Caesar E. Ordonez

Doppler broadening of energy spectra in Compton cameras

Compton cameras utilize the Compton effect to achieve directional localization of gamma rays. The accuracy of the localization depends, in part, on the uncertainties in the measurement of energies. These uncertainties have conventionally been assumed to be due to the effects of finite detector energy resolution alone. There is another source of energy uncertainty that none of the Compton cameras proposed or built thus far accounts for the Doppler broadening of energy spectra that arises from the Compton interaction between gamma rays and moving electrons bound to atoms. The authors have used Monte Carlo simulations and direct calculations to demonstrate that the energy uncertainties due to Doppler broadening are non-negligible. For low-energy gamma rays, these uncertainties are generally more significant than those due to the finite energy resolutions of semiconductor detectors (for example, germanium and silicon). Expressions for estimating the angular uncertainties in Compton cameras due to energy resolution and Doppler broadening are derived. The authors conclude that the accuracy of the calculation of the scatter angle can be improved if the electron pre-collision momentum is known. However, even without such knowledge. Compton cameras still have the potential to achieve spatial resolutions that are comparable to those of mechanically collimated systems.

C-SPECT—A Clinical Cardiac SPECT/Tct Platform: Design Concepts and Performance Potential

Because of scarcity of photons emitted from the heart, clinical cardiac SPECT imaging is mainly limited by photon statistics. The sub-optimal detection efficiency of current SPECT systems not only limits the quality of clinical cardiac SPECT imaging but also makes more advanced potential applications difficult to be realized. We propose a high-performance system platform - C-SPECT, which has its sampling geometry optimized for detection of emitted photons in quality and quantity. The C-SPECT has a stationary C-shaped gantry that surrounds the left-front side of a patients thorax. The stationary C-shaped collimator and detector systems in the gantry provide effective and efficient detection and sampling of photon emission. For cardiac imaging, the C-SPECT platform could achieve 2 to 4 times the system geometric efficiency of conventional SPECT systems at the same sampling resolution. This platform also includes an integrated transmission CT for attenuation correction. The ability of C-SPECT systems to perform sequential high-quality emission and transmission imaging could bring cost-effective high-performance to clinical imaging. In addition, a C-SPECT system could provide high detection efficiency to accommodate fast acquisition rate for gated and dynamic cardiac imaging. This paper describes the design concepts and performance potential of C-SPECT, and illustrates how these concepts can be implemented in a basic system.

Dependence of angular uncertainties on the energy resolution of Compton cameras

One of the factors that affect the accuracy of the directional localization in Compton cameras is the energy resolution of the detectors. In a two-detector camera that consists of scatter and absorption detectors, analytic expressions that relate the angular uncertainty to the energy resolution of either the scatter or absorption detector have been suggested before. Here, the authors compare these models for the uncertainty in the scatter angle that are dependent on both the scatter and absorption detectors. In these two-detector models, they relax the fixed-energy condition for the incident gamma ray. Their analysis indicates that the energy resolution performance of the absorption detector must also be an important consideration in the design of Compton cameras.

A concept of cylindrical Compton camera for SPECT

A novel concept of a Compton camera for SPECT is proposed. The camera consists of two concentric cylindrical detectors that operate in coincidence. The camera is triggered by Compton scatter events in the inner (scatter) detector. The scattered photons are then detected in the outer (absorption) detector. If both detectors have excellent energy and three-dimensional (3D) spatial resolutions, such a camera could make 3D imaging possible with appropriate reconstruction algorithms. In this paper, the authors estimate the potential performance of the cylindrical Compton camera that is made up of argon-gas or silicon-based scatter detector and a xenon-gas absorption detector.

The geometric response function for convergent slit-slat collimators

We have derived an analytic geometric transfer function (GTF) for a convergent slit-slat collimator that treats the parallel slit-slat collimator as a special case. The effective point spread function (EPSF) is then derived from the GTF through the Fourier transform. The results of these derivations give an accurate description of the complete geometric response for a slit-slat collimator that includes the effects of the shape and orientation of the slit and slats. We have also derived exact and approximate sensitivity formulae and spatial resolution formulae using the EPSF.

Comparison of wavelength-shifting fiber types and methods of ribbon assembly for the depth-encoding Anger detector

We are developing a depth-encoding Anger detector for imaging 511 keV annihilation photons. Our design integrates wavelength-shifting (WLS) fibers onto an Anger-type detector to allow measurement of depth-of-interaction (DOI). As few as 0.3% of blue scintillation photons created in NaI(Tl) contribute to detectable signal at the end of the WLS fiber ribbon. Thus, maximizing signal output from the fibers is important for achieving the best possible DOI measurement. We have investigated the effects of two design options on the signal output from the WLS fibers: fiber geometry and methods of assembling fibers into a ribbon. A blue-to-green WLS fiber (Saint Gobain BCF-91A) was chosen because its absorption spectrum matches well the emission spectrum of NaI(Tl). Its emissions, peaked at 500 nm, can be detected with reasonable quantum efficiency by enhanced-green-response PMTs. These fibers are available in several configurations (round or square cross-section, single or multiple cladding layers) and sizes.

A comparison of the performance of high QE photomultiplier tubes to conventional photomultiplier tubes

We have carried out experiments to assess the performance advantage of High Quantum Efficiency (HQE) Photomultiplier Tubes (PMTs) over conventional PMTs for possible use in a cardiac SPECT system. In particular, we have conducted tests of pulse height resolution and position sensitivity of HQE PMTs and compared their performance to conventional PMTs. Furthermore, we have conducted a cathode sensitivity scan and pulse height resolution scan of four HQE PMTs and four conventional PMTs. The HQE PMTs outperformed the conventional PMTs in both pulse height resolution and position sensitivity. Finally, we tested the four HQE PMTs on a 3 mm × 3 mm pixellated NaI(Tl) detector module and were able to resolve the pixels with 8.3% average energy resolution.

Experimental measurement of the energy-dependent point-source response function in Tc-99m SPECT imaging

An experimental measurement of the /sup 99m/Tc point-source response function (PSRF) for a SPECT system is reported in 40 energy windows (80-160 keV). Many researchers have proposed methods for improving SPECT reconstruction by using scattered radiation. Implementation of such reconstruction algorithms requires detailed knowledge of the energy-dependent PSRF, including both the scattering within the patient body and the imaging characteristics of the camera. The authors have measured this PSRF experimentally using a cylindrical phantom filled with water and having a movable point-source immersed inside. The measurements were made using a gamma-camera with a special xyE acquisition interface card that provided both the x-y coordinates and the energy of each event. A spherical capsule filled with /sup 99m/Tc was mounted on a geared armature which moved the source without opening the phantom. Measurements, exceeding 4 million counts at each source position, were made at radial intervals of 1 cm (0-16 cm) and at angular separations of 11.25 degrees. This idealized phantom (cylindrical symmetry and uniform attenuating medium) approximates whole-body imaging in SPECT and provides data for validating Monte Carlo simulations and testing reconstruction algorithms. The authors report fit parameters for an empirical analytic model of the PSRF. The singular value decomposition (SVD) of this SPECT imaging system is computed. From the SVD, energy weighting functions are derived as an alternative to the usual energy windows.

Analytic derivation of pinhole collimator sensitivity for a general source distribution

Pinhole collimators are widely used for SPECT imaging of small organs and animals. There also has been renewed interest in using pinhole collimation for clinical cardiac SPECT imaging, which uses multiple pinhole arrays to achieve high sensitivity and complete data sampling. Overall sensitivity of a pinhole array is critical in determining a systems performance. Conventionally, a point source model has been used to evaluate the overall sensitivity and optimize the system design. This model is simple but far from realistic. This work addresses the use of more realistic source models to assess the sensitivity performance of pinhole systems. We derived an analytical formula for the sensitivity of pinhole collimation with a general source distribution model using spherical harmonics. As special cases of this general model, we also provide the pinhole sensitivity formulas for line segment, disk and sphere sources. These results show that the point source model is just the zero-order approximation of the other source models. The point source overestimates or underestimates the sensitivity when the line segment or disk source lies in a plane that is parallel or perpendicular to the pinhole aperture plane, respectively. The sphere source yields the same sensitivity as a point source at the center of the sphere when attenuation is not considered. The calculated sensitivities based on these formulas show good agreement with separate Monte Carlo simulations in simple cases. The general and special sensitivity formulas derived here are useful for the design and optimization of SPECT systems that utilize pinhole collimators.

Experimental and simulation results with the Depth-encoding Anger Detector

Poor image quality limits clinical utility of dual-head gamma cameras for coincidence imaging. The use of thick crystals imposes one limitation. Conventional camera design results in parallax error due to an inability to measure depth-of-interaction (DOI). We are developing a modified gamma camera that measures DOI. The Depth-encoding Anger Detector incorporates a wavelength-shifting (WLS) optical fiber ribbon onto the face of an otherwise-conventional thick-crystal camera design. The conventional components compute x-y interaction coordinates and total energy signal. The WLS fiber ribbon measures DOI. We previously reported on the theoretical basis of the Depth-encoding Anger Detector and a prototype design. The diameter of a critical cone, defined by the indices of refraction of the NaI(Tl) and the WLS fibers, determines DOI. Scintillations within the cone propagate from the NaI(Tl) into the fibers; photons outside of the cone reflect back into the crystal. The Depth-encoding Anger Detector measures the cones diameter to determine DOI. In this work, we report on simulation and experimental results from the Depth-encoding Anger Detector project. This includes the results of Detect2000 simulations used to examine different detector designs and the results of experimental measurements with the prototype system. While the simulations indicate feasibility of the concept, the simulations and experiments demonstrate that proper design and manufacture of the system are crucial for success.