Magnetic Nanoparticles for Early Detection of Cancer by Magnetic Resonance Imaging
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Nanoparticles for Early Detection of Cancer by Magnetic Resonance Imaging Wenbin Lin, Taeghwan Hyeon, Gregory M. Lanza, Miqin Zhang, and Thomas J. Meade
Abstract This article provides a brief overview of recent progress in the synthesis and functionalization of magnetic nanoparticles and their applications in the early detection of malignant tumors by magnetic resonance imaging (MRI). The intrinsic low sensitivity of MRI necessitates the use of large quantities of exogenous contrast agents in many imaging studies. Magnetic nanoparticles have recently emerged as highly efficient MRI contrast agents because these nanometer-scale materials can carry high payloads while maintaining the ability to move through physiological systems. Superparamagnetic ferrite nanoparticles (such as iron oxide) provide excellent negative contrast enhancement. Recent refinement of synthetic methodologies has led to ferrite nanoparticles with narrow size distributions and high crystallinity. Target-specific tumor imaging becomes possible through functionalization of ferrite nanoparticles with targeting agents to allow for site-specific accumulation. Nanoparticulate contrast agents capable of positive contrast enhancement have recently been developed in order to overcome the drawbacks of negative contrast enhancement afforded by ferrite nanoparticles. These newly developed magnetic nanoparticles have the potential to enable physicians to diagnose cancer at the earliest stage possible and thus can have an enormous impact on more effective cancer treatment.
Introduction Magnetic resonance imaging (MRI) is a noninvasive imaging technique whereby images are generated based on the nuclear magnetic resonance signals of the water proton (1H) nuclei in the specimen.1 The spin-lattice (longitudinal) relaxation time (T1) and the spin-spin (transverse) relaxation time (T2) of the proton spins along with the proton density determine the MR signal intensity from a particular tissue. T1-weighted MR images are generated based on the rate of longitudinal relaxation (proportional to 1/T1) of the water
1H
nuclei. When the protons resonating at an equilibrium frequency are excited with a radio frequency pulse, a change in their net magnetization will occur as a result of the overpopulation of the higher energy states of the nuclear spins. Protons having more rapid longitudinal relaxation (i.e., shorter T1) will relax back to their equilibrium state faster, yielding higher net electromagnetic signals to afford positive contrast enhancement in the MR image. T2-weighted MR images, on the other hand, are generated based on the rate of
MRS BULLETIN • VOLUME 34 • JUNE 2009 • www.mrs.org/bulletin
transverse relaxation (proportional to 1/T2) of the water 1H nuclei. A faster transverse relaxation leads to more rapid dephasing of individual spins, resulting in reduced signal intensity (negative contrast enhancement) in the MR image. Owing to its incredibly high spatial resolution, excellent soft tissue contrast, and superior depth of penetration, MRI has become a
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