coloured tag for MRI

drgmscn · 发表于 2008-7-8 12:53:57

Not everything is black and white, perhaps with the exception of MRI. Aside from the artificial colours that can be added by computer, MRI is a technique of contrasts and greyscales. However, that could all change soon thanks to the ongoing development of microscopic magnetic particles by researchers in the US who hope to bring a little colour to MRI.

MRI contrast agents have been around for decades, and while they may produce some incredibly powerful results available to researchers and clinicians alike, they essentially stick to their name in what they do - enhancing contrast in an MRI scan. Now, collaborators from the National Institute of Standards and Technology (NIST) and the Neurological Disorders and Stroke section of the National Institutes of Health (NIH) report new work on microscopic magnets that could complement contrast agents by allowing different regions and targeted areas to show up with different colours in an MRI scan.

Writing in the journal Nature the researchers explain that the new MRI colour agents could also enhance overall sensitivity and improve the quality of information available from MRI.

The properties of the new micro-magnets mean they could act as "smart tags" to target and identify particular cells, tissues, or physiological conditions, for medical research or diagnostic purposes. However, the colour agents differ markedly in how they work when compared with conventional MRI contrast agents and biological markers, in that it is not their chemistry, but their physical shape that affects their properties. Physical shape is a precisely tuneable feature of the micro-magnets and affects the radio-frequency (RF) signal in such a way that the resulting information can be formatted to show the full spectrum of visible colours in the final image.

"Current MRI technology is primarily black and white," says lead author Gary Zabow, "this is like a coloured tag for MRI." He points out that sets of different magnets designed to appear as different colours could, for example, be coated to attach to different cell types, such as cancerous versus normal. The cells then could be identified by tag colour. These colour agents might be compared to a micro version of radiofrequency identification (RFID) tags used in tracking deliveries and warehouse inventories.

Each micro-magnet consists of two round, vertically stacked magnetic discs a few micrometres in diameter, separated by a small gap. The researchers have borrowed microlithography fabrication techniques common to the microelectronics industry to create the micro-magnets and point out that they are entirely compatible with standard MRI hardware. This top-down microfabrication approach gives them more control than they would have with a bottom-up chemical synthesis approach, explains Zabow, although it is obviously more complicated. "In the end, we hope that we get a bit more bang for the buck because we can engineer really precise shapes and geometries into the magnetic particles," Zabow told Nature.

They created a customized magnetic field for each tag by making it from particular materials and tweaking the geometry, perhaps by widening the gap between the discs or changing the thickness or diameter of the discs. Protons in the water in a sample flowing between the discs are thus affected by the magnetic fields and affect the MRI radio signal in a predictable way. Initial tests show that changing the magnet geometry causes a significant shift in frequency signals. The magnetic particles have another advantage over chemical agents in that they could potentially be detected individually for imaging purposes or even switched on or off using a soluble material to fill that gap that is released only under specific physiological conditions of temperature or pH perhaps.

The micro-magnets might make medical diagnostic images as information-rich as the kinds of optical images of tissue samples now common in biomedical and biotechnological research. Those fields have a plethora of coloured markers, such as fluorescent proteins and tuneable quantum dots, with which to tag cells and even individual biomolecules.

Zabow's micro-magnets will, however, need extensive further engineering and testing, including clinical studies, before they could be used in people undergoing MRI examination, not least because the prototypes are composed of toxic nickel. Zabow suggests that the clinical versions of the micro-magnets will most likely be made from an iron-based, and medically approved, material.

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