## 2 Dynamic Nuclear Polarization in Semiconductor Nano-and Microstructures
### Microwave-Irradiation-Induced Nuclear Polarization
#### 2.2.6 Si Nanoparticles for Hyperpolarized MRI  
MRI of hyperpolarized nuclei was shown to provide a very high image contrast with little or no background signal [11]. To date, in vivo applications of pre-hyperpolarized materials have been limited by relatively short nuclear spin relaxation times. Si NPs were proposed as a new type of hyperpolarized MRI agent. Nuclear spin relaxation times for a variety of Si NPs were found to be extremely long, ranging from many minutes to hours at room temperature (Figure 17), allowing hyperpolarized NPs to be transported, administered and imaged on practical time scales [11]. Additionally, Si NPs can be surface functionalized using techniques common to other biologically targeted NP systems. These results suggest that Si NPs can be used as a targetable, hyperpolarized MRI agent (Figure 18) with a large range of potential applications. Figure 16: In vivo applications of \({}^{29}\)Si MRI using hyperpolarized Si MPs. (**a–c**) In vivo co-registered \({}^{1}\)H:\({}^{29}\)Si MRI of hyperpolarized Si particles administered intragastrically (**a**), intraperitoneally (**b**) and intravenously (**c**) via a tail-vein catheter. (**d**) Demonstration of perfusion imaging using \({}^{29}\)Si MRI of hyperpolarized Si MPs injected into the tumor of a prostate cancer (TRAMP) animal. In panels (**b**) and (**d**), the green circle shows the approximate location of the catheter. The color scale bars show \({}^{29}\)Si signal intensity in arbitrary units. (Reprinted by permission from Springer Nature [25], © (2013)). Micrometer-sized Si MPs, which reveal the longest known relaxation times up to 136 min at room temperature at 2.9 T [20], are difficult to administer and impossible to handle in-vivo. A new window for MRI applications was opened in Reference [52], where it was demonstrated that \({}^{29}\)Si NPs in powder synthesized from a gas phase using a laser-assisted technique, with average particle size of \(55\pm 12\) nm can be hyperpolarized with superior properties, that is, maximal polarization about 12.6% with a long \(T_{1}\) relaxation time of \(42.3\pm 0.1\) min at room temperature (Figure 19A). Such a long depolarization time enabled the observation of \({}^{29}\)Si NMR signal more than 5 h after transfer to the imaging system (Figure 19B,C). The presence of PEG polymer on the surface of the nanoparticles resulted in shortening of \(T_{1}\) relaxation times at room temperature from \(42.3\pm 0.1\) min to \(34.0\pm 0.3\) min (Figure 19A), while no change in the polarization level was observed. A further significant increase in the polarization level is proposed by lowering the temperature (to <2 K) in the course of the DNP process, using a dedicated long-run helium bath or helium-free cryostat, boosting the obtainable \({}^{29}\)Si polarization. Conducting the DNP process at higher magnetic fields is also expected to increase final nuclear polarization. The limited availability of high-power microwave sources operating in the frequency range above 100 GHz is presently the major difficulty. Figure 18: \({}^{29}\)Si MRI of hyperpolarized Si NPs. (**A**) H-shaped phantom filled with high-resistivity Si particles (\(d_{0}=1.6\) μm) pre-polarized at low temperature (\(T=4.2\) K) and high magnetic field (\(B=5\) T) for 60 h and warmed and transferred to a 4.7 T imager. (**B**) single \({}^{29}\)Si image of phantom in (**A**). No \({}^{29}\)Si image could be obtained without hyperpolarization using the same sequence. (Reprinted with permission from Reference [11]. © (2009) American Chemical Society). Optimal physical and chemical conditions, including surface-defect concentration adjusted to the particle size, are necessary to achieve a high level of DNP enhancement [10]. Real-time MRI-guided catheter tracking using hyperpolarized \({}^{29}\)Si particles (polycrystalline/amorphous; average mean diameter \(\sim\)2 \(\upmu\)m) was demonstrated [53].