Hyperpolarising 13C-labelled molecules and imaging their fate in vivo enables insights into metabolic processes in the body minimally invasive. Magnetic resonance imaging is intrinsically SNR limited due to the fact that only a few ppm of spins are polarised in strong magnetic fields such as 3T. Different ways exist to polarise spins outside of the MRI scanner using hyperpolarisation techniques such as spin-exchange optical pumping (SEOP), parahydrogen-induced polarisation (PHIP) or dynamic nuclear polarisation (DNP). Most commonly used for polarising molecules and observing their fate in vivo is DNP, because of its versatility and the commercial availability of polarisers. SEOP is mainly used for polarising gas for lung MRI, while PHIP is still limited in its in vivo applications.

Dynamic nuclear polarisation (DNP) in itself is already fairly old, with publications dating back >30 years. The main innovation, pioneered by a group of researchers in Malmö, is that it is possible to dissolve a polarised sample, which is an amorphous glass at 1K, in hot water solution while retaining its polarisation [1]. The method was optimised over the years, hence nowadays obtaining polarisation levels of the dissolved solution of up to ~50%. DNP works by doping a sample (eg [1-13C]pyruvic acid) with an electron paramagnetic agent, which contains an unpaired electron in the inner shell. This sample is placed into a strong magnetic field (typically 3.35T or 5T) in a liquid Helium bath. The Helium temperature is lowered by pumping a vacuum to go down to temperatures of typically 0.8K-1.4K. Electron spins are polarised by nearly 100% at this field and temperature. The polarisation is transferred to 13C by irradiating with microwaves on the resonance frequency of the electron spins. After typically 1-3 hours of polarisation, the sample is moved out of the liquid Helium bath and dissolved with a hot water-based solution. This solution decays now rapidly with T1 and can be injected into animals or (after quality assurance) into humans for various studies.

While a few groups built their own polarisers, the commercial availability of dissolution DNP polarisers greatly eased entry to the field. Most groups use one of the following two polarisers. The HyperSense is optimised for in vitro and small-animal research due to its relatively simple sample preparation and relatively small sample size. It was produced and sold by OxfordInstruments (Oxford, UK). The SpinLab is prepared for human experiments, due to sterility, larger dose, multiple-sample polarisation, and is commercially available from GE (Milwaukee, WI, USA). It is not a clinical device and research groups need to apply for their own approvals from the respective health authorities, such as the FDA in the US.

In principle any nucleus in any molecule exhibiting a magnetic spin can be polarised. However, there are practical limitations. First of all, the longitudinal T1 relaxation must be long in order for the polarisation to be retained in vivo; the gyromagnetic ratio should not be too low to ease the imaging part; the substance must not be toxic for in vivo studies; if metabolism is to be observed, it must be faster or at least in the range of T1 and resonances should not overlap to be distinguishable by spectroscopy; and the substance must form an amorphous glass to be polarisable.

The most common substance for metabolic imaging currently is [1-13C]pyruvate, which is a non-toxic, endogenous substance, easily polarisable, with a long T1 (20s-80s), and a rapid and relevant metabolism [2]. It is converted intra-cellularly into lactate, alanine and bicarbonate, hence yielding valuable insights into metabolic processes, and alterations during disorders, such as cancer or ischemia. Although [1-13C]pyruvate is the clinically most advanced substance, many others were investigated pre-clinically as well for detecting a big variety of other processes [3,4]: fumarate (cell necrosis), urea (perfusion + kidney function), acetate (metabolism), bicarbonate (pH),etc. Furthermore, hyperpolarisation is not limited to 13C and nuclei such as 15N [5] or 29Si [6] have been polarised and studied in vivo using dissolution DNP as well.

Various things have to be taken into consideration for the MRI side of hyperpolarisation. Most MRI scanners are optimised for 1H imaging. Special hard- and software is required to image non-proton nuclei (commonly called X-nuclei or multi-nuclear spectroscopy (MNS)). The MRI scanner needs broadband capabilities for transmit and receive chain. Dedicated MNS coils tuned to 13C and ideally also 1H for good anatomical scans are required. Because of the lower gyromagnetic ratio of 13C, which is a quarter of 1H, gradient encoding is challenging and more RF power is needed, while coil loading is worse.

Special MRI pulse sequences are required for metabolic imaging. Boundary constraints are the non-recoverably (with T1 and excitation) disappearing polarisation and the requirement to encode ideally five dimensions: 3D spatial, 1D spectral and the temporal dynamics. Many different techniques with their unique advantages and disadvantages have been developed by multiple groups over the years. In general, the more encoding efficient a sequence is, the less robust it becomes [7]. While many fast sequences exist, which can potentially encode really high resolutions, the main limitation is still SNR, hence leading to fairly low matrix sizes. Typical resolutions are in the order of (½ cm)³. The choice of sequence as of today is mostly subjective, and there is no consensus yet on which sequence is ideal. After the acquisition, the data has to be reconstructed and quantified. For [1-13C]pyruvate, most people commonly use the downstream metabolite to pyruvate ratio, as this yields a semi-quantitative number related to the conversion rate.

The field started its in vivo studies mostly in rat experiments. Over the years, many different diseases (mostly in oncology, cardiology and neurology) were investigated in various studies with many different substances. The field is now moving towards clinical applications, with first human studies being performed with hyperpolarised [1-13C]pyruvate at multiple sites. The various in vivo applications of hyperpolarisation will be the topic of the next educational talk.