Euromedim 2006 :1st European Conference on Molecular Imaging Technology

A new multimodal and quantitative approach for in vivo small animal brain studies : combination of the Nuclear Magnetic Resonance and the radiosensitive Beta-MicroProbe

11 May 2006, 14:00

1h

Palais du Pharo, Marseille

poster • Biomedical perspectives and technical challenges for morpho-functional imaging (multimodality : PET/CT,SPECT/CT,PET/MRI,SPECT/MRI ...) Poster session : Imaging systems, Molecular Imaging

Speaker

Ms Aurélie Desbrée (Laboratoire Imagerie et Modélisation en Neurobiologie et Cancérologie)

Description

Elucidating physiological mechanisms in small animal in vivo requires the development of new techniques including imaging with multiple modalities. Combining exploratory techniques has the tremendous advantage to record simultaneously complementary parameters on the same living animal. In this field, an exciting challenge remains in the combination of Nuclear Magnetic Resonance (RMN) and Positron Emission Tomography (PET) since small animals studies are limited by strict technical constraints in vivo. Therefore, we proposed to couple NMR techniques to the radiosensitive Beta-MicroProbe, that showed to be an elegant and versatile alternative to PET measurements. This coupling offers the possibility to obtain complementary and simultaneous biological informations and to realise an absolute quantification of the measured radioactive signal in a brain structure of interest. Previous works allowed us to validate the physical feasibility of this combination. Indeed, we demonstrated experimentally and by Monte Carlo simulations that, although the detection volume of the probe could be significantly reduced because of the influence of the intense magnetic field on the positrons range, the sensitivity is only slightly reduced (<7% for a 500 µm diameter probe in 18F). In fact, two effects have been pointed out. First, the magnetic field improves the detection of the local efficiency for the positrons close to the probe, compensating partially the non-detection of the positrons far from the probe. Second, an increase in the deposited energy in the scintillating fiber leads to an increase of the light output. Based on our physical results, we evaluated the feasibility of this new dual- modality system in a biological context. To this aim, we decided to reproduce pharmacological measurements with the probe on anesthetized rat with [18F]-MPPF, an antagonist of the 5-HT1A receptors, but with animals placed during the acquisition inside the 7-T magnet. Two probes were chronically implanted for each rat, one in the hippocampus (specific area) and one in the cerebellum (nonspecific area). We attested the stability of [18F]-MPPF kinetics on both brain areas compared to experiments achieved without magnetic field. We thus demonstrated the possibility to realise a tracer biokinetic measurement simultaneously to the acquisition of anatomical images of the hippocampus and cerebellum. Finally, this study raised a question of quantification particularly interesting since the detection volume of the probe is not entirely included in the hippocampus. Therefore, using a voxelised rat brain phantom and the precise knowledge of the probes positions attested with aMRI images, the present experiments were also conducted to improve the absolute quantification of the tracer concentration in the hippocampus.