Mitsuru Ohnuma

Open-type magnetocardiograph with cylindrical magnetic shield

We have developed an open-type magnetocardiograph (MCG) with a cylindrical magnetic shield—having an auxiliary magnetic shield—and 64 gradiometers based on low-transition temperature (Tc) superconducting quantum interference devices (SQUIDs). A revolving door is applied to the magnetic shield with the aim of providing easy access to a subject and easy operation. The cryostat attached to the MCG contains 64 low-Tc SQUIDs and is about 1m in height, so there must be an opening through the cylindrical surface of the magnetic shield. The properties of the magnetic shield were simulated using finite-element-method simulations and experiments. The results of the simulations indicated that the auxiliary magnetic shield recovers enough of the shielding performance of a magnetic shield with an opening that it performs like a magnetic shield without an opening. The shielding factor of the developed magnetic shield is above 46dB in the vicinity of the magnetic shield. Accordingly, the open-type MCG can clearly measur...

Unshielded fetal magnetocardiography system using two-dimensional gradiometers

We performed fetal magnetocardiography (fMCG) measurements in an unshielded hospital environment to demonstrate our developed fMCG system using two-dimensional (2D) gradiometers based on low-T c superconducting quantum interference devices. The subjects were 25 pregnant women, including two subjects with twin pregnancies, or 27 fetuses, in this study. The 2D gradiometer was optimized for fMCG measurements and detected both the axial second-order gradient and planar-first-order gradient of a magnetic field. Depth of the fetal heart was also measured by using ultrasonography before fMCG measurement. As a result, the QRS peaks of the fMCG waveform were detected in real time for 15 cases including early fetal gestational periods as 24 and 25 weeks. Moreover, the P and T waves were also detected in some cases by averaging. These results demonstrate that the developed unshielded fMCG system can detect fMCG waveforms in an unshielded hospital environment.

Demonstration of Unshielded Fetal Magnetocardiography System Using Two-Dimensional Gradiometers

We performed fetal magnetocardiography (fMCG) measurements in an unshielded hospital environment to demonstrate our developed fMCG system using two-dimensional (2D) gradiometers based on low-T c superconducting quantum interference devices. The subjects were 25 pregnant women, including two subjects with twin pregnancies, or 27 fetuses, in this study. The 2D gradiometer was optimized for fMCG measurements and detected both the axial second-order gradient and planar-first-order gradient of a magnetic field. Depth of the fetal heart was also measured by using ultrasonography before fMCG measurement. As a result, the QRS peaks of the fMCG waveform were detected in real time for 15 cases including early fetal gestational periods as 24 and 25 weeks. Moreover, the P and T waves were also detected in some cases by averaging. These results demonstrate that the developed unshielded fMCG system can detect fMCG waveforms in an unshielded hospital environment.

Note: Unshielded bilateral magnetoencephalography system using two-dimensional gradiometers

Magnetoencephalography (MEG) noninvasively measures neuronal activity with high temporal resolution. The aim of this study was to develop a new type of MEG system that can measure bilateral MEG waveforms without a magnetically shielded room, which is an obstacle to reducing both the cost and size of an MEG system. An unshielded bilateral MEG system was developed using four two-dimensional (2D) gradiometers and two symmetric cryostats. The 2D gradiometer, which is based on a low-T(c) superconducting quantum interference device and wire-wound pickup coil detects a magnetic-field gradient in two orthogonal directions, or ∂/∂x(∂(2)B(z)/∂z(2)), and reduces environmental magnetic-field noise by more than 50 dB. The cryostats can be symmetrically positioned in three directions: vertical, horizontal, and rotational. This makes it possible to detect bilateral neuronal activity in the cerebral cortex simultaneously. Bilateral auditory-evoked fields (AEF) of 18 elderly subjects were measured in an unshielded hospital environment using the MEG system. As a result, both the ipsilateral and the contralateral AEF component N100m, which is the magnetic counterpart of electric N100 in electroencephalography and appears about 100 ms after the onset of an auditory stimulus, were successfully detected for all the subjects. Moreover, the ipsilateral P50m and the contralateral P50m were also detected for 12 (67%) and 16 (89%) subjects, respectively. Experimental results demonstrate that the unshielded bilateral MEG system can detect MEG waveforms, which are associated with brain dysfunction such as epilepsy, Alzheimers disease, and Down syndrome.

Performance of open cylindrical magnetic shield for magnetocardiograph using low-Tc SQUIDs

We evaluated the performance of an open cylindrical magnetic shield for a magnetocardiograph using low-transition temperature (low-Tc) superconducting quantum interference devices (SQUIDs) so that we could miniaturize an entire magnetocardiograph system. The open magnetic shield was about 1 m in diameter and 2 m in axial length, and had a revolving door that provided easy access inside the shield as well as easy operation. The shielding factor for our magnetic shield was above 46 dB, and its distribution was uniform in the vicinity of the shield. The magnetocardiogram including the P-wave peak could clearly be measured with 64 axial gradiometers based on low-Tc SQUIDs in the open magnetic shield. Typical distributions of myocardial ion currents of a normal subject were also obtained with the system.