James Hugg

CZT gamma camera with pinhole collimator: Spectral measurements

To date, CZT has been investigated for a variety of nuclear imaging applications using mostly parallel hole collimators. The image quality realized has evolved to make these configurations competitive with the mature conventional configurations using scintillators such as NaI. However the full potential of CZT detectors is still evolving. In our investigations we have found that there are significant additional advantages of the combination of monolithic CZT detectors with pinhole collimators over the known advantages of CZT detectors by themselves. In particular we show here the improved energy response spectrum for a selection of common nuclear medicine isotopes. The improved energy resolution is important to quality control, image fidelity and stability, and new applications such as dual isotope imaging.

Effect of detector energy response on image quality of myocardial perfusion SPECT

The goal of this study was to evaluate the effect of the detector energy response on the quality of 99mTc myocardial perfusion SPECT images. A Data Spectrum torso phantom was prepared to model the low-dose rest portion of a standard one-day myocardial perfusion protocol. Projection data were acquired with a recently developed Ultra-Fast Cardiac SPECT System (UFC, GE Healthcare). UFC utilizes an array of CZT detector modules and pinhole collimators. A point source in air was used to measure the 99mTc spectrum in CZT. In addition to acquiring emission data, the phantom was scanned with high resolution CT and converted into a 3D model for the SimSET Monte Carlo simulation package, which was then used to generate photon history files. We developed a collimator-detector response module that operates on the SimSET photon history files. This module performs multi-pinhole collimation followed by a stochastic energy blurring operation and generates projection data. The simulated CZT detector response was derived from a measured spectrum, and ideal energy response served as reference. Simulation results were compared to actual torso phantom acquisitions. Components of the resulting projection data (amount of primary and scattered photons) and reconstructed slices were compared. For fixed energy acceptance windows, the asymmetric CZT energy response shape leads to a 30% reduction of the scatter component in measured data and contributes to superior reconstructed image quality.

High performance SPECT system for simultaneous SPECT-MR imaging of small animals

Our goal is to develop a high performance SPECT system for simultaneous SPECT-MR imaging of small animals (SA). The SPECT system has inner diameter (ID) of 15.4 cm and outer diameter of 19.8 cm. It comprises five seamless cylindrical detectors, each with 19 CZT modules (2.54×2.54 cm2, 16×16 pixels). The SPECT system can be operated either stand-alone or as an insert into an MRI system with a minimum 20 cm bore. Cylindrical multipinhole (MPH) collimator sleeves (CSs), made with tungsten powder and solid tungsten pinhole apertures, were designed to provide maximum geometric efficiency under the systems geometric constraints. Different MPH collimators were designed for mouse or rat imaging, and for static high-resolution or dynamic imaging without CS rotation. Sparse-view image reconstruction methods reduce CS rotation. Monte Carlo simulations confirm the SPECT imaging characteristics of 2 MPH CSs that have 18 and 36 pinholes with 1 mm and 1.5 mm system resolution, respectively. Sparse-view 3D MPH image reconstruction with system response modeling indicates that 36 pinholes are sufficient to provide artifact-free images at 1.5 mm resolution without CS rotation. The SPECT system with the 2 MPH CSs, the RF coil, and all mechanical and electronics components have been constructed. Initial experimental phantom and small animal studies demonstrated the high performance and imaging characteristics of the SPECT system. In conclusion, a high performance small animal (SA) SPECT system has been designed and constructed for simultaneous SA SPECT-MRI. Initial subsystem testing has demonstrated excellent SPECT and MRI imaging performance that matches design predictions.

Simultaneous in vivo dynamic contrast-enhanced magnetic resonance and scintigraphic imaging

In this study, we investigated the in vivo application of an integrated small-animal magnetic resonance (MR) and gamma-ray imaging system that consists of a semiconductor-based radiation detector, a parallel-hole collimator, and a specialized radiofrequency coil. Gadodiamide and (99m)Tc sestimibi agents were injected simultaneously into a mouse, and simultaneous dynamic contrast-enhanced MR and scintigraphic images of the kidneys were acquired. The time curves of both the MR signal intensity and radioactivity indicate a rapid uptake of the agents followed by a more gradual excretion, consistent with the previously reported literature. Our results demonstrate the feasibility of measuring multiple biological processes at the same time using both MR contrast agents and radiotracers.

Feasibility study of a unilateral RF array coil for MR-scintimammography.

Despite its high sensitivity, the variable specificity of magnetic resonance imaging (MRI) in breast cancer diagnosis can lead to unnecessary biopsies and over-treatment. Scintimammography (SMM) could potentially supplement MRI to improve the diagnostic specificity. The synergistic combination of MRI and SMM (MRSMM) could result in both high sensitivity from MRI and high specificity from SMM. Development of such a dual-modality system requires the integration of a radio frequency (RF) coil and radiation detector in a strong magnetic field without significant mutual interference. In this study, we developed and tested a unilateral breast array coil specialized for MRSMM imaging. The electromagnetic field, specific absorption ratio and RF coil parameters with cadmium-zinc-telluride detectors encapsulated in specialized RF and gamma-ray shielding mounted within the RF coil were investigated through simulation and experimental measurements. Simultaneous MR and SMM images of a breast phantom were also acquired using the integrated MRSMM system. This work, we feel, represents an important step toward the fabrication of a working MRSMM system.

Uniformity correction using non-uniform floods

Calibration and quality control (QC) of gamma cameras with fixed pinhole collimators presents unique challenges since the usual assumption of uniform flood intensity at the surface of the detector is violated. For the GE Discovery NM 530c∗, a fast cardiac SPECT system with multiple pinholes and CZT detectors focused on the heart, the problem is further complicated by the fact that the plane of the pinhole may not be parallel to either the detector or the flood source. To address this, we derived an expression for the geometrical response to an ideal uniform flood source, then added further terms to approximate the attenuation and scatter behavior of a real flood source. The model was validated with Monte Carlo simulations for a range of angles for both flood and pinhole. Accurate knowledge of the flood source orientation is essential for good uniformity correction; we developed a jig that permits repeatable flood positioning for rapid daily QC. Alternatively we have shown that the angle of the flood can be calculated from observed systematic flood non-uniformity. Experimental measurements show that variations in pinhole penetration as a function of angle can be detected in the residual error of the floods; the magnitude of the effect agrees very well with predictions from a simple model of the knife edge collimator. Uniformity effects attributable to the finite stopping power of CZT were also observed. The methods described in this paper have been implemented in the GE Discovery NM 530c and Discovery NM/CT 570c imaging systems, and portions of the technology are patent pending.

Continuing evaluation of an MR compatible SPECT insert for simultaneous SPECT-MR imaging of small animals

The goal of this conference record is to present the experiments conducted to evaluate a magnetic resonance (MR) compatible, simultaneous SPECT-MR insert for small animal (SA) imaging that we have been working on for the last three years. As previously demonstrated, the insert consists of 5 rings of 19 MR compatible CZT detectors connected seamlessly. A multi-pinhole (MPH) collimator with focused pinholes was built using a plastic shell filled with high-density coated tungsten powder and fitted with solid tungsten pinhole inserts. To acquire SPECT and MR data simultaneously, a shielded transmit/receive radio frequency (RF) coil is inserted into the MPH collimator. The insert was evaluated as a standalone SA SPECT system using both a hot-rod resolution phantom (HR-RP) experiment and SA imaging studies. Furthermore, simultaneous SPECT-MR phantom and SA imaging experiments were conducted by placing the SPECT-MR insert inside a 3T clinical MRI system. From acquired experimental data, SPECT images were reconstructed using an in-house developed 3D MPH ML-EM method with pinhole collimator detector response model and compensation. As a standalone SA SPECT system, the insert provided good quality images in phantom and SA studies. The quality of the SPECT images of the HR-RP acquired during the simultaneous SPECT-MR imaging experiment was comparable to the quality of the images acquired during standalone SPECT acquisition. Despite the fact that the SPECT insert degraded the MR image signal-to-noise-ratio (SNR) and caused field distortions, MR imaging was possible within the specific field-of-view of interest. When registered and fused, the SPECT and MR HR-RP images were found to be in acceptable geometric agreement. Data acquired during the simultaneous SPECT-MR SA study was used to obtain a dynamic SPECT image and Time Activity Curve showing initial uptake and later washout of 99mTc MAG3 into and out of a mouses kidneys. Furthermore, a SPECT kidney image was fused with the simultaneously acquired MR image and showed acceptable geometric agreement. Fast dynamic MR imaging was not possible due to severe geometric distortions present when using high speed sequences. These experimental results demonstrate the ability of the insert to obtain SA SPECT dynamic studies and the feasibility of simultaneous SA SPECT-MR imaging. Further engineering improvements remain to be explored to reduce the interaction between the SPECT and MR systems and to eventually enable simultaneous fast dynamic MR studies over the full volume of subject animals.

The design of optimal multipinhole collimators for a seamless SPECT detector ring

The purpose of the study was to systematically design two multipinhole (MPH) collimators for a MR compatible seamless small animal SPECT detector ring (DR) that, for a given system resolution (SR), maximized detector area usage and achieved highest possible geometric efficiency (GE) within the systems design constraints. The SPECT DR consists of 5 rings of 19 2.56×2.56cm2 individual CZT detectors connected seamlessly with a total area of 62,259mm2. A set of multipinhole apertures was arranged on a cylindrical collimator sleeve (CS) and had a common-volume-of-view (CVOV) of 50mm diameter.

Modeling spectral distortions in energy resolved photon-counting x-ray detector

Conventional x-ray detectors integrate the photon energy flux, losing individual photon energy information. By contrast, energy resolved photon-counting x-ray detectors (PCXDs) count photons in energy windows, thus retaining some energy information. This provides a number of advantages, including the use of energy information to aid in material discrimination. However, this capability relies on accurately measuring changes in the energy spectrum as the x-ray beam passes through the object. Several effects, including characteristic x-ray effects and charge sharing between detector pixels, result in distortions in the energy spectrum that complicate measuring the attenuating effects of the object on the energy spectrum. Our goal was to investigate and develop models for these effects that would be useful in compensating for them in applications involving spectral analysis. We used a previously developed 6-threshold CdTe-based PCXD to validate the models. Previously with this detector we observed higher than predicted counts at low energies. Characteristic x-rays emitted in the detector can distort the spectrum in a pixel via x-ray escape either out of the detector or into adjacent pixels, giving rise to a count with a reduced energy. The escaped x-rays can also produce reduced-energy counts in adjacent pixels. The second effect, charge sharing, results since x-ray interactions in the detector produce a charge cloud with finite size. If close to a pixel boundary and combined with charge diffusion, reduced-energy counts in both pixels can be produced. In this study, we developed a fast Monte Carlo method for modeling characteristic x-ray effects and an analytic method for modeling charge sharing effects. The models produced energy spectra in good agreement with those measured by the PCXD. These models can be used to improve the performance of energy-based composition estimation and ring correction methods by modeling the spectral distortions present in real detectors.

Affordable CZT SPECT with dose-time minimization (Conference Presentation)

PURPOSE Pixelated CdZnTe (CZT) detector arrays are used in molecular imaging applications that can enable precision medicine, including small-animal SPECT, cardiac SPECT, molecular breast imaging (MBI), and general purpose SPECT. The interplay of gamma camera, collimator, gantry motion, and image reconstruction determines image quality and dose-time-FOV tradeoffs. Both dose and exam time can be minimized without compromising diagnostic content. METHODS Integration of pixelated CZT detectors with advanced ASICs and readout electronics improves system performance. Because historically CZT was expensive, the first clinical applications were limited to small FOV. Radiation doses were initially high and exam times long. Advances have significantly improved efficiency of CZT-based molecular imaging systems and the cost has steadily declined. We have built a general purpose SPECT system using our 40 cm x 53 cm CZT gamma camera with 2 mm pixel pitch and characterized system performance. RESULTS Compared to NaI scintillator gamma cameras: intrinsic spatial resolution improved from 3.8 mm to 2.0 mm; energy resolution improved from 9.8% to <4 % at 140 keV; maximum count rate is <1.5 times higher; non-detection camera edges are reduced ~3-fold. Scattered photons are greatly reduced in the photopeak energy window; image contrast is improved; and the optimal FOV is increased to the entire camera area. CONCLUSION Continual improvements in CZT detector arrays for molecular imaging, coupled with optimal collimator and image reconstruction, result in minimized dose and exam time. With CZT cost improving, affordable whole-body CZT general purpose SPECT is expected to enable precision medicine applications.