Lars-Eric Adam

Optimization of a fully 3D single scatter simulation algorithm for 3D PET

We describe a new implementation of a single scatter simulation (SSS) algorithm for the prediction and correction of scatter in 3D PET. In this implementation, out of field of view (FoV) scatter and activity, side shields and oblique tilts are explicitly modelled. Comparison of SSS predictions with Monte Carlo simulations and experimental data from uniform, line and cold-bar phantoms showed that the code is accurate for uniform as well as asymmetric objects and can model different energy resolution crystals and low level discriminator (LLD) settings. Absolute quantitation studies show that for most applications, the code provides a better scatter estimate than the tail-fitting scatter correction method currently in use at our institution. Several parameters such as the density of scatter points, the number of scatter distribution sampling points and the axial extent of the FoV were optimized to minimize execution time, with particular emphasis on patient studies. Development and optimization were carried out in the case of GSO-based scanners, which enjoy relatively good energy resolution. SSS estimates for scanners with lower energy resolution may result in different agreement, especially because of a higher fraction of multiple scatter events. The algorithm was applied to a brain phantom as well as to clinical whole-body studies. It proved robust in the case of large patients, where the scatter fraction increases. The execution time, inclusive of interpolation, is typically under 5 min for a whole-body study (axial FoV: 81 cm) of a 100 kg patient.

Investigation of scattered radiation in 3D whole-body positron emission tomography using Monte Carlo simulations

The correction of scattered radiation is one of the most challenging tasks in 3D positron emission tomography (PET) and knowledge about the amount of scatter and its distribution is a prerequisite for performing an accurate correction. One concern in 3D PET in contrast to 2D PET is the scatter contribution from activity outside the field-of-view (FOV) and multiple scatter. Using Monte Carlo simulations, we examined the scatter distribution for various phantoms. The simulations were performed for a whole-body PET system (ECAT EXACT HR+, Siemens/CTI) with an axial FOV of 15.5 cm and a ring diameter of 82.7 cm. With (without) interplane septa, up to one (two) out of three detected events are scattered (for a centred point source in a water-filled cylinder that nearly fills out the patient port), whereby the relative scatter fraction varies significantly with the axial position. Our results show that for an accurate scatter correction, activity as well as scattering media outside the FOV have to be taken into account. Furthermore it could be shown that there is a considerable amount of multiple scatter which has a different spatial distribution from single scatter. This means that multiple scatter cannot be corrected by simply rescaling the single scatter component.

Fast implementation of the single scatter simulation algorithm and its use in iterative image reconstruction of PET data

In positron emission tomography (PET), scatter correction is usually performed prior to image reconstruction using a more or less exact model of the scatter processes. These models require estimates of the true activity and object density distributions of the imaged object. The problem is that these estimates are computed from measured data and, therefore, already contain scattered events. The purpose of this work was to overcome this problem by incorporating scatter characteristics directly into the process of iterative image reconstruction. This could be achieved by an optimized implementation of the single scatter simulation (SSS) algorithm, which results in a significant speed-up of the scatter estimation procedure. The scatter simulation was then included in the forward projection step of maximum likelihood image reconstruction. The results demonstrate that this approach leads to a more exact estimation of the scatter component which cannot be obtained by a simple sequential data processing strategy.

Performance measurements for the GSO-based brain PET camera (G-PET)

Performance measurements on the high sensitivity, high resolution G-PET scanner have been completed. This scanner with a diameter of 42.0 cm and axial field-of-view of 25.6 cm was designed for brain receptor imaging, as well as regular clinical and blood perfusion studies in the brain. The transverse and axial resolution near the center are 4.0 and 5.0 mm (fwtm of 8.0 and 10.0 mm), respectively. At a radial offset of 10 cm these numbers deteriorate by less than 12%. The absolute sensitivity of this scanner measured with a 70 cm long line source is 4.57 cps/kBq. Scatter fraction measured with a line source in a 20 cm diameter by 19 cm long cylinder is 34.5%. For the same cylinder, the peak NEC rate is measured to be 75 kcps at an activity concentration of 12.95 kBq/ml (0.35 /spl mu/Ci/cc), while the peak true coincidence rate is 200 kcps. Image contrast measured with six spheres placed in the cylinder with an activity concentration ratio of 8:1 is 21-24% for the smallest sphere (diameter of 10 mm). However, due to the loss of counts near the sphere edges, the image contrast for this sphere is as high as 36% when using a region half the size of the sphere diameter and a 30 minutes acquisition time. We also show results from the 3D Hoffman brain phantom as well as /sup 18/F-FDG patient scans. These images illustrate the high visual quality of images acquired on the G-PET scanner.

Performance of a GSO brain PET camera

A high sensitivity, high resolution brain PET scanner has been developed. The scanner comprises 58 rows of 320 4/spl times/4/spl times/10 mm/sup 3/ gadolinium orthosilicate (GSO) crystals, coupled to a continuous light guide that is sampled by 288 39-mm photomultiplier tubes in a hexagonal grid. A distortion removal algorithm has been developed to remove position non-linearities and to identify individual crystal regions in a flood image with physical crystal locations on the scanner. A central profile through a sinogram of a point source in the center of the scanner shows that the spatial resolution (fwhm) is 3.5 mm (fwtm=8.4 mm). The Hoffman brain phantom has been successfully imaged to demonstrate the capability of the scanner.

Implementation of a single scatter simulation algorithm for 3D PET: application to emission and transmission scanning

We describe a new implementation of a Single Scatter Simulation (SSS) algorithm for the prediction and correction of scatter in 3D PET. Comparison of SSS predictions to Monte Carlo simulations and experimental data from uniform, line and cold-bar phantoms showed that the code is accurate for uniform as well as asymmetric objects and can model different energy-resolution crystals and low level discriminator (LLD) settings. The method was also extended to the case of transmission scanning and applied to patient studies. Ongoing work aims at incorporating the algorithm in routine clinical image reconstruction, at evaluating its accuracy in absolute quantitation studies, and its robustness in the case of large patients, where the scatter fraction increases.

Monte Carlo simulation of the scatter contribution in a 3D whole-body PET

Scatter contamination is one of the main reasons for image degradation in 3D Positron Emission Tomography (PET). The knowledge about the amount of scatter and its distribution is a prerequisite for performing an accurate scatter correction. One concern is the scatter contribution from activity outside the field-of-view (FOV) and multiple scatter. We examined the scatter distribution for various phantoms using Monte Carlo simulations. The simulations were performed for a whole-body PET system (ECAT EXACT HR/sup +/, Siemens/CTI). The scanner has an axial FOV of 15.5 cm and a ring diameter of 82.7 cm. With (without) interplane septa the scatter contribution is up to 40(65)% (for a line source in a 40 cm cylinder) of the total counts. The scatter fraction varies significantly with the axial position. The results show also that for an accurate scatter correction, both activity and scatter media outside the FOV have to be taken into account. Furthermore it could be shown that there is a considerable amount of multiple scatter which has a different spatial distribution from single scatter. Therefore multiple scatter cannot be corrected by simply rescaling the single scatter component.

A high-resolution GSO-based brain PET camera

A high-resolution GSO-based PET camera is being developed for brain imaging. The system is based upon a detector that uses Anger-logic positioning with 4/spl times/4/spl times/10 mm/sup 3/ crystals coupled to a continuous light-guide and an array of 39-mm diameter photomultiplier tubes. Measurements of a small crystal array have demonstrated that individual crystals can be resolved. The system is 3D (no septa) with a diameter of 42 cm and an axial field-of-view of 25 cm. The detector and overall scanner design has been guided by Monte Carlo simulations. The GSO PET scanner will have improved spatial resolution and higher count-rate capability than the NaI(TI) HEAD Penn-PET scanner that was built previously. GSO was chosen because of its higher stopping power, faster decay, and excellent energy resolution, which is critical for good scatter rejection.

Performance evaluation of the whole-body PET scanner ECAT EXACT HR/sup +/

The performance parameters of the whole-body PET scanner ECAT EXACT HR/sup +/ (CTI/Siemens, Knoxville, TN) were determined following the standard proposed by the International Electrotechnical Commission (IEC). The tests were expanded by some measurements concerning the accuracy of the correction algorithms and the geometric fidelity of the reconstructed images. The scanner consists of 32 rings, each with 576 BGO detectors (4.05/spl times/4.39/spl times/30 mm/sup 3/) covering an axial field-of-view of 15.5 cm and a patient port of 56.2 cm. The transaxial resolution in the 2D (3D) mode is 4.5 (4.3) mm at the center. It increases to 8.9 (8.3) mm radially and to 5.8 (5.2) mm tangentially at a radial distance of r=20 cm. The average axial resolution varies between 4.9 (4.1) mm FWHM at the center and 8.8 (8.1) mm at r=20 cm. The system sensitivity for true events is 5.85 (26.4) cps/Bq/ml (measured with a 20 cm cylinder phantom). The 50% dead-time losses where reached for a true event count rate of 286 (500) kcps at an activity concentration of 74 (25) kBq/ml. The system scatter fraction is 0.24 (0.35). The correction algorithms work reliable, except for the 3D attenuation correction. The ECAT EXACT HR/sup +/ has a good and nearly isotropic spatial resolution. Due to the small detector elements, however, it has a low slice sensitivity which is a limiting factor for image quality.

Methods to optimize whole body surveys with the C-PET camera

An optimized FDG PET survey of the human body should acquire data near peak camera performance. This is achieved by a knowledge of patient activity distributions and tumor uptake as well as camera performance characteristics. The C-PET camera has been used to perform both patient-matched phantom and clinical measurements. Phantom measurements of camera resolution, countrates, noise, signal-to-noise ratio (SNR) and contrast have been performed as a function of activity and scan duration. Measurements of body countrates and normal organ, whole body and tumor activity have been performed on patient data as a function of dose, uptake period and volume in the FOV. Peak NEC is reached at a singles countrate of 5 Mcps. However, image nonuniformity, contrast and SNR are optimal at 3 Mcps. At this countrate camera resolution is not significantly degraded while measured SNR and contrast for a torso-sized phantom are near their optimal values. Noise for head and leg matched phantoms are lower due to the reduced attenuation and scatter for these objects. Emission scan durations of 4 mins for the head and torso and 2 min for the legs result in only moderately reduced SNR compared to 10 min scans. Torso SUVs are constant across patients and uptake periods. Normal lung, liver, heart and kidney uptake remain constant to /spl plusmn/10% between 70 and 120 mins post-injection, while tumor uptakes increase by 30% over this period, showing the value of delayed imaging. In conclusion, an optimized FDG PET survey requires injection of 2.6 MBq/kg of /sup 18/F, a 60 minute uptake period and survey from knees to head (140 cm axially) within 60 minutes. In this way overall activities, countrates and SNRs are optimized. At the same time tumor uptakes are sampled near their peak while background activities are saturated or declining.