The ability to track the distribution and differentiation of progenitor and stem cells by high-resolution in vivo imaging techniques would have significant clinical and research implications. We have developed a cell labeling approach using short HIV-Tat peptides to derivatize superparamagnetic nanoparticles. The particles are efficiently internalized into hematopoietic and neural progenitor cells in quantities up to 10-30 pg of superparamagnetic iron per cell. Iron incorporation did not affect cell viability, differentiation, or proliferation of CD34+ cells. Following intravenous injection into immunodeficient mice, 4% of magnetically CD34+ cells homed to bone marrow per gram of tissue, and single cells could be detected by magnetic resonance (MR) imaging in tissue samples. In addition, magnetically labeled cells that had homed to bone marrow could be recovered by magnetic separation columns. Localization and retrieval of cell populations in vivo enable detailed analysis of specific stem cell and organ interactions critical for advancing the therapeutic use of stem cells.

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A quantum computer (QC) can operate in parallel on all its possible inputs at once, but the amount of information that can be extracted from the result is limited by the phenomenon of wave function collapse. We present a new computational model, which differs from a QC only in that the result of a measurement is the expectation value of the observable, rather than a random eigenvalue thereof. Such an expectation value QC can solve nondeterministic polynomialtime complete problems in polynomial time. This observation is significant precisely because the computational model can be realized, to a certain extent, by NMR spectroscopy on macroscopic ensembles of quantum spins, namely molecules in a test tube. This is made possible by identifying a manifold of statistical spin states, called pseudo-pure states, the mathematical description of which is isomorphic to that of an isolated spin system. The result is a novel NMR computer that can be programmed much like a QC, but in other respects more closely resembles a DNA computer. Most notably, when applied to intractable combinatorial problems, an NMR computer can use an amount of sample, rather than time, which grows exponentially with the size of the problem. Although NMR computers will be limited by current technology to exhaustive searches over only 15 to 20 bits, searches over as much as 50 bits are in principle possible, and more advanced algorithms could greatly extend the range of applicability of such machines.Several physical implementations of computational models other than the standard von Neumann model have recently been proposed, which in principle scale better on certain types of computational problems. Most notably, Adleman (1) has solved a traveling salesman problem by DNA computing, and Shor (2) has shown theoretically that a quantum computer (QC) should be able to factorize integers in polynomial time. Nondeterministic polynomial-time complete (NP-complete) problems are a class of computationally intractable problems of particular interest, both because they are ''polynomially equivalent'' to one another and because they are encountered in many important applications (3-5). The traveling salesman problem includes a set of NP-complete instances, which indicates that DNA computing may someday be useful in solving NP-complete problems. In contrast, it is widely believed that a QC cannot solve NP-complete problems in polynomial time (6). Until now, however, only very small problems have been solved by DNA computing (7), and no one has yet succeeded in building a QC able to handle more than two bits of information (8-10).In this paper, we consider another physical mechanism that is capable of computation, namely NMR spectroscopy. This approach is based on the fact that the spins in each molecule of a liquid sample are largely isolated from the spins in all other molecules. As a result, each molecule is effectively an independent QC. Via radio-frequency electromagnetic pulses, it is straightforward to manipulate each of the spins in every molecule...