NMR spectroscopy and CEST imaging

Metabolites of low molecular mass (e.g. creatine and glucose), proteins, and other macromolecular structures carry weakly bound protons (¹H nuclei) at their surface which can exchange with protons in neighboring bulk water molecules. This process, named chemical exchange (CE), occurs spontaneously and depends on concentration, pH, temperature, and other properties of the solution.

Exchanging protons can resonate at different frequencies ("chemical shifts") in the ¹H NMR spectrum. Hence, they can be labeled selectively (e.g. by resonant radiofrequency irradiation) to induce equal population of the two ¹H spin states in a magnetic field – this technique is called saturation. Chemical exchange pumps this information into the water pool. Ongoing irradiation accumulates saturation in the water pool and produces the CEST effect (chemical exchange saturation transfer), i.e. a detectable reduction of the NMR signal of water protons. The amplification effect can make up several orders of magnitude. Note that in contrast to conventional NMR spectroscopy (MRS), which acquires the signal of tightly bound, hence non-exchanging nuclei (¹H, ¹³C, ¹⁹F, ³¹P...), CEST employs the water signal to indirectly detect the signal of weakly bound protons in biomolecules.

When CEST is combined with MR imaging (MRI) techniques, different entities of cellular compounds – as illustrated in the Figure – can be scanned in living tissue (in vivo) with a sensitivity comparable to conventional MRI. Since the discovery of the phenomenon in the year 2000, CEST-MRI has been applied to diagnostic imaging of various diseases (e.g. tumors, stroke, neurodegenerative disorders). These studies revealed new information about those pathologies on a molecular level.

The NMR signal in living tissue is fundamentally limited by the available thermal polarization of P ≈ 10^-6. Outside tissues, though, polarization levels can be manipulated to values of P ≈ 0.5 using hyperpolarization techniques, e.g. dynamic nuclear polarization (DNP), yielding a tremendous gain in NMR signal by about five orders of magnitude. Following the administration of substances carrying hyperpolarized spins, highest NMR signal is also exploitable in living tissue for the lifetime of polarized states (on the order of minutes), enabling NMR applications of unprecedented sensitivity.

One such application is metabolic MR using hyperpolarized ¹³C (see Figure): ¹³C-enriched, endogenous substrates (e.g. 1-¹³C-Pyruvate) are hyperpolarized and subsequently administered into living beings, where cellular metabolism 'transfers' the hyperpolarized ¹³C nuclei to other substances. As a consequence, the NMR signals of downstream metabolic products (e.g. 1-¹³C-Lactate) are also enhanced and become detectable. Utilizing dynamic ¹³C NMR techniques, each hyperpolarized metabolite can be identified by its individual ¹³C resonance frequency (chemical shift), and tracking the time course of hyperpolarized signals enables to quantify enzymatic activity (e.g. Lactate Dehydrogenase) in real time.
This way, the metabolic alterations of cancer tissues can be assessed, e.g., the increased activity of Lactate Dehydrogenase leading to an increased flux of Pyruvate to Lactate compared to healthy tissues. To realize assessment of cancer metabolism via hyperpolarized ¹³C also in humans, our research group currently establishes this cutting-edge technology using a SPINlab^TM DNP system for clinical application.