Gerd Muehllehner

Benefit of Time-of-Flight in PET: Experimental and Clinical Results

Significant improvements have made it possible to add the technology of time-of-flight (TOF) to improve PET, particularly for oncology applications. The goals of this work were to investigate the benefits of TOF in experimental phantoms and to determine how these benefits translate into improved performance for patient imaging. Methods: In this study we used a fully 3-dimensional scanner with the scintillator lutetium-yttrium oxyorthosilicate and a system timing resolution of ∼600 ps. The data are acquired in list-mode and reconstructed with a maximum-likelihood expectation maximization algorithm; the system model includes the TOF kernel and corrections for attenuation, detector normalization, randoms, and scatter. The scatter correction is an extension of the model-based single-scatter simulation to include the time domain. Phantom measurements to study the benefit of TOF include 27-cm- and 35-cm-diameter distributions with spheres ranging in size from 10 to 37 mm. To assess the benefit of TOF PET for clinical imaging, patient studies are quantitatively analyzed. Results: The lesion phantom studies demonstrate the improved contrast of the smallest spheres with TOF compared with non-TOF and also confirm the faster convergence of contrast with TOF. These gains are evident from visual inspection of the images as well as a quantitative evaluation of contrast recovery of the spheres and noise in the background. The gains with TOF are higher for larger objects. These results correlate with patient studies in which lesions are seen more clearly and with higher uptake at comparable noise for TOF than with non-TOF. Conclusion: TOF leads to a better contrast-versus-noise trade-off than non-TOF but one that is difficult to quantify in terms of a simple sensitivity gain improvement: A single gain factor for TOF improvement does not include the increased rate of convergence with TOF nor does it consider that TOF may converge to a different contrast than non-TOF. The experimental phantom results agree with those of prior simulations and help explain the improved image quality with TOF for patient oncology studies.

An Iterative Image Space Reconstruction Algorthm Suitable for Volume ECT

The trend in the design of scanners for positron emission computed tomography has traditionally been to improve the transverse spatial resolution to several millimeters while maintaining relatively coarse axial resolution (1-2 cm). Several scanners are being built with fine sampling in the axial as well as transverse directions, leading to the possibility of the true volume imaging. The number of possible coincidence pairs in these scanners is quite large. The usual methods of image reconstruction cannot handle these data without making approximations. It is computationally most efficient to reduce the size of this large, sparsely populated array by back-projecting the coincidence data prior to reconstruction. While analytic reconstruction techniques exist for back-projected data, an iterative algorithm may be necessary for those cases where the point spread function is spatially variant. A modification of the maximum likelihood algorithm is proposed to reconstruct these back-projected data. The method, the iterative image space reconstruction algorithm (ISRA), is able to reconstruct data from a scanner with a spatially variant point spread function in less time than other proposed algorithms. Results are presented for single-slice data, simulated and actual, from the PENN-PET scanner.

Positron emission tomography

The developments in positron emission tomography (PET) are reviewed with an emphasis on instrumentation for clinical PET imaging. After a brief summary of positron imaging before the advent of computed tomography, various improvements are highlighted including the move from PET scanners with septa to fully 3D scanners, changes in the preferred scintillators, efforts to improve the energy discrimination, and improvements in attenuation correction. Time-of-flight PET imaging is given special attention due to the recent revival of this technique, which promises significant improvement. Besides technical instrumentation efforts, other factors which influenced the acceptance of clinical PET are also discussed.

Accelerated Iterative Reconstruction for Positron Emission Tomography Based on the EM Algorithm for Maximum Likelihood Estimation

The EM method that was originally developed for maximum likelihood estimation in the context of mathematical statistics may be applied to a stochastic model of positron emission tomography (PET). The result is an iterative algorithm for image reconstruction that is finding increasing use in PET, due to its attractive theoretical and practical properties. Its major disadvantage is the large amount of computation that is often required, due to the algorithms slow rate of convergence. This paper presents an accelerated form of the EM algorithm for PET in which the changes to the image, as calculated by the standard algorithm, are multiplied at each iteration by an overrelaxation parameter. The accelerated algorithm retains two of the important practical properties of the standard algorithm, namely the selfnormalization and nonnegativity of the reconstructed images. Experimental results are presented using measured data obtained from a hexagonal detector system for PET. The likelihood function and the norm of the data residual were monitored during the iterative process. According to both of these measures, the images reconstructed at iterations 7 and 11 of the accelerated algorithm are similar to those at iterations 15 and 30 of the standard algorithm, for two different sets of data. Important theoretical properties remain to be investigated, namely the convergence of the accelerated algorithm and its performance as a maximum likelihood estimator.

Singles transmission in volume-imaging PET with a 137Cs source

The feasibility of a new method of attenuation correction in PET has been investigated, using a single-photon emitter for the transmission scan. The transmission scan is predicted to be more than a factor of ten faster with the singles method than the standard coincidence method, for comparable statistics. Thus, a transmission scan be completed in 1-2 min, rather than 10-20 min, as is common practice with the coincidence method. In addition, a potential advantage of using the single-photon source 137Cs, which has an energy of 662 keV, is that postinjection transmission studies can be performed using energy discrimination to separate the transmission from the emission data at 511 keV. In order to compensate for the energy difference of the attenuation coefficients at 662 keV compared to 511 keV, the transmission images are segmented into two compartments, tissue and lung, and known values (for 511 keV) of attenuation are inserted into these compartments. This technique also compensates for the higher amount of scatter present with the singles method, since it is not possible to use a position gate (based on collinearity of the source and two detector positions) as is commonly done with a positron-emitting source. We have demonstrated, with experimental phantom studies, that the singles transmission method combined with segmentation gives results equivalent both qualitatively and quantitatively to the coincidence method, but requires significantly less time.

Design of a lanthanum bromide detector for time-of-flight PET

Recent improvements in the growth and packaging of lanthanum bromide (LaBr/sub 3/), in addition to its superb intrinsic properties of high light output, excellent energy resolution, and fast decay time, make it a viable detection material for a positron emission tomography (PET) scanner based on time-of-flight (TOF). We have utilized theoretical simulations and experimental measurements to investigate the design and performance of pixelated LaBr/sub 3/ Anger-logic detectors suitable for use in a TOF PET scanner. Our results indicate that excellent energy resolution can be obtained from individual as well as multicrystal arrays of LaBr/sub 3/ in a 4 mm/spl times/4 mm /spl times/ 30 mm geometry. Measured energy resolutions (at 511 keV) of 4.1% for a single crystal and an average of 5.1% for an array of 100 crystals have been achieved with our best samples. Both simulations and experimental measurements of an Anger-logic based detector consisting of the LaBr/sub 3/ crystal array coupled to a continuous light guide and seven photomultiplier tubes (PMTs), have resulted in the ability to clearly discriminate 511 keV interactions in each crystal. We have measured coincidence time resolutions for both 0.5% and 5.0% cerium-doped LaBr/sub 3/ and found that the higher level of Ce-doping yielded superior results with little to no degradation in light output or energy resolution. The time resolution for a single 5.0% Ce-doped LaBr/sub 3/ crystal (4 mm /spl times/ 4 mm /spl times/ 30 mm) coupled directly to a PMT was measured to be 275 ps full-width at half-maximum (FWHM). With an array of 100 crystals coupled to a light guide and seven PMT cluster an average time resolution of 290 ps FWHM was obtained by summing the signals from the PMT cluster. Ultimately, two 5.0% Ce-doped LaBr/sub 3/ Anger-logic detectors placed in coincidence yielded a time resolution of 313 ps FWHM.

Design evaluation of A-PET: A high sensitivity animal PET camera

In recent years it has been shown that PET is capable of obtaining in vivo metabolic images of small animals. These serve as models to study the development and progress of diseases within humans. Imaging small animals requires not only image resolution better than 2 mm, but also high sensitivity in order to image ligands with low specific activity or radiochemical yields. Toward achieving these goals, we have developed a discrete 2 /spl times/ 2 /spl times/ 10 mm/sup 3/ GSO Anger-logic detector for use in a high resolution, high sensitivity, and high count-rate animal PET scanner. This detector uses relatively large 19 mm diameter photomultiplier tubes (PMT), but nevertheless achieves good spatial and energy resolution. The scanner (A-PET) has a port diameter of 21 cm, transverse field-of-view of 12.8 cm, axial length of 11.6 cm, and operates in 3-D volume imaging mode. The absolute coincidence sensitivity is 1.3% for a point source. Due to the use of large PMTs in an Anger design, the encoding ratio (number of crystals/PMT) is high, which reduces the complexity and leads to a cost-effective scanner. Simulation results show that this scanner can achieve high NEC rates for small cylindrical phantoms due to its high sensitivity and low dead-time. Initial measurements show that our design goals for spatial resolution and sensitivity were realized in the prototype scanner.

Three-dimensional image reconstruction for PET by multi-slice rebinning and axial image filtering

A fast method is described for reconstructing volume images from three-dimensional (3D) coincidence data in positron emission tomography (PET). The reconstruction method makes use of all coincidence data acquired by high-sensitivity PET systems that do not have inter-slice absorbers (septa) to restrict the axial acceptance angle. The reconstruction method requires only a small amount of storage and computation, making it well suited for dynamic and whole-body studies. The method consists of three steps: (i) rebinning of coincidence data into a stack of 2D sinograms; (ii) slice-by-slice reconstruction of the sinogram associated with each slice to produce a preliminary 3D image having strong blurring in the axial (z) direction, but with different blurring at different z positions; and (iii) spatially variant filtering of the 3D image in the axial direction (i.e. 1D filtering in z for each x-y column) to produce the final image. The first step involves a new form of the rebinning operation in which multiple sinograms are incremented for each oblique coincidence line (multi-slice rebinning). The axial filtering step is formulated and implemented using the singular value decomposition (SVD). The method has been applied successfully to simulated data and to measured data for different kinds of phantom (multiple point sources, multiple discs, a cylinder with cold spheres, and a 3D brain phantom).

Constrained Fourier space method for compensation of missing data in emission computed tomography

A method is introduced to compensate for missing projection data that can result from gas between detectors or from malfunctioning detectors. This method uses constraints in the Fourier domain to estimate the missing data, thus completing the data set so that the filtered backprojection algorithm can be used to reconstruct artifact-free images. The image reconstructed from estimates using this technique and a data set with gaps is nearly indistinguishable from an image reconstructed from a complete data set without gaps, using a simulated brain phantom.

Imaging performance of a-PET: a small animal PET camera

The evolution of positron emission tomography (PET) imaging for small animals has led to the development of dedicated PET scanner designs with high resolution and sensitivity. The animal PET scanner achieves these goals for imaging small animals such as mice and rats. The scanner uses a pixelated Anger-logic detector for discriminating 2 /spl times/ 2 /spl times/ 10 mm/sup 3/ crystals with 19-mm-diameter photomultiplier tubes. With a 19.7-cm ring diameter, the scanner has an axial length of 11.9 cm and operates exclusively in three-dimensional imaging mode, leading to very high sensitivity. Measurements show that the scanner design achieves a spatial resolution of 1.9 mm at the center of the field-of-view. Initially designed with gadolinium orthosilicate but changed to lutetium-yttrium orthosilicate, the scanner now achieves a sensitivity of 3.6% for a point source at the center of the field-of-view with an energy window of 250-665 keV. Iterative image reconstruction, together with accurate data corrections for scatter, random, and attenuation, are incorporated to achieve high-quality images and quantitative data. These results are demonstrated through our contrast recovery measurements as well as sample animal studies.