Design principles of highly sensitive microwave measuring devices with waveguide/waveguide resonator sensors of nonelectrical quantities are proposed. These sensors contain passive microwave components for controlling propagation characteristics of guided microwaves. Such sensors are characterized by increased measurement accuracy and sensor sensitivity to measured parameters in comparison with prototypes without such components. Examples of schemes for separate single/multiple interaction of oppositely-directed electromagnetic waves with monitored objects in waveguide resonators are described. Applications of these principles for the design of various microwave measuring devices are considered.

The effective aperture of the antenna is determined using measured backscattering data. An oscillator with a known port impedance is connected to the antenna under test to modulate the backscattered radiation. The measurement is based on measuring the signal in the modulation sidebands. Using a simple circuit model for the antenna-oscillator system, the antenna effective aperture, bandwidth and radiation pattern can be determined. In this paper, theory for antenna aperture measurement is developed, and measurements with error considerations of an antenna with a highly reactive port impedance at the application frequency are presented. The results are compared to a traditional transmission measurement results.

The effect of alfentanil on suxamethonium‐induced muscle fasciculations was studied in a double‐blind study in 34 children (mean age 6.8 years) and in 30 adults (mean age 20 years). After pretreatment with either alfentanil 50 μg kg‐1 or saline, each patient was anaesthetized with a sleep dose of thiopental followed by suxamethonium 1.5 mg kg‐1 for endotracheal intubation. Compared to the control groups, alfentanil significantly decreased the intensity of visible muscle fasciculations caused by suxamethonium. In children, the duration of muscle fasciculations was shorter in the alfentanil than in the control group. In adults, the intensity rather than the duration of fasciculations was attenuated by alfentanil. The inhibition of fasciculations caused by alfentanil was also demonstrated in children in the surface electromyogram recorded on the biceps. There was no circulatory response to endotracheal intubation in the groups pretreated with alfentanil.

The origin of the magnetic susceptibility signal caused by mechanical activity of the heart is studied. Magnetic signals were measured with a second order SQUID (superconductor quantum interface device) gradiometer by applying a homogeneous external magnetic field with magnitude up to 260 A/m. Three experimental methods were used to separate the different components in the magnetic susceptibility signal. First, the thorax of the subject was surrounded by water solution of MnCl/sub 2/ with various magnetic susceptibilities, then a plastic bag containing a similar solution was placed on the chest of the subject, and finally the chest wall of the subject was immobilized during the measurements by using a special vacuum casting. The results suggest that usually about 65-80% of the signal measured near the heart originates directly from cardiac volume changes. This cardiac plethysmogram can be separated easily by immobilizing the subject's chest or by surrounding the chest with the liquid, whose magnetic susceptibility is about -5*10/sup -6/.< >

A model is developed for the source of magnetic field fluctuations emanating from thermal agitation of electric charge in conductors. The calculation of the thermal magnetic noise with the model involves the solution of a general volume conductor forward problem. Frequency-dependent equations for this problem are derived from Maxwell’s equations. The model is applied to calculate the magnetic noise generated by infinite conducting slabs. A good agreement between theoretical predictions and experimental results were found. Thermal magnetic noise fields inside magnetically shielded rooms are estimated and noise in biomagnetic measurements is discussed.

The sources of auditory evoked electric and magnetic responses were modelled with multiple current dipoles. The activity at the auditory cortices, 2.6 cm beneath the skull, explains the observed\(\overline {N100m}\) field distribution. An additional current source near the vertex is needed to explain the corresponding potential distributions. The locations of the current sources of\(\overline {N100m}\) did not show any systematic dependence on the tonal frequency.

Magnetic-susceptibility plethysmography (MSPG) is a new method proposed for monitoring cardiac-volume changes. The measurement principle and recent work by other investigators are reviewed. The effect of the cardiac-related motion of the anterior chest wall on the MSPG signal is studied. The results suggest that the motion of the chest-wall-air boundary may contribute significantly (∼75%) to the MSPG signal when recorded near the heart.

Voluntary limb movements are preceded by MEG shifts beginning even as early as 1.2 s before the movement. These shifts reverse polarity above the cortical motor representation area of the limb concerned.

Somatosensory evoked magnetic fields were compared with corresponding electric scalp potentials in order to study their neural sources. Mappings of the magnetic responses showed activity at the primary and secondary somatosensory cortices. Additional sources besides these areas seem to be involved in the generation of the somatosensory vertex scalp potentials.

We have constructed a three-channel differential SQUID magnetometer for the simultaneous measurement of the three orthogonal components of the magnetic field in the same location. The structure of the device is described and MCG data measured are shown. Interference effects between the channels are discussed.

The MCGs measured in a case of an anterolateral myocardial infarction were compared with the corresponding simulated MCGs. The most distinctive changes in the measured MCGs were observed in the upper and lower parts of the thorax as predicted by the simulation.

Magnetic susceptibility plethysmography (MSPG) is a non-invasive technique for detecting changes in the total cardiac volume. The MSPG signal is observed with a sensitive magnetometer when an external magnetic field is applied to the thorax. The signal is due to the differences in magnetic susceptibilities of intracardiac blood, heart muscle, and surrounding tissues. In this work the spatial distribution of the MSPG signal across the chest of normal subjects is presented. The ventricular ejection of blood was clearly exhibited in signals measured above the heart. Relatively large MSPG signals were also observed on the upper and lower thorax during the ventricular ejection, related to the blood flow in the aortic arch and in the abdominal aorta, respectively tively. The temporal behavior of the MSPG is compared with the results obtained from existing clinical methods. The origin of the MSPG signal is analyzed theoretically. When a fixed cardiac magnetic dipole model was used, overall change in cardiac volume as a function of time could be evaluated with reasonable accuracy when the proximity effect of the torso was taken into account.

Auditory vertex responses elicited by short tone bursts were compared with their magnetic counterparts. Special attention was paid to the behaviour of the N100 deflection of the response. Electrical responses were recorded from scalp locations Fp2, Fz, Cz, Pz, C4 and T4 and the magnetic responses half way between P4 and T6, at a point where the response has one of its amplitude extrema. Different ISIs (from 1 to 16 sec) were applied in order to differentiate specific and nonspecific evoked potential components from each other. The main results were as follows: 1.(1) The scalp distribution of the electrical vertex response depends on the ISI used: with frequent stimulation there are no marked differences in the amplitudes of N100 between frontal and central areas but with long ISIs the amplitude maxima move to the vertex.2.(2) The magnetic responses also show a clear ISI dependence. The magnetic counterpart of N100 saturates at shorter ISIs than N100 recorded from the vertex. Independent of the ISI the magnetic counterpart of P200 is constantly very small.

The retina and the pigment epithelium of the human eye contain sources of bioelectric currents, which give rise to intra- and extraocular electric current densities. This paper first gives a review on the magnetic fields arising from ocular currents: the magneto-oculogram (MOG) and the magnetoretinogram (MRG). A mathematical model is presented for calculation of the field distributions and a comparison with experimental data is made. Finally results from experiments on motion and light induced changes in ocular magnetic fields are used to determine retinal current densities.