Chien-Min Kao

Potentials of Digitally Sampling Scintillation Pulses in Timing Determination in PET

We investigate the potentials of digitally sampling scintillation pulses techniques for positron emission tomography (PET) in this paper, focusing on the determination of the event time. We have built, and continue building, a digital library of PET event waveforms generated with various combinations of photo-detectors and scintillator materials, with various crystal sizes. Events in this digital library are obtained at a high sampling of 20 GSps (Giga-samples per second) so that their waveforms are recorded with high accuracy. To explore the potential advantages of digitally sampling scintillation pulses, we employ a dataset in the above-mentioned library to evaluate two methods for digitizing the event pulses and linear interpolation techniques to analyze the resulting digital samples. Our results show that the two digitization methods that we studied can yield a coincidence timing resolution of about 300 ps FWHM when applied to events generated by a pair of LSO + PMT detector units. This timing resolution is comparable with that is achieved by the same detector pair with a constant fraction discriminator (CFD). As a benchmark, regular-time sampling (RTS) method, usually implemented with very fast traditional analog-to-digital converters (ADCs) for digitizing scintillation pulses, is not feasible for a multi-channel system like a PET system. Digitizing scintillation pulses with multi-voltage threshold (MVT) method could be implemented at a reasonable cost for a PET system. With digitized PET event samples, various digital signal processing (DSP) techniques can be implemented to determine event arrival time. Our results have therefore demonstrated the promising potentials of digitally sampling scintillation pulses techniques in PET imaging.

A new approach for pulse processing in positron emission tomography

In this paper, we propose a new technique for positron emission tomography (PET) pulse processing that has the potential to overcome design limitations in PET arising from the need to use fast, high-cost analog-to-digital converters (ADCs). In this technique, we consider the voltage pulse derived from the charge pulse generated by a scintillator/photomultiplier tube (PMT) detector upon a detection event and model the voltage pulse by a fast, linearly rising edge followed by a slower exponential decay. Based on this model, we show that the basic parameters of the voltage pulse that are relevant for PET event detection can be determined from a trigger time and four time intervals measured on the pulse. We also discuss an electronic implementation in which the required measurements can be obtained by using clocks, comparators, and registers, thereby eliminating the use of ADCs and constant fraction discriminators in PET front-end electronics. We conduct computer simulation studies to evaluate the performance of the proposed technique and our results are promising. When used with Cerium Doped Lutetium Oxyorthosilicate (LSO)/PMT, the proposed technique can generate accurate results for the photon energy, the event time, and the decay-time constant. In addition, it can support an energy resolution of about 30% and a coincidence window of about 10 ns. On the other hand, in its present form the proposed technique is not suitable for use with BGO/PMT due to its low light yield.

A High-Sensitivity Small-Animal PET Scanner: Development and Initial Performance Measurements

An important challenge in positron emission tomography (PET) is the development of dedicated small-animal PET (muPET) systems having high sensitivity. In this paper, we investigate the use of an opposing pair of large-area detectors having high detection efficiency and moderate depth-of-interaction resolution in a compact scanner geometry for achieving a high system sensitivity. Substantial resolution degradations created by this hardware configuration, on the other hand, are corrected for by applying model-based image reconstruction. We have developed a prototype scanner adopting this design and evaluated its performance properties. Our results show that the prototype can achieve a central sensitivity of ~28.2% when using an energy window (EW) of 250-750 keV and a coincidence time window (CW) of 10 ns. When decreasing the EW to 400-750 keV, the central sensitivity still remains high at ~14.4%. Over the typical ranges of radioactivity used in PET rodent imaging with [18F]fluorodeoxyglucose (FDG), the prototype provides noise-equivalent count rates (NECRs) that are considerably higher than other muPET systems. For a mouse/rat-sized cylinder, we obtain a peak NECR of ~ 1.86/1.2 Mcps at ~32/40 MBq when using a 250/350-750 keV EW and a 6 ns CW. Using a resolution phantom, we obtain an average FWHM (full width at the half-maximum) of 1.79 mm for 1.35 mm-diameter rod sources when the axis of the phantom is vertical to the detectors of the prototype. This is consistent with the 1.2 mm FWHM image resolution previously obtained by using simulation data. The prototype also generates FDG rat images having good visual quality.

A family of π-scheme exponential Radon transforms and the uniqueness of their inverses

We propose a family of π-scheme exponential Radon transforms (ERTs) of an object function, which characterize the data functions that arise in π-scheme single-photon emission computed tomography (SPECT) with uniform attenuation. Single-interval 180° acquisition in SPECT can be interpreted as a special case of the proposed π-scheme SPECT. We show the existence and uniqueness of the inverse π-scheme ERT and provide an iterative algorithm for obtaining the object function from the π-scheme ERT. Numerical results in our simulation studies confirm the mathematical results.

Effect of the data constraint on few-view, fan-beam CT image reconstruction by TV minimization

Application of the total variation (TV) minimization algorithm is applied to image reconstruction in few-view fan-beam computed tomography (CT). In particular, the impact of noise is investigated for different levels of tolerance on the data error. The artifacts due to noise are seen to vary dramatically in spatial frequency as the data error tolerance changes.

Non-iterative methods incorporating a priori source distribution and data information for suppression of image noise and artefacts in 3D SPECT

Non-iterative methods have been developed for image reconstruction in 3D SPECT with uniform attenuation and distance-dependent spatial resolution. It was observed that these methods can, in general, be susceptible to data noise and other errors, yielding conspicuous image artefacts. In this work, we developed and evaluated a regularized inverse-filtering approach for effective suppression of noise and artefacts in 3D SPECT images without significantly compromising image resolution. The proposed approach allows the incorporation of a priori random image field and data information and can thus robustly control the degree of suppression of noise and artefacts in 3D SPECT images. Using computer simulations, we evaluated and compared quantitatively images reconstructed from data sets of various noise levels by the use of the proposed methods and the existing non-iterative methods. These numerical results clearly demonstrated that the proposed regularized inverse-filtering approach can effectively suppress image noise and artefacts that plague the existing non-iterative methods, thus yielding quantitatively more accurate 3D SPECT images. The proposed regularized inverse-filtering approach can also be generalized to other imaging modalities.

Potential advantages of digitally sampling scintillation pulses in timing determination in PET

We investigate the use of digital signal processing (DSP) techniques for event processing in positron emission tomography (PET). In this work, we focus on the determination of the event time. We are building a digital library of pulse waveforms generated by using different photo-detectors, scintillator materials, crystal sizes, and read-out electronics of interest in PET. We employ a dataset from our library and investigate two methods for obtaining digital samples of an event pulse and several DSP algorithms for estimating the event time. Our first results, by using non-optimized DSP methods, are encouraging.

Transmission image reconstruction and redundant information in SPECT with asymmetric fanbeam collimation

Two novel approaches for the reconstruction of asymmetric fanbeam transmission computed tomography data are discussed. The first, called the hybrid approach, involves a Fourier-based rebinning of the fanbeam data into parallel-beam data, Reconstruction then proceeds by use of filtered backprojection (FBP). The second approach, called generalized fanbeam filtered backprojection (GFFBP), involves direct fanbeam FBP reconstruction of a modified fanbeam sinogram. In both cases, the data are multiplied by weight functions that seek to appropriately normalize redundant data while exploiting them for noise reduction. The GFFBP approach is found to have resolution-noise tradeoffs superior to those of the hybrid approach for low degrees of smoothing, although for the higher levels of smoothing likely to be of interest in practical situations, the difference between the approaches is negligible. However, GFFBPs distance-dependent fanbeam backprojection factor also produced a high-intensity peripheral artifact that impinged slightly upon the object of interest. Because it ultimately makes use of parallel-beam FBP for reconstruction, the hybrid approach avoids this artifact.

Image restoration and reconstruction with a Bayesian approach

We have extended Johnsons Bayesian method for image restoration and reconstruction by introducing diagonal line sites, using symmetric neighborhood configurations, and employing an additional hyperparameter for estimation of line sites. A general formulation for arbitrary neighborhood configurations was derived. The major part of this paper deals with the conduct of computer simulations intended to examine the effect of the hyperparameters, the diagonal line sites, and the size of the neighborhood configuration on the performance of the proposed Bayesian method. We show that, for optimal performance, distinct hyperparameters should be used for the intensity sites and line sites. The results also suggest that a large neighborhood configuration should be used. By comparing the near-optimal restored images, we demonstrated that the use of diagonal line sites, along with the symmetric configurations thus made possible, can effectively remove the blocky edge artifacts and produce images of better quality. When the method was applied to positron emission tomography (PET) image reconstruction, our results showed that the quality of the reconstructed images was improved for both computer-simulated and real patient PET data.

Implementation of LYSO/PSPMT Block Detector With All Digital DAQ System

We are developing, at the Huazhong University of Science and Technology, a small-animal positron emission tomograph (PET) scanner that has two new features. Firstly, it uses detector modules that are based on a new digital readout electronics previously investigated and reported by us. Secondly, it can arrange the detector modules into various scanner geometries for different imaging needs. In this paper, we will describe the design and implementation of the detector module and report its performance properties. The detector module has a total detection sensitive area of 5.3 × 5.3 cm2, an overall energy resolution of 14.1% FWHM at 511 keV, and a block-level coincidence timing resolution of 1.5 ns FWHM. In addition, our preliminary imaging study shows that our small-animal PET scanner can resolve 1.0 mm-diameter rods separated by 2.0 mm.