"History, of course, is difficult to write, if for no other reason, than that it has so many players and so many authors." -P. J. Keating (former Australian Prime Minister)Starting with post-war developments in nuclear magnetic resonance (NMR) a race for stronger and stronger magnetic fields has begun in the 1950s to overcome the inherently low sensitivity of this promising method. Further challenges were larger magnet bores to accommodate small animals and eventually humans. Initially, resistive electromagnets with small pole distances, or sample volumes, and field strengths up to 2.35 T (or 100 MHz 1 H frequency) were used in applications in physics, chemistry, and material science. This was followed by stronger and more stable (Nb-Ti based) superconducting magnet technology typically implemented first for small-bore systems in analytical chemistry, biochemistry and structural biology, and eventually allowing larger horizontal-bore magnets with diameters large enough to fit small laboratory animals. By the end of the 1970s, first low-field resistive magnets big enough to accommodate humans were developed and superconducting whole-body systems followed. Currently, cutting-edge analytical NMR systems are available at proton frequencies up to 1 GHz (23.5 T) based on Nb 3 Sn at 1.9 K. A new 1.2 GHz system (28 T) at 1.9 K, operating in persistent mode but using a combination of low and high temperature multi-filament superconductors is to be released. Preclinical instruments range from small-bore animal systems with typically 600-800 MHz (14.1-18.8 T) up to 900 MHz (21 T) at 1.9 K. Human whole-body MRI systems currently operate up to 10.5 T. Hybrid combined superconducting and resistive electromagnets with even higher field strength of 45 T dc and 100 T pulsed, are available for material research, of course with smaller free bore diameters. This rather costly development toward higher and higher field strength is a consequence of the inherently low and, thus, urgently needed sensitivity in all NMR experiments. This review particularly describes and compares the developments in superconducting magnet technology and, thus, sensitivity in three Moser et al.UHF MR-Magnet Technology fields of research: analytical NMR, biomedical and preclinical research, and human MRI and MRS, highlighting important steps and innovations. In addition, we summarize our knowledge on safety issues. An outlook into even stronger magnetic fields using different superconducting materials and/or hybrid magnet designs is presented.

PurposeTo overcome current limitations in combined transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) studies by employing a dedicated coil array design for 3 Tesla.MethodsThe state‐of‐the‐art setup for concurrent TMS/fMRI is to use a large birdcage head coil, with the TMS between the subject's head and the MR coil. This setup has drawbacks in sensitivity, positioning, and available imaging techniques. In this study, an ultraslim 7‐channel receive‐only coil array for 3 T, which can be placed between the subject's head and the TMS, is presented. Interactions between the devices are investigated and the performance of the new setup is evaluated in comparison to the state‐of‐the‐art setup.ResultsMR sensitivity obtained at the depth of the TMS stimulation is increased by a factor of five. Parallel imaging with an acceleration factor of two is feasible with low g‐factors. Possible interactions between TMS and the novel hardware were investigated and were found negligible.ConclusionThe novel coil array is safe, strongly improves signal‐to‐noise ratio in concurrent TMS/fMRI experiments, enables parallel imaging, and allows for flexible positioning of the TMS on the head while ensuring efficient TMS stimulation due to its ultraslim design. Magn Reson Med 74:1492–1501, 2015. © 2014 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.

We demonstrate the feasibility of dynamic perfusion quantification in skeletal muscle at 7 Tesla using PASL. This may help to better investigate the physiological processes in the skeletal muscle and also in diseases such as diabetes mellitus and peripheral arterial disease.

Skeletal muscle metabolism is impaired in disorders like diabetes mellitus or peripheral vascular disease. The skeletal muscle echo planar imaging (EPI) signal (SEPI) and its relation to energy metabolism are still debated.Localised 31P MRS and SEPI data from gastrocnemius medialis of 19 healthy subjects were combined in one scanning session to study direct relationships between phosphocreatine (PCr), pH kinetics and parameters of time courses. Dynamic spectroscopy (semi-LASER) and EPI were performed immediately before, during and after 5 min of plantar flexions. Data were acquired in a 7 T MR scanner equipped with a custom-built ergometer and a dedicated 31P/1H radio frequency (RF) coil array.Using a form-fitted multi-channel 31P/1H coil array resulted in high signal-to-noise ratio (SNR). PCr and pH in the gastrocnemius medialis muscle were quantified from each 31P spectrum, acquired every 6 s. During exercise, SEPI(t) was found to be a linear function of tissue pH(t) (cross-correlation r = –0.85 ± 0.07). Strong Pearson's correlations were observed between post exercise time-to-peak (TTP) of SEPI and (a) the time constant of PCr recovery τPCr recovery (r = 0.89, p < 10− 6), (b) maximum oxidative phosphorylation using the linear model, Qmax, lin (r = 0.65, p = 0.002), the adenosine-diphosphate-driven model, Qmax,ADP (r = 0.73, p = 0.0002) and (c) end exercise pH (r = 0.60, p = 0.005).Based on combined accurately localised 31P MRS and weighted MRI, both with high temporal resolution, strong correlations of the skeletal muscle SEPI during exercise and tissue pH time courses and of post exercise SEPI and parameters of energy metabolism were observed. In conclusion, a tight coupling between skeletal muscle metabolic activity and tissue signal weighting, probably induced by osmotically driven water shift, exists and can be measured non-invasively, using NMR at 7 T.

Simultaneous acquisition of PCr and Pi images with high temporal resolution, suitable for measuring PCr and pH kinetics in exercise-recovery experiments, was demonstrated at 7T. This study presents a fast alternative to MRS for quantifying energy metabolism of posterior muscle groups of the lower leg. Magn Reson Med 75:2324-2331, 2016. © 2015 Wiley Periodicals, Inc.