We developed a SQUID biomagnetic measurement system for magnetospinography/magnetoneurography (MSG/MNG). Biomagnetic measurement is a promising method to noninvasively obtain functional information of neurons or muscles in a living body. Weak magnetic fields elicited by their electric activities were captured by highly sensitive magnetic flux sensors arranged along the body surface. The current distribution was estimated by appropriate source reconstruction algorithm applied to the obtained magnetic field distribution. The estimated current distribution overlaied on the anatomical image acquired by magnetic resonance imaging (MRI) or radiographic imaging is informative in terms of neuromuscular function and contributes a lot to the accurate diagnoses of neurological disorders.

The intensity of biomagnetic fields observed on the body surface are quite small and ranging from several fT to several tens pT. Therefore, superconducting quantum interference device (SQUID)-based magnetic flux sensors are practically applied to the detection of the biomagnetic fields. Various SQUID-based measurement systems for magnetoencephalograph (MEG) and magnetocardiograph (MCG) are already commercialized and introduced to hospitals for diagnoses of the brain/heart diseases.

As well as the conventional SQUID-based biomagnetic measurement systems such as MEG or MCG, the measurement system for MSG/MNG is equipped with an array of SQUID sensors, a cryostat to keep superconducting state of the sensors, electronics to linearization and dynamic range enhancement of the sensor outputs, a digital data acquisition unit with filters, and PCs for control and analysis.

However, to detect MSG/MNG signals that are smaller and faster than MEG signals, various improvements were added to the measurement system; the sensor array composed of vector-type SQUID gradiometers, the uniquely-shaped cryostat, and a digital data acquisition unit with higher sampling rate. Especially, the cryostat design was optimized for fitting the sensor array to the posterior of a supine subject. The cryostat composed of a cylindrical main body to reserve liquid helium (LHe) and the horizontal protrusion from its side surface. The SQUID sensor array oriented vertically was installed in the protrusion along its upper surface. The magnetic signals from any part of the body can readily be obtained as long as the subject placed a certain body part on the protrusion. Additionally, owing to the structure of the cryostat, on-site X-ray imaging became applicable to obtain the skeletal structure of the subject from lateral and anterior sides. The obtained X-ray image was effective for positioning a subject and further magnetic source analysis.

Whenever we introduced a SQUID-based system to a hospital, the operational cost of the LHe consumption became a large inevitable problem so far. To solve the problem of the LHe cost, we combined the MSG/MNG system with a closed-cycle helium recondensing system, which could recycle almost 100% of LHe. It enabled the system to work without refilling LHe for more than one year and drastically suppressed its operational cost.

The prototype of the MSG/MNG system is continuously being operated at the Tokyo Medical and Dental University hospital for years under the cooperation between medicine and engineering. In addition to the development of the hardware of the system, various software for analysis and visualization were also developed based on innovative ideas proposed from the medical staffs operating the MSG/MNG system. In particular, "the virtual electrodes" arranged along the neural pathways in the computer can display the behavior of the current distribution estimated from the magnetic field in waveforms. The waveforms from the virtual electrodes make the data obtained by the MSG/MNG easy to understand for physicians familiar with conventional electrophysiological testing and effective in diagnosing the site of the lesion.

Through our recent studies, we have successfully observed and evaluated nerve activities in the cervical, lumbar, thoracic, shoulder, elbow, and palmar areas, using the MSG/MNG system. In the future, the MSG/MNG is expected to spread as a new diagnostic tool for neurological diseases obtaining the functional information of nerves in every body part.

Keywords: SQUID, Biomagnetism, Magnetospinograph, Magnetoneurograph