Herfried Wieczorek

Analytical model for SPECT detector concepts

Pixellated cadmium-zinc telluride (CZT) detectors, providing higher spatial resolution and energy resolution than current gamma cameras, will improve the image quality of SPECT detector systems. Their performance has to be evaluated in terms of resolution, detector efficiency and image quality in a similar way as has been done earlier for NaI detectors. We have developed an analytical model for spatial resolution and geometric efficiency of collimators specifically for pixellated CZT based detectors. We derive an exact description for a variety of static and rotating detector concepts, use standard performance criteria for detection efficiency, and adapt measures for spatial resolution of pixellated detectors, based on the sampling of the single pixel response function (SPRF). The concept is extended to enable a comparative description of continuous scintillator based SPECT cameras. Tradeoffs among resolution, efficiency, signal-to-noise ratio (SNR) and minimum detectable contrast have been investigated for different detector concepts. Our analysis shows that concepts using rotating collimators suffer from noise accumulation, except for purely hot spot imaging. A finite optimization of current SPECT systems can be done based on this analytical model, and a further increase in image quality will be achieved by the higher spatial resolution and energy resolution of solid-state detectors.

The image quality of FBP and MLEM reconstruction

We have developed an image quality theory for filtered back-projection (FBP) and maximum likelihood expectation maximization (MLEM) based on quality measures like signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), signal and noise power spectra and the detective quantum efficiency (DQE) concept. Analytic expressions are derived for unattenuated SPECT reconstruction with ideal collimation to obtain the fundamental performance parameters of different reconstruction algorithms. This theory is verified by measurements of signal and noise power on simulated phantoms. We demonstrate that the noise power of reconstructed images is proportional to the diameter of the object given as a number of voxels. This is analytically proven for FBP and clarified by assessment of the convergence properties for MLEM. The latter technique is shown to be superior to FBP in terms of a noise level at least two times lower. The free choice of MLEM reconstruction parameters and correction for physical effects in the image acquisition enables quantitative evaluation of SPECT and PET images.

Physical aspects of detector design

A new generation of digital radiation detectors is described which is based on solid state technology. The detector front-end consists of two stages, the conversion and the readout part. For the conversion stage there are two different concepts: direct conversion using heavy semiconductor layers, or indirect conversion by combination of scintillators and photodiodes. The readout stage is based on amorphous silicon large-area technology and charge sensitive readout amplifiers. In this paper the properties of different detector materials, system aspects of detector design, and the impact of physical properties on image quality are discussed.

Measurement and simulation of the dynamic performance of a-Si:H image sensors

Abstract A conclusive model has been developed that describes the retardation in the signal rise and decay of a-Si:H image sensors. The transient currents are well described by deep trapping and thermal emission of photogenerated electrons. The model can be applied to the correction of image sequences.

SPECT Image Quality and Quantification

Accurate reconstruction methods for SPECT are a pre-requirement for absolute tracer quantification. Exact activity numbers can be extracted from SPECT images by use of iterative reconstruction with appropriate corrections for attenuation, scatter and detector resolution. We have developed an analytical model for SPECT image generation, including attenuation, scatter and reconstruction by filtered back-projection (FBP). Image quality is described in terms of signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). MatLab simulations were added to include an evaluation of ordered-subset expectation maximization (OSEM) as an example of statistical reconstruction, and for a quantitative assessment of lesion signal recovery. As expected, classical reconstruction without attenuation or scatter correction provides incorrect contrast values, and standard corrections like the Chang method cannot mitigate this problem. Statistical reconstruction allows implementing all corrections and results in better image quality than FBP. Our analysis shows, however, that for adequate signal recovery the contrast-to-noise ratio for OSEM is the same as for FBP. We explain the trade-off between SPECT image quality and quantification by describing reconstruction in terms of signal power spectra (SPS), noise power spectra (NPS) and detective quantum efficiency (DQE).

Transient currents in a-Si:H diodes

The decay of stationary photocurrents on a-Si:H Schottky and nip diodes is investigated. The transient current is attributed to deep trapping and thermal emission of electrons. A good fit of decay curves is obtained from Shockley-Read statistics. Injection and recombination are shown to be minor effects under normal conditions.

Collimator spatial resolution

Pixellated detectors based on cadmium-zinc telluride (CZT) are a promising alternative to NaI-based SPECT cameras. The performance of either system is quantitatively described in terms of resolution and sensitivity. This investigation is based on an analytical model published earlier, describing SPECT collimators according to the NEMA definitions of planar sensitivity and system spatial resolution without scatter. We have extended the model to include the effect of collimator hole shape and orientation as well as collimator and detector penetration. Equations for spatial resolution are derived for NaI and CZT based SPECT systems with hexagonal, round or square parallel-hole collimators and with pinhole collimators. Standard equations for spatial resolution are valid only for square hole collimators along one of the detector axes. In any other direction, spatial resolution is higher. The exact values depend on the hole shape and on the ratio of object-detector distance to collimator length. For pixellated pinhole detectors, similar corrections depending on the ratio of detector pixel size to the diameter of a point source projected on the detector apply. In scintillation cameras, spatial resolution is not only influenced by the average absorption depth in the scintillator, but it is strongly affected by the parallax effect of the collimator hole. Spatial resolution is therefore lower than usually thought. We present a model that describes the performance of current SPECT collimators and permits to optimize future detector systems for high image quality.

Electron traps in Gd 3 Ga 3 Al 2 O 12 :Ce garnets doped with rare-earth ions

The curves of thermally stimulated luminescence of Gd3Ga3Al2O12:Ce3+ ceramics (a nominally pure sample and samples doped with rare-earth ions) are measured in the temperature range of 80–550 K. The depth and the frequency factor of electron traps established by Eu and Yb impurities are determined. An energy-level diagram of rare-earth ions in the bandgap of Gd3Ga3Al2O12 is presented.

Source of rectangular X-ray pulses for studying scintillators

A pulsed X-ray source has been designed and constructed with the following parameters: X-ray tube voltage, 10–50 kV; anode current, up to 2 A; pulse duration (smooth control), 80 ns–2 μs; and leading and trailing front width, 6 ns. The source can operate in both single-pulse and pulse-train mode with a repetition frequency of up to 2 kHz. The X-ray pulse has a rectangular shape with a relative amplitude instability below 0.8%. In combination with appropriate detection system, the proposed X-ray source allows the kinetics of X-ray-induced luminescence growth and decay to be measured in a six-decimal-order dynamic range with respect to time and amplitude. Examples of X-ray-induced luminescence kinetics measured in scintillators are presented.

Analytical model of coincidence resolving time in TOF-PET.

The coincidence resolving time (CRT) of scintillation detectors is the parameter determining noise reduction in time-of-flight PET. We derive an analytical CRT model based on the statistical distribution of photons for two different prototype scintillators. For the first one, characterized by single exponential decay, CRT is proportional to the decay time and inversely proportional to the number of photons, with a square root dependence on the trigger level. For the second scintillator prototype, characterized by exponential rise and decay, CRT is proportional to the square root of the product of rise time and decay time divided by the doubled number of photons, and it is nearly independent of the trigger level. This theory is verified by measurements of scintillation time constants, light yield and CRT on scintillator sticks. Trapping effects are taken into account by defining an effective decay time. We show that in terms of signal-to-noise ratio, CRT is as important as patient dose, imaging time or PET system sensitivity. The noise reduction effect of better timing resolution is verified and visualized by Monte Carlo simulation of a NEMA image quality phantom.