Biomagnetism

INSTRUMENTATION

The most sensitive instrument for measuring magnetic fields is based on the superconductivity quantum interference device SQUID. A SQUID is a mechanism used to measure extremely weak signals, such as subtle changes in the human body’s electromagnetic energy field. Using a device called a Josephsons junction, a SQUID can detect a change of energy as much as 100 billion times weaker than the electromagnetic energy.

Theory of Josephson junction

The Josephson junction is composed of two superconductors separated by a thin insulating layer. The junction is connected in a circuit with an external voltage applied across a resistor, the current; I and the voltage drop, V across the junction can be measured.

Super conductivity and Super conductors

When certain substances are cooled below a critical temperature, the electrical temperature, the electrical resistance becomes very small, effectively vanishing. The superconductor is not a perfect conductor, but a perfect diamagnetic material with zero electrical resistance. This is known as Meissner Effect. The high sensitivity of SQUID is made possible by superconductivity.


WORKING

What is SQUID magnetometer?

Superconducting Quantum Interference Device can detect magnetic fields generated by electrical activity in smooth muscle. The SQUID MAGNETOMETER, a popular and extremely useful device, uses the interaction between magnetic flux and josephson junction.

How does a SQUID work?

The SQUID has as its active elements one or more josephson junctions. A josephson junction is a weak link between two superconductors that can support supercurrent below a critical value. The special properties the josephson junction cause the impedance of the SQUID loop to be a periodic function of the magnetic flux threading the SQUID so that a modulation signal applied to the bias current is used with a lock in detector to measure the impedance and to linearism the voltage to flux relationship. The net result is that a SQUID functions as a flux to voltage converter with unrivaled energy sensitivity.Figure5 shows the structure of SQUID

Figure 5

The SQUID is located inside a small cylindrical, superconducting magnetic shield in the middle of liquid helium dewar as shown in fig: 5 This minimizes helium evaporation due to excessive heating. The special geometry of the detection coil is less sensitive to the magnetic field from distant sources. The coil is mounted at the bottom of the dewar, close to the head. The rest of the hardware is designed to minimize helium boil off, eliminate RF interference, and to not contribute Johnson noise or distort any external ac fields.

TYPES OF SQUIDS

Based on the applied bias there are two types

1. AC or RF
2. DC SQUIDS

DC SQUIDS are easier to analyze and understand, but until the past decade, AC SQUIDS were more popular as they were easier to construct and use.

DC SQUIDS

The main part of a dc SQUID is the dual junction superconducting loop as shown in figure6

Figure 6

I0-critical supercurrent carried by the loop
I1&I2-junction current
∂1&∂2 -phase difference across the junction

The SQUID loop has an inductance, L, which means that any change in the magnetic flux through the loop will result in a current being induced to produce opposing flux. The induced current is the shielding current, is, and circulates around the loop. A dc SQUID is designed such that when a dc current is sent to one end of the device, the current divides into two parts to take different routes to the other end. At this end, interference takes place between the two waves. When a magnetic field is applied to the region separating the paths, it accelerates the flow of electrons in one path and retards the electrons in other path. Because of this the interference conditions are changed. This can be detected by incorporating a wave link in each of the paths. This effect is very sensitive to the presence of magnetic field and it is for this reason that SQUIDS are useful as good magnetic sensors.The figure7 shows the arrangement of dc SQUID system

Figure 7

Since the signal out of a SQUID is compared with the magnitude of normal electronic signals, amplification is necessary. The dc SQUID is supplied with a constant current. The flux through the loop is modulated by a 100 KHz flux from the feedback coil. The greater magnitude would mask any signal that one would wish to detect. Lock in amplifier amplifies the difference between the SQUID loop’s signal and the 100 KHz signal and this is fed back to the feedback coil, the feedback coil adjusts until the feedback flux cancels the input flux. The SQUID loop has a small inductance, so flux is usually collected using a large input coil, and mutually induced into the SQUID loop via another coil.

MORE ABOUT SQUIDS

SQUID materials

SQUIDS have been fabricated from pure Nb and from Pb alloys containing about 10%Au or In. Pure lead is not used because of its instability in thermal cycling. The Nb-Pb alloy structure is preferred because it has better properties than all Pb or all Nb alloy structure. It has extra hardness and high tensile strength.

APPLICATIONS OF SQUIDS

SQUIDS have been used for a variety of testing purposes that demand extreme sensitivity; including engineering, medical and geological equipment. A SQUID is capable of detecting a change in magnetic field without any extra equipment.

SQUIDS IN BIOMAGNETISM

The measurements of biologically produced magnetic fields were unknown before the invention of SQUIDS. There are many magnetic fields which have been measured, ranging from the susceptibility of tissue to applied magnetic fields to ionic healing currents and those associated with neural or muscle activity

BRAIN IMAGING

By far the largest area of study within Biomagnetism is brain imaging. Most existing non invasive brain imaging methods such as Computerized Tomography (CT), Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET), measure the distribution of some kind of matter and are therefore primarily a measure of structure.

MAGNETOENCEPHALOGRAPHY (MEG)

It utilizes SQUIDS to measure the magnetic fields produced in the brain by ionic current flow arising from neural activity. Brain generates rhythmical potentials which originate in the individual neurons of the brain. The potentials get summated as millions of cells developing synchronously and appear as a surface waveform. Current MEG techniques have spatial resolutions of 1-5mm but the time resolution\ions is between 1-5ms allowing real time imaging. This real time imaging allows research into epileptic seizures and other psychological disorders.figure8 shows the comparison of the spatial and time resolution of MEG, MRI and PET imaging.

Figure 8

OTHER APPLICATIONS

MAGNETOCARDIOGRAPH (MCG)

SQUIDS are used in popularly for heart monitoring. The figure9 shows the MCG measurement system.

Figure 9.1

Figure 9.2: Schematic diagram of magneto cardiogram measurement system

Heart produces a rhythmically synchronized signal. This signal is described by means of a current dipole. SQUIDS pick up the magnetic signal. Signal is then amplified and sampled through an RF amplifier. Controller interconnects the magnetic signal arising from the heart to the processor via A\D converter. Processor is connected to printers and to display monitors.


ADVANTAGES OF MAGNETIC MEASUREMENT

§ Magnetic measurement of nerve function can be made directly in the conducting fluid.
· The use of split torrid allows measurement without puncture or intrusive contact with nerve axon.
§ Measures the current density directly and allows determination of current profiles (MCG).
§ MCG exceeds the diagnostic value of ECG.
§ Good spatial and temporal resolution.
· Magnetic field is less than distorted by tissues than the electric potential.
· Localization is more exact especially for multiple dipole sources.
§ Quick measurement
§ Non- invasivenessFunctional diagnostic information(MRI,CT,PET)

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