Electrical measurements of physiological potentials are well established in clinical diagnostics. The flow of an electrical current creates a corresponding magnetic field. Thus bioelectric activity will generate biomagnetic fields. These fields can be detected with Tristan SQUID magnetometers. For many of the classical electrical techniques such as ECG and EEG, there are corresponding magnetic techniques (magnetocardiography or MCG and magnetoencephalography or MEG). In some cases (magnetopneumography, hepatic iron store measurements, intestinal ischemia, peripheral nerve and muscle activity), there is no corresponding electrical technique and biomagnetic measurements offer unique measurement capabilities. Unlike CT or MRI imaging which yield structural information, biomagnetic measurements offer real-time functional imaging.

The strength of biomagnetic signals is many orders of magnitude smaller than even the earth’s magnetic field, which is 1/2 Gauss or 50 microtesla.  The signal strengths associated with biomagnetism (Fig. 1) require the use of extremely sensitive detection systems. The units in this figure are femtotesla, 1 fT = 10^-15 tesla.  The only instrument with the required sensitivity and bandwidth is the SQUID magnetometer.


Figure 1: Typical amplitudes and frequencies of various biomagnetic signals.                                                           The shaded regions indicate the limits of currently available SQUID sensors.

The components of a SQUID magnetometer (Fig. 2) typically consist of the following: a detection coil, which senses changes in the external magnetic field and transforms them into an electrical current; an input coil which transforms the resulting current into a magnetic flux in the SQUID sensor; electronics which transform the applied flux into a room temperature voltage output; and acquisition hardware and software for acquiring, storing and analyzing data.  Both the SQUID amplifier and the detection coils are superconducting devices. Thus some type of refrigerant (liquid helium or liquid nitrogen) or refrigeration device (cryocooler) is needed to maintain the SQUID and detection coil in the superconducting state. Additional signal conditioning electronics may be needed to improve signal-to-noise.

Figure 2: Block diagram of a SQUID magnetometer

The SQUID sensor and electronics package can be considered as a black box that acts like a magnetic field-to-voltage converter and amplifier with extremely high gain. In addition, it offers extremely low noise, high dynamic range, excellent linearity, flat phase response and a bandwidth that can extend from dc to beyond 100 kHz, capabilities that no other single sensor offers.

The type of SQUID sensor and detection coil configuration is dependent on what is to be measured. Figure 1 also shows the capability of both low temperature (requiring liquid helium temperatures, and referred to as LTS) and high temperature (requiring liquid nitrogen temperatures, and referred to as HTS) SQUID magnetometers. Tristan biomagnetic measurement systems make use of either Tristan’s LSQ/20 LTS dc SQUID sensor or the HTM-8 HTS dc SQUID sensor. The input coil for an LTS SQUID is normally fabricated from flexible superconducting NbTi wire. The inherent anisotropic nature of HTS SQUIDs requires that the input coils be planar. Typically HTS magnetometers are available only as pure magnetometers.

Another factor to be considered is the detection coil configuration. Conceptually, the easiest input circuit to consider for detecting changes in magnetic fields is a pure magnetometer (Fig. 2). However, magnetometers are extremely sensitive to all magnetic signals in the environment. This may be acceptable if one is measuring ambient fields. However, if the magnetic signal of interest is weak, then environmental magnetic interference may prevent measurements. If the signal source is close to the detection coil, then a gradiometer coil may allow a weak signal to be measured. Figure 3 shows the relative noise rejection for 1st and 2nd derivative gradiometers.  The figure insert shows a first order gradiometer, consisting of two coils connected in series but wound in opposite senses, and separated by a distance "b", called the gradiometer baseline.  A uniform magnetic field (e.g., from a distant environmental source) would couple equal but opposite quantities of flux into the two coils, resulting in zero net flux in the gradiometer, or zero signal.  However, signal sources that are close to the lower coil (relative to the baseline, or separation between coils) would couple significantly more flux into the lower coil than into the upper coil; this would result in a net flux in the gradiometer and hence the signal would be detected.

Figure 3: Response of gradient coils relative to magnetometer response (1/r3 suppressed)

For objects objects that are close (relative to the gradiometer baseline), the gradiometer acts as a pure magnetometer, while rejecting more than 99% of the magnetic signals coming from distant objects. In essence, the gradiometer acts as a "compensated" magnetometer.

Normally, SQUID magnetometers (and gradiometers) map the axial (B_Z) component of the magnetic field. Obviously, using three sensors, it is possible to monitor all three vector components of the magnetic field. Additional channels of SQUID sensors can be used to provide reference channels for electronic balancing. Portions of the reference magnetometer responses are summed electronically with the detection coil(s) output to reject common mode signals from distant noise sources. Electronic balancing can be used to create an HTS axial gradiometer from two HTS magnetometers.

Series 600 LTS Systems

Model 601 Single Channel Gradiometer System

The 601 is a single channel LTS (liquid helium) SQUID gradiometer system. Its components consist of a Cryogenic Probe with liquid helium level sensor, a 1st order axial (dB_Z/dz) detection coil, iMAG^® LTS SQUID and electronics (1 channel) and a Model BMD-6 Liquid Helium Dewar. With a 1 cm detection coil, sensitivities approaching 10 fT/ÖHz are possible. The BMD-6 dewar allows the detection coils to be placed within 10 mm of room temperature.  System components:

• 1^st order axial detection coil, nominal 1 cm diameter, 2% balance
• Cryogenic Probe with liquid helium level sensor
• Model LSQ/20 LTS dc SQUID
• Model BMD-6 Liquid Helium Dewar
• Model iMC-303 Cryogenic Control Unit
• Model iFL-301-L Flux-Locked Loop
• Model CC-60 six meter fiber-optic composite connecting cable
• Manual and accessory pack

Model 603 Three Channel Gradiometer

The 603 is a three channel system with the detection coils oriented orthogonally. To reduce external noise, the coils are configured as one axial and two planar gradiometers (dB_Z/dz, dB_X/dz, dB_Y/dz). With 1 cm detection coils, sensitivities approaching 10 fT/ÖHz are possible. The 603 can also be ordered with three axial detection coils.  System components:

Model 606 Noise Cancellation Gradiometer

Figure 4: Model 606 with BMD-10M Dewar

The 606 (Fig. 4) is an extension of the Model 603 with the addition of three channels (BX, BY, BZ) for electronic noise cancellation. System noise is dependent on the electronic noise cancellation implementation, but 10 fT/ÖHz is expected. The 606 can also be ordered with six axial detection coils.

Model 612 Twelve Channel Gradiometer

The 612 is a medium channel count axial gradiometer system (Fig. 5) designed for sub-10 mm detection coils. With 5 mm detection coils, sensitivities approaching 20 fT/ÖHz are possible.

Figure 5:  Model 612 peripheral nerve system

Options for the Series 600 LTS magnetometers include 2 mm detection coils, 1st and 2nd order axial and planar detection coils, improved uniform field noise rejection, an adjustable tail to permit 2 mm tail gaps, helium level monitors and a flexible transfer tube.

Series 700 HTS Systems

For biomagnetic measurements not requiring the sensitivities of LTS systems, Tristan offers systems based on Tristan’s HTM-8 HTS (liquid nitrogen temperature) SQUID sensor, which can operate in ambient and millitesla fields.

The 701 is a compact single channel magnetometer system. It includes iMAG^® electronics and a 7.6 cm diameter liquid nitrogen dewar with a hold time > 16 hours. The NLD-310 dewar allows the detection coil to be within 5 mm of room temperature

The 703 is a moderate sized (12.7 cm diameter) three channel magnetometer system. The detection coils can be configured axially, orthogonally (B_X, B_Y, B_Z) or for electronic noise cancellation. A custom version of the 703 (Fig. 6) was used to make the first unshielded measurements of magnetocardiograms using an HTS system.

Options for the Series 700 HTS magnetometers include tailed dewars, modified inserts that allow ±90° dewar orientation, an adjustable tail to permit < 5 mm tail gaps, external magnets, additional sensors to form electronic gradiometers and LN2 level readouts.


Figure 6.  Model 703 HTS magnetocardiography system

Model 5700 SQUID Biosusceptometer

The Model 5700 system is designed for measuring fields from paramagnetic materials in the body such as hepatic iron stores in the liver. Measurements are made by determining the change in magnetic field at the detector as the subject is moved into and away from the sensitive region of the detector. A small magnetic field is applied during these measurements by a self contained superconducting magnet. To simulate the presence of the body during the measurements, a water filled bag, approximating the natural diamagnetism of the body, is located between the sensor and the body.

The system includes dual channel axial gradiometers (3rd channel optional), superconducting magnet and power supply, a Model BMD-25 liquid helium dewar and gantry, water bag and reservoir, movable bed and a data acquisition and analysis system and all necessary accessories. As with all Tristan systems, an on-site training course in the proper use of the Model 5700 system is available.  Form more information, including a patient measurement video, visit the Model 5700 site.

Choose from existing designs, modified versions of standard systems, or fully customized systems for your specific needs. Tristan will supply individual components or complete systems.

Examples of custom systems include a 29 channel Intestinal Ischemia gradiometer, a dual cryostat (magnet and dual gradiometer) system for Magnetopneumography, and multiple other systems for biomagnetic measurements.

Be it standard or custom, many choices are available. We can supply systems with:

Applications

• Magnetoencephalography
• Magnetocardiography
• Fetal Magnetocardiography
• Peripheral Nerve and Muscle Activity
• Liver Iron (hepatic iron stores) Assessment
• Intestinal Ischemia
• Magnetopneumography
• Single Neuron Activity Studies