TEP/IRM hybride en neuro-imagerie

The hybrid Positron Emission Tomography/Magnetic Resonance Imaging (PET/MRI) is a newly available imaging modality combining the molecular and metabolic PET information with the morphological and functional data provided by MRI. Integrated PET/MRI tomographs were conceived in analogy to the current PET/Computed Tomography (PET/CT) technology, with specific properties linked to the intrinsic differences of MRI and CT imaging. In the field of neuro-imaging, in particular, MRI provides a larger panel of information, as compared with CT, and is already systematically fused and used as a support for PET images for diagnostic and research purposes. We summarize here our current experience with the first integrated PET/MRI tomograph installed in Switzerland, concerning specifically three clinical applications: brain tumors characterization, the diagnosis of neurodegenerative dementias and the presurgical evaluation of pharmaco-resistant epilepsy. With this sequential tomograph, we could combine the full range of diagnostic MR sequences (including diffusion tensor imaging, tractography, spectroscopy, functional MR) with PET imaging of brain glucose metabolism (by ¹⁸F-Fluorodeoxyglucose–FDG) and of amino acid transport (by ¹⁸F-Fluoroethyltyrosine–FET). We also summarize the main results obtained in neuro-imaging by the different groups working with these new hybrid tomographs. These data show that PET/MRI, acquired in a single imaging session, may represent the modality of choice for neuro-imaging.     

Tagged MRI and PET in severe CAD: discrepancy between preoperative inotropic reserve and intramyocardial functional outcome after revascularization

In severe coronary artery disease (CAD), it has been shown that intramyocardial inotropic reserve as assessed with tagged magnetic resonance imaging (MRI) is uniformly distributed among positron emission tomography (PET) patterns reflecting normal or concomitant reductions in perfusion and glucose metabolism. This preliminary study aimed to delineate the relationship between preoperative values of intramyocardial inotropic reserve (in different PET patterns of perfusion and glucose uptake) and intramyocardial functional outcome after surgical revascularization in severe CAD. Twelve patients underwent preoperative tagged MRI (baseline, 10 microg.kg(-1).min(-1) of dobutamine), H2 15O/[18F]fluorodeoxyglucose PET imaging, and postoperative resting tagged MRI. Regional midmyocardial circumferential shortening (Ecc, in %) and PET patterns (normal, match viable, mismatch viable, and infarcted) were assessed in three tagged MRI/PET short-axis slices. Ecc at baseline ranged from 12 +/- 6 to 8 +/- 5 and 4 +/- 4% in normal, match-viable, and infarcted regions, respectively (P <0.05) and was 8 +/- 5% in mismatch-viable regions. Of the 429 regions studied, 187 showed preoperative inotropic reserve with dobutamine, but 238 showed postoperative functional improvement. Postoperative functional improvement was less common in infarcted regions (41 vs. approximately 60% in the other PET patterns), but the extent of improvement was similar among PET patterns (approximately 6%). Postoperative functional improvement occurred in 53% of all (normal, match viable, and mismatch viable) regions without inotropic reserve. In severe CAD, revascularization affords greater intramyocardial functional benefit than expected from the evaluation of intramyocardial inotropic reserve with low-dose dobutamine. Postoperative functional improvement in PET-viable regions without inotropic reserve suggests that factors other than regionally enhanced perfusion contribute to such functional improvement.     

Simultaneous PET-MRI: a new approach for functional and morphological imaging

Noninvasive imaging at the molecular level is an emerging field in biomedical research. This paper introduces a new technology synergizing two leading imaging methodologies: positron emission tomography (PET) and magnetic resonance imaging (MRI). Although the value of PET lies in its high-sensitivity tracking of biomarkers in vivo, it lacks resolving morphology. MRI has lower sensitivity, but produces high soft-tissue contrast and provides spectroscopic information and functional MRI (fMRI). We have developed a three-dimensional animal PET scanner that is built into a 7-T MRI. Our evaluations show that both modalities preserve their functionality, even when operated isochronously. With this combined imaging system, we simultaneously acquired functional and morphological PET-MRI data from living mice. PET-MRI provides a powerful tool for studying biology and pathology in preclinical research and has great potential for clinical applications. Combining fMRI and spectroscopy with PET paves the way for a new perspective in molecular imaging.     

Imaging neurodegeneration in Parkinson's disease

Neuroimaging techniques have evolved over the past several years giving us unprecedented information about the degenerative process in Parkinson's disease (PD) and other movement disorders. Functional imaging approaches such as positron emission tomography (PET) and single photon emission computerised tomography (SPECT) have been successfully employed to detect dopaminergic dysfunction in PD, even while at a preclinical stage, and to demonstrate the effects of therapies on function of intact dopaminergic neurons within the affected striatum. PET and SPECT can also monitor PD progression as reflected by changes in brain levodopa and glucose metabolism and dopamine transporter binding. Structural imaging approaches include magnetic resonance imaging (MRI) and transcranial sonography (TCS). Recent advances in voxel-based morphometry and diffusion-weighted MRI have provided exciting potential applications for the differential diagnosis of parkinsonian syndromes. Substantia nigra hyperechogenicity, detected with TCS, may provide a marker of susceptibility to PD, probably reflecting disturbances of iron metabolism, but does not appear to correlate well with disease severity or change with disease progression. In the future novel radiotracers may help us assess the involvement of non-dopaminergic brain pathways in the pathology of both motor and non-motor complications in PD.     

La stimulation électrique pallidale dans le traitement des mouvements anormaux : l’expérience montpelliéraine dans l’utilisation de l’imagerie fonctionnelle comme critères de sélection des patients

Magnetic resonance anatomical imaging gives to the practician an important set of criteria for selecting patients with dystonic and dyskinetic syndromes who can get benefice from a treatment by electrical deep brain stimulation. However, anatomical anomalies seen on MRI, whatever their number and morphological extension, do not reflect the effective (residual) functional activity of the corresponding structures. We describe, in this short report, the way how, in our experience, functional imaging techniques used for measuring cerebral perfusion and neurotransmission in Single Photon Emission Computerized Tomography (SPECT), cerebral metabolism in Positron Emission Tomography (PET) and motor neuroactivations in functional Magnetic Resonance Imaging (fMRI) are capable of progressively fulfil this deficiency.     

Comparaison de l’IRM de diffusion et de la TEP/TDM au FDG dans le bilan des lymphomes

PurposeDiffusion weighted MRI (DW-MRI) sequences appear as a promising functional technique supplementary to morphologic MRI for oncology purposes. We evaluated the results of DW-MRI for the staging of lymphomas, compared to FDG PET/CT.MethodsTwenty-seven patients with lymphoma referred for FDG PET/CT (initial staging, relapse or treatment evaluation) were prospectively included. They underwent MRI including free breathing DW and T2 weighted imaging. Lymph node areas and organs involvement were listed for each modality and compared using Cohen's kappa (κ) test. MRI performances were evaluated using FDG PET as the gold standard. The results of PET and MRI were compared (with respect to the final staging by the haematologist).ResultsRegarding the lymph nodes, 154 involved areas were detected by MRI out of the 184 detected by PET, that is an excellent concordance (κ = 0.87), sensitivity of 0.84 and specificity of 1. Concordance and sensitivity were inferior for extranodal disease (notably bone lesions) with 27 lesions detected by MRI out of the 40 viewed with PET. Regarding pre-treatment evaluation, two patients were understaged both with PET and MRI (bone marrow involvement); assessment of stage was concordant for both modalities in 18 patients out of 21.ConclusionsPerformance of MRI including DW images was close to that of FDG PET/CT for lymph node areas involvement. Further studies are needed to assess its sensitivity for extranodal lesions, and its accuracy for determining the stage of the disease.     

Molecular imaging for mitochondrial metabolism and oxidative stress in mitochondrial diseases and neurodegenerative disorders

BackgroundIncreasing evidence from pathological and biochemical investigations suggests that mitochondrial metabolic impairment and oxidative stress play a crucial role in the pathogenesis of mitochondrial diseases, such as mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome, and various neurodegenerative disorders. Recent advances in molecular imaging technology with positron emission tomography (PET) and functional magnetic resonance imaging (MRI) have accomplished a direct and non-invasive evaluation of the pathophysiological changes in living patients.Scope of reviewIn this review, we focus on the latest achievements of molecular imaging for mitochondrial metabolism and oxidative stress in mitochondrial diseases and neurodegenerative disorders.Major conclusionsMolecular imaging with PET and MRI exhibited mitochondrial metabolic changes, such as enhanced glucose utilization with lactic acid fermentation, suppressed fatty acid metabolism, decreased TCA-cycle metabolism, impaired respiratory chain activity, and increased oxidative stress, in patients with MELAS syndrome. In addition, PET imaging clearly demonstrated enhanced cerebral oxidative stress in patients with Parkinson's disease or amyotrophic lateral sclerosis. The magnitude of oxidative stress correlated well with clinical severity in patients, indicating that oxidative stress based on mitochondrial dysfunction is associated with the neurodegenerative changes in these diseases.General significanceMolecular imaging is a promising tool to improve our knowledge regarding the pathogenesis of diseases associated with mitochondrial dysfunction and oxidative stress, and this would facilitate the development of potential antioxidants and mitochondrial therapies.     

A Dual Tracer PET-MRI Protocol for the Quantitative Measure of Regional Brain Energy Substrates Uptake in the Rat

We present a method for comparing the uptake of the brain''s two key energy substrates: glucose and ketones (acetoacetate [AcAc] in this case) in the rat. The developed method is a small-animal positron emission tomography (PET) protocol, in which ¹¹C-AcAc and ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) are injected sequentially in each animal. This dual tracer PET acquisition is possible because of the short half-life of ¹¹C (20.4 min). The rats also undergo a magnetic resonance imaging (MRI) acquisition seven days before the PET protocol. Prior to image analysis, PET and MRI images are coregistered to allow the measurement of regional cerebral uptake (cortex, hippocampus, striatum, and cerebellum). A quantitative measure of ¹¹C-AcAc and ¹⁸F-FDG brain uptake (cerebral metabolic rate; μmol/100 g/min) is determined by kinetic modeling using the image-derived input function (IDIF) method. Our new dual tracer PET protocol is robust and flexible; the two tracers used can be replaced by different radiotracers to evaluate other processes in the brain. Moreover, our protocol is applicable to the study of brain fuel supply in multiple conditions such as normal aging and neurodegenerative pathologies such as Alzheimer''s and Parkinson''s diseases.     

Accuracy of [18F]FDG PET/MRI for the Detection of Liver Metastases

Background

The aim of this study was to compare the diagnostic accuracy of [¹⁸F]FDG-PET/MRI with PET/CT for the detection of liver metastases.

Methods

32 patients with solid malignancies underwent [¹⁸F]FDG-PET/CT and subsequent PET/MRI of the liver. Two readers assessed both datasets regarding lesion characterization (benign, indeterminate, malignant), conspicuity and diagnostic confidence. An imaging follow-up (mean interval: 185±92 days) and/-or histopathological specimen served as standards of reference. Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) were calculated for both modalities. Accuracy was determined by calculating the area under the receiver operating characteristic (ROC) curve. Values of conspicuity and diagnostic confidence were compared using Wilcoxon-signed-rank test.

Results

The standard of reference revealed 113 liver lesions in 26 patients (malignant: n = 45; benign: n = 68). For PET/MRI a higher accuracy (PET/CT: 82.4%; PET/MRI: 96.1%; p<0.001) as well as sensitivity (67.8% vs. 92.2%, p<0.01) and NPV (82.0% vs. 95.1%, p<0.05) were observed. PET/MRI offered higher lesion conspicuity (PET/CT: 2.0±1.1 [median: 2; range 0–3]; PET/MRI: 2.8±0.5 [median: 3; range 0–3]; p<0.001) and diagnostic confidence (PET/CT: 2.0±0.8 [median: 2; range: 1–3]; PET/MRI 2.6±0.6 [median: 3; range: 1–3]; p<0.001). Furthermore, PET/MRI enabled the detection of additional PET-negative metastases (reader 1: 10; reader 2: 12).

Conclusions

PET/MRI offers higher diagnostic accuracy compared to PET/CT for the detection of liver metastases.     

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.     

Apport de la TEP/IRM dans la prise en charge des cancers gynécologiques pelviens

Hybrid PET/MRI imaging is a technological innovation allowing simultaneous acquisition of metabolic targets and fine tissue characterization. It is particularly indicated in pelvic gynecological cancers, for which FDG PET and MRI with diffusion sequences are recommended, performed here according to the one stop shop principle. Based on an analysis of the literature, we share in this article our local experience (65 examinations in 47 patients).     

ClearPEM,un TEP dédié au sein

Conventional breast imaging (mammography, ultrasound, MRI) relies on the analysis of anatomical characteristics. Whole-body positron emission tomography (PET) allows for the acquisition of a metabolic image, yet is limited by its poor spatial resolution. The Crystal Clear Collaboration developed a PET dedicated for breast imaging, the ClearPEM, in order to offer a high-resolution nuclear imaging technique. The patient is installed in the prone position on the exam bed, with two detector plates rotating around to breast to acquire a 3-dimensional image. Two prototypes were built and installed in hospitals. We summarize the technical solutions necessary for the development of this system and present a summary of its performances as well as an outlook on preclinical and clinical tests.     

PET for diagnosis and therapy of brain tumors

Main contribution of PET in the management of brain tumors is at the therapeutic level. Specific reasons explain this role of molecular imaging in the therapeutic management of brain tumors, especially gliomas. Gliomas are by nature infiltrating neoplasms and the interface between tumor and normal brain tissue may not be accurately defined on CT and MRI. Also, gliomas are often histologically heterogeneous with anaplastic areas evolving within a low-grade tumor, and the contrast-enhancement on CT or MRI does not represent a good marker for anaplastic tissue detection. Finally, assessment of tumor residue, recurrence or progression may be altered by different signals related to inflammation or adjuvant therapies, even on contrast-enhanced CT and MRI. These limitations of the conventional neuroimaging in delineating tumor and detecting anaplastic tissue lead to potential inaccuracy in lesion targeting at different steps of the management (diagnostic, surgical, and post-therapeutic stages). Molecular information provided by PET has proved helpful to supplement morphological imaging data in this context. ¹⁸F-FDG (FDG) and amino-acid tracers such as ¹¹C-methionine (MET), provides complementary metabolic data that are independent from the anatomical MR information. These tracers help in the definition of glioma extension, in the detection of anaplastic areas and in the postoperative follow-up. Additionally, PET data have an independent prognostic value. To take advantage of PET data in glioma treatment, PET might be integrated in the planning of image-guided biopsies, radiosurgery and resection.     

Functional mapping in the human brain using high magnetic fields.

An avidly pursued new dimension in magnetic resonance imaging (MRI) research is the acquisition of physiological and biochemical information non-invasively using the nuclear spins of the water molecules in the human body. In this trial, a recent and unique accomplishment was the introduction of the ability to map human brain function non-invasively. Today, functional images with subcentimetre resolution of the entire human brain can be generated in single subjects and in data acquisition times of several minutes using 1.5 tesla (T) MRI scanners that are often used in hospitals for clinical purposes. However, there have been accomplishments beyond this type of imaging using significantly higher magnetic fields such as 4 T. Efforts for developing high magnetic field human brain imaging and functional mapping using MRI (fMRI) were undertaken at about the same time. It has been demonstrated that high magnetic fields result in improved contrast and, more importantly, in elevated sensitivity to capillary level changes coupled to neuronal activity in the blood oxygenation level dependent (BOLD) contrast mechanism used in fMRI. These advantages have been used to generate, for example, high resolution functional maps of ocular dominance columns, retinotopy within the small lateral geniculate nucleus, true single-trial fMRI and early negative signal changes in the temporal evolution of the BOLD signal. So far these have not been duplicated or have been observed as significantly weaker effects at much lower field strengths. Some of these high-field advantages and accomplishments are reviewed in this paper.     

68Ga-PSMA-HBED-CC PET/MRI is superior to multiparametric magnetic resonance imaging in men with biochemical recurrent prostate cancer: A prospective single-institutional study

BackgroundThe primary objective was to compare the overall diagnostic performance, presented as detection rate of ⁶⁸Ga-PSMA-HBED-CC positron emission tomography/magnetic resonance imaging (PSMA PET/MRI) versus conventional, multiparametric MRI (mpMRI) in a population of patients with biochemically recurrent prostate cancer. In conjunction with this analysis, secondary objectives included the evaluation of the detection rate stratified by PSA levels and primary treatment modality.MethodsA total of 165 PSMA PET MRI were performed from April 2018 to May 2021, of whom 108 were presenting for biochemical recurrent disease. The PSMA PET vertex to thigh were read by two different board-certified nuclear medicine physicians while the MRI head and neck, chest, abdomen, and pelvis (with dedicated, PI-RADS compliant multiparametric prostate MRI) were read by two board certified diagnostic radiologists.AnalysisPSMA PET/MRI had a higher detection rate than mpMRI when evaluating patients with biochemical recurrence (BCR) with similar results demonstrated when sub-analysis was performed using PSA levels, primary treatment modality, and time since androgen deprivation therapy. Our study also showed PSMA PET/MRI had a higher sensitivity than mpMRI.DiscussionOur findings demonstrate that PSMA PET/MRI is a better imaging modality in the detection of disease in the setting of BCR when compared to MRI alone. Combined utility with PSMA PET/MRI is a powerful tool which can aid in not only the detection of disease, but also guide in treatment planning for prostate cancer patients.     

Place de la TEP au FDG dans le bilan des épilepsies pharmacorésistantes

The work-up of drug-resistant partial epilepsy is intended to localize epileptogenic foci in the purpose of a possible surgery. We aimed to assess the role of fluorodeoxyglucose Positron Emission Tomography (FDG PET) in this scope. This study involved 34 patients who underwent brain FDG PET, with a final diagnosis in 21. The value of FDG PET for lateralisation and localisation of the epileptogenic focus was evaluated by a blinded interpretation, and compared with the value of standard investigations. The impact of FDG PET was assessed by the means of a questionary intended for the neurologist in each case. All the epilepsy types together, FDG PET lateralised and localised the foci in 65 and 47% of the 34 subjects respectively. Among the 19 subjects with final diagnosis (patients with bilateral foci excluded), lateralisation was correct in 84% and localisation in 63% (and the values were greater for temporal epilepsy than for extratemporal foci). PET frequently provided additional information compared with MRI, but not with EEG. FDG PET was useful in 82.5% of cases (confirming management in 65% and changing it in 17.5% of the patients). Our experience corroborated the value of FDG PET for lateralisation and localisation of epileptogenic foci, and its role in case of normal MRI. However, FDG PET appears as a confirmation tool rather than an examination resulting in a change in management rate.     

MRI/PET nonrigid breast-image registration using skin fiducial markers

We propose a finite-element method (FEM) deformable breast model that does not require elastic breast data for nonrigid PET/MRI breast image registration. The model is applicable only if the stress conditions in the imaged breast are virtually the same in PET and MRI. Under these conditions, the observed intermodality displacements are solely due the imaging/reconstruction process. Similar stress conditions are assured by use of an MRI breast-antenna replica for breast support during PET, and use of the same positioning. The tetrahedral volume and triangular surface elements are used to construct the FEM mesh from the MRI image. Our model requires a number of fiducial skin markers (FSM) visible in PET and MRI. The displacement vectors of FSMs are measured followed by the dense displacement field estimation by first distributing the displacement, vectors linearly over the breast surface and then distributing them throughout the volume. Finally, the floating MRI image is warped to a fixed PET image, by using an appropriate shape function in the interpolation from mesh nodes to voxels. We tested our model on an elastic breast phantom with simulated internal lesions and on a small number of patients imaged, with FMS using PET and MRI. Using simulated lesions (in phantom) and real lesions (in patients) visible in both PET and MRI, we established that the target registration error (TRE) is below two pet voxels.     

In vivo micro-CT imaging of liver lesions in small animal models

Three-dimensional micro computed tomography (microCT) offers the opportunity to capture images liver structures and lesions in mice with a high spatial resolution. Non-invasive microCT allows for accurate calculation of vessel tortuosity and density, as well as liver lesion volume and distribution. Longitudinal monitoring of liver lesions is also possible. However, distinguishing liver lesions from variations within a normal liver is impossible by microCT without the use of liver- or tumor-specific contrast-enhancing agents. The combination of microCT for morphologic imaging with functional imaging, such as positron emission tomography (PET) or single photon emission tomography (SPECT), offers the opportunity for better abdominal imaging and assessment of structure discrepancies visible by functional imaging.This paper describes methods of current microCT imaging options for imaging of liver lesions compared to other imaging techniques in small animals.     

Positron emission tomography: a clinical and biological research tool

Medical and biological imaging has undergone a revolution in the past decade. Positron emission tomography (PET) has been developed to visualize biochemical and physiological phenomena in living humans and animals. For instance, blood flow, blood volume, glucose metabolism, amino acid metabolism, can be quantitatively estimated by means of PET with various radioactive tracers. This functional and molecular imaging technique has progressed rapidly from being a research technique in laboratories to a routine clinical imaging modality. The most widely used radiotracer in routine is 18F-fluorodeoxyglucose (18FDG), which is an analogue of glucose. Since glucose metabolism is increased many fold in malignant tumors, PET has a major role in the field of clinical oncology and recently in cardiology and neurology. PET is also a valuable tool to study cerebral or cardiac binding sites and to image the expression of reporter genes in small animals. In this review, we summarize the most recent developments in PET imaging with particular reference to the radiotracers available and their application.     

Sparse/Low Rank Constrained Reconstruction for Dynamic PET Imaging

In dynamic Positron Emission Tomography (PET), an estimate of the radio activity concentration is obtained from a series of frames of sinogram data taken at ranging in duration from 10 seconds to minutes under some criteria. So far, all the well-known reconstruction algorithms require known data statistical properties. It limits the speed of data acquisition, besides, it is unable to afford the separated information about the structure and the variation of shape and rate of metabolism which play a major role in improving the visualization of contrast for some requirement of the diagnosing in application. This paper presents a novel low rank-based activity map reconstruction scheme from emission sinograms of dynamic PET, termed as SLCR representing Sparse/Low Rank Constrained Reconstruction for Dynamic PET Imaging. In this method, the stationary background is formulated as a low rank component while variations between successive frames are abstracted to the sparse. The resulting nuclear norm and l ₁ norm related minimization problem can also be efficiently solved by many recently developed numerical methods. In this paper, the linearized alternating direction method is applied. The effectiveness of the proposed scheme is illustrated on three data sets.