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Microtesla MRI of the Human Brain with Simultaneous MEG

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Microtesla MRI of the Human Brain with Simultaneous MEG

November 16, 2007 (H14).  In the past few years, low-field (microtesla level) magnetic resonance imaging (MRI) using SQUIDs as detectors has been pioneered by John Clarke’s Group at the University of California at Berkeley, CA, USA.  One of the consequences has been the proposal to integrate in one instrument the anatomical imaging of the brain by MRI with the functional imaging by magnetoencephalography (MEG).  This could create a new and especially powerful tool for brain research and diagnostics.  Work in this direction was initiated, among others, at the Los Alamos National Laboratory (LANL), Applied Modern Physics Group.  Thus far, no comparable activity has been reported on in Europe.  Therefore, this Forum would like to highlight an important milestone in that LANL R&D activity: Vadim S. Zotev, Andrei N. Matlashov, Petr L. Volegov, Igor M. Savukov, Michelle A. Espy, John C. Mosher, John J. Gomez, and Robert H. Kraus, Jr, submitted to the Experimental Nuclear Magnetic Resonance (ENC) Conference the abstract reproduced below.  Under the title above, it reports on the first ever low-field MRI of the human head and subsequent MEG using the same SQUID array.  We hope that this result will stimulate analogous R&D activity also in Europe, for example in the framework of a European (EU) research project.  The text of the abstract follows below.   
  
Magnetic resonance imaging at ultra-low fields (ULF MRI) is a promising new imaging method [1] that uses measurement fields in the microtesla range. In this method, sample magnetization is enhanced by a pre-polarization field substantially larger than the measurement field, and MRI signals are subsequently measured at an ultra-low field using SQUIDs (superconducting quantum interference devices) – the most sensitive detectors of magnetic flux. Unlike conventional high-field imaging, ULF MRI is compatible with SQUID-based techniques for biomagnetic measurements, such as magnetoencephalography (MEG) [2]. The combination of MEG and ULF MRI capabilities in one multi-channel instrument would eliminate the MEG/MRI co-registration errors and allow simultaneous functional (MEG) and anatomical (ULF MRI) imaging of the human brain.

Recently, we reported the first multi-channel SQUID system specially designed for both ULF MRI and MEG [3,4]. The seven 37 mm diameter second-order SQUID gradiometers have magnetic field resolutions of 1.2 to 2.8 fT/√Hz. Five sets of magnetic field and gradient coils are used for 3D Fourier imaging using pre-polarization. The system was used to acquire ULF images of a human hand and record auditory MEG [3]. It was also used to demonstrate benefits of multi-channel imaging at ULF, including imaging acceleration with the sensor array [5].

In this work, we report the first MR images of the human head acquired at an ultra-low (46 microtesla) measurement field (Figure 1). The subject’s head was placed under the bottom of the cryostat. A pre-polarization field of 30 mT was applied for 1 sec prior to each imaging step. The image was acquired at 46 µT field according to 3D Fourier protocol with 3 mm × 3 mm × 6 mm resolution. A multiple-echo technique was used, and three echoes with echo tops at 65 ms, 137 ms, and 210 ms were recorded. The images in Figure 1 are composite seven-channel T2 weighted images corresponding to the first echo. They were obtained by averaging six consecutive scans with 81×61×11 image matrix size. Only four horizontal layers of the 3D image (out of 11 acquired) are shown in the figure. The images corresponding to the second and third echoes (not shown) exhibit stronger T2 contrast. Using the multiple-echo data, we were able to estimate values of the transverse relaxation time T2 for different parts of the human head at ULF for the first time.

Immediately following the ULF MRI experiment, and without removing the human subject from inside the system, we also performed auditory MEG measurements. 
Our results demonstrate feasibility and potential of the human brain imaging at microtesla magnetic fields. They also show that SQUID-based instrumentation com­bining MEG and ULF MRI for advanced brain studies is practical.

References:
[1] R. McDermott et al. Proc. Natl. Acad.  Sci. 101, 7857 (2004).
[2] P. Volegov et al. Magn. Reson. Med. 52, 467 (2004).
[3] V.S. Zotev et al. IEEE Trans. Appl.  Supercond. 17, 839 (2007).
[4] V.S. Zotev et al. Supercond. Sci.Technol. 20, S367 (2007).
[5] V.S. Zotev at al. Preprint physics/0701188 (2007).

Fig. 1. 3D image of the human head acquired at 46 µT field.
Fig. 1. 3D image of the human head acquired at 46 µT field.

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