Lana Volokh

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.

Myocardial perfusion imaging with an ultra-fast cardiac SPECT camera - a phantom study

Myocardial perfusion imaging is currently the most frequently used nuclear medicine test. Recently, several alternative approaches to myocardial perfusion SPECT imaging have been evaluated. In this study, we compare imaging capabilities of the new ultra-fast cardiac (UFC) system developed by GE Healthcare to a conventional cardiac SPECT camera (Ventri, GE Healthcare). The UFC system uses an array of Cadmium Zinc Telluride (CZT) pixilated detectors simultaneously imaging all cardiac views with no moving parts during acquisition. High system sensitivity together with high intrinsic resolution of the CZT detectors allow acquiring high quality SPECT studies within short time. Image data were acquired experimentally using a Data Spectrum anthropomorphic torso phantom and simulating realistic male and female anatomy, uptake distribution and perfusion defect variations typical for rest and stress Tc99m imaging. Post-reconstruction, estimated count values on the mid-myocardial surface were assessed to calculate the normalized standard deviation (NSD) for the uniform (healthy) myocardium and the normalized contrast (NC) for known defect volumes. Segmental uptake for a 17-segment heart model was used to calculate agreement between reconstructed distributions obtained with the novel and conventional systems.

Singular Value Decomposition of Pinhole SPECT Systems.

A single photon emission computed tomography (SPECT) imaging system can be modeled by a linear operator H that maps from object space to detector pixels in image space. The singular vectors and singular-value spectra of H provide useful tools for assessing system performance. The number of voxels used to discretize object space and the number of collection angles and pixels used to measure image space make the matrix dimensions H large. As a result, H must be stored sparsely which renders several conventional singular value decomposition (SVD) methods impractical. We used an iterative power methods SVD algorithm (Lanczos) designed to operate on very large sparsely stored matrices to calculate the singular vectors and singular-value spectra for two small animal pinhole SPECT imaging systems: FastSPECT II and M3R. The FastSPECT II system consisted of two rings of eight scintillation cameras each. The resulting dimensions of H were 68921 voxels by 97344 detector pixels. The M3R system is a four camera system that was reconfigured to measure image space using a single scintillation camera. The resulting dimensions of H were 50864 voxels by 6241 detector pixels. In this paper we present results of the SVD of each system and discuss calculation of the measurement and null space for each 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.