3D PET Research Articles

Accuracy of 3D acquisition mode for myocardial FDG PET studies using a BGO-based scanner

The aim of the present study was to evaluate the quantitative and qualitative accuracy of 3D PET acquisitions for myocardial FDG studies. Phantom studies were performed with both a homogeneous and an inhomogeneous phantom. Activity profiles were generated along the phantoms using 2D and several 3D reconstructions, varying the 3D scaling value to adjust the scatter correction algorithm. Furthermore, ten patients underwent a dynamic myocardial FDG PET scan, using an interleaved protocol consisting of frames with alternating 2D and 3D acquisition. For each myocardial study, 13 volumes of interest were defined, representing 13 myocardial segments. First, the optimal scaling value for the scatter correction algorithm was determined using data from the phantom and four patient studies. This scaling value was then applied to all ten patients. 2D and 3D acquisitions were compared for both static (i.e. activity concentrations in the last 2D and 3D frames) and dynamic imaging (calculation of the metabolic rate of glucose). For both phantom and patient studies, suboptimal results were obtained when the default scaling value for the scatter correction algorithm was used. After adjusting the scaling value, for all ten myocardial FDG studies, a very good correlation (r2=0.99) was obtained between 2D and 3D data. With the present protocol no significant differences were observed in qualitative interpretation. The 3D FDG acquisition mode is accurate and has clear advantages over the 2D mode for myocardial FDG studies. A prerequisite is, however, optimisation of the 3D scatter correction algorithm.

A New Component Approach to Efficiency Normalization for 3D PET

Efficiency normalization corrects the non-uniform response of the 3D PET detector due to the scanner's geometry, non-uniformity of the crystals, and gain variation in the photomultiplier tubes. In reviewing the literature, most component approaches to detector efficiency normalization are applied to the block design ring scanner. In this paper, we propose a new component normalization method for Philips' non-block design ring scanner, where the normalization mainly consists of two components, the crystal efficiency and the detector geometry. The derivation is based on the physics model of the measured data, along with a wheel-sum assumption, so that the crystal efficiency is separated from the model and both terms are estimated independently. The crystal efficiency is calibrated using a uniform cylinder phantom. A minimum least-square estimation method is used to smooth the noisy crystal efficiency. The detector geometry is calibrated using a stationary uniform plane phantom, since it is independent of the projection ray's azimuthal angle. The result is then applied with a radial solid angle correction to compensate for the unusual geometry of the plane source. Finally, this technique is validated using a uniform cylinder phantom reconstruction along with scatter and attenuation correction

High accuracy multiple scatter modelling for 3D whole body PET

A new technique for modelling multiple-order Compton scatter which uses the absolute probabilities relating the image space to the projection space in 3D whole body PET is presented. The details considered in this work give a valuable insight into the scatter problem, particularly for multiple scatter. Such modelling is advantageous for large attenuating media where scatter is a dominant component of the measured data, and where multiple scatter may dominate the total scatter depending on the energy threshold and object size. The model offers distinct features setting it apart from previous research: (1) specification of the scatter distribution for each voxel based on the transmission data, the physics of Compton scattering and the specification of a given PET system; (2) independence from the true activity distribution; (3) in principle no scaling or iterative process is required to find the distribution; (4) explicit multiple scatter modelling; (5) no scatter subtraction or addition to the forward model when included in the system matrix used with statistical image reconstruction methods; (6) adaptability to many different scatter compensation methods from simple and fast to more sophisticated and therefore slower methods; (7) accuracy equivalent to that of a Monte Carlo model. The scatter model has been validated using Monte Carlo simulation (SimSET).

Digimouse: a 3D whole body mouse atlas from CT and cryosection data

We have constructed a three-dimensional (3D) whole body mouse atlas from coregistered x-ray CT and cryosection data of a normal nude male mouse. High quality PET, x-ray CT and cryosection images were acquired post mortem from a single mouse placed in a stereotactic frame with fiducial markers visible in all three modalities. The image data were coregistered to a common coordinate system using the fiducials and resampled to an isotropic 0.1 mm voxel size. Using interactive editing tools we segmented and labelled whole brain, cerebrum, cerebellum, olfactory bulbs, striatum, medulla, masseter muscles, eyes, lachrymal glands, heart, lungs, liver, stomach, spleen, pancreas, adrenal glands, kidneys, testes, bladder, skeleton and skin surface. The final atlas consists of the 3D volume, in which the voxels are labelled to define the anatomical structures listed above, with coregistered PET, x-ray CT and cryosection images. To illustrate use of the atlas we include simulations of 3D bioluminescence and PET image reconstruction. Optical scatter and absorption values are assigned to each organ to simulate realistic photon transport within the animal for bioluminescence imaging. Similarly, 511 keV photon attenuation values are assigned to each structure in the atlas to simulate realistic photon attenuation in PET. The Digimouse atlas and data are available at http://neuroimage.usc.edu/Digimouse.html.

AN EXACT ANALYTIC APPROACH TO 3D PET IMAGE RECONSTRUCTION

We investigate the development of a new analytic approach for 3D PET reconstruction by extension of the chord-based approach for X-ray cone-beam tomography (XCBT). The extension is made possible by recognizing a connection that allows one to reformat 3D PET data into effective cone-beam data so that the chord-based approach can be applied. An important advantage of the new approach is that it can work with general PET system geometries, not limited to the conventional circular geometry. In this paper, we consider a rectangular PET system consisting of four flat-panel detectors. We derive exact analytic reconstruction algorithms for this system and evaluate their performances in numerical studies. Our preliminary results indicate that the derived algorithms can yield accurate reconstructions when applied to noiseless PET data that are generated by assuming the ideal projection model. The algorithms are also observed to respond reasonably well to data noise. Currently, our algorithms do not utilize the entire 3D PET data. Potential modifications to the algorithms for enabling the inclusion of the entire 3D PET data for reconstruction, as well as the theoretical significances and potential practical advantages of this new analytic reconstruction approach, are discussed.

Dopamine D 2 receptor binding in drug-naïve patients with schizophrenia examined with raclopride-C11 and positron emission tomography

The aim was to test the dopamine hypothesis of schizophrenia in a further analysis of D 2-like dopamine binding using the radioligand [ 11C]raclopride and high resolution 3-dimensional (3D) PET. Eighteen drug-naive patients with schizophrenia and seventeen control subjects were examined. The D 2 binding potential (BP) in the putamen, the caudate and the thalamus was calculated using the simplified reference tissue model. The volume of regions of interest was controlled for by MRI. Symptoms were rated with the Positive and Negative Syndrome Scale for Schizophrenia (PANSS). No significant group differences were found for D 2 BP in the putamen or in the caudate and there was no significant hemispheric difference for any region. In the right thalamus the D 2 BP was significantly lower in patients as compared to control subjects, whereas a numerical difference did not reach statistical significance for the left thalamus. There was no significant correlation between D 2 BP and total PANSS score in any region. There was a highly significant age effect in the caudate and in the putamen, but not in the thalamus. In this relatively large PET study of exclusively drug-naive schizophrenic patients, a lower D 2 BP in the right thalamus was found in the patient group. This finding is in agreement with two previous studies in Sweden and in Japan using the high-affinity radioligand [ 11C]FLB 457 and provide further support for a role of dopamine in the thalamus related to the pathophysiology of schizophrenia.

Validation on an anthropomorphic phantom of FORE optimization in 3D PET

A study for validating a proposed optimization of Fourier Rebinning (FORE) is presented. FORE is the most widely used rebinning algorithm for 3D PET data and operates into sinograms Fourier Transform domain. The frequency–distance relation (FDR) on which FORE is based is valid at high frequencies; low frequencies are usually rebinned with single slice rebinning (SSRB). A square low-frequency region is commonly defined on empirical basis and implies an abrupt transition of rebinning strategy. We proposed a simple index able to map the validity of the FDR in the sinogram frequency space and a consequent gradual transition mask for an optimized FORE. In this study we tested the method on an anthropomorphic phantom filled with activities simulating the in vivo 18F-FDG uptake and containing spherical lesions of different radii. Both abrupt and gradual partitions were implemented with different parameters corresponding to a different extension of the low-frequency SSRB region. Results show that, in the considered condition, the performances of the two methods are similar. Theoretically, a larger robustness of the gradual transition is foreseen on more complex structures and in more critical conditions characterized by appreciable discrepancies between Fourier Transform of rebinned sinograms obtained with FORE and SSRB.

Noise Properties of Four Strategies for Incorporation of Scatter and Attenuation Information in PET Reconstruction Using the EM-ML Algorithm

Conventional methods for dealing with attenuation and scatter in PET can limit the reconstructed image quality, particularly if the attenuating medium is large (as in whole body 3D PET). In such cases, often a substantial scatter subtraction is performed followed by amplification of the remaining data (to correct for attenuation) resulting in noisy reconstructions. More recent iterative reconstruction methods include the attenuation in the system model in conjunction with either pre-scatter subtraction or a separate addition of the scatter component after each application of the forward model. This work compares these more conventional approaches of including attenuation and scatter to the case where attenuation and scatter information are both included within the system matrix used by the expectation maximization maximum likelihood (EM-ML) algorithm. For this case all acquired data are used and regarded as a source of information by the reconstruction algorithm. Multiple realisations of simulated data sets have been used to compare the performance of the unified attenuation and scatter model with other methods. For a large attenuating medium and low counts there are notable differences between the four main ways of including attenuation and scatter within the reconstruction-with full pre-correction of the data being inferior compared to all the other methods, and the method which models scatter and attenuation within the system matrix showing some advantages. This work suggests that if regularisation of the EM algorithm is carried out by early termination of the iterative process, the subtraction method is the better approach among the techniques considered. In contrast, if a post-reconstruction smoothing approach to regularisation is to be used (whereby highly iterated, noisy image estimates are smoothed), the full modeling method for attenuation and scatter yields the better results, albeit at the computational cost of many more iterations being required

島津製3D PETの散乱補正・吸収補正の定量評価

島津製3D PETの散乱補正・吸収補正の定量評価

Processing of transmission data from an uncollimated single photon source

The EXACT 3D PET scanner uses a Cs-137 single photon rotating point source for the transmission scan. As the source is un-collimated, the transmission data are contaminated by scatter. It has been suggested that segmentation of the reconstructed image can restore the quantitative information in the image. We study here if the results can be further improved by the application of a scale factor for every transaxial plane.

Optimization of temporal basis functions in dynamic PET imaging

In this paper we introduce and evaluate a completely data-driven method to optimize both the linear expansion coefficients and the used temporal basis functions in dynamic PET reconstruction from list-mode data. We present the first results of our method using simulated 2D PET data. The time activity curves are modeled as a conic combination of B-splines. The B-splines are optimized with respect to their knot locations. We have combined the newly introduced method with our previously developed dynamic MLEM algorithm in order to further improve the likelihood. The results show that even one iteration of our newly developed algorithm can drastically increase the likelihood and the resulting images better represent the underlying TACs.

A motion-incorporated reconstruction method for gated PET studies

Cardiac and respiratory motion artefacts in PET imaging have been traditionally resolved by acquiring the data in gated mode. However, gated PET images are usually characterized by high noise content due to their low photon statistics. In this paper, we present a novel 4D model for the PET imaging system, which can incorporate motion information to generate a motion-free image with all acquired data. A computer simulation and a phantom study were conducted to test the performance of this approach. The computer simulation was based on a digital phantom that was continuously scaled during data acquisition. The phantom study, on the other hand, used two spheres in a tank of water, all of which were filled with 18F water. One of the spheres was stationary while the other moved in a sinusoidal fashion to simulate tumour motion in the thorax. Data were acquired using both 4D CT and gated PET. Motion information was derived from the 4D CT images and then used in the 4D PET model. Both studies showed that this 4D PET model had a good motion-compensating capability. In the phantom study, this approach reduced quantification error of the radioactivity concentration by 95% when compared to a corresponding static acquisition, while signal-to-noise ratio was improved by 210% when compared to a corresponding gated image.

Assessment of the short-lived non-pure positron-emitting nuclide 120I for PET imaging

The non-pure positron-emitting iodine isotope (120)I (T(1/2)=81 min) is a short-lived alternative to (124)I. (120)I has a positron abundance more than twice that of (124)I and a maximum positron energy of 4 MeV. This study was undertaken to evaluate and characterise the qualitative and quantitative PET imaging of (120)I. (120)I was produced via the (120)Te(p,n) reaction on highly enriched (120)Te. The measurements were done with the Siemens scanner HR+ and the 2D PET scanner GE PC4096+. A cylinder containing three cold inserts and a phantom resembling a human brain slice were used to evaluate half-life, positron abundance and background correction. To analyse the image resolution, a -mm tube placed in water was filled with (120)I and (18)F. Comparisons with (18)F, (124)I and (123)I (measured with SPECT) were made using the Hoffman 3D brain phantom. The half-life of 81.1 min was reproduced by the PET measurements. The PET-based positron abundance ranged from 47.9% to 55.0%. The reconstructed image resolution found with the HR+ was 5.4 mm FWHM (12.3 mm FWTM), in contrast to 4.6 mm (8.6 mm) when using (18)F. Erroneous positive and negative numbers of radioactivity found in the cold inserts became nearly zero when the background of gamma-coincidences was corrected for. Images of the Hoffman phantom were inferior to those obtained when (18)F or (124)I was applied but superior to the (123)I-SPECT images. Our data show that (120)I of high radionuclidic purity can be regarded as a suitable nuclide for the PET imaging of radioiodine-labelled pharmaceuticals.

P2-374: MR-guided 3D PET mapping of longitudinal changes in regional cerebral metabolism of normal subjects

P2-374: MR-guided 3D PET mapping of longitudinal changes in regional cerebral metabolism of normal subjects

IC-P-085: MR-guided 3D PET mapping of longitudinal changes in regional cerebral metabolism of normal subjects

IC-P-085: MR-guided 3D PET mapping of longitudinal changes in regional cerebral metabolism of normal subjects

Optimization of the effective light attenuation length of YAP:Ce and LYSO:Ce crystals for a novel geometrical PET concept

The effective light attenuation length in thin bars of polished YAP:Ce and LYSO:Ce scintillators with lengths of the order of 10 cm has been studied for various wrappings and coatings of the crystal lateral surfaces. This physical parameter plays a key role in a novel 3D PET concept based on axial arrays of long scintillator bars read out at both ends by Hybrid Photodetectors (HPDs) since it influences the spatial, energy and time resolutions of such a device. In this paper we show that the effective light attenuation length of polished crystals can be reduced by wrapping their lateral surfaces with Teflon, or tuned to the desired value by depositing a coating of Cr or Au of well-defined thickness. The studies have been carried out with YAP and LYSO long scintillator bars, read out by standard photomultiplier tubes. Even if the novel PET device will use different scintillators and HPD readout, the results described here prove the feasibility of an important aspect of the concept and provide hints on the potential capabilities of the device.

TH‐D‐330A‐07: Model‐Based Statistical Image Reconstruction for 4D PET

Purpose: Four‐dimensional (4D) PET can be acquired with gated, dynamic or list mode. In reality, a major problem limiting its clinical application is the poor statistics, since the total coincidence counts in conventional 3D PET are divided into several phase bins and each of them is treated as an independent entity in 4D image reconstruction. We develop a mathematically rigorous system approach that allows one to maximally enhance the signal‐to‐noise ratio (SNR) of 4D PET by simultaneously considering the coincidences acquired at all time points when reconstructing the phase‐resolved images. Method and Materials: A GE Discovery‐ST PET/CT scanner was used to acquire 4D‐CT/PET images. A Real‐time Position Management (RPM) system was used to determine the respiration phases and to correlate temporally the PET and CT images. By deformable registration of the 4D‐CT images, a patient‐specific motion model was derived and incorporated into our “spatial‐temporal PET reconstruction” algorithm based on the maximum likelihood principle. The approach was quantitatively evaluated with numerical and physical phantom experiments. Five clinical studies of pancreatic, lung and liver cancer patients were then carried out. Results: Via a novel concept of “virtual curved line‐of‐response”, we proved that the PET “4D likelihood” can be maximized with a modified expectation‐maximization algorithm. Numerical/physical phantom experiments and patient studies showed that the algorithm converged monotonically. In the former two cases, the “ground truths” were reached within 40 iterations, and the SNRs were enhanced by more than 80% over the regular 4D PET and 35% over 3D PET. Similar level of improvement was observed for the patient studies. Conclusion: A spatial‐temporal reconstruction formalism has been established to fully take advantage of the coincidence information acquired in the 4D acquisition process when reconstructing phase‐resolved PET images. It allows us to obtain the statistically optimal 4D solution and substantially improved SNRs in 4D PET.

WE-D-ValB-02: Thresholding of PET Target Volumes for Treatment Planning and Response Monitoring: Measurement and Modelling Approaches

Purpose: To describe a method to determine the thresholds of different sized objects in PETimages for the purpose of target segmentation for radiotherapytreatment planning and response monitoring and compare the measurements to a modelling approach using point spread functions. Method and Materials:PETimages of the IEC image quality phantom containing 6 spheres of different size containing a known activity concentration of an [F‐18] solution were repeatedly imaged at different times in both 2D and 3D PET acquisition modes. Nominal thresholds were found for each sphere and plotted against the mean uptake values in each sphere. The point spread function (PSF) of the PET scanner was measured by imaging a 1 □l drop of high activity concentration. The full‐width‐at‐half‐maximum (FWHM) of these PSFs was then used to generate simulated PETimages by convolving a binary mask of the 6 spheres with the PSF in 3 dimensions. Results: Nominal thresholds and the mean uptake scale linearly with activity concentration for a given sphere size. The slope of these threshold‐uptake lines are larger for smaller spheres. The deviation from the slope 0.5 indicates the degree of partial volume effect or non‐uniform uptake in the target. Once these lines are quantified the actual threshold for a target of unknown size (approximately spherical in shape) can be found by an iterative procedure. The FWHM of the measured PSFs depends on acquisition mode and PET reconstruction parameters. For the 3D mode, the PSF model excellently predicts the nominal thresholds for all sphere sizes, but not for the 2D mode. Conclusion: Segmentation of targets for treatment response monitoring requires an adaptation of the thresholds if the target size is shrinking. The PSF has the potential to be used for pre‐processing the PETimage before segmentation.

MO‐E‐330A‐01: Joint Estimation of Motion and Activity Concentration for 4D Gated PET Studies

Purpose: Gated PET acquisition has been traditionally used to address the respiratory/cardiac motion induced artifacts. However, gated PET images usually have poor image quality due to insufficient photon counts. In this regard, various efforts have been made to estimate the motion information from the gated PET images first, and then use this information to produce motion‐free, high quality images. The purpose of this study is to combine the two inter‐related tasks—motion estimation and image reconstruction, into one single process. A potential advantage of such an approach is a more accurate estimation of the motion information, as well as a better quality of the reconstructed image. Method and Materials: Motion estimation and image reconstruction are jointly performed by maximizing a modified likelihood function that incorporates motion information. A computer simulation was performed on a 2D digital phantom placed in a simulated PET scanner to test the feasibility of this approach. The digital phantom consisted of a hot sphere moving sinusoidally within a warm background. The motion of the sphere was discretized into 10 steps. This simulated a tumor in the patient body influenced by the respiratory motion during a gated data acquisition using 10 time frames. The simulated PET scanner consisted of two parallel gamma cameras each having 140 detectors, and could rotate in 280 angular steps. Projection data were modeled to follow a Poisson statistics, with 20% of random counts. Results: The jointly estimated image recovered the shape of the moving object while still preserved a good image quality. Quantification error was reduced by 21.2% when compared to the static reconstruction, and SNR was improved by 185% when compared to the gated image. Conclusion: The joint estimation approach we have proposed has a good potential in motion artifacts correction for 4D PET images.

MO‐A‐330D‐01: Advances in PET Technology ‐ New Crystals and Detector Designs

Over the past ten years, the use of clinical PET, particularly in the field of oncology, has increased dramatically. More recently, the introduction of hybrid PET/CT scanners has led to an enhanced ability to provide anatomical correlation to the functional findings on the PET scan. And lastly, the growing field of molecular imaging has led to the development of a variety of dedicated PET systems for small animal imaging. In the clinical arena, the goal is to develop scanners with higher sensitivity and count rate capability to be able to acquire whole body scans more efficiently. Efficiency is essential with respect to small animal imaging as well, but it must be accomplished with very high spatial resolution. This presentation will review the basics of PET imaging with a look towards the advances being made to address the needs of these two very different imaging tasks. The use of 2D versus 3D will be discussed as well as the effect using different crystal materials. Other advances being actively pursued such as the use of time‐of‐flight PET will also be discussed.Educational Objectives:After attending this presentation, the attendee will be able to list 2 advantages and 2 disadvantages of 3D PET compared to 2D PET for clinical, whole body imaging, name 3 different materials used in state‐of‐the‐art PET scanners and list 2 advantages of each, and discuss two potential advantages for time‐of‐flight PET.