A. Pasquarelli

28-channel hybrid system for neuromagnetic measurements

Progress toward the development of a 28-multichannel system for neuromagnetic measurements is described. A novel hybrid design consisting of 16 first-order axial gradiometers and 12 first-order planar gradiometers was chosen, which optimize the use of the available cylindrical volume of the dewar tail. This configuration maintains the symmetry of the detected pattern with respect to rotation of a biomagnetic source located under the center of the array and features a localization power considerably better than an array of all first-order planar gradiometers. The detecting array permits simultaneous magnitude measurements over a circular scalp region of 16-cm diameter. The magnetic sensors used are Nb/PbAuIn DC SQUIDs fabricated at IBM. The devices incorporate resonant damping resistors shunting the inductance, resulting in smooth flux-voltage characteristics and, consequently, very low noise figures in a flux-locked loop configuration. A simple and low-cost electronic system has been designed and fabricated for the DC SQUID sensors, consisting of a compact head mounted on top of the cryogenic probe and a control unit where all the DC remote controls are performed. The dynamic range of 10/sup 7/ square root Hz and the bandwidth of 50 kHz are specifically designed to work in multichannel DC SQUID instrumentation.

Measurement of Segmental Transit Through the Gut in Man A Novel Approach by the Biomagnetic Method

The techniques commonly used to evaluate the transit of contents through the gut feature some limitations for being either inaccurate, invasive, inconvenient, or potentially dangerous for the subjects. Aim of this study was to establish a safe, noninvasive and accurate technique for the measurement of segmental oroanal transit time. We localized an orally ingested magnetic marker by means of a biomagnetic instrumentation that allows us to identify in a three-dimensional pattern the position of a biomagnetic source inside the body. The biomagnetic localizations were compared with the anatomical data obtained by magnetic resonance imaging investigations. The study was performed in 12 healthy subjects, and scans were taken every hour up to the arrival of the marker into the cecum; thereafter, scans were taken every 4 hr up to the elimination of the marker. In 99% of the isofield maps obtained from each field scan, the marker was localized within the bowel walls. The mean oroanal transit time was 56±5 hr, the mouth-to-cecum transit time was 13±1.7 hr, and the total colonic transit time was 43.5±5 hr (mean±sem). Segmental colon transit did not show major differences among the regions considered, although most of the time was spent in the right colon. In fact, a good correlation was found between transit time through the right colon and oroanal and total colonic transit (r=0.77,P<0.02,r=0.79,P<0.02 respectively). In conclusion, this method might be a safe alternative to the techniques presently used in the clinical setting for the measurement of intestinal transit.

Magnetocardiography and exercise testing

Twenty healthy male subjects (age range, 15-25 years; median, 21 years) underwent magnetocardiography during physical exercise. Significant ST-segment displacements of the magnetic signal were found during exercise at a heart rate of 120 beats/min compared to the magnetic signal at rest (P < .001). Since no significant ST-segment changes were found in the electrocardiogram recorded simultaneously with the magnetocardiogram, it is concluded that the magnetocardiogram shows junctional ST-T segment changes earlier than the electrocardiogram.

A SQUID based AC susceptometer for the investigation of large samples.

We developed a SQUID based susceptometer with a large available magnetized volume for the investigation of large objects. The magnetizing field is generated by a pair of Helmoltz coils. To achieve a high signal-to-noise ratio, the susceptometer is operated in a lock-in mode with an AC magnetizing field. A negative feedback control allows the rejection of the applied field with a relative residual of 1 x 10(-7). The apparatus was tested with substances of known magnetic susceptibility. The overall sensitivity, stated in terms of the magnetic moment, is better than 7 x 10(-11) A m2 for small samples.

The study of steady magnetic fields associated with primary and secondary ST shift in ischaemic rabbit hearts

The study of injury potentials associated with DC currents that generate the primary or secondary ST shifts during cardiac ischaemia is possible only through the invasive technique of the DC electrogram. Clinical surface ECG recordings are AC coupled and cannot be used. This paper reports the use of non-invasive and unshielded magnetocardiographic measurements to evaluate the DC injury currents associated with ST shifts during coronary artery occlusions in the isolated rabbit heart. The effect on the magnetic ST shift is studied under different ischaemic conditions including regional ischaemia, global ischaemia, global ischaemia following long periods of regional ischaemia, regional ischaemia after repeated episodes of reversible global ischaemia, and bilateral regional ischaemia. Recording of DC magnetic fields allows the characterization of primary and secondary ST displacement for each induced ischaemic condition. Our measurements show that the ST shift starts earlier when inducing ischaemia in hearts previously subjected to ischaemic episodes than in hearts where the ischaemia was produced for the first time.

A Nine Channel DC SQUID System for Biomagnetism

In the last few years several multi-channel systems for biomagnetic measurements have been developed. The first examples featured four-five sensors (Ilmoniemi et al., 1984; Romani, 1984; Williamson et al., 1984), whereas more recently a seven-channel system became commercially available, also in a configuration which endowes simultaneous investigation at 14 scalp sites with two twin systems (Biomagnetic Technologies Inc.). However several projects aimed to develop larger systems are being carried on in several countries both by groups involved in fundamental research and by industries. In general, these systems are designed to be used for neuromagnetic studies, even if efforts are being made also to setup large size cardiomagnetometers (Erne’, personal communication). A common feature is that all systems are planned to work in shielded environments.

Measurement of gastrointestinal transit time by means of biomagnetic instrumentation: preliminary results

The measurement of gastrointestinal transit time, GITT, is of particular interest in the study of gastrointestinal activity and in the diagnosis of some pathologies. In the present work we report the measurements of GITT obtained by applying the biomagnetic method.

The clinical magnetocardiographer in Ulm

We present a magnetocardiographic system developed in the framework of an international cooperation and installed at the University of Ulm. The system design is conceived for clinical and routine use. The main sensor consists of a hexagonal 55 channels planar system with dc SQUID magnetometers contained in a low noise dewar. Nineteen reference channels are used to form software gradiometers. The patient handling setup and the dewar support are designed to easily allow the positioning procedure and the interaction with the patient. The sensor is operated in a high quality magnetically shielded room made of three soft magnetic metal layers and an external aluminum layer. The low frequency performances can be enhanced by means of an active compensation technique. A set of biopreamplifiers for 128 channels BSPM measurements is included. The data acquisition system allows to perform MCG and BSPM measurement simultaneously with sampling frequencies up to 10 kHz. All basic preprocessing, bandpass filtering, decimation and noise compensation are performed on the fly using an array of DSPs. The data analysis session provides both time domain approach and source identification approach. Direct integration of the results with CT or MRI data is also available.

Biomagnetic measurements utilising a superparamagnetic marker: a feasibility study

SummaryA preliminary study is here reported on a new potential marker for biomagnetic measurements. The marker consists of superparamagnetic polymer microspheres which were detected in the presence of external steady magnetic fields by means of an r.f.-SQUID magnetometer. The particles were prepared in samples differing in the concentration value and immersed in a homogeneous magnetic field of variable intensity. A simple model was taken into account for the distribution of the microspheres in the samples, so that the theoretical values were compared to the marker field values measured by the biomagnetic sensor. The overall sensitivity of the experimental apparatus and the minimum concentration value of the marker were then estimated.

11 channel magnetometer for biomagnetic measurements in unshielded environments

The authors describe a novel 11-channel magnetometer designed for operation in unshielded environments. The planar geometry of the system is particularly suitable for recording biomagnetic signals over a flat surface, as required for cardiac and gastrointestinal applications. The specific configuration of the sensor array also makes it possible to use the system for reliable and fast cardiac clinical measurements. Ambient noise rejection was achieved by the use of second-order gradiometers, optimized by means of additional fine balancing, whereas a sensitivity of less than 10 fT/Hz/sup 1/2/ was obtained by low-noise DC-SQUIDs (superconducting quantum interference devices), using Nb/Nb/sub 2/O/sub 5//PbAuIn technology for the Josephson junctions. The system is presently utilized for a systematic investigation of the localization properties of cardiac sources from data obtained during single-shot measurements, which are believed to be suitable for clinical studies. The results of localization are compared with anatomical information from magnetic resonance imaging data.<<ETX>>