Hybrid PET-MR Research Articles

Design and Characterization of a Gradient-Transparent RF Copper Shield for PET Detector Modules in Hybrid MR-PET Imaging

This paper focuses on the design and the characterization of a frequency-selective shield for positron emission tomography (PET) detector modules of hybrid magnetic resonance-PET scanners, where the shielding of the PET cassettes is located close to the observed object. The proposed shielding configuration is designed and optimized to guarantee a high shielding effectiveness (SE) of up to 60 dB for ${B} _{1}$ -fields at the Larmor frequency of 64 MHz, thus preventing interactions between the radio-frequency (RF) coil and PET electronics. On the other hand, the shield is transparent to the gradient fields with the consequence that eddy-current artifacts in the acquired EPI images are significantly reduced with respect to the standard solid-shield configuration. The frequency-selective behavior of the shield is characterized and validated via simulation studies with CST MICROWAVE STUDIO in the megahertz and kilohertz range. Bench measurements with an RF coil built in-house demonstrated the high SE at the Larmor frequency. Moreover, measurements on a 4-T human scanner confirmed the abolishment of eddy current artifact and also provided an understanding of where the eddy currents occur with respect to the sequence parameters. Simulations and measurements for the proposed shielding concept were compared with a solid copper shielding configuration.

Multi-atlas attenuation correction supports full quantification of static and dynamic brain PET data in PET-MR

In simultaneous PET-MR, attenuation maps are not directly available. Essential for absolute radioactivity quantification, they need to be derived from MR or PET data to correct for gamma photon attenuation by the imaged object. We evaluate a multi-atlas attenuation correction method for brain imaging (MaxProb) on static [18F]FDG PET and, for the first time, on dynamic PET, using the serotoninergic tracer [18F]MPPF.A database of 40 MR/CT image pairs (atlases) was used. The MaxProb method synthesises subject-specific pseudo-CTs by registering each atlas to the target subject space. Atlas CT intensities are then fused via label propagation and majority voting. Here, we compared these pseudo-CTs with the real CTs in a leave-one-out design, contrasting the MaxProb approach with a simplified single-atlas method (SingleAtlas). We evaluated the impact of pseudo-CT accuracy on reconstructed PET images, compared to PET data reconstructed with real CT, at the regional and voxel levels for the following: radioactivity images; time-activity curves; and kinetic parameters (non-displaceable binding potential, BPND).On static [18F]FDG, the mean bias for MaxProb ranged between 0 and 1% for 73 out of 84 regions assessed, and exceptionally peaked at 2.5% for only one region. Statistical parametric map analysis of MaxProb-corrected PET data showed significant differences in less than 0.02% of the brain volume, whereas SingleAtlas-corrected data showed significant differences in 20% of the brain volume. On dynamic [18F]MPPF, most regional errors on BPND ranged from -1 to +3% (maximum bias 5%) for the MaxProb method. With SingleAtlas, errors were larger and had higher variability in most regions. PET quantification bias increased over the duration of the dynamic scan for SingleAtlas, but not for MaxProb. We show that this effect is due to the interaction of the spatial tracer-distribution heterogeneity variation over time with the degree of accuracy of the attenuation maps.This work demonstrates that inaccuracies in attenuation maps can induce bias in dynamic brain PET studies. Multi-atlas attenuation correction with MaxProb enables quantification on hybrid PET-MR scanners, eschewing the need for CT.

P.1.i.019 - Simultaneous imaging of task-specific glucose metabolism and functional connectivity with hybrid PET-MR

A pruning mechanism is developed for the identification of polynomial NARX/NARMAX models. in order to systematically delete redundant terms inserted in the structure selection phase. The proposed approach is capable of overcoming somelimitations met by classical regressor deletion criteria, especially when the data available for identitication are not adequately exciting or the model family does not include all the correct regressors. The effectiveness of the algorithm is shown by comparison analysis on some simulation examples.

Abstract 19147: Fast, Free-breathing, Heart-rate Independent Quantitative Myocardial Oxygenation MRI at 3T for Detecting Ischemic Heart Disease Without Contrast Agents With Simultaneous 13N-Ammonia PET Validation

Introduction: Myocardial BOLD MRI is an emerging non-contrast approach for the assessment of ischemic heart disease. Current BOLD MR methods are limited in part by poor spatial coverage, heart rate dependency and image artifacts, particularly at 3T. To address these limitations, we developed a heart-rate independent, free-breathing 3D T2 mapping technique at 3T that utilizes near perfect imaging efficiency, which can be completed within 3 minutes. We tested our approach in a canine model and validated our findings with simultaneously acquired 13N-ammonia PET perfusion data in a clinical hybrid PET-MR system. Methods: Healthy dogs (n=8) were studied with a state of the art PET-MR system (Siemens Healthcare). For validation, dynamic 13N-ammonia PET scans were simultaneously acquired with BOLD MR data. PET images were analyzed using qPET (Cedars-Sinai Medical Center). T2 values were measured from basal, mid and apical slices at rest and adenosine stress in BOLD images. Myocardial BOLD reserve (T2(stress):T2(rest)) and perfusion reserve (Q(stress):Q(rest)) were computed on a slice basis and regressed. Results: A representative set of BOLD and PET images acquired at rest and adenosine stress are shown in Fig. 2. T2 measures at stress were significantly greater than at rest (T2: 39.5±2.5 ms (rest) vs. 44.0±3.3 ms (stress), p<0.05)); similar results were observed for Q (Q: 0.8±0.1 ml/mg/min (rest) vs. 2.0±0.9 ml/mg/min (stress); p<0.05). Linear regression of BOLD and perfusion reserves showed high correlation (R2=0.6, p<0.05). Conclusions: This is the first study to examine myocardial BOLD MRI with simultaneously acquired PET perfusion data. Our findings showed that BOLD response is highly correlated with PET perfusion suggesting that the proposed BOLD MR method is a viable non-contrast approach for imaging myocardial perfusion. Further studies are required to examine its utility in the setting of ischemic heart disease.

Simultaneous functional imaging using fPET and fMRI.

Brain mapping of task-associated changes in metabolism with PET has been accomplished by subtracting scans acquired during two distinct static states. We have demonstrated that PET can provide truly dynamic information on cerebral energy metabolism using constant infusion of FDG and multiple stimuli in a single experiment. We demonstrate here that the functional PET (fPET-FDG) method accomplished simultaneously with fMRI, can enable the first direct comparisons in time, space and magnitude of hemodynamics and oxygen and glucose consumption. The imaging studies were performed on a 3T Tim-Trio MR scanner modified to support an MR-compatible BrainPET insert. Ten healthy subjects were included. The total PET acquisition and infusion time was 90 minutes. We did 3 blocks of right hand fingers tapping for 10 minutes at 30, 50 and 70 minutes after the beginning of the PET acquisition. ASL and BOLD imaging were acquired simultaneously during the motor paradigm. Changes in glucose utilization are easily observed as changes in the TAC slope of the PET data (FDG utilization rate) and in the derivative signal during motor stimuli in the activated voxels. PET and MRI (ASL, and BOLD) activations are largely colocalized but with very different statistical significance and temporal dynamic, especially in the ipsilateral side of the stimuli. This study demonstrated that motor activation can be measured dynamically during a single FDG PET scan. The complementary nature of fPET-FDG to fMRI capitalizes on the emerging technology of hybrid MR-PET scanners. fPET-FDG, combined with quantitative fMRI methods, allow us to simultaneously measure dynamic changes in glucose utilization and hemodynamic, addressing vital questions about neurovascular coupling.

MR-PET evaluation of 1-month post-ablation therapy for hepatocellular carcinoma: preliminary observations.

The purpose of the study was to evaluate the feasibility and protocol optimization of whole-body hybrid MR-PET system performed 1-month after post-locoregional thermoablative procedures for hepatocellular carcinomas (HCCs). Eight patients (6 men and 2 women; mean age, 56.6 ± 5.5 years) with 9 ablated HCCs constituted our study population. Three readers interpreted the studies to determine the presence or absence of residual malignancy. Two readers independently assessed the fused MR-PET images to compare registration accuracy of two types of T2-weighted (triggered T2 half-Fourier acquisition single-shot turbo spin-echo and turbo spin-echo) and T1-weighted [Cartesian and radial 3D gradient echo (GRE)]. Image quality evaluation of both 3D-GRE T1-weighted sequences was evaluated. Kappa statistics were used to measure inter-observer agreement. Non-parametric Kruskal-Wallis and Wilcoxon signed-rank tests were used for qualitative data analysis. Definite residual tumor was observed in 3/9 ablations; two were PET positive. All residual tumors were isovascular on MRI. Radial 3D-GRE demonstrated significantly superior MR-PET subjective co-registration in comparison with the remaining sequences and showed a non-significant trend toward higher image quality scores than Cartesian GRE. Whole-body hybrid MR-PET is feasible as a part of 1-month follow-up post-locoregional thermoablative treatment for HCC. Radial 3D-GRE offers improved co-registration with PET data, with overall good image quality.

Whole-Body Positron Emission Tomography-Magnetic Resonance in Breast Cancer

Whole-BodyMagnetic Resonance Technology Hybrid positron emission tomography (PET)-magnetic resonance (MR) technology has recently been introduced into the market, appearing in the clinical setting in 2007. Whether designed as a sequential or simultaneous system, the hybrid PET-MR produces high-resolution anatomical, biological, and functional imaging. Given the limited approved indications of PET/computed tomography (CT) in patients with breast cancer, it is understandable that the potential role of PET-MR in such patients remains to be determined. The impetus to develop whole-body MR imaging (WB-MRI) scanning techniques lies in its advantages over CT. Namely, MRI makes no use of ionizing radiation, provides exceptional soft tissue contrast, and offers the ability to perform multisequence andmultiplanar imaging, which allows for improved lesion characterization. The feasibility of a hybrid PET-MR scanner to evaluate the WB was dependent on the development of WB-MR sequences that could be conducted within a reasonable period while providing diagnostic images. The development of such sequences has been made possible by hardware innovations including multireceiver channel WB scanners as well as acquisition acceleration techniques. Such advances allow the assessment of multiple organ systems during 1 scan to be conducted in an efficient and clinically applicable manner. In terms of clinical applications, WBMRI holds promise in evaluating tumors with frequent metastatic spread to the bone, liver, and central nervous system, such as lung cancer, colorectal cancer, prostate cancer, melanoma, and definitively breast cancer. The recent advent

Highly accelerated Point-Spread Function mapping based on Finite Rate of Innovation for EPI distortion correction.

Hybrid MR-PET scans enable acquisition of both types of images within a session. Despite consistent subject positioning, image alignment can still be challenging. Functional MR images rely on echo-planar imaging (EPI) and present geometric distortions due to static B0 field inhomogeneities. Direct B0 [1] and Point Spread Function (PSF) mapping [2] (Figure ​(Figure1)1) have been proposed for distortion correction. The PSF method is more robust [3], but acquisition times are long even with previous acceleration approaches [4–7]. Figure 1 The PSF mapping sequence is repeated while varying the amplitude of the prewinder phase encode gradient (in red). We used the Finite Rate of Innovation (FRI) framework [8, 9] to detect the PSF peak position to sub-pixel precision using as few ks samples as possible. Images were acquired on a 3.0T Philips Achieva: 2.5×2.5×4.0 mm3, matrix 96×95, 95 ks steps with under-sampling retrospectively performed. PSF peak position was estimated using both the fully-sampled (zero-filled by a factor of 1000) and highly-undersampled data. To determine the PSF peak location, pattern matching was performed. The signal measured at each spatial location was compared to a predicted signal pattern accounting for the ks sampling scheme and the search progressively refined up to the intended precision. EPI images were undistorted as in [10]. Figure ​Figure22 shows example PSF peak shift maps (relative to expected undistorted positions). Figure ​Figure3A3A shows the original EPI slice matching Figure ​Figure2,2, and Figures 3B-E show the corrected images using each displacement map. Comparison with the GE image (Figure ​(Figure3F)3F) confirms accurate geometrical corrections. Figure 2 Maps of the PSF shifts (in pixels) estimated with: A0 2; B) 3; C) 4; D) all 95 ks Figure 3 EPI image (with corresponding outer contour in yellow); Undistorted EPI images estimating the displacement field from: B) 2; C) 3; D) 4 and E) all 95 ks samples and F) GE image (corresponding outer contour in yellow propagated to all undistorted EPI images) ... Using the proposed approach the position of the PSF peak can be estimated from a very small number of samples. In the future distortion map estimation could easily be incorporated into standard preparation phases. Making distortion correction of EPI images more practical would facilitate combining functional PET and MR information as well as structural connectivity information from diffusion-weighted MR images.

Dynamic analysis of MR-PET data on brain tumors.

The introduction of hybrid MR-PET scanners offers new perspectives to better correlate MR and PET data with respect to time and space domain. In case of brain tumor patients, dynamic susceptibility contrast (DSC)-MRI is often used to measure perfusion levels of brain, while dynamic [18F]-fluoro-ethyl-tyrosine (FET)-PET provides additional functional information. The dynamic analysis of data is becoming more relevant, rather than static analysis, and with this the extraction of parametric images. In this context, the aim of this work is to compare dynamic MR-PET data based on different features. A FET-PET scan was carried out on the 3T-MR-BrainPET [1] on each patient. Data was acquired for 60 min after injection in list-mode and reconstructed using OSEM3D software (4 subsets, 32 iterations). All MRI sequences were acquired simultaneous to the PET measurement. In this work, MPRAGE and DSC-MRI based on an EPI sequence images are used. For the dynamic analysis, data was co-registered and the same parameters were extracted, with Matlab, from MR-PET data to produce peak, time to peak (TTP), area under the curve (AUC) and wash-in parametric images. Results from 3 of the analyzed datasets from patients with brain tumors are presented on Figure ​Figure1.1. MPRAGE, DSC-MRI EPI sequence and FET-PET images and the extracted parametric images are shown. Figure 1 Analyzed datasets (i, ii and iii). MRI images (a) MPRAGE (post contrast), (b) DSC-MRI EPI, and the corresponding extracted parametric images: (c) peak and (d) AUC. (e) FET-PET image (summed image of 20-40 min p.i., previously filtered with a 2 mm Gaussian ... In FET-PET parametric images an uptake area can be identified in the region of morphological changes in anatomical MR. The parametric images provide extra information in addition to the summed FET-PET images. The time curves obtained from a gray-matter region defined on MPRAGE are presented on Figure ​Figure22. Figure 2 MRI and PET contrast time curves. The FET-PET data was truncated (the total acquisition time was 3600 s) in order to merge the two curves in one graph. The MRI and PET contrast time curves were normalized to the maximum to be able to compare the shape ... The extracted parametric images from dynamic MR-PET showed good spatial and temporal agreement (Figure ​(Figure1).1). These results suggest that dynamic MR-PET may have extra information about biology and that the combination of simultaneous acquisition and analysis may be beneficial.

Attenuation correction synthesis for hybrid PET-MR scanners: validation for brain study applications.

In this work, we further validate a CT and attenuation map (μ-map) synthesis algorithm [1]. The CT synthesis method relies on a pre-acquired set of aligned MRI/CT pairs from multiple subjects. Each MRI from the database is non-rigidly registered to the target MRI. The CTs in the database are then mapped using the same transformation to the target MRI. A local image similarity measure between the target MRI and the set of registered MRIs is used as a surrogate of the underlying morphological similarity. Finally, the synthetic CT is generated using a voxel-wise weighting scheme, and converted to linear attenuation coefficients by a piecewise linear transformation. Following the proposed method, a pseudo CT (pCT) was generated using only the MRI of the subject and compared to the ground truth CT, validating the accuracy of the CT synthesis. A PET image (PETpCT) was then reconstructed with an off-line version of the Siemens Healthcare reconstruction software using the pCT μ-map, and compared with the gold standard PET reconstructed using the CT μ-map. We validated our method for brain-related applications with 16 subjects and compared our solution to: a simpler atlas-based method, named the best-atlas method,

Attenuation correction synthesis for hybrid PET-MR scanners: application to brain studies.

Attenuation correction is an essential requirement for quantification of positron emission tomography (PET) data. In PET/CT acquisition systems, attenuation maps are derived from computed tomography (CT) images. However, in hybrid PET/MR scanners, magnetic resonance imaging (MRI) images do not directly provide a patient-specific attenuation map. The aim of the proposed work is to improve attenuation correction for PET/MR scanners by generating synthetic CTs and attenuation maps. The synthetic images are generated through a multi-atlas information propagation scheme, locally matching the MRI-derived patient's morphology to a database of MRI/CT pairs, using a local image similarity measure. Results show significant improvements in CT synthesis and PET reconstruction accuracy when compared to a segmentation method using an ultrashort-echo-time MRI sequence and to a simplified atlas-based method.

Multimodal neuroimaging in humans at 9.4 T: a technological breakthrough towards an advanced metabolic imaging scanner.

The aim of this paper is twofold: firstly, to explore the potential of simultaneously acquiring multimodal MR-PET-EEG data in a human 9.4 T scanner to provide a platform for metabolic brain imaging. Secondly, to demonstrate that the three modalities are complementary, with MRI providing excellent structural and functional imaging, PET providing quantitative molecular imaging, and EEG providing superior temporal resolution. A 9.4 T MRI scanner equipped with a PET insert and a commercially available EEG device was used to acquire in vivo proton-based images, spectra, and sodium- and oxygen-based images with MRI, EEG signals from a human subject in a static 9.4 T magnetic field, and demonstrate hybrid MR-PET capability in a rat model. High-resolution images of the in vivo human brain with an isotropic resolution of 0.5 mm and post-mortem brain images of the cerebellum with an isotropic resolution of 320 µm are presented. A (1)H spectrum was also acquired from 2 × 2 × 2 mm voxel in the brain allowing 12 metabolites to be identified. Imaging based on sodium and oxygen is demonstrated with isotropic resolutions of 2 and 5 mm, respectively. Auditory evoked potentials measured in a static field of 9.4 T are shown. Finally, hybrid MR-PET capability at 9.4 T in the human scanner is demonstrated in a rat model. Initial progress on the road to 9.4 T multimodal MR-PET-EEG is illustrated. Ultra-high resolution structural imaging, high-resolution images of the sodium distribution and proof-of-principle (17)O data are clearly demonstrated. Further, simultaneous MR-PET data are presented without artefacts and EEG data successfully corrected for the cardioballistic artefact at 9.4 T are presented.

Effects of magnetic fields of up to 9.4 T on resolution and contrast of PET images as measured with an MR-BrainPET.

Simultaneous, hybrid MR-PET is expected to improve PET image resolution in the plane perpendicular to the static magnetic field of the scanner. Previous papers have reported this either by simulation or experiment with simple sources and detector arrangements. Here, we extend those studies using a realistic brain phantom in a recently installed MR-PET system comprising a 9.4 T MRI-scanner and an APD-based BrainPET insert in the magnet bore. Point and line sources and a 3D brain phantom were filled with 18F (low-energy positron emitter), 68Ga (medium energy positron emitter) or 120I, a non-standard positron emitter (high positron energies of up to 4.6 MeV). Using the BrainPET insert, emission scans of the phantoms were recorded at different positions inside and outside the magnet bore such that the magnetic field was 0 T, 3 T, 7 T or 9.4 T. Brain phantom images, with the ‘grey matter’ compartment filled with 18F, showed no obvious resolution improvement with increasing field. This is confirmed by practically unchanged transaxial FWHM and ‘grey/white matter’ ratio values between at 0T and 9.4T. Field-dependent improvements in the resolution and contrast of transaxial PET images were clearly evident when the brain phantom was filled with 68Ga or 120I. The grey/white matter ratio increased by 7.3% and 16.3%, respectively. The greater reduction of the FWTM compared to FWHM in 68Ga or 120I line-spread images was in agreement with the improved contrast of 68Ga or 120I images. Notwithstanding elongations seen in the z-direction of 68Ga or 120I point source images acquired in foam, brain phantom images show no comparable extension. Our experimental study confirms that integrated MR-PET delivers improved PET image resolution and contrast for medium- and high-energy positron emitters even though the positron range is reduced only in directions perpendicular to the magnetic field.

NOVEL HYBRID POSITRON EMISSION TOMOGRAPHY- MAGNETIC RESONANCE (PET-MR) MULTI-MODALITY INFLAMMATORY IMAGING HAS IMPROVED DIAGNOSTIC ACCURACY FOR DETECTING CARDIAC SARCOIDOSIS

Cardiac sarcoidosis (CS) is associated with poor outcomes, but detection remains difficult. Few studies evaluate 18-fluorodeoxyglucose (18-FDG) positron emission tomography (PET) and cardiac magnetic resonance imaging (MRI) for CS diagnosis. None examine a novel hybrid PET-MR approach. We sought to

Fibrin-Targeted PET Probes for the Detection of Thrombi

There is an ongoing effort to develop better methods for noninvasive detection and characterization of thrombi. Here we describe the synthesis and evaluation of three new fibrin-targeted positron emission tomography (PET) probes (FBP1, FBP2, FBP3). Three fibrin-specific peptides were conjugated as 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-monoamides at the C- and N-termini and chelated with (64)CuCl2. Probes were prepared with a specific activity ranging from 10 to 130 μCi/nmol. Both the peptides and the probes exhibited nanomolar dissociation constants (Kd) for the soluble fibrin fragment DD(E), although the Cu-DOTA derivatization resulted in a 2-3 fold loss in affinity relative to the parent peptide. Biodistribution and imaging studies were performed in a rat model of carotid artery thrombosis. For FBP1 and FBP2 at 120 min post injection, the vessel containing the thrombus showed the highest concentration of radioactivity after the excretory organs, that is, the liver and kidneys. This was confirmed ex vivo by autoradiography, which showed >4-fold activity in the thrombus-containing artery compared to the contralateral artery. FBP3 showed much lower thrombus uptake, and the difference was traced to greater metabolism of this probe. Hybrid MR-PET imaging with FBP1 or FBP2 confirmed that these probes were effective for the detection of an arterial thrombus in this rat model. A thrombus was visible on PET images as a region of high activity that corresponded to a region of arterial occlusion identified by simultaneous MR angiography. FBP1 and FBP2 represent promising new probes for the molecular imaging of thrombi.

Inital experience of imaging cardiac sarcoidosis using hybrid PET-MR - a technologist's case study

Background Cardiac imaging has been identified as a potential use of hybrid PET-MRI. This new technology allows simultaneous Positron Emission Tomography (PET) and MR scanning to occur. This case study features a 34 year old male with known pulmonary sarcoidosis which was diagnosed in 2006 and initially treated with immunosuppressants. He presented with NYHA functional class 2 symptoms. ECHO showed asymmetrical LV anterior and lateral wall hypertrophy with a maximum wall thickness of 18mm in the basal and mid anterior and lateral wall. The patient was referred for a cardiac18F Fluorodeoxyglucose (FDG) PET-MR study to determine the cause of the hypertrophy. The possible differential diagnoses included hypertrophic cardiac myopathy with pulmonary sarcoidosis or cardiac sarcoidosis.

Advances in multimodal neuroimaging: Hybrid MR–PET and MR–PET–EEG at 3 T and 9.4 T

Multi-modal MR–PET–EEG data acquisition in simultaneous mode confers a number of advantages at 3T and 9.4T. The three modalities complement each other well; structural–functional imaging being the domain of MRI, molecular imaging with specific tracers is the strength of PET, and EEG provides a temporal dimension where the other two modalities are weak. The utility of hybrid MR–PET at 3T in a clinical setting is presented and critically discussed. The potential problems and the putative gains to be accrued from hybrid imaging at 9.4T, with examples from the human brain, are outlined. Steps on the road to 9.4T multi-modal MR–PET–EEG are also illustrated. From an MR perspective, the potential for ultra-high resolution structural imaging is discussed and example images of the cerebellum with an isotropic resolution of 320μm are presented, setting the stage for hybrid imaging at ultra-high field. Further, metabolic imaging is discussed and high-resolution images of the sodium distribution are presented. Examples of tumour imaging on a 3T MR–PET system are presented and discussed. Finally, the perspectives for multi-modal imaging are discussed based on two on-going studies, the first comparing MR and PET methods for the measurement of perfusion and the second which looks at tumour delineation based on MRI contrasts but the knowledge of tumour extent is based on simultaneously acquired PET data.

MR-Based PET Motion Correction Procedure for Simultaneous MR-PET Neuroimaging of Human Brain

Positron Emission Tomography (PET) images are prone to motion artefacts due to the long acquisition time of PET measurements. Recently, simultaneous magnetic resonance imaging (MRI) and PET have become available in the first generation of Hybrid MR-PET scanners. In this work, the elimination of artefacts due to head motion in PET neuroimages is achieved by a new approach utilising MR-based motion tracking in combination with PET list mode data motion correction for simultaneous MR-PET acquisitions. The method comprises accurate MR-based motion measurements, an intra-frame motion minimising and reconstruction time reducing temporal framing algorithm, and a list mode based PET reconstruction which utilises the Ordinary Poisson Algorithm and avoids axial and transaxial compression. Compared to images uncorrected for motion, an increased image quality is shown in phantom as well as in vivo images. In vivo motion corrected images show an evident increase of contrast at the basal ganglia and a good visibility of uptake in tiny structures such as superior colliculi.

Advances in hybrid MR–PET at 3 T and 9.4 T in humans

Hybrid MR–PET data acquisition in simultaneous mode confers a number of advantages at 3T and 9.4T. From an MR perspective, the potential for ultra-high resolution structural imaging is discussed and example images of the cerebellum with an isotropic resolution of 320μm are presented. Further, metabolic imaging is discussed and high-resolution images of the sodium distribution are demonstrated. Examples of tumour imaging on a 3T MR–PET system are included and discussed.

Implementation and analysis of list mode algorithm using tubes of response on a dedicated brain and breast PET

In this work we present an innovative algorithm for the reconstruction of PET images based on the List-Mode (LM) technique which improves their spatial resolution compared to results obtained with current MLEM algorithms. This study appears as a part of a large project with the aim of improving diagnosis in early Alzheimer disease stages by means of a newly developed hybrid PET-MR insert. At the present, Alzheimer is the most relevant neurodegenerative disease and the best way to apply an effective treatment is its early diagnosis. The PET device will consist of several monolithic LYSO crystals coupled to SiPM detectors. Monolithic crystals can reduce scanner costs with the advantage to enable implementation of very small virtual pixels in their geometry. This is especially useful for LM reconstruction algorithms, since they do not need a pre-calculated system matrix. We have developed an LM algorithm which has been initially tested with a large aperture (186mm) breast PET system. Such an algorithm instead of using the common lines of response, incorporates a novel calculation of tubes of response. The new approach improves the volumetric spatial resolution about a factor 2 at the border of the field of view when compared with traditionally used MLEM algorithm. Moreover, it has also shown to decrease the image noise, thus increasing the image quality.