Biomagnetism

By Dr. Amr A. Abd-Elghany

MagnetismMagnetism is a fundamental property of matter; it is generated by moving charges, usually electrons.

Atoms and molecules have electron orbitals that can be paired (an even number of electrons cancels the magnetic field) or unpaired (the magnetic is present).

Magnetic fields exist as dipoles, where the north pole is the origin of the magnetic field lines and the south pole is the return. One pole cannot exist without the other.

Magnetic susceptibility describes the extent to which a material becomes magnetized when placed in a magnetic field .

Three categories of susceptibility are defined: diamagnetic, paramagnetic, and ferromagnetic.

Diamagnetic materials have slightly negative susceptibility (the induced internal magnetization oppose the external magnetic field and lower the local magnetic field surrounding the material). Examples of diamagnetic materials are calcium, water, and most organic materials C&H.

Graphite sheet placed overa strong magnet

Frog placed in a magnetic field of 16 tesla

Paramagnetic materials have slightly positive susceptibility and enhance the local magnetic field. Examples of paramagnetic materials are molecular oxygen O2, some blood degradation products, and gadolinium-based contrast agents.

Ferromagnetic materials are "superparamagnetic"-that is, they augment the external magnetic field substantially. Examples are iron, cobalt, and nickel.

As with electric charges, "like" magnetic poles repel and "opposite" poles attract.

The magnetic field strength, B, (also called the magnetic flux density) can be calculated as the number of magnetic lines of force per unit area.

The SI unit for B is the tesla (T), and as a benchmark, the earth's magnetic field is about 1/20,000 T. An alternate unit is the gauss (G), where 1 T = 10,000 G.

Magnetic fields can be induced by a moving charge in a wire.

The direction of the magnetic field depends on the sign and the direction of the charge in the wire, as described by the "right hand rule": The fingers point in the direction of the magnetic field when the thumb points in the direction of a moving positive charge (i.e., opposite the direction of electron movement).

Wrapping the current-carrying wire many times in a coil causes a superimposition of the magnetic fields, augmenting the overall strength of the magnetic field inside the coil, with a rapid falloff of field strength outside the coil .

This cardiac MRI picture shows the 4 chambers of the heart using what is called a "bright blood" technique. Blood within the heart shows up as white, and the heart muscle shows up as dark gray. MRI is a scan that lets doctors see inside the body without having to perform surgery. The test is painless, and uses no radiation. Cardiac MRI is a test that gives doctors a detailed picture of the heart, including the chambers and valves, without patients having to undergo cardiac catheterization. 

Biomagnetism

• Is the phenomenon of magnetic fields produced by living organisms; it is a subset of bioelectromagnetism.

• In contrast, organisms' use of magnetism in navigation is magnetoception and the study of the magnetic fields' effects on organisms is magnetobiology such as Magnetocardiography, Magnetoencephalography, Magnetogastrography using SQUID can measure fields as low as 5x10-18 Tesla.

The most important application of Biomagnetism

Magnetic Resonance Imaging

Why MRI?• Which one is Brain MRI?

Which one is Brain CT?

CT axial only then reconstruct the slicesMRI real images not reconstructed

MRI device at 1979 by Damadian

MRI Scan is a radiology technique that uses magnetism, radio waves & computer to produce image of body structures.

MRI scan uses radiofrequency waves & strong magnetic field rather than x-rays to provide remarkably clear & detailed pictures of internal organs & tissues.

MRI test done with a large machine that uses a magnetic field & pulses of radio wave energy to make pictures of organs & structures inside the body. In many cases MRI gives information that cannot be seen as well as with an x-ray, ultrasound or CT scan.

•

What is MRI?

Basic principle of MRI

Principle• Images are created by the interaction between

the external magnet in the machine and the internal magnet in the human body.

• There are 3 physical principles concerning MRI:1. The moving charged body is surrounded by

magnetic field directly proportional with the size of the body.

2. The proton is largely greater in size and number (80% body weight is water) than the electron.

3. The small magnets will take the direction of the larger magnets.

• Radio waves generators emit radio waves having the same frequency as the frequency of the protons. The hydrogen protons will be moved.

• The protons will be returned back with different speeds loosing the energy gained by radio waves giving a signal.Proton alignment with magnetic field

Protons after RF (resonance)

T1 & T2 weighted image

• T1 T2

The resultant image depends on

• Motion: non mobile hydrogen-----no signal.

• Amount: minimal hydrogen (air) ----no signal.

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How Does an MRI Work - MRI Questions - MRI Answers - MRI Maryland.flv

Types of MRI Machines

1. Closed MRI

2. Open MRI

Disadvantages of open MRIThe magnet is weak so the scanning time is relatively long but the resolution of the images the same as closed one.

3. Extremity MRI

advantages of Extremity MRIYou can investigate the hand or leg (only small parts wrist, elbow, ankle, joints) to reduce the load on the big device.

4. Dynamic MRI

• The bed rotates from upright to recumbent (lie down position), stopping at any angle in between.

Patient with low back pain after surgery

Does a lie down only scanner see the patient problem? No EX: Spinal instability on standing position

• (such as Fonar, Toshiba Access, and Hitachi MRP-5000).• Always stay on and cannot be turned off.• They have the advantage of lower cost and lower

maintenance (they require no cryogens for cooling).

MRI instrumentationCategorization of magnets according to their action

1. Permanent magnets

• Such as Siemens OPEN and Picker OUTLOOK.• are based on the electromagnetic principle that electric

current running through a coil produces a magnetic field.• These magnets can be turned off and on.

2. Resistive magnets

• Form of electromagnets.• These magnets operate near absolute zero temperature (e.g.,

4.2oK or -270oC).• There would be almost no resistance in their wires.• This in turn allows us to use very strong electric currents to

generate a high magnetic field without generating significant heat (hence the name superconducting).

• To achieve these ultra low temperatures, cryogens (such as liquid nitrogen) are required (which are very expensive).

• The majority of the available scanners today are superconducting magnets.

3. Superconducting magnets

Categorization of magnets according to field strength

• Ultra high field (3.0 to 4.0 T): these are mainly used for research and MR spectroscopy.

• High field (1.0 to 2.0 T).• Mid field (0.3 to 1.0 T).• Low field (less than 0.1 to 0.2 T).• Ultra low field (less than 0.1 T).

The receiver and transmitter coils

• The surface coils have 2 conditions: 1. it has to surround the part of interest. 2. this part should not be moved during the imaging

process even due to breathing.

Head coil Breast surface coil

Chest surface coil Knee surface coil

Advantages of MRI

• No radiation hazards (EM waves).• Superior soft tissue contrast.• Multi-planner imaging capability (we

can take images for the patients at any position lie down, upright, oblique).

• Absence of artifact from adjacent bones as CT (no lines appears as CT).

• Tissue characterization based on signal.

Contraindications of MRIMagnetic field strength=120 times gravity

• Cardiac pacemaker.• Metal objects: • Within the eye.• Near the spinal cord.• Cochlear implants.• Insulin pumps.• Neurostimulators.

MRI magnet room is extremely powerful. The magnetic will bring all magnetic metal toward it, sometimes at high speed. Leave any metal object outside of the building. Check with MRI Operator if access is needed in the room with the magnet.

Zone 1 : This is a public access area with no restrictions.

Zone 2 : This is a semi restricted area where patients and hospital staff can interact.

Zone 3 : This area is completely physically restricted from non MR personnel especially the general public.

Safety Zones