IEEE CME 2012 Conference
Plenary Talk 5

Ultra-low field NMR/MRI

Lutz Trahms, Ph.D
Director and Professor
Department of Biosignals
Physikalisch-Technische Bundesanstalt
Abbestr. 2-12, D-10587 Berlin, Germany
E-mail:lutz.trahms@ptb.de

Abstract:
Nuclear magnetic resonance (NMR) is a well established spectroscopic tool in chemistry, physics and biology. In medical diagnostics, magnetic resonance imaging (MRI) has become an indispensable radiologic tool, as well. Driven by the desire to improve spectral resolution and signal-to-noise ratio, the strength of the static magnetic field of NMR spectrometers has increased from a few hundred millitesla in the early pioneering years up to more than ten Tesla today. On the other hand, it became evident during the last few years that NMR in ultra-low fields, i.e. at Larmor frequencies around or even below one kilohertz, opens a window to information that is difficult or even impossible to obtain in high fields. In this audiofrequency range, NMR reflects slow molecular dynamics that could be studied before only by using the technically most demanding field cycling technique. By exploiting the wide band characteristics of superconducting quantum interference devices (SQUIDs) one can record in one spectrum the resonance lines of different nuclei, having Larmor frequencies that differ considerably, such as, e.g., ³¹P and ¹H. By varying the detection field strength, the impact of heteronuclear J-coupling between such nuclei can easily be modulated, thus enabling a detailed study of their intramolecular interaction.

Also for MRI there are a number of expectations that motivate the use of ultra-low fields. T₁ and T₂ relaxation times of different tissue types may show contrast in low fields where conventional high field MRI fails. This could particularly apply to can-cerous tissue. The biomagnetic fields generated by bioelectric cur-rents in the brain are no longer negligible against the static detection field of ultra-low field MRI. This may of-fer the chance to observe the effect of brain function by a frequency shift of the NMR signal of the protons sur-rounding the active neurons. While it is impossible to measure MEG in the presence of a static field in the tesla range, this is no longer true for the microtesla field of ultra-low field MRI. Thus, the combination of MEG with low field MRI may re-sult in a hybrid device that may ease the application and interpretation of MEG, and foster its dissemination even further.

In this contribution I will describe the physical and technical background of ultra-low field NMR and MRI. For illustration, I will present measurement examples that demonstrate the performance of low field NMR and MRI in terms of the above mentioned new potentialities.

Lutz Trahms took physics at the Technical University of Aachen and the Free University of Berlin, receiving his Ph.D. in 1982. During his post doc time at FU Berlin he investigated the molecular structure of biomembranes by nuclear magnetic resonance. In 1986 he joined the Physikalisch-Technische Bundesanstalt Berlin, where he now is the head of the Biosignal Department. During the last 25 years at PTB he explored the potential of SQUID based magnetic measurement technique for medical diagnostics. His R&D activity focussed on the development of diagnostic tools for interdisciplinary research in Cardiology, Neuroscience, Radiology, and Oncology. His current research interests are biosignal processing, biomedical applications of magnetic nanoparticles, and low field magnetic resonance. He has worked with many academic and industrial partners in the frame of various national and European funding programs. L.T. is author or co-author of more than 150 publications in international peer-reviewed journals. From 1999 to 2011 he has been the chair of the section “Magnetic Methods in Medicine” of the German Association of Biomedical Engineering.