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Improved contrast in ultra-low-field mri with time-dependent bipolar prepolarizing fields: theory and nmr demonstrations

Nieminen J. O., Voigt J., Hartwig S., Scheer H. J., Burghoff M., Trahms L., Ilmoniemi R. J.

Metrology and Measurement Systems

2013

Vol. 20, nr 3

327--336

The spin-lattice (T1) relaxation rates of materials depend on the strength of the external magnetic field in which the relaxation occurs. This T1) dispersion has been suggested to offer a means to discriminate between healthy and cancerous tissue by performing magnetic resonance imaging (MRI) at low magnetic fields. In prepolarized ultra-low-field (ULF) MRI, spin precession is detected in fields of the order of 10-100 μT. To increase the signal strength, the sample is first magnetized with a relatively strong polarizing field. Typically, the polarizing field is kept constant during the polarization period. However, in ULF MRI, the polarizing-field strength can be easily varied to produce a desired time course. This paper describes how a novel variation of the polarizing-field strength and duration can optimize the contrast between two types of tissue having different T1) relaxation dispersions. In addition, NMR experiments showing that the principle works in practice are presented. The described procedure may become a key component for a promising new approach of MRI at ultra-low fields.

Squid system for meg and low field magnetic resonance

Burghoff M., Albrecht H. H., Hartwig S., Hilschenz I., Körber R., Sander Thömmes T., Scheer H. J., Voigt J., Trahms L.

Metrology and Measurement Systems

2009

Vol. 16, nr 3

371-375

A SQUID magnetometer system was developed for measuring sustained brain activity by magnetoencephalography (DC-MEG) and to record the free precession decay of protons (FPD) of the human brain at very low fields. The SQUID system has a white noise level of about 4 fT/√Hz. To generate the MR signal, two magnetic fields are used: a static polarisation field of a few mT and a static detection field of a few microtesla. To test the spectral resolution of the system, we measured the FPD of protons in distilled water having a spectral line width of about 156 mHz with an instrumental resolution of 2 mHz. The proton resonance line width of the human brain was found to be about 3.0 Hz. Using the same SQUID system we recorded a DC-MEG signal with an amplitude of about 1.5 pT upon motor stimulation. On the basis of these data, we discuss the possibility of detecting a shift of the resonance line due to the superposition of the neuromagnetic field generated by sustained brain activity.