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Honolulu, Hawaii, United States, 2006/11/06 - A new imaging technique allows detection of signals from molecules present at 10,000 times lower concentrations than conventional MRI techniques.
Conventional diagnostic imaging is mainly based on morphological contrast that is a result of different general tissue characteristics. Molecular imaging is a new approach for detecting diseases much earlier, visualizing biological processes at the cellular and molecular level in living organisms, and detecting changes in biochemistry. Corresponding molecular markers appear in quite low concentrations. Hence, the imaging technique must be very sensitive. Magnetic resonance imaging (MRI) has some significant advantages in terms of using non-ionizing radiation (in contrast to x-rays) and giving high resolution tomographies for any arbitrary position and orientation. However, conventional MRI suffers from inherent low sensitivity. A new method, using xenon as the signal source, was developed by researchers in California and will make MRI an important technique in molecular imaging, offering a huge potential for specific detection of disease markers. The new technique allows detection of signals from molecules present at 10,000 times lower concentrations than conventional MRI techniques. Called HYPER-CEST, for hyperpolarized xenon chemical exchange saturation transfer, this new technique could become a valuable tool for medical diagnosis, including the early detection of cancer.
In vivo molecular imaging requires specific sensor molecules that exclusively target a biomolecule of interest. Such sensors are difficult to develop for MRI with sufficient sensitivity because the technique detects tiny magnetizations of molecules that are exposed to a magnetic field. There are many molecular imaging agents – MRI contrast agents, PET and SPECT probes, fluorescence probes like quantum dots – and there is already a huge variety of targeting groups used for these probes.
"The motivation for our work was to combine two clever detection methods to boost the MRI sensitivity" Leif Schröder explains to Nanowerk: "First, our xenon biosensor concept makes use of transferring the molecular information onto nuclei that are encapsulated in molecular cages and that can be detected at very low concentrations because they are hyperpolarized. This means that they have a much larger detectable magnetization. Second, we used an indirect detection assay that makes optimized use of all the xenon atoms. One biosensor molecule can be used to label thousands of xenon nuclei and to detect a significantly increased signal."
Schröder, a member of the Pines Lab at UC Berkeley, is first author of a recent paper, titled "Molecular Imaging Using a Targeted Magnetic Resonance Hyperpolarized Biosensor", that was published in the October 20, 2006 edition of Science.
The xenon biosensor project is a cooperation between the Pines Lab and the Wemmer Group at Lawrence Berkeley National Laboratory that started several years ago. The first paper on this biosensor concept was published in PNAS in 2001. Subsequently, several important steps towards the imaging application were presented, including work on optimized xenon delivery and the first one-dimensional imaging profiles.
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By Michael Berger, Copyright 2006 Nanowerk LLC