Magnetoencephalography: Theory, Instrumentation, and Applications to Noninvasive Studies of the Working Human Brain

We also refer to the article when discussing SQUID gradiometers, which they discuss in detail. Russ and I have a figure in IPMB showing two types of gradiometers; here I show a color version of this figure adapted from Hämäläinen et al.

In Chapter 11 of IPMB, Russ and I reproduce my favorite figure from Hämäläinen et al.: their Fig. 8, showing the spectrum of several magnetic noise sources. Earlier in our book, Russ and I warn readers to beware of log-log plots in which the distance spanned by a decade is not the same on the vertical and horizontal axes. Below I redraw Hämäläinen et al.’s figure with the same scaling for each axis. The advantage of this version is that you can easily estimate the power law relating noise to frequency from the slope of the curve. The disadvantage is that you get a tall, skinny illustration.

I like many things about Hämäläinen et al.’s the review. They present some lovely pictures of neurons drawn by Ramon Cajal. There’s a detailed discussion of the magnetic inverse problem, and a long analysis of evoked magnetic fields. In IPMB, Russ and I mention using a magnetically shielded room to reduce the noise in MEG data, but don’t give details. Hämäläinen et al. describe their shielded room:

The room is a cube of 2.4-m inner dimensions with three layers of μ-metal, which are effective for shielding at low frequencies of the external magnetic noise spectrum (particularly important for biomagnetic measurements), and three layers of aluminum, which attenuate very well the high-frequency band. The shielding factor is 10³—10⁴for low-frequency magnetic fields and about 10⁵ for fields of 10 Hz and above.

They show a nice photo of a subject having her MEG measured in this room; I hope she’s not claustrophobic.

The authors were members of a leading biomagnetism group in the 1990s. Matti Hämäläinen is now with the Athinoula A. Martinos Center for Biomedical Imaging at Massachusetts General Hospital and is a professor at Harvard. Rita Hari is an emeritus professor at Aalto University (formerly the Helsinki University of Technology). Risto Ilmoniemi is now head of the Department of Neuroscience and Biomedical Engineering at Aalto. Olli Lounasmaa (1930—2002), the leader of this impressive group, was known for his research in low temperature physics. In 2012 the Low Temperature Laboratory at Aalto was renamed the O. V. Lounasmaa Laboratory in his honor.

What do I like best about the Finn’s landmark review? They cite me! In particular, the experiment I performed as a graduate student working with John Wikswo to measure the magnetic field of a single axon.

Wikswo et al. (1980) reported the first measurements of the magnetic field of a peripheral nerve. They used the sciatic nerve in the hip of a frog; the fiber was threaded through a toroid in a saline bath. When action potentials were triggered in the nerve, a biphasic magnetic signal of about 1 ms duration was detected. Later, the magnetic field of an action potential propagating in a single giant crayfish axon was recorded as well (Roth and Wikswo, 1985). The measured transmembrane potential closely resembled that calculated from the observed magnetic field. From these two sets of data, it was possible to determine the intracellular conductivity.

The videos below, presented by several of the authors, augment the discussion of biomagnetism in Intermediate Physics for Medicine and Biology, and provide a short course in magnetoencephalography. Enjoy!