The short story about hyperpolarization

The invisible becomes visible – a new MR-scanning technology

Ground-breaking hyperpolarization magnetic resonance imaging at Aarhus University Hospital in Denmark reveals new metabolic details in cancer cells, in atherosclerotic heart muscles, in cerebral stroke and in organs of the diabetic patient.

Today, doctors use different imaging technologies – CT, PET, SPECT/PT, and MR – to obtain the best knowledge of the patient’s disease. But, in the future the so-called hyperpolarization MR spectroscopy will become a new effective way of increasing knowledge of what goes on in the cells of the body –in both healthy persons as well as in patients.

Very shortly described hyperpolarisation or Dynamic Nuclear Polarization (DNP) is a new MR-scanning technology for in-vivo quantification of metabolic processes with an extremely high sensitivity.  The DNP technology allows rapid and high-sensitivity in vivo detection of pre-polarized 13C compounds (bio-probes) with a signal enhancement of more than 10.000 (Figure 1). This substantially increases the detection limit for quantitative measurements of specific metabolic fluxes involved in processing important intermediates in lipid, sugar and amino acid metabolism.

Figure 1

A solution (few ml) of a bio-active molecule (bio-probe), where one of the 12C atoms is exchanged with a 13C atom, is hyperpolarized in the polarisator. The solution is injected into the patient a few seconds after finishing the polarization process. The MR-spectroscopic scanning process runs through the next few minutes with a time resolution of about 1 sec.

The injected bio-probes are designed to trace specific metabolic fluxes in living tissue by exchanging selected 12C atoms with 13C atoms that can be hyperpolarized. Injection of a bio-probe results in a very strong MR-signal from the main molecule and its breakdown products, thus being quantitated with extremely high sensitivity.               

Even though [1-13C]pyruvate is the only bio-probe employed in humans until now, there is already an availability of a number of bio-probe molecules for tracing various metabolic processes in animals.
[1,4-13C]fumerate might be the next one introduced for human use due to its potential for monitoring metabolic effects of therapy in for example tumours. Other bio-probes tested right now are lactate, acetate, leucine and glutamine, but the range of theoretically available bio-probe designs is long. Figure 2 illustrates how the various steps in glycolysis can be quantified dependent on which atoms in the bio-probe (e.g. glucose) are exchanged with 13C. In contrast to the static quantification in standard spectroscopy a highly dynamic measure of metabolic fluxes are provided by the DNP technique. This is due to the highly enhanced signal to noise ratio implying a very high time resolution in the recordings.

Figure 2

Different 12C atoms in e.g. glucose or in pyruvate can be exchanged with 13C for hyperpolarization and traced through the various steps of the glycolysis. The colouring of individual carbon atoms allows tracing them through the individual glycolytic steps in the figure. 13C exchange on specific carbon positions allows for quantification of both intermediary and end product when injected into a living organism. Other steps in the TCA flux can be traced as well.

Examples of research fields and clinical applications of the DNP technology

Metabolic changes detected by DNP have, in experimental tumours, been shown to correlate with response to chemotherapy. Therefore, the DNP technology is expected to provide an enhanced prediction of therapy effects in pancreatic cancer in its potential for detecting possible metabolic markers of response or resistance in patients.

 

Another oncological application area of the DNP technology is the combination with hadron therapy. Commercially available proton accelerators allow deposit of particle (hadron) radiation energy very locally in a solid tumour thus minimizing radiation damage to nearby healthy tissue and additionally minimizing the risks of secondary cancers following years later. The DNP technology can identify areas in e.g. the prostate gland that present the most abnormal metabolism and thus the highest malignity. Thus DNP might be used for guidance of the hadron radiation.

 

The DNP methodology can also be used for in vivo quantification of fluxes of lipid, sugar and amino acid metabolism in lifestyle related diseases. It allows possible acquisition changes in metabolic fluxes in liver and muscles following a dietary shift to proteins with varying amino acid composition and load (e.g. low glycine) in nutrients. Other examples are dietary programs with focus on branched-chain amino acids (Leucine, Isoleusine and Valine).  The study of enzymatic controlled metabolic fluxes is a final example of DNP application in the studies of the metabolic inflexibility in diabetic or obese persons switch (metabolic flexibility) between utilization of carbohydrates and lipid derivatives. This includes ketone bodies as fuel substrates and preventive interventions like exercise, high glucose diet, high fat diet or anti-inflammatory treatment affecting the dys-metabolic state of obese subjects, with or without diabetes or metabolic syndrome.

A metabolic and pharmacological study in cells cultured in MR-compatible bio-reactors is a future application of DNP due to the high sensitivity of this technology. Regarding surface adherent cells a substantial concentration of cells can be achieved in a reactor. An example is endothelial progenitor cells that are believed to be influential on tissue regeneration by enhancing the formation of new blood vessels in ischemic tissue. Other relevant cell types are various cancer cell lines. A design example of an MR-compatible bio-reactor is shown in figure 3. The bio-reactor is located deeply in the system circumvented by the MR-electronics (RF-coils) and connected to various fluid lines.

Figure 3
Upper: a vertical MR-compatible bio-reactor prepared for 13C DNP metabolic measurements in cells grown on a scaffold located inside the reactor.
Lower: microscopy of progenitor cells. They adhere to hydrophilic surfaces of 3D printed scaffold with predefined spaces between fibres. Adherent cells on the fibres (stained for nuclei) are shown to the right.  With the cells in position inside the bio-reactor this is submerged into the RF circuitry and pushed forward to the iso-centre of a high field horizontal magnet.