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Bioelectromagnetism

(deoxyhemoglobin) afer oxygen is released in the capillaries. Tis deoxyhemoglobin causes a slight

distortion of the magnetic felds around intravenous blood vessels. Tis distortion weakens the proton

signal in the vicinity. Tis phenomenon is called the blood oxygenation level dependent (BOLD) efect.

Using this efect, it is possible to image dynamic changes in local oxygen metabolism and regional

blood fow. As a more advanced imaging technology, functional fMRI was introduced in 1990, due to

the blood-oxygenation level-dependent efect, which was discovered by Seiji Ogawa, now at Tohoku

Fukushi University, and his co-workers at Bell Laboratory (Ogawa et al., 1990a, b, 1992). Hemoglobin

is diamagnetic, and deoxyhemoglobin is paramagnetic which means that the magnetic susceptibility

of these two materials is slightly diferent. Focusing on the NMR signal caused by this slight diference

in magnetic susceptibility, the measurement using the BOLD efects was proposed. Since Ogawa’s pro­

posal, fMRI was widely spread interest and applications in neurocognitive and neurophysiological stud­

ies. fMRI depends also on the basis of NMR. Since fMRI can image brain activity with high resolution

in a non-invasive manner, fMRI is being used to elucidate the mechanisms of functional brain activity

in the felds of medicine, physiology, cognitive science, and education.

As mentioned above, fMRI focuses on the slight diferences in magnetic susceptibility of materi­

als in the blood. Historically, in 1846, Michael Faraday investigated the magnetic properties of dried

blood and found that blood was not magnetic (Faraday, 1846). Pauling pointed out that if he deter­

mined the magnetic susceptibilities of atrial and venous blood, he would have found them to difer by

a large amount (as much as 20 per cent) for completely oxygenated and completely deoxygenated blood

(Pauling and Coryell, 1936a).

Aferward, Pauling focused on the oxygen saturation in the blood and the magnetism of the blood.

Linus Carl Pauling (1901–1994), professor at California Institute of Technology, and his student, Charles

Dubois Coryell (1912–1971), later professor at the Massachusetts Institute of Technology (MIT), reported

that oxygenated as well as carbonmonoxy forms of hemoglobin were diamagnetic, but discovered that

the deoxygenated protein was magnetic (Pauling and Coryell, 1936a, b). Te magnetic susceptibility of

blood hemoglobin changed as a function of whether it was bound to oxygen or not. Pauling and Coryell

reported that ferrohemoglobin (hemoglobin) is strongly paramagnetic and contains four unpaired elec­

trons per heme. Te oxygen molecule, O2, contains two unpaired electrons and is also paramagnetic.

However, when O2 is combined with ferromagnetic to form oxyferrohemoglobin (also called oxyhe­

moglobin) with no unpaired electrons and is diamagnetic. Pauling received in 1954 the Nobel Prize in

Chemistry for his research into the nature of the chemical bonds and its application to the elucidation

of the structure of complex substances. He was awarded the Nobel Peace Prize in 1962 for his opposition

to weapons of mass destruction.

2.5.2.3 Bioelectric-Generated Magnetism

Biomagnetism is found in electric current changes. Tis means that biomagnetic felds can be produced

by electric current fow in biological systems. Recording of biomagnetic activity of weak signals is dif­

ferent from the recording of bioelectric signals. Te former needs the use of sensitive magnetometers.

Te technical development of sensitive magnetometers requires detecting very weak signals of electrical

phenomena. ECG and EEG measures directly and non-invasively the endogenous oscillatory, electrical

activity in the heart and brain. Magnetocardiography (MCG) and magnetoencephalography can mea­

sure indirectly and non-invasively the magnetic felds generated by the changes of electrical activity. Te

time course of ECG and MCG signals are basically similar.

In 1963, Gerhard Baule and Richard McFee, both at Syracuse University, USA, detected for the frst

time MCG of electrical activity in a human heart with an induction coil magnetometer outdoors in an

open feld (Baule and McFee, 1963). Te magnetometer was made by winding two million turns of cop­

per wire on a dumbbell-shaped ferrite core (about 30 cm in length and 9 cm in diameter). Te two copper

pick-up coils were connected in opposition so that they canceled the induced voltage from the uniform

magnetic background fuctuations. Later, a group from the USSR repeated the Baule-McFee’s experi­

ment and confrmed their experimental results (Cohen, 1969). Cohen pointed out that the measurement