Philippe Laniece

SIC, an intracerebral radiosensitive probe for in vivo neuropharmacology investigations in small laboratory animals: theoretical considerations and practical characteristics

Although high-resolution tomographs provide a new approach that strongly simplifies the measurement of in vivo tracer biodistribution and kinetics in small animals, they suffer from an important drawback: the need for animal anesthesia or immobilization, which restricts the neurophysiological investigations. Furthermore, quantitative in vivo experiments realized on the brain sometimes only require a simple measurement of the radioactivity achieved on a few local points and do not necessarily imply the use of a tomograph, which is a detector of high cost. These constraints led the authors to develop an interacerebral /spl beta/ sensitive probe, sonde intracerebrate (SIC) (French acronym of intracerebral probe) that will allow chronic measurements of the neurophysiological activity in awake and unrestrained small animals. The volume to which the probe is sensitive and the noise contributions to the relevant signal have been evaluated through Monte Carlo simulations. Characterizations of a first prototype based on a small piece of scintillating fiber (500-/spl mu/m diameter and 1-mm length) fused to a same diameter optical fiber coupled in turn to a photomultiplier are also presented. A first configuration of the detector is finally proposed.

A new high resolution radioimager for the quantitative analysis of radiolabelled molecules in tissue section

We present a high-speed, high-resolution imager of beta particles. It is devoted to be used in autoradiography experiments such as receptor binding or in situ hybridization experiments, either instead of, or in complement with autoradiographic film and emulsions. It allows the user to locate and perform quantitative analyses of (3H, 14C, 35S, 33P, 32P, 125I) labelled molecules with a 15 microm spatial resolution on a 0.9 x 1.3 cm2 sensitive area. Combining recent techniques (specific scintillator thin sheets and intensified charge-coupled device (CCD)) this imager offers a wide dynamic range and real-time acquisition.

In vivo quantification of localized neuronal activation and inhibition in the rat brain using a dedicated high temporal-resolution β+-sensitive microprobe

Understanding brain disorders, the neural processes implicated in cognitive functions and their alterations in neurodegenerative pathologies, or testing new therapies for these diseases would benefit greatly from combined use of an increasing number of rodent models and neuroimaging methods specifically adapted to the rodent brain. Besides magnetic resonance (MR) imaging and functional MR, positron-emission tomography (PET) remains a unique methodology to study in vivo brain processes. However, current high spatial-resolution tomographs suffer from several technical limitations such as high cost, low sensitivity, and the need of restraining the animal during image acquisition. We have developed a β+-sensitive high temporal-resolution system that overcomes these problems and allows the in vivo quantification of cerebral biochemical processes in rodents. This β-MICROPROBE is an in situ technique involving the insertion of a fine probe into brain tissue in a way very similar to that used for microdialysis and cell electrode recordings. In this respect, it provides information on molecular interactions and pathways, which is complementary to that produced by these technologies as well as other modalities such as MR or fluorescence imaging. This study describes two experiments that provide a proof of concept to substantiate the potential of this technique and demonstrate the feasibility of quantifying brain activation or metabolic depression in individual living rats with 2-[18F]fluoro-2-deoxy-d-glucose and standard compartmental modeling techniques. Furthermore, it was possible to identify correctly the origin of variations in glucose consumption at the hexokinase level, which demonstrate the strength of the method and its adequacy for in vivo quantitative metabolic studies in small animals.

Monte Carlo simulation of the microPET FOCUS system for small rodents imaging applications

In this paper, the authors validated the microPET FOCUS modelling with GATE. The performances of this Monte Carlo platform were presented on voxelized rat brain phantom, specially for the simulation of realistic [C-11]Raclopride exam with a realistic modelling of the injected dose and acquisition time. These results are the consequence of complete installation of this Monte Carlo platform simulation on a cluster computing architecture. Finally, the authors showed a first preliminary approach to improve the quantitative analysis in rat brain dopaminergic studies with an implementation of corrections about the positron range and gamma accolinearity effects. In the future, these corrections could be included as priors in a maximum a posteriori reconstruction algorithm. Moreover, the simulation of whole body exams can be planned in order to optimize the quantitative analysis for small animal PET imaging

Performance characterization and first clinical evaluation of a intra-operative compact gamma imager

The growing interest of cancer surgeons for intra-operative probes has led to the development of several prototypes of high resolution mini gamma cameras. The aim of this paper is to present a global characterization of the one the authors developed and the corresponding first evaluation in a clinical context. The current prototype of POCI (peroperative compact imager) is a 24 mm diameter intensified position sensitive diode. In order to face the various clinical situations, two sets of collimator/scintillator imaging head have been developed either for high spatial resolution or high efficiency purposes, Both of them have been first optimized for /sup 99m/Tc labeled tumor detection. Intrinsic performances are the following: the spatial resolution ranges from 1 mm up to 1.9 mm (without significant distortion) and the corresponding efficiency ranges from 6.10/sup -h/ up to 2.10/sup -4/. Phantom studies illustrating these results are proposed. First clinical evaluation of POCI concerned sentinel lymph node imaging which is included in melanoma and breast cancer staging protocols. Preliminary results already show that performances of POCI are compatible with intra-operative imaging purposes and suggest how such mini-cameras can improve the success rate of tumor removal surgeries.

Simultaneous in vivo magnetic resonance imaging and radioactive measurements with the β-MicroProbe

PurposeMultimodal instrumentation is a new technical approach allowing simultaneous and complementary in vivo recordings of complementary biological parameters. To elucidate further the physiopathological mechanisms in intact small animal models, especially for brain studies, a challenging issue is the actual coupling of magnetic resonance imaging (MRI) techniques with positron emission tomography (PET): it has been shown that running the technology for radioactive imaging in a magnet alters the spatiotemporal performance of both modalities. Thus, we propose an alternative coupling of techniques that uses the β-MicroProbe instead of PET for local measurements of radioactivity coupled with MRI.MethodsWe simultaneously recorded local radioactivity due to [18F]MPPF (a 5-HT1A receptor PET radiotracer) binding in the hippocampus with the β-MicroProbe and carried out anatomical MRI in the same anaesthetised rat.ResultsThe comparison of [18F]MPPF kinetics obtained from animals in a magnet with kinetics from a control group outside the magnet allowed us to determine the stability of tracer biokinetic measurements over time in the magnet. We were thus able to show that the β-MicroProbe reliably measures radioactivity in rat brains under an intense magnetic field of 7 Tesla.ConclusionThe biological validation of a β-MicroProbe/MRI dual system reported here opens up a wide range of future multimodal approaches for functional and pharmacological measurements by the probe combined with various magnetic resonance technologies, including anatomical MRI, functional MRI and MR spectroscopy.

Combining the radiosensitive Beta MicroProbe to Nuclear Magnetic Resonance: theoretical approach for in vivo studies in small animals

In vivo small animal imaging with multiple modalities has become an important tool in modern biomedical research. Indeed, combining exploratory techniques allows simultaneous recording of complementary data, which is required to elucidate complex physiopathological mechanisms. In this field, because of strict technical constraints in vivo, an exciting challenge remains in the combination of Nuclear Magnetic Resonance (NMR) and Positron Emission Tomography (PET). Coupling NMR with a radiosensitive Beta MicroProbe offers therefore a very interesting technical alternative. Here, we assessed the feasibility of this new combination by theoretically evaluating the ability of the Beta MicroProbe to monitor radioactivity in a magnet. To that aim, we modelled with Geant4 the effect of an intense magnetic field on the probe field of view and showed that the field should not have an impact on the global efficiency of the probe.

PIXSIC: A Wireless Intracerebral Radiosensitive Probe in Freely Moving Rats.

The aim of this study was to demonstrate the potential of a wireless pixelated β+-sensitive intracerebral probe (PIXSIC) for in vivo positron emission tomographic (PET) radiopharmacology in awake and freely moving rodents. The binding of [11C]raclopride to D2 dopamine receptors was measured in anesthetized and awake rats following injection of the radiotracer. Competitive binding was assessed with a cold raclopride injection 20 minutes later. The device can accurately monitor binding of PET ligands in freely moving rodents with a high spatiotemporal resolution. Reproducible time-activity curves were obtained for pixels throughout the striatum and cerebellum. A significantly lower [11C]raclopride tracer-specific binding was observed in awake animals. These first results pave the way for PET tracer pharmacokinetics measurements in freely moving rodents.

The Potential of a Radiosensitive Intracerebral Probe to Monitor 18F-MPPF Binding in Mouse Hippocampus In Vivo

As mouse imaging has become more challenging in preclinical research, efforts have been made to develop dedicated PET systems. Although these systems are currently used for the study of physiopathologic murine models, they present some drawbacks for brain studies, including a low temporal resolution that limits the pharmacokinetic study of radiotracers. The aim of this study was to demonstrate the ability of a radiosensitive intracerebral probe to measure the binding of a radiotracer in the mouse brain in vivo. Methods: The potential of a probe 0.25 mm in diameter for pharmacokinetic studies was assessed. First, Monte Carlo simulations followed by experimental studies were used to evaluate the detection volume and sensitivity of the probe and its adequacy for the size of loci in the mouse brain. Second, ex vivo autoradiography of 5-hydroxytryptamine receptor 1A (5-HT1A) receptors in the mouse brain was performed with the PET radiotracer 2′-methoxyphenyl-(N-2′-pyridinyl)-p-18F-fluorobenzamidoethylpiperazine (18F-MPPF). Finally, the binding kinetics of 18F-MPPF were measured in vivo in both the hippocampus and the cerebellum of mice. Results: Both the simulations and the experimental studies demonstrated the feasibility of using small probes to measure radioactive concentrations in specific regions of the mouse brain. Ex vivo autoradiography showed a heterogeneous distribution of 18F-MPPF consistent with the known distribution of 5-HT1A in the mouse brain. Finally, the time–activity curves obtained in vivo were reproducible and validated the capacity of the new probe to accurately measure 18F-MPPF kinetics in the mouse hippocampus. Conclusion: Our results demonstrate the ability of the tested radiosensitive intracerebral probe to monitor binding of PET radiotracers in anesthetized mice in vivo, with high temporal resolution suited for compartmental modeling.

PIXSIC: A Pixellated Beta-Microprobe for Kinetic Measurements of Radiotracers on Awake and Freely Moving Small Animals

We present a design study of PIXSIC, a new β+ radiosensitive microprobe implantable in rodent brain dedicated to in vivo and autonomous measurements of local time activity curves of beta radiotracers in a small (a few mm3) volume of brain tissue. This project follows the initial β microprobe previously developed at IMNC, which has been validated in several neurobiological experiments. This first prototype has been extensively used on anesthetized animals, but presents some critical limits for utilization on awake and freely moving animals. Consequently, we propose to develop a wireless setup that can be worn by an animal without constraints upon its movements. To that aim, we have chosen a Silicon-based detector, highly β sensitive, which allows for the development of a compact pixellated probe (typically 600 × 200 × 1000 μm3), read out with miniaturized wireless electronics. Using Monte-Carlo simulations, we show that high resistive Silicon pixels are appropriate for this purpose, assuming that the pixel dimensions are adapted to our specific signals. More precisely, a tradeoff has to be found between the sensitivity to β+ particles and to the 511 keV j background resulting from annihilations of β+ with electrons. We demonstrate that pixels with maximized surface and minimized thickness can lead to an optimization of their β+ sensitivity with a relative transparency to the annihilation background.