SQUIDs in biomagnetism: a roadmap towards improved healthcare

Abstract

Globally, the demand for improved health care delivery while managing escalating costs is a major challenge. Measuring the biomagnetic fields that emanate from the human brain already impacts the treatment of epilepsy, brain tumours and other brain disorders. This roadmap explores how superconducting technologies are poised to impact health care. Biomagnetism is the study of magnetic fields of biological origin. Biomagnetic fields are typically very weak, often in the femtotesla range, making their measurement challenging. The earliest in vivo human measurements were made with room-temperature coils. In 1963, Baule and McFee (1963 Am. Heart J. 55 95−6) reported the magnetic field produced by electric currents in the heart (‘magnetocardiography’), and in 1968, Cohen (1968 Science 161 784−6) described the magnetic field generated by alpha-rhythm currents in the brain (‘magnetoencephalography’). Subsequently, in 1970, Cohen et al (1970 Appl. Phys. Lett. 16 278–80) reported the recording of a magnetocardiogram using a Superconducting QUantum Interference Device (SQUID). Just two years later, in 1972, Cohen (1972 Science 175 664–6) described the use of a SQUID in magnetoencephalography. These last two papers set the scene for applications of SQUIDs in biomagnetism, the subject of this roadmap. The SQUID is a combination of two fundamental properties of superconductors. The first is flux quantization—the fact that the magnetic flux Φ in a closed superconducting loop is quantized in units of the magnetic flux quantum, Φ0 ≡ h/2e, ≈ 2.07 × 10−15 Tm2 (Deaver and Fairbank 1961 Phys. Rev. Lett. 7 43–6, Doll R and Näbauer M 1961 Phys. Rev. Lett. 7 51–2). Here, h is the Planck constant and e the elementary charge. The second property is the Josephson effect, predicted in 1962 by Josephson (1962 Phys. Lett. 1 251–3) and observed by Anderson and Rowell (1963 Phys. Rev. Lett. 10 230–2) in 1963. The Josephson junction consists of two weakly coupled superconductors separated by a tunnel barrier or other weak link. A tiny electric current is able to flow between the superconductors as a supercurrent, without developing a voltage across them. At currents above the ‘critical current’ (maximum supercurrent), however, a voltage is developed. In 1964, Jaklevic et al (1964 Phys. Rev. Lett. 12 159–60) observed quantum interference between two Josephson junctions connected in series on a superconducting loop, giving birth to the dc SQUID. The essential property of the SQUID is that a steady increase in the magnetic flux threading the loop causes the critical current to oscillate with a period of one flux quantum. In today’s SQUIDs, using conventional semiconductor readout electronics, one can typically detect a change in Φ corresponding to 10−6 Φ0 in one second. Although early practical SQUIDs were usually made from bulk superconductors, for example, niobium or Pb-Sn solder blobs, today’s devices are invariably made from thin superconducting films patterned with photolithography or even electron lithography. An extensive descriptio...

Dynamic nuclear polarisation of liquids at one microtesla using circularly polarised RF with application to millimetre resolution MRI

“…Nuclear magnetic resonance (NMR) at ultra-low fields has attained interest in several fields such as combining magnetic resonance imaging (MRI) with magnetoencephaolography (MEG) [1,2], resolving J-coupling structures in molecules [3,4,5], enhancing MRI contrast between certain biological tissues [6], obtaining MR image of samples enclosed in metal [7], detecting neuronal currents directly [8,9] and measuring fundamental quantities [10]. The detection of NMR signals in ultra-low fields, typically below the earth's magnetic field, requires highly sensitive sensors, such as superconducting quantum interference devices (SQUID) [11,12] or optically-pumped atomic magnetometers [13,14].…”

Miniaturized Magnetic Sensors for Implantable Magnetomyography

“…The SQUIDs are widely used in many applications including in biomedical engineering for sensing MCG and MEG signals. Such sensitivity levels of SQUIDs require them to remain in a magnetically shielded room that is equipped with an appropriate cooling system for operation at liquid‐helium temperature 4.2 K, [ 38 ] which also increases the cost of SQUIDs to several thousands of dollars.…”

Feasibility of functional MRI at ultralow magnetic field via changes in cerebral blood volume

“…It has been suggested that the weak fluctuating magnetic fields observed with MEG might be detected directly with ULFMRI from the tiny frequency shift in the signal (Neuronal Current Imaging, NCI) (Kraus et al, 2008;Cassara et al, 2008;Cassara et al, 2009;Körber et al, 2013). Although recent advances in SQUID technology have reduced the noise closer to the predicted required level (Körber et al, 2016), so far there has been no demonstration of NCI in a human brain.…”