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ABSTRACT: Early clinical results with time-of-flight (TOF) positron emission tomography (PET) systems have demonstrated the advantages of TOF information in PET reconstruction. Reconstruction approaches in TOF-PET systems include list-mode and binned iterative algorithms as well as confidence-weighted analytic methods. List-mode iterative TOF reconstruction retains the resolutions of the data in the spatial and temporal domains without any binning approximations but is computationally intensive. We have developed an approach [DIRECT (direct image reconstruction for TOF)] to speed up TOF-PET reconstruction that takes advantage of the reduced angular sampling requirement of TOF data by grouping list-mode data into a small number of azimuthal views and co-polar tilts and depositing the grouped events into histo-images, arrays with the sampling and geometry of the final image. All physical effects are included in the system model and deposited in the same histo-image structure. Using histo-images allows efficient computation during reconstruction without ray-tracing or interpolation operations. The DIRECT approach was compared with 3-D list-mode TOF ordered subsets expectation maximization (OSEM) reconstruction for phantom and patient data taken on the University of Pennsylvania research LaBr (3) TOF-PET scanner. The total processing and reconstruction time for these studies with DIRECT without attention to code optimization is approximately 25%-30% that of list-mode TOF-OSEM to achieve comparable image quality. Furthermore, the reconstruction time for DIRECT is independent of the number of events and/or sizes of the spatial and TOF kernels, while the time for list-mode TOF-OSEM increases with more events or larger kernels. The DIRECT approach is able to reproduce the image quality of list-mode iterative TOF reconstruction both qualitatively and quantitatively in measured data with a reduced time.
IEEE transactions on medical imaging. 03/2012; 31(7):1461-71.
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ABSTRACT: Introduction — During proton therapy, positron emitting radio‐isotopes are produced along the track where incident proton beam interacts with human tissue. The isotope activity profile is related to the proton dose distribution and can be imaged using a PET scanner. Many factors affect the obtained PET images, such as various patient geometry, time interval between proton therapy and PET scan, the PET scan duration, and PET scanner design. In order to quantitatively relate the obtained PET image to the dose delivered in patient, an understanding of these factors in the PET image is necessary. This work is to develop a simulation pathway to facilitate such study. Method — The simulation starts with proton beam irradiating the patient geometry, obtains isotopes distribution, records in list‐mode gammas from positron annihilations arriving at PET ring geometry, and finally reconstructs the PET images. The patient DICOM CT are input to GEANT4 code to construct the geometry. Experimental isotope cross‐section data are built into the simulation. The positron emitter 3D map is used as the source in the PET scanner simulation. The PET scanner geometry, crystal detector type and its cross‐section are the input parameters. The received signals in PET crystals are output in list‐mode. Image reconstruction is performed with iterative TOF image reconstruction together with data corrections for scatter and attenuation. Results — The 3‐D proton dose and isotope distributions are obtained. The reconstructed isotope distribution from PET simulation correlates with the original isotope map generated from proton beam irradiation. Some of the factors affecting the obtained PET images will be addressed. In the future, the simulation can be used to quantitatively correlate to the proton dose. It can also facilitate the design of a PET system that can yield an accurate and efficient reconstruction of the isotope activities and verify the proton dose.
Medical Physics 05/2010; 37(6):3179-3179. · 2.83 Impact Factor
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ABSTRACT: A prototype time-of-flight (TOF) PET scanner based on cerium-doped lanthanum bromide [LaBr(3) (5% Ce)] has been developed. LaBr(3) has a high light output, excellent energy resolution and fast timing properties that have been predicted to lead to good image quality. Intrinsic performance measurements of spatial resolution, sensitivity and scatter fraction demonstrate good conventional PET performance; the results agree with previous simulation studies. Phantom measurements show the excellent image quality achievable with the prototype system. Phantom measurements and corresponding simulations show a faster and more uniform convergence rate, as well as more uniform quantification, for TOF reconstruction of the data, which have 375 ps intrinsic timing resolution, compared to non-TOF images. Measurements and simulations of a hot and cold sphere phantom show that the 7% energy resolution helps to mitigate residual errors in the scatter estimate because a high energy threshold (>480 keV) can be used to restrict the amount of scatter accepted without a loss of true events. Preliminary results with incorporation of a model of detector blurring in the iterative reconstruction algorithm not only show improved contrast recovery but also point out the importance of an accurate resolution model of the tails of LaBr(3)'s point spread function. The LaBr(3) TOF-PET scanner demonstrated the impact of superior timing and energy resolutions on image quality.
Physics in Medicine and Biology 11/2009; 55(1):45-64. · 2.83 Impact Factor
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ABSTRACT: For modern time-of-flight (TOF) positron emission tomography (PET) systems, in which the number of possible lines of response and TOF bins is much larger than the number of acquired events, the most appropriate reconstruction approaches are considered to be list-mode methods. However, their shortcomings are relatively high computational costs for reconstruction and for sensitivity matrix calculation. Efficient treatment of TOF data within the proposed DIRECT approach is obtained by 1) angular (azimuthal and co-polar) grouping of TOF events to a set of views as given by the angular sampling requirements for the TOF resolution, and 2) deposition (weighted-histogramming) of these grouped events, and correction data, into a set of "histo-images," one histo-image per view. The histo-images have the same geometry (voxel grid, size and orientation) as the reconstructed image. The concept is similar to the approach involving binning of the TOF data into angularly subsampled histo-projections-projections expanded in the TOF directions. However, unlike binning into histo-projections, the deposition of TOF events directly into the image voxels eliminates the need for tracing and/or interpolation operations during the reconstruction. Together with the performance of reconstruction operations directly in image space, this leads to a very efficient implementation of TOF reconstruction algorithms. Furthermore, the resolution properties are not compromised either, since events are placed into the image elements of the desired size from the beginning. Concepts and efficiency of the proposed data partitioning scheme are demonstrated in this work by using the DIRECT approach in conjunction with the row-action maximum-likelihood (RAMLA) algorithm.
IEEE transactions on medical imaging. 02/2009; 28(5):739-51.
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ABSTRACT: The next generation human PET scanners will possess the capability to measure the time-of-flight (TOF) information due to the availability of faster scintillators like LaBr<sub>3</sub> and LSO. These systems will be able to measure timing resolutions well below 1 ns. The shorter point spread function (compared to conventional PET) allows reconstruction from fewer angles. We show that with improved TOF-resolution, the number of angles needed to obtain artifact free reconstruction decreases with improving TOF resolution. In a ring scanner this property can be exploited by mashing the data from adjacent angles using the TOF information. Based on the angular sampling criterion for PET we propose an angular sampling criterion for TOF-PET. Simulated data of 2D TOF-PET systems were reconstructed from a varying number of angles with iterative reconstruction. The comparison with the listmode reconstruction confirmed the predicted relationship. Simulated 3D TOF-PET data were rebinned into a 2D data. The relationship between the number of angles and the TOF resolution also determines the maximum axial acceptance angle where rebinning can be done without contrast loss. The angular sampling criterion for TOF-PET can will be useful to determine the number of mashing angles and axial tilts for a certain timing resolution
Nuclear Science Symposium Conference Record, 2005 IEEE; 11/2005
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ABSTRACT: [99mTc]TRODAT-1 was the first 99mTc-labeled imaging agent to show specific binding to dopamine transporters (DAT) in the striatum (STR) of human brain. Additionally, in vitro binding and autoradiographic experiments demonstrated that this tracer also binds to serotonin transporters (SERT) in the midbrain/hypothalamus (MB) area. In this study, [99mTc]TRODAT-1 was investigated as a potentially useful ligand to image SERT in the MB of living brain. A total of eight single-photon emission tomography (SPET) scans were performed in two baboons (Papio anubis) after intravenous (i.v.) injection of 740 MBq (20 mCi) of [99mTc]TRODAT-1 using a triple-head gamma camera equipped with ultra-high-resolution fan-beam collimators (scan time: 0-210 min). In four blocking studies, baboons were pretreated with (+)McN5652 (1 mg/kg, i.v.) or methylphenidate (1 mg/kg, i.v.) to specifically block SERT or DAT, respectively. After co-registration with magnetic resonance images of the same baboon, a region of interest analysis was performed using predefined templates to calculate specific uptake in the midbrain area and the striatum, with the cerebellum as the background region [(MB-CB)/CB, (STR-CB)/CB]. Additionally, two PET scans of the same baboons were performed after i.v. injections of 74-111 MBq (2-3 mCi) of [11C](+)McN5652 to identify the SERT sites. In [99mTc]TRODAT-1/SPET scans, the SERT sites in the MB region were clearly visualized. Semiquantitative analysis revealed a specific uptake in MB ([MB-CB]/CB) of 0.30+/-0.02, which was decreased to 0. 040+/-0.005 after pretreatment with nonradioactive (+)McN5652, a selective SERT ligand. Pretreatment with methylphenidate reduced the specific binding of [99mTc]TRODAT-1 to DAT sites [(STR-CB)/CB] from 2.45+/-0.13 to 0.32+/-0.04 without any effect on its binding to SERT sites [(MB-CB)/CB], which was confirmed by the co-registration of the [11C](+)McN5652/PET scans. This preliminary study suggests that specific binding of [99mTc]TRODAT-1 to SERT sites can be detected by in vivo SPET imaging despite the low target to background ratio. These findings provide impetus for further development of similar compounds with improved binding affinity and selectivity to SERT sites.
European Journal of Nuclear Medicine 05/1999; 26(4):342-7.
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ABSTRACT: Spinal bone density was measured in eight patients with osteopetrosis to assess the natural history of the disease and to monitor the response to therapy. Quantitative computed tomographic scans of the lumbar vertebra were obtained in seven patients, and dual photon absorptiometric scans were obtained when the technique became available. Six children were afflicted with the infantile malignant recessive condition and two with the less severe dominant condition. In all cases, bone densitometry values ranged from four to five times higher than the mean for normal age and gender-matched controls. In four children with recessive osteopetrosis, quantitative computed tomographic and dual photon absorptiometric scans showed an excellent correlation (R = 0.93) between the methods. Quantitative computed tomographic values ranged from 597 to 730 mg/cm3 (mean = 655 mg/cm3) in children with osteopetrorickets and from 901 to 1000 mg/cm3 (mean = 980 mg/cm3) in the same children when the rickets was cured. In two children treated with bone marrow transplantation, bone densitometry values returned to normal within three years. Bone densitometry provides a safe and noninvasive method for observing the natural history and therapeutic response of the osteopetrotic syndromes.
Clinical Orthopaedics and Related Research 10/1993; · 2.53 Impact Factor
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ABSTRACT: To determine if calcium status is associated with blood lead levels and behavior, 64 black urban children aged 18 to 47 months were studied. Twenty-seven controls (blood lead levels, less than 1.45 mumol/L) were compared with 37 cases (blood lead levels, greater than or equal to 1.45 mumol/L) with respect to four calcium measures (calcium intake, serum calcium level, 1,25-dihydroxyvitamin D level, and bone densitometric findings) and three behavioral scores. Levels of 25-hydroxyvitamin D provided a measure of vitamin D sufficiency. As expected, blood lead level was associated with pica scores. However, none of the calcium measures differed between cases and controls. Controlling for four confounders (season, pica score, maternal education, and sex), yielded no significant differences between the two groups in the mean values of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D level. There was no interaction between blood lead level and the four covariates. No relationship could be demonstrated between calcium status and the pica scores.
American journal of diseases of children (1960) 03/1990; 144(2):186-9.
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ABSTRACT: Fifty-nine children aged 18 to 47 months with normal and elevated blood lead levels had bone mineral density (BMD in gm/cm2) and bone mineral content (BMC in gm/cm) measured using the technique of single photon absorptiometry of the radius shaft. No normative data are available for black children of this age group. Moderate elevations of blood lead were not found to influence bone mineralization. The BMC of the study children was significantly higher than the published values for white children of the same age. We found no statistical difference between the bone mineral density of males and females in this age group.
Pediatric Radiology 02/1990; 20(6):461-3. · 1.67 Impact Factor
