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 ...

The use of high-relaxivity, intracellular contrast agents has enabled MRI monitoring of cell migration through and homing to various tissues, such as brain, spinal cord, heart, and muscle. Here it is shown that MRI can detect single cells in vivo, homing to tissue, following cell labeling and transplantation. Primary mouse hepatocytes were double-labeled with green fluorescent 1.63-m iron oxide particles and red fluorescent endosomal labeling dye, and injected into the spleens of recipient mice. This is a common hepatocyte transplantation paradigm in rodents whereby hepatocytes migrate from the spleen to the liver as single cells. One month later the animals underwent in vivo MRI and punctuated, dark contrast regions were detected scattered through the livers. MRI of perfused, fixed samples and labeled hepatocyte phantoms in combination with histological evaluation confirmed the presence of dispersed single hepatocytes grafted into the livers. Appropriate controls were used to determine whether the observed contrast could have been due to dead cells or free particles, and the results confirmed that the contrast was due to disperse, single cells. Detecting single cells in vivo opens the door to a number of experiments, such as monitoring rare cellular events, assessing the kinetics of stem cell homing, and achieving early detection of metastases. Magn Reson Med 55:242-249,