MRI detection of single particles for cellular imaging

Abstract

There is rapid growth in the use of MRI for molecular and cellular imaging. Much of this work relies on the high relaxivity of nanometer-sized, ultrasmall dextran-coated iron oxide particles. Typically, millions of dextran-coated ultrasmall iron oxide particles must be loaded into cells for efficient detection. Here we show that single, micrometer-sized iron oxide particles (MPIOs) can be detected by MRI in vitro in agarose samples, in cultured cells, and in mouse embryos. Experiments studying effects of MRI resolution and particle size from 0.76 to 1.63 m indicated that T 2 * effects can be readily detected from single MPIOs at 50-m resolution and significant signal effects could be detected at resolutions as low as 200 m. Cultured cells were labeled with fluorescent MPIOs such that single particles were present in individual cells. These single particles in single cells could be detected both by MRI and fluorescence microscopy. Finally, single particles injected into singlecell-stage mouse embryos could be detected at embryonic day 11.5, demonstrating that even after many cell divisions, daughter cells still carry individual particles. These results demonstrate that MRI can detect single particles and indicate that single-particle detection will be useful for cellular imaging.N umerous recent studies (1-4) indicate that there is a wide range of applications for MRI in molecular and cellular imaging. A key requirement for these applications is the availability of high-relaxivity MRI contrast agents that have a large effect on the MRI signal. One of the agents with very high relaxivity is nanometer-sized, ultrasmall dextran-coated iron oxide particles (USPIOs). These nanometer-sized particles have a large effect on MRI signal intensities due to the fact that they are superparamagnetic and disrupt magnetic field homogeneity to an extent much larger than their size. A growing number of studies have demonstrated the usefulness of USPIOs and MRI to detect receptors (3, 5-8) and monitor cell migration (9-11). Indeed, when a cell is labeled with millions of USPIOs, single cells can be detected by MRI even though the MRI is acquired at low resolution (50-100 m) compared with the size of the cells (5-20 m; refs. 12-14).A drawback of techniques that use USPIOs is that for significant signal changes, many particles need to be within an imaging voxel. Recently, we have shown that micrometer-sized iron oxide particles (MPIOs), which are commercially available, are efficiently endocytosed by a variety of cells, and these particles can be used for cellular imaging by MRI (14). Because these particles are polymer-coated and are impregnated with a fluorescent agent, it becomes possible to do both fluorescence microscopy and MRI on cells labeled with such particles. Empirical observations suggest that an iron oxide particle disrupts the magnetic field enough for MRI detectability for a distance at least 50 times its size, ¶ leading to the conclusion that cells harboring single, micrometer-sized particles should be detectable by ...

Statistical diffusion tensor histology reveals regional dysmyelination effects in the shiverer mouse mutant

“…Second, although the acquisition time reported here is relatively long in comparison to in vivo MRI protocols, this time is still significantly shorter than that required for embedding, sectioning and staining material for histology, even without additional 3D reconstructions. Third, the lack of dehydration and sectioning artifacts in MR images assists the subsequent conventional histological analysis of the same brain, since the two modalities are not mutually exclusive and, as shown here and in other studies (Bobinski et al, 2000;Shapiro et al, 2004), can be applied sequentially to a single specimen. Thus, structures identified in each type of image can be correlated, with ex vivo MR microscopy providing a natural link between low resolution in vivo MRI and highresolution conventional histology.…”

Magnetic Resonance Imaging of Single Co-Labeled Mesenchymal Stromal Cells after Intracardial Injection in Mice

“…The use of these large iron oxide mirco-particles for magnetic cell labeling was initially described by Shapiro et al [7]. The crucial advantage of these micro-particles is their pronounced T2* effect due to the large iron oxide core, allowing even single micro-particles to be detected by MRI [40] which is preferable for single cell imaging. In addition to the fluorescent micro-particles, MSC were also labeled with CFSE, a common fluorescent dye for cytoplasmic labeling of cells.…”

Magnetic resonance imaging of the migration of neuronal precursors generated in the adult rodent brain

“…Each particle contains 1-10 pg of iron, enabling very high levels of iron loading. Indeed, even single MPIO particles have been detected in single cells in culture (Shapiro et al, 2005) and in fixed mouse embryos (Shapiro et al, 2004). Furthermore, single cells labeled with MPIOs have been detected, in vivo, by MRI.…”