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Hyperpolarized MRI

Mar

Hyperpolarized 13C MRI and Molecular Imaging

Illes Muller

PET/MRI

Hyperpolarized 13C imaging approach increases the MR signal more than 20,000 times for studying real-time metabolism of disease. Metabolic MRI with hyperpolarized agents shows promise by helping support the differentiation of benign and malignant lesions, separating aggressive from slow-growth tumors and facilitating non-invasive treatments.

The Need for Speed

Molecular Imaging describes techniques that directly or indirectly visualize, characterize, and measure the distribution of molecular or cellular processes at the molecular and cellular levels in humans and other living systems.

The most suitable modalities for small-animal in vivo imaging applications are based on nuclear medicine techniques (essentially, positron emission tomography [PET] and single photon emission computed tomography [SPECT]), optical imaging (OI), computed tomography (CT), magnetic resonance imaging (MRI), magnetic resonance spectroscopy imaging (MRSI), and ultrasound.

Conventional magnetic resonance imaging (MRI) relies on magnetic resonance (MR) signal from proton nuclei of water within the body. The MR signal is encoded with magnetic field gradients for 2D and 3D imaging with no fundamental barriers to spatial resolution as long as sufficient MR signal is available.MRI provides excellent contrast and spatial resolution without radiation exposure - however one limitation of MRI in particular is low sensitivity, especially when compared to PET or SPECT.

Hyperpolarization

Hyperpolarization may address this problem by polarizing spins of a nucleus by several orders of magnitude that seen at thermodynamic equilibrium. However this technique practically doesn't work in water, because spins return back to their equilibrium state, i.e. very low polarization, within seconds. ³He, ¹³C, ¹⁵N, ¹²⁹Xe and other nuclear spins can be hyperpolarized to the order of near unity resulting in signal enhancement by 4-6 orders of magnitude. Moreover, the decay of their hyperpolarized spin state can be as long as several hours - making useful chemical compounds as hyperpolarized contrast agents. These agents are prepared by physical and/or chemical manipulations followed by administration of these contrast agents in living organisms and their MRI or MRSI imaging.

Hyperpolarized (HP) ¹²⁹Xe and ³He have been achieved by optical pumping, with potential for low-radiation imaging of the lungs. For nuclei found in endogenous molecules (in particular carbon and nitrogen), the dynamic nuclear polarization (DNP) technique has emerged as a way to polarize small-molecule metabolites. Briefly, ¹³C-labeled molecules, doped with small quantities of a stable radical, are cooled to approximately 1 K in a magnetic field; microwave irradiation transfers polarization from the fully polarized electron spins on the radical to the ¹³C nuclei. The sample is then rapidly dissolved using a hot pressurized solution, which can be injected into an animal (or human) in a separate imaging magnet.

Opening the fourth dimension by Chemical Shift Imaging

This approach increases the MR signal more than 20,000 times, thus increasing the biological sensitivity of hyperpolarized MR imaging. Hyperpolarized contrast agents are similar to radioactive tracers in that their signal- generating capability decays exponentially with time - similar to SPECT and PET tracers. The dramatic signal enhancements obtained allow not only the detection of the introduced metabolic agent, but also its metabolic products in real-time. This enabled by magnetic resonance spectroscopic imaging (MRSI) offering the fourth dimension of chemical shift reporting on composition of tissue, i.e. imaging of protons of metabolites in tumors, cardiac tissue and brain, in addition to three spatial dimensions. Its biggest application so far has been in imaging the glucose consumption in tumors — glucose and lactate for the localization of benign and malignant prostate cancer. this concept has a lot of potential for other kinds of metabolic applications, too, most notably diabetes imaging.

Despite signal boost by several orders of magnitude, hyperpolarized MRI relies on signal from relatively dilute spins of administered hyperpolarized contrast agents. For example, hyperpolarized ¹³C-lactate concentration in vivo is on the order of a few mM, which is several orders of magnitude lower than proton concentration of tissue water. As a result, SPECT and PET are inherently significantly more sensitive (by orders of magnitude) imaging modalities when accounting for contrast agent quantity. When comparing hyperpolarized MRI to PET imaging, it should also be noted that the vast majority of hyperpolarized contrast agents have significantly shorter lifetime on the order, of 0.5-5 minutes in vivo. This double-edged sword limits the use of hyperpolarized contrast agents from the perspective of metabolic pathways penetration, contrast agent in vivo delivery, pharmaceutical preparation and imaging site distribution. On the other hand, it offers an opportunity to perform a repeat scan within minutes after initial hyperpolarized scan, because there is no background signal from the first initially administered dose.

Bringing it into one system

PET/MR imaging is just a phenomenal tool — it combines two very strong technologies. This field however opens even more new opportunities by potentially combining the power of molecular imaging of hyperpolarized MRI and high sensitivity PET. While the main advantage of hyperpolarized MRI is the large sensitivity boost enabled by increased nuclear spin polarization, this increase is not endowed by the magnetic field of the MRI scanner. As a result, it is possible to perform MRI of hyperpolarized contrast agents in very low magnetic fields. The nanoScan PET/MRI is equipped with a permanent 1T magnet which is seamlessly integrated and automated into the equipment. Our advantage is the inherently low cost maintenance, because the need for a high-field cryogenic magnet is eliminated and also no other site preparation and supportive maintenance, like water cooling is required. The combination of low cost and sub-second scan speed is a clear advantage.

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