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Magnets Magnets are objects that
generate a magnetic field, a force-field that either pulls or repels certain materials, such
as nickel and iron.
• Permanent MagnetsThere are typically four categories of permanent magnets:• Neodymium Iron Boron (NdFeB)
• Temporary Magnets material that behaves like a permanent
magnets when in the presence of a magnetic field. Soft iron devices, such as paper clips, are often temporary magnets.
• Electromagnets
Electromagnets are made by winding a wire into multiple loops around a core material, this formation is known as a solenoid.
MEGMagnetoencephalography • is a functional neuroimaging technique to form a map for
brain activity by recording magnetic fields.
• Magnetic fields ,produced by electrical currents that occur naturally in the brain , can be measured using very sensitive magnetometers
History of MEG:• first ,a copper induction coil used as the detector. and To reduce the
magnetic background noise, the measurements were made in a magnetically shielded room, The coil detector was hardly sensitive enough, resulting in poor, noisy MEG measurements that were difficult to use.
• At first, a single SQUID detector was used to successively measure the magnetic field at a number of points around the subject’s head. and, in the 1980s, MEG manufacturers started to arrange many sensors , to cover a larger area of the head.
The basis of MEG signals:
• Neural cells produce a current that produce a weak magnetic field , that approx. equal to 10 fT (Femto Tesla) of cortical alctivity , and 103 fT for human alpha rhythm.
• according to the Maxwell equation , any electrical current will produce perpendicular oriented magnetic field.
• The axons of neurons located horizontally to the scalp surface, and this bundle located in the sulci, researchers experiment various methods to detect deep brain (i.e., non-cortical) signal, but no clinically useful method is currently available.
Magnetic shielding:• External signals affect on magnetic of brain as Earth's magnetic field, To
shield the fields by obtain constructing rooms made of aluminium and mu-metal for reducing high-frequency and low-frequency noise, respectively.
• MSR is A magnetically shielded room model that consists of three main layers. Each of these layers is made of a pure aluminium layer, plus a high-permeability ferromagnetic layer.
Source localization• The challenge posed by MEG is to determine the location of
electric activity within the brain from the induced magnetic fields outside the head. There is mane technology to use:
The inverse problem:• It is called an inverse problem because it starts with the results
and then calculates the causes.
The inverse problem:• The defects of use the technology :
1. there are infinite possible "correct" answers.
2. the problem of defining the "best" solution is itself the subject of intensive research.
Magnetic source imaging
• The estimated source locations can be combined with magnetic resonance imaging (MRI) images to create magnetic source images (MSI).
• The problem of use this technique in clinical practice is that it produces colored areas with definite boundaries superimposed upon an MRI scan
Dipole model source localization• A widely accepted source-modeling technique for MEG
involves calculating a set of equivalent current dipoles (ECDs), which assumes the underlying neuronal sources to be focal
Dipole model source localization
• The limitations:• (1) Difficulties in localizing extended sources with ECDs.• (2) Problems with accurately estimating the total number of
dipoles in advance.• (3) Dependency on dipole location, especially depth in the
brain.
Distributed Source Models
• Unlike multiple-dipole modeling, distributed source models divide the source space into a grid containing a large number of dipoles
Independent component analysis (ICA)
• Independent component analysis (ICA) is another signal processing solution that separates different signals that are statistically independent in time