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Strong emitter for high field

Especially interesting for
  • neuroscientists
  • physicians
  • manufacturers of MR tomographs

The trend towards higher magnetic fields in magnetic resonance (MR) diagnostics is of major benefit to MR spectroscopy. However, to exploit its potential for the non-invasive quantification of metabolite concentrations in the human organism, new high-frequency coils with high local transmit power are needed. PTB has developed such a coil for 7-tesla tomographs and characterized it by means of numerical simulations and measurements in such a way that it can be used in vivo.

MR-spectroscopic target volume (voxel) in the visual cortex of a volunteer (left) with an HF transmit field generated by the coil (top). The lines in the MR spectrum from the voxel (right) represent metabolites.

With magnetic resonance spectroscopy (MRS), biochemical substances such as neurotransmitters and amino-acids can be differentiated due to the protons that are bound in them, and quantified in vivo and non-invasively. This measurement technique benefits from the increasingly high magnetic fields of modern MR tomographs. Their sensitivity and their spectral resolution, as well as their capacity to identify an increasing number of metabolites, increase nearly proportionally to the field strength. Hence, attempts are currently being made to make MRS exploitable at ultrahigh-field tomographs at 7 tesla. But although being well-suited for imaging, the radiofrequency (RF) excitation coils currently used for such field strengths generate transmit fields that are too weak for MRS. In order to attain the necessary excitation of the spin system to be measured, the high-frequency pulses thus have to be transmitted for a relatively long time. The resulting low spectral bandwidth causes a strong “chemical shift artefact”, causing the measuring volumes for the various metabolites of an MR spectrum to be clearly spatially shifted against each other.

MR tomograph – which PTB operates together with partners at the Berlin Ultrahigh Field Facility (B.U.F.F.) in Berlin-Buch –, a two-channel RF excitation coil has been developed for MRS in the occipital lobe of the human brain; thanks to stronger transmit fields, this coil succeeds in minimizing this undesirable effect. In order to maximize the transmit field of the coil in the target volume, simulations of the field distribution were carried out and validated by means of field measurements in the tomograph. The optimum coil geometry was determined using a multichannel transmit array with which pulse amplitudes and phases can be adjusted independently of each other. This allows a circularly polarized magnetic RF field of 45 μT to be generated in the target volume – the visual cortex. In addition, the results of the simulations and field measurements allowed the specific absorption rate to be limited to such an extent that hazards for test persons due to tissue heating are ruled out. With this coil, MRS pulse sequences can be applied which reduce the chemical shift artefact to a few percent, so that the measuring volumes for all metabolites are practically congruent with each other.

The low pulse durations also allow pulse sequences with very short echo times to be used, so that the resulting MR spectrum includes the resonance signals of a large number of metabolites. Tests with volunteers have shown that the concentrations of 18 metabolites can be determined with an acceptable measurement uncertainty. A Notified Body has certified the technical safety of the new coil, so that it can now be used for measurements on external volunteers. First application studies dealing with neurological issues have been started.