The introduction of the superconducting quantum interference device (SQUID) – an extremely sensitive magnetometer – into the medical field has enabled the non-invasive detection of such small magnetic fields. SQUIDs are now used in magnetoencephalography (MEG) to measure biomagnetic fields that emanate from the human brain, and in magnetocardiography (MCG) to measure fields in the heart.

The journal Superconductor Science and Technology has published a roadmap exploring the applications of SQUIDs in biomagnetism. The article, SQUIDs in biomagnetism: a roadmap towards improved healthcare, brings together eight chapters that discuss different aspects of how superconducting technologies are poised to impact clinical care (Supercond. Sci. Technol. 29 113001).

Technology advances

The roadmap begins with a review of state-of-the-art SQUID-based magnetometers and gradiometers for biomagnetic measurements, with a focus on the approaches employed to reduce various sources of noise. The second chapter reviews the neuroscientific and clinical use of MEG – by far the most widespread biomagnetism application. MEG systems, which contain an array of SQUIDs cooled using liquid helium to about 4 K, are employed for clinical applications including pre-surgical mapping of focal epilepsy and identification of eloquent cortex in patients with brain tumours or vascular malformations.

Chapter three discusses high-transition-temperature (Tc) SQUID sensors for MEG. Operation at liquid-nitrogen temperature, 77 K, could enable sensors to be placed substantially closer to the scalp. This, in theory, would improve the MEG system's signal-to-noise ratio and spatial resolution. Chapter four introduces the concept of SQUID-based ultra-low-field MRI, which could potentially provide higher image contrast than conventional MRI. Possible clinical uses include screening for cancer without a contrast agent, and detecting traumatic brain injury, stroke and degenerative diseases such as Alzheimer's.

 The fifth chapter examines the combination of MEG and ultra-low-field MRI into a single multimodal system, based on a shared array of SQUIDs. This combination would enable structural imaging of the head concurrently with the recording of brain activity. Chapter six describes another application of ultra-low-field MRI: neuronal current imaging (NCI). Neuronal imaging requires measurements with high temporal and spatial resolution, but existing non-invasive techniques such as MEG and functional MRI cannot offer both. NCI combines the direct measurement of MEG with the spatial precision of MRI. While NCI cannot currently achieve sufficient sensitivity, it is close, and its realization could revolutionize functional brain imaging.

Chapter seven introduces the topic of magnetic nanoparticles for immunoassay. These particles are biofuntionalized, for example with an antibody that binds to its corresponding antigen, and any resulting changes in the properties of the nanoparticles are detected with a SQUID.

The roadmap concludes with a look at the commercial market for MEG systems. Despite huge advances since MEG's introduction, the number of commercial systems deployed around the world remains small (around 250 units employing about 50,000 SQUIDs), in part due to the high cost and the need to surround the entire system in an expensive magnetically shielded room. However, recent developments such as automatically refilling liquid-helium systems, reductions in sensor noise and the potential availability of high-Tc sensors, among others, could help increase the market size in the near future.

In the roadmap's abstract, John Clarke from UC Berkeley and Risto Ilmoniemi from Aalto University School of Science highlight the particular need for improved non-invasive technologies to measure brain function. With hundreds of millions of people worldwide suffering from brain disorders such as epilepsy, stroke, dementia or depression, earlier and more accurate diagnosis could reduce the enormous cost to society of these diseases.

"Once the challenges outlined in this roadmap have been met and the outstanding problems have been solved, the potential demand for SQUID-based health technology can be expected to increase by ten- if not hundred-fold," they write.

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