From bench to imagingMagnetic resonance molecular imaging with nanoparticles
Section snippets
The NMR phenomenon
An understanding of MR contrast agents is founded upon a rudimentary understanding of MR imaging and the NMR (nuclear magnetic resonance) phenomenon. The basic principle of NMR states that intrinsic angular momentum or spins of protons (ie, hydrogen nuclei) and electrons, 1, 2 when placed in a strong external magnetic field (B0), orientate themselves either parallel (ie, spin up) or antiparallel (ie, spin down) to B0. The overall impact, which is a function of B0, is minute, about 0.01 to 0.1
Passive targeting
Contrast agents can concentrate within a pathologic site by either passive or active targeting mechanisms. Passive targeting agents primarily highlight phagocytic cells and organs naturally responsible for particle clearance within the body. Macrophages, the primary phagocytic cell type responsible for the removal of foreign materials, originate in the bone marrow as pre-monocytes, circulate as monocytes, and localize into connective tissue (histiocytes), liver (Kupffer’s cells), lung (alveolar
Iron oxide–based agents
Superparamagnetic metals, such as iron oxides, exert influence well beyond their immediate size, which accounts for their ease of detection on T1/T2 or highly T2-weighted images. A wide variety of iron oxide–based nanoparticles have been developed that differ in hydrodynamic particle size and surface coating material (dextran, starch, albumin, silicones). 4 In general terms, these particles are categorized based on nominal diameter into superparamagnetic iron oxides (SPIOs) (50-500 nm) and
Paramagnetic gadolinium-based nanoparticles
USPIOs provide adequate circulation time for tissue penetration but also require significant time, usually 24 hours, for sufficient background signal clearance. Because these agents create an extended image void, proximate anatomy surrounding key targeted points of interest maybe obscured. This is particularly manifested in high-resolution imaging. The interest in paramagnetic contrast agents for molecular imaging reflects an alternative desire to create a “bright” contrast signal rapidly,
Summary
MR is emerging as an advantageous technique for molecular imaging, given its high spatial resolution and unique capability to elicit both anatomic and physiologic information simultaneously. Superparamagnetic agents take advantage of the wide-ranging effects of susceptibility artifacts produced by iron oxides, which appear as dark contrast voids with T2-weighted imaging. Alternatively, development of ultraparamagnetic nanoparticles or liposomes, which carry high paramagnetic payloads, exert a
Acknowledgment
The authors have indicated they have no financial conflicts of interest.
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