It not surprising that many developments in the field of magnetic resonance are aimed to improve sensitivity. One can blame NMR spectroscopy’s inherently poor sensitivity on the very small population differences between magnetically induced nuclear spin states. The extent of these population differences (also referred to as the degree of polarization) is normally dictated by the Boltzmann distribution and is dependent on magnetic field strength, temperature, and the gyromagnetic ratio of the observed nucleus. Methods have emerged and recently refined that transfer nuclear polarization from a highly polarized species to the nuclei of interest such that the degree of polarization is increased well beyond what is allowed by the Boltzmann distribution. These procedures are referred to as “hyperpolarization methods” and may result in a dramatic >104-fold increase in signal strength over what can be achieved at thermal equilibrium.
For organic molecules, the two general methods of hyperpolarization are dynamic nuclear polarization (DNP) and parahydrogen induced polarization (PHIP). The greatly enhanced sensitivity obtained using these techniques can substantially reduce detection limits, thereby increasing the power of magnetic resonance to detect trace components. The greater signal-to-noise ratio (SNR) may also allow for rapid, sequential spectral acquisition and therefore is well suited for studying kinetics of many different chemical and biochemical processes.
Dynamic Nuclear Polarization (DNP)
DNP is now a well-established technique that is facilitated by the transfer of the polarization of unpaired electrons to the nuclei under study. Dissolution DNP methods require the sample be kept cold (e.g., <4 K) while irradiated by microwaves in a strong magnetic field (e.g., 3 T). After irradiation, the sample is rapidly dissolved using hot pressurized solvent and transferred to an NMR tube or an injection syringe for immediate NMR or MRI data acquisition. A free-radical doping agent must be present in the sample at mM concentrations during microwave irradiation in order to provide the needed nuclear polarization for intermolecular transfer. Generally, acceptable levels of polarization are achieved using irradiation times that are several tens of minutes to several hours in length. Low-temperature DNP, a method pioneered by the laboratory of Robert Griffin, however, is a slightly different technique than dissolution DNP. Both hyperpolarization and NMR acquisition take place in the solid state, where the hyperpolarized signal is effectively continually regenerated which allows for data acquisition from hours to many days.
Parahydrogen-Induced Polarization (PHIP)
PHIP is a fairly well-established technique that transfers polarization directly from parahydrogen (para-H2) to nearby nuclei of interest or by using RF-based magnetization transfer methods. For organic molecules, para-H2 is either added directly across unsaturated carbon bonds or mixed with the sample under conditions such that polarization can transfer from para-H2 to sites within the molecule. Some advantages of PHIP over DNP are that a polarized sample may be attained quite quickly (on the order of seconds or minutes) and that free-radical doping agents are not required.
Perhaps the most exciting consequence of signal enhancement obtained using DNP and PHIP is the potential for in vivo 1H, 13C and 15N monitoring of metabolism. In particular, 13C MRI imaging using hyperpolarized 13C-enriched organic molecules offers significant advantages over 1H-based imaging techniques, because background signals are not detected and the large chemical shift range for 13C leads to increased molecular selectivity. Currently there is interest in the use of isotopically enriched hyperpolarized substrates for medical imaging because detailed, metabolic information (substrate localization and biochemical transformations) and physiological information (e.g., intracellular pH) may be obtained. Although the signal enhancement of hyperpolarized spin one-half nuclei decays with T1, research is under way to establish long-lived nuclear states with the promise that metabolism may be studied over time scales of minutes, or even hours, instead of seconds.
➤ Development of Hyperpolarized Metabolic Contrast Agents Using PASADENA
➤ Using Stable Isotopes to Decipher Systems-Level in vivo Metabolism with MS and MRI