate PSFs and absolute detection efficiency of pinhole collimators in radionuclide imaging. The PSFs were calculated for user defined pinhole and source geometries with multiple rays to account for collimator penetration. For validation we compared our simulations to analytical models, Monte Carlo simulations from literature, and experiments with 99 mTc sources using a variety of pinhole geometries including knife and keel edges. We find that shape and magnitude of the simulated PSFs are in very good agreement with analytical and experimental results. Importantly, our simulations show that the absolute detection efficiency of pinhole systems can be computed with an accuracy error of less than 10% using the ray-tracing approach. In contrast to Monte Carlo simulations ray-tracing simulations are computational very efficient and therefore very fast. In conclusion, we developed a ray-tracing method that calculates PSFs and detection efficiencies for different pinhole and source geometries quickly and reliably

Evaluation of a Large Pixellated Cadmium Zinc Telluride Detector for Small Animal Radionuclide Imaging

Small animal microSPECT systems require high resolution and efficiency to faithfully image biodistributions of molecular reporters and radiopharmaceuticals in a short time. Because of their variable magnification, pinhole and multipinhole cameras are particularly well suited for small animal imaging systems. These cameras nonetheless require a high degree of sampling to achieve high resolution in tomographic images. In order to construct a high resolution pinhole and multipinhole gamma camera for small animal imaging, we developed a large cadmium zinc telluride (CZT) detector array. This detector, having 128times128 1.5times1.5 mm2 pixels, is one of the largest of its kind in terms of number of pixels and readout channels. The CZT detector crystal array and application-specific integrate circuits (ASICs) are embedded in an aluminum enclosure to form a compact cassette unit. The signals generated by gamma interactions in the CZT crystal are amplified, shaped and multiplexed within the detector unit, and thereafter read by a computer-based data acquisition system. A high-energy keel edge pinhole collimator was coupled to this detector and used to image photons with energies up to 250 keV. This new CZT gamma detector was characterized using Tc-99m (140 keV) and In-111 (171 keV and 245 keV). Specifically, we measured the dead pixel fraction, the uniformity, the intrinsic spatial resolution, and the energy resolution of this detector. Furthermore, we assessed the sensitivity of the pinhole camera. The detector was shown to have fewer than 1% of dead pixels, and also demonstrated energy resolutions of 6.8 % at 140keV (Tc-99m), 6.2 % at 171 keV, and 6.0 % at 245 keV (In-111). Using a microsphere phantom at 3 cm from a 0.5 mm pinhole, a sensitivity of 20 cps/MBq (Tc-99m) was achieved. This new detector will be integrated into our recently developed microSPECT/microCT small animal scanner to increase the overall system sensitivity and image resolution.

Evaluation of a MR-compatible CZT detector

Multimodality imaging is increasingly relevant in many clinical and preclinical fields. PET/CT and SPECT/CT studies, for instance, generally provide more information than the same imaging modalities taken separately. This synergistic effect, obtained by combining anatomical and functional information, also would be desirable for magnetic resonance imaging (MRI), a nonionizing modality that provides excellent spatial resolution and soft-tissue contrast. The development of a PET/MRI or SPECT/MRI system however represents a significant technological challenge, in part due to the effect of the strong MRI magnetic field on the PET/SPECT detection subsystem. This environment complicates the use of photomultiplier tubes (PMTs) and rather favors solid-state detection technologies. As a proof of principle, we have studied the behavior of a MR-compatible CdZnTe (CZT) detector, a room-temperature semiconductor already used in some SPECT systems. The detector, made from a single 5x5x2 mm3 CZT crystal, was coupled to a Cremat CR-110 preamplifier with nonmagnetic connectors. The whole detector/preamplifier assembly was tested within a static magnetic field while the radiation detector electronics remained outside the magnet room. We measured the energy resolution of the detector from 57Co spectra both inside and outside the magnet. The results suggest that the static magnetic field does not affect the energy resolution of the CZT detector, making it a good candidate for the development of a MRI device combining a nuclear medicine imaging subsystem.

A small animal helical SPECT scanner

Small animal imaging relies on cone-beam reconstruction geometries to achieve high spatial resolution for both microSPECT and microCT. However, normal circular-orbit cone-beam geometries offer a restricted field of view which for microSPECT has nonuniform sensitivity and spatial resolution. These limitations can be addressed by using a more complex detector orbit such a helix. In this paper, we present a small animal helical SPECT system which uses CZT detectors to acquire radionuclide data with excellent energy resolution and spatial resolution. It incorporates a gantry with integrated slip-ring for continuous detector rotation and an automated animal bed for translation. Both simulation studies and experimental data show this scanner improves the axial spatial resolution and lengthens the field of view in the axial direction

Dual modality micro-SPECT and micro-CT for small animal imaging: technical advances and challenges

Small animal dual modality microSPECT-micro CT has seen many technological advances during recent years. The design of small animal dual modality scanners is a multidisciplinary field, where several interrelated technological problems must be integrated in a complex instrument. This article describes the general concepts that must be taken into consideration during the design process of dual modality microSPECT- microCT scanners. A description of the contemporary scanner technology is presented using the recently designed dual modality micro SPECT -microCT at the Physics Research Laboratory at UCSF. The technology is described with a simple approach to introduce the reader to the complex process of the dual modality scanner design. This article includes a discussion of current technological challenges that have potential to improve or expand the microSPECT-microCT performance and its applications.

Feasibility study of multipinhole collimators for high resolution small animal imaging

Multipinhole SPECT has the potential to overcome the inherent low efficiency of single pinhole gamma cameras typically used in small animal radionuclide imaging. By increasing the detector efficiency, multipinhole SPECT cameras allows to use reduced radionuclide dose and shorter scanning times than single pinhole gamma cameras. Reduced scanning times are advantageous for longitudinal studies where small animal models must be imaged several times. In addition, a spatial resolution of 1 mm or better is often required in drug delivery research and myocardial perfusion studies. In this article, we evaluate multipinhole collimator designs to obtain the best pinhole configuration that will increase the detector efficiency and maximize the use of a recently developed CZT with 128 times128 pixels. Additionally the multipinhole design minimize the overlap of the projection data to avoid artifacts in the tomographic reconstructions. For this purpose, we simulated the acquisition of projection data from point sources, microspheres phantoms, and resolution phantoms, by using a in house developed Monte Carlo program which accounts for the gamma photons interactions with the pinhole and detector crystal. The generated projection data was reconstructed using a standard ML-EM algorithm adapted for multipinhole radionuclide imaging. We determined the multipinhole collimator efficiency, resolution, and signal to noise ratio for several possible multipinhole configurations. A design of five pinhole arrays allowed high efficiency imaging (3.93 fold) while achieving a spatial resolution of 1 mm or better. Reconstruction of hot rod phantom data showed that a contrast of 90% between the hot rods and cold background was obtained after 45 iterations of the ML-EM algorithm.

High resolution clinical and preclinical SPECT-CT

Dual modality preclinical and clinical SPECT/CT scanners have been under constant technological advance since the development of the first prototype dual modality clinical scanner in 1990. Dual modality SPECT/CT has rapidly become an established imaging tool in clinical and preclinical settings. In this article, we describe the present technology of dual modality SPECT/CT scanners used for clinical and preclinical imaging studies