H. Herzog

Expectation maximization reconstruction of positron emission tomography images using anatomical magnetic resonance information

Using statistical methods the reconstruction of positron emission tomography (PET) images can be improved by high-resolution anatomical information obtained from magnetic resonance (MR) images. The authors implemented two approaches that utilize MR data for PET reconstruction. The anatomical MR information is modeled as a priori distribution of the PET image and combined with the distribution of the measured PET data to generate the a posteriori function from which the expectation maximization (EM)-type algorithm with a maximum a posteriori (MAP) estimator is derived. One algorithm (Markov-GEM) uses a Gibbs function to model interactions between neighboring pixels within the anatomical regions. The other (Gauss-EM) applies a Gauss function with the same mean for all pixels in a given anatomical region. A basic assumption of these methods is that the radioactivity is homogeneously distributed inside anatomical regions. Simulated and phantom data are investigated under the following aspects: count density, object size, missing anatomical information, and misregistration of the anatomical information. Compared with the maximum likelihood-expectation maximization (ML-EM) algorithm the results of both algorithms show a large reduction of noise with a better delineation of borders. Of the two algorithms tested, the Gauss-EM method is superior in noise reduction (up to 50%). Regarding incorrect a priori information the Gauss-EM algorithm is very sensitive, whereas the Markov-GEM algorithm proved to be stable with a small change of recovery coefficients between 0.5 and 3%.

High resolution BrainPET combined with simultaneous MRI.

UNLABELLEDnAfter the successful clinical introduction of PET/CT, a novel hybrid imaging technology combining PET with the versatile attributes of MRI is emerging. At the Forschungszentrum Jülich, one of four prototypes available worldwide combining a commercial 3T MRI with a newly developed BrainPET insert has been installed, allowing simultaneous data acquisition with PET and MRI. The BrainPET is equipped with LSO crystals of 2.5 mm width and Avalanche photodiodes (APD) as readout electronics. Here we report on some performance characteristics obtained by phantom studies and also on the initial BrainPET studies on various patients as compared with a conventional HR+ PET-only scanner.nnnMATERIAL, METHODSnThe radiotracers [18F]-fluoro-ethyl-tyrosine (FET), [11C]-flumazenil and [18F]-FP-CIT were applied.nnnRESULTSnComparing the PET data obtained with the BrainPET to those of the HR+ scanner demonstrated the high image quality and the superior resolution capability of the BrainPET. Furthermore, it is shown that various MR images of excellent quality could be acquired simultaneously with BrainPET scans without any relevant artefacts.nnnDISCUSSION, CONCLUSIONnInitial experiences with the hybrid MRI/BrainPET indicate a promising basis for further developments of this unique technique allowing simultaneous PET imaging combined with both anatomical and functional MRI.

PET quantitation and imaging of the non-pure positron-emitting iodine isotope 124I.

A series of PET studies using phantoms is presented to characterize the imaging and quantitative performance of the positron-emitting iodine isotope 124I. Measurements were performed on the 2D-PET scanner GE 4096+ as well as on the Siemens PET scanner HRR+ operated in both 2D and 3D modes. No specific correction was applied for the gamma-rays emitted together with the positrons. As compared to 18F, in studies with 124I there is a small loss of image resolution and contrast, and an increase in background. The quantitative results varied between different scanners and various acquisition as well as reconstruction modes, with an average relative difference of -6 +/- 13% (mean+/-SD) in respect of the phantom radioactivity as measured with gamma-ray spectroscopy. We conclude that quantitation of a radiopharmaceutical labelled with 124I is feasible and may be improved by the development of specific corrections.

Fully-3D PET Image Reconstruction Using Scanner-Independent, Adaptive Projection Data and Highly Rotation-Symmetric Voxel Assemblies

For iterative, fully 3D positron emission tomography (PET) image reconstruction intrinsic symmetries can be used to significantly reduce the size of the system matrix. The precalculation and beneficial memory-resident storage of all nonzero system matrix elements is possible where sufficient compression exists. Thus, reconstruction times can be minimized independently of the used projector and more elaborate weighting schemes, e.g., volume-of-intersection (VOI), are applicable. A novel organization of scanner-independent, adaptive 3D projection data is presented which can be advantageously combined with highly rotation-symmetric voxel assemblies. In this way, significant system matrix compression is achieved. Applications taking into account all physical lines-of-response (LORs) with individual VOI projectors are presented for the Siemens ECAT HR+ whole-body scanner and the Siemens BrainPET, the PET component of a novel hybrid-MR/PET imaging system. Measured and simulated data were reconstructed using the new method with ordered-subset-expectation-maximization (OSEM). Results are compared to those obtained by the sinogram-based OSEM reconstruction provided by the manufacturer. The higher computational effort due to the more accurate image space sampling provides significantly improved images in terms of resolution and noise.

Analysis and Correction of Count Rate Reduction During Simultaneous MR-PET Measurements With the BrainPET Scanner

In hybrid magnetic resonance-positron emission tomography (MR-PET) studies with the Siemens 3T MR-BrainPET scanner an instantaneous reduction of the PET sensitivity was observed during execution of certain MR sequences. This interference was investigated in detail with custom-made as well as standard clinical MR sequences. The radio-frequency pulses, the switched gradient fields and the constant magnetic field were examined as the relevant parameters of the magnetic resonance imaging (MRI) system as well as the air temperature within the PET detectors. Our investigation comprised the analysis of the analog PET signals, the total count rates, the geometric distribution of the count rate reduction within the BrainPET detector as well as reconstructed images. The fast switching magnetic field gradients were identified to distort the analog PET detector signals. The measured count rate reduction was found to be less than 3%, but only up to 2% in the case of echo planar imaging sequences, as applied in functional MRI. For clinical sequences routinely used in hybrid MR-BrainPET measurements, a correction method has been designed, implemented, and evaluated .

Acute S-ketamine application does not alter cerebral [18F]altanserin binding : a pilot PET study in humans

SummaryModeling short-term psychotic states with subanaesthetic doses of ketamine provides substantial experimental evidence in support of the glutamate hypothesis of schizophrenia. Ketamine exerts its pharmacological effects both directly via interactions with glutamate receptors and indirectly by stimulating presynaptic release of endogenous serotonin (5-HT). The aim of this feasibility study was to examine whether acute ketamine-induced 5-HT release interferes with the binding of the 5-HT2A receptor (5-HT2AR) radioligand [18F]altanserin and positron emission tomography (PET). Two subjects treated with ketamine and one subject treated with placebo underwent [18F]altanserin PET at distribution equilibrium conditions. Robust physiological, psychopathological and cognitive effects were present at ketamine plasma concentrations exceeding 100u2009µg/l during >70u2009min. Notwithstanding, we observed stable radioligand binding (changes ±95% CI of −1.0 ± 1.6% and +4.1 ± 1.8% versus −1.2 ± 2.6%) in large cortical regions presenting high basal uptake of both, [18F]altanserin and ketamine. Marginal decreases of 4% of radioligand binding were observed in the frontal lobe, and 8% in a posteriorily specified frontomesial subregion. This finding is not compatible with a specific radioligand displacement from 5-HT2AR which should occur proportionally throughout the whole brain. Instead, the spatial pattern of these minor reductions was congruent with ketamine-induced increases in cerebral blood flow observed in a previous study using [15O]butanol PET. This may caused by accelerated clearance of unspecifically bound [18F]altanserin from cerebral tissue with increased perfusion. In conclusion, this study suggests that [18F]altanserin PET is not sensitive to acute neurotransmitter fluctuations under ketamine. Advantageously, the stability of [18F]altanserin PET towards acute influences is a prerequisite for its future use to detect sub-acute and chronic effects of ketamine.

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

PurposeThe non-pure positron-emitting iodine isotope 120I (T1/2=81xa0min) is a short-lived alternative to 124I. 120I has a positron abundance more than twice that of 124I and a maximum positron energy of 4xa0MeV. This study was undertaken to evaluate and characterise the qualitative and quantitative PET imaging of 120I.Methods120I was produced via the 120Te(p,n) reaction on highly enriched 120Te. 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 1-mm tube placed in water was filled with 120I and 18F. Comparisons with 18F, 124I and 123I (measured with SPECT) were made using the Hoffman 3D brain phantom.ResultsThe half-life of 81.1xa0min 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 5.4xa0mm FWHM (12.3xa0mm FWTM), in contrast to 4.6xa0mm (8.6xa0mm) when using 18F. Erroneous positive and negative numbers of radioactivity found in the cold inserts became nearly zero when the background of γ-coincidences was corrected for. Images of the Hoffman phantom were inferior to those obtained when 18F or 124I was applied but superior to the 123I-SPECT images.ConclusionOur data show that 120I of high radionuclidic purity can be regarded as a suitable nuclide for the PET imaging of radioiodine-labelled pharmaceuticals.

Motion correction of head movements in PET: realisation for routine usage

With the increase of scanner resolution head motion in PET brain studies becomes an increasingly serious limitation. Methods to correct for motion have been proposed. In this work the realisation of a motion tracking system in a PET environment and the motion correction of list mode data with the MAF method is presented. In a phantom study the method is validated and the loss in image quality is documented in a phantom with simulated movements. The relevance of motion correction for patient data above the level of system resolution is studied. In a real patient study we show the effect of motion and the applicability of the presented system.

Quantification of the whole-body distribution of PET radiopharmaceuticals, applied to 3-N-([18F]fluoroethyl) spiperone

Using a multi slice whole body PET scanner PC4096-15WB, diagnostic measurements of the cerebral distribution of the D2 receptor ligand 3-N-([18F]fluoroethyl)spiperone were extended to quantify the biodistribution of this PET radiopharmaceutical. As a rotating line source was used for measured attenuation correction, transmission scans could be combined with emission scans even after injection of the tracer. Only 1 of the total administered dose (TAD) was found in the whole brain at 180 min, but the striatum and pituitary were still excellently delineated. Urinary bladder, gall bladder, and liver were the organs with the highest TAD ranging from 6% to 25%. The gall bladder is the critical organ with an absorbed dose of about 200 mGy/kBq followed by the urinary bladder and liver with 83 and 66 mGy/kBq, respectively. In the rest of the body radioactivity was evenly distributed. The total body dose was found to be 11.9 mGy/kBq.

Maximum a Posteriori Reconstruction using PRESTO and PET/MR data acquired Simultaneously with the 3TMR-BrainPET

In this paper, a Maximum a Posteriori Reconstruction algorithm is applied for the first time to the Siemens 3TMR BrainPET scanner. The implementation of this algorithm is done in the PET REconstruction Software TOolkit. This software is able to cope with 3D PET listmode data and can deal with the specificities of the Siemens 3TMR BrainPET scanner. Simulated data was used to assess the performance of the algorithm. Several a priori distributions were tested and the results are compared to OSEM reconstruction. Simultaneously acquired PET and MR data of a human patient was used to show the feasibility of the Maximum a Posteriori algorithm. The results are promising and expected to improve clinical applications.