Joseph Anthony Heanue

A bound on the energy resolution required for quantitative SPECT

Scattered radiation is one of several physical perturbations that limit the accuracy of quantitative measurements in single-photon emission computed tomography (SPECT). Improvement in detector energy resolution leads to a reduction of scatter counts and a corresponding improvement in the quantitative accuracy of the SPECT measurement. In this study, simulated SPECT projections of a simple myocardial perfusion phantom were used to investigate the effect of detector energy resolution on the data. The phantom consists of a spherical shell of radionuclide within a 15 cm radius water-filled cylinder. Each projection contains on the order of 3 x 10(5) counts. The results demonstrate that a full-width, half-maximum energy resolution of 3-4 keV is sufficient to render the error due to scatter insignificant compared to the uncertainty due to photon statistics in this case. Further simulations verify that because smaller objects produce less scatter, they can be imaged accurately with degraded energy resolution. These results are useful when designing prototype systems that utilize solid-state detectors and low-noise electronics to achieve improved energy resolution.

The use of CdTe or CdZnTe for pulse-counting and current-mode medical imaging applications

Abstract Recent progress in CdTe and CdZnTe detector research has made these detectors appear attractive for medical imaging. As part of a feasibility study, Monte Carlo simulations have been developed to investigate the detection efficiency and scatter rejection capabilities of these materials at diagnostic energies. We have also analyzed the count rate limitations and current mode capabilities of Cd(Zn)Te. This preliminary work indicates that the 3–4 keV FWHM energy resolution and the 10 5 cps/channel of which these detectors are capable should be adequate for most applications. In addition, although further experimental measurements are needed, we expect that the current mode operation of these detectors is acceptable for many systems. The major drawback of Cd(Zn)Te detectors is the low photopeak efficiency at 140 keV. In order to improve this efficiency, we have examined both a novel singular-value-decomposition (SVD) algorithm and a hardware-based technique to correct for spectral distortion arising from charge trapping.

CMOS detector readout electronics for an emission-transmission medical imaging system

We have built a prototype medical imaging system in which transmission X-ray data provide anatomical information while emission radionuclide data yield physiological information. We have developed a CMOS integrated circuit for readout of the systems high-purity germanium detector. The circuit can be operated in three different modes. The first is a slow pulse-counting mode for detection of radionuclide events at 10/sup 4/ cps with 1-2 keV energy resolution. Next, there is a fast pulse-counting mode for simultaneous acquisition of emission and transmission data or for acquisition of dual-energy X-ray data at 10/sup 6/ cps with 5-7 keV energy resolution. Finally, current mode operation allows fast acquisition of X-ray transmission data. The circuit architecture includes a common preamplifier, selectable pulse shaping stages, and a common output buffer. >

The relative importance of energy resolution for quantitative /sup 99m/Tc SPECT imaging

The authors seek to determine the desired energy resolution for quantitative SPECT imaging. As the energy resolution of the system is improved, the relative error due to scatter decreases. Yet, at some point the improvement becomes inconsequential since the scatter error is small compared to the other physical perturbations in the radionuclide measurement. In order to estimate the energy resolution at which this condition becomes true, the authors used a Monte Carlo code to simulate the emission data from a myocardial perfusion phantom. The data were reconstructed using a maximum likelihood code, and the images were analyzed to determine the relative effects of attenuation correction, collimator response compensation, noise, and scatter rejection on image quantitation. The simulations showed that improving the system energy resolution beyond 5 keV offers little benefit for myocardial perfusion studies as judged by the noise in a 9-pixel ROI. The relevance of this result to other applications is also discussed.

Improvements in CdZnTe detection system for combined X-ray CT and SPECT

Combined emission-transmission imaging systems that perform both X-ray CT and SPECT have the potential to correlate anatomical and physical data, and to improve the accuracy of in vivo radiopharmaceutical quantitation. A detection system suitable for the combined CT/SPECT system must achieve both excellent energy resolution at low count rates for radionuclide imaging and must be compatible with the high flux rates and wide dynamic range encountered in X-ray imaging. We therefore are investigating the use of a pixellated CdZnTe for CT/SPECT imaging. Alternative electrode geometries of CdZnTe detectors for SPECT have been investigated with numerical simulation.

CMOS readout electronics for an emission-transmission medical imaging system

We have developed a CMOS chip for readout of a germanium detector in an emission-transmission medical imaging system. The chip can be operated in three different modes. First, there is a slow pulse counting mode for acquisition of radionuclide data at 10/sup 4/ cps/channel and 1-2 keV energy resolution. Next, there is a fast pulse counting mode for collection of dual-energy X-ray data or for simultaneous accumulation of radionuclide emission and X-ray transmission counts at 10/sup 6/ cps and 5-7 keV energy resolution. Finally, there is a current mode readout for fast acquisition of X-ray transmission data. The new readout electronics offer the potential for improved quantitation of SPECT data. Monte Carlo simulations indicate that 2 keV energy resolution is sufficient to reduce the uncertainty due to scatter to below the level of uncertainty due to photon statistics. In addition, the X-ray data can be used to generate an object-specific attenuation map at the radionuclide energy. Thus, the new electronics offer the possibility of virtually scatter-free, attenuation-corrected quantitative SPECT images.<<ETX>>

Energy-dependent quantum detective efficiency (QDE) measurements of a photon-counting CdTe detector array used for the scanning-beam digital x-ray (SBDX) system

The Scanning-Beam Digital X-ray (SBDX) system utilizes a scanning x-ray pencil beam and a small-area detector array for low-dose cardiac angiography with tomographic imaging capabilities. For the system to provide adequate signal-to-noise ratios, the multi-element detector must be highly efficient and capable of high photon count rates. Cadmium telluride (CdTe) is well suited to these purposes. The CdTe SBDX detector is a direct-conversion photon-counting device consisting of 2304 elements. The efficiency of the detector is a function of several factors including the incident photon energy, the fluorescence properties of CdTe, and the discriminator threshold that determines whether sufficient energy was deposited in an element to register a count. For maximum efficiency, the discriminator threshold must be set low enough to detect CdTe k-fluorescence photons (23-31 keV), but not so low as to register false counts from electronic noise. The purpose of this investigation was to evaluate the energy-dependent quantum detective efficiency (QDE) of a new lower-noise SBDX detector design and to determine whether adequately low thresholds can be achieved. Experiments were performed using metal fluorescer foils to generate quasi-monochromatic x-ray beams with energies of 17.5, 25.3, and 46.0 keV. The resulting spectral purities were high, although fluence rates were low. The measured QDE values at 17.5, 25.3, and 46.0 keV were 60%, 76%, and 86% repsectively.

Multi-mode digital detector readout for solid-state medical imaging detectors

A multipurpose digital detector readout for medical imaging applications is presented. The readout is capable of measuring both current and charge, allowing a single detector array to perform imaging functions previously accomplished with two separate machines. The circuit employs a variable rate analog-to-digital converter (ADC) to measure current over a 130dB dynamic range in a 1 kHz band and resolve charge pulses down to 360e at 100 000 events/s. Detector currents of up to 7 A and charge pulses as large as 25 fC can be measured. A low-noise charge sensing amplifier (CSA) is combined with digital pulse shaping to optimize the noise performance and flexibility of the charge measurements. Fabricated in an 1.2 m complimentary metal-oxide-semiconductor (CMOS), the circuit occupies 1.5 mm and dissipates 11 mW/channel from a 5 V supply.