DNP in MRI: An in-bore approach at 1.5 T Jan G. Krummenacker a,b , Vasyl P. Denysenkov a , Maxim Terekhov b , Laura M. Schreiber b , Thomas F. Prisner a,⇑ a Institute of Physical and Theoretical Chemistry and Center of Biomolecular Magnetic Resonance, Goethe University Frankfurt, Max-von-Laue-Str. 7, 60438 Frankfurt, Germany b Section of Medical Physics, Department of Diagnostic and Interventional Radiology, Johannes Gutenberg University Medical Center, Langenbeckstraße 1, 55131 Mainz, Germany article info Article history: Received 18 October 2011 Revised 16 December 2011 Available online 30 December 2011 Keywords: Dynamic Nuclear Polarization (DNP) MRI abstract We have used liquid state (‘‘Overhauser’’) Dynamic Nuclear Polarization (DNP) to significantly enhance the signal to noise ratio (SNR) of Magnetic Resonance Imaging (MRI). For the first time this was achieved by hyperpolarizing directly in the MRI-scanner field of 1.5 T in continuous flow mode and immediately delivering the hyperpolarized substance to the imaging site to ensure maximum contrast between hyper- polarized sample and sample at thermal polarization. We achieve a maximum absolute signal enhance- ment factor of 98; while the hyperpolarized sample is transported at a flow rate of up to 30 ml/h yielding an average flow speed up to 470 mm/s over a distance of approximately 80 mm. A spatial imaging reso- lution of 100 lm with a signal to noise ratio of 25 was achieved on the flowing sample. Application to MRI contrast enhancement or microfluidic imaging can be envisaged immediately. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Magnetic Resonance Imaging (MRI) provides spatially and tem- porally resolved information about spin density, e.g. proton ( 1 H) or carbon ( 13 C) spins, and their chemical environment. Because of the low energy of the excitation (in the radiofrequency range) the method is noninvasive and does not require chemical alteration of the sample or application of ionizing radiation. On the other hand this leads to low sensitivity and contrast of the method, lim- iting the spatial and temporal resolution of the method. Typically, contrast is enhanced by administering specific contrast agents, which significantly shortens the longitudinal and transversal relax- ation times T 1 and T 2 of the nuclei close to it. Sensitivity can be im- proved by different techniques, such as optimization of hardware and acquisition parameters, or the use of higher magnetic field, which increases the energy splitting of the involved nuclear spins and therefore the Boltzmann polarization. For larger objects this is technically very challenging and thus increasingly expensive, but also practically limited by the effects of the large magnetic fields and high-frequency excitations, for example on the human body or on neuronal transmission [1,2]. A different approach tack- ling both issues simultaneously is hyperpolarization, i.e. creating a non-Boltzmann nuclear polarization. Hyperpolarization can be achieved by several methods and mechanisms, such as Para-Hydrogen Induced Polarization (PHIP) [3,4], optical polariza- tion of, e.g. hyperpolarized xenon [5] or DNP [6–8]. DNP is a tech- nique, in which hyperpolarization of nuclear spins is achieved by microwave irradiation of unpaired electron spins in radicals, which are coupled to these nuclei, e.g. 1 H, 13 C, 15 N. The electron spin pop- ulation is perturbed if the microwave irradiation is resonant with the electron spin transition, which affects the polarization of hyperfine-coupled close nuclei. For large microwave power (i.e. saturating the electron spin transition) the order of magnitudes larger thermal electron spin polarization is effectively transferred to these nuclear spins in the sample. For proton spins the maxi- mum polarization gain amounts to 660, whereas for 13 C the sensi- tivity gain can be as large as 2600. This enormous potential in prospective signal enhancements is exploited for MRI applications by several technical approaches. Dissolution DNP is an approach, where hyperpolarization is achieved in an external magnet (typically 3.4 T, 95 GHz for EPR excitation), i.e. spatially separated from the imaging magnet, in the solid state at low temperatures [9–12]. After a typical polariza- tion buildup time of more than 30 min, the sample is quickly heated to room temperature and dissolved in about a second. As a liquid sample it is then shuttled into the imaging magnet for the MRI application, e.g. metabolic imaging [9]. Performing the hyperpolarization under these conditions is optimal for high signal enhancements, e.g. of carbons, because additionally to the DNP ef- fect a Boltzmann enhancement from the temperature jump is ob- tained. Therefore, enormous signal enhancements (>10,000 [6]) can be achieved on relatively large sample volumes (up to 100 ml [12]) on a broad range of target molecules, creating entirely new possibilities and fields of application. 1090-7807/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2011.12.015 ⇑ Corresponding author. Fax: +49 69 798 29404. E-mail addresses: krummenacker@prisner.de (J.G. Krummenacker), vasyl@ prisner.de (V.P. Denysenkov), terekhov@uni-mainz.de (M. Terekhov), lschreib@ uni-mainz.de (L.M. Schreiber), Prisner@Chemie.Uni-Frankfurt.de (T.F. Prisner). Journal of Magnetic Resonance 215 (2012) 94–99 Contents lists available at SciVerse ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr