Magnetic Resonance Imaging

How The Development of Magnetic Resonance Imaging (MRI) Illustrates What's Possible With Magnetic Fields.

The breakthrough in the discovery of both the MRI for imaging and Q magnets for pain relief is found in magnetic field gradients. While the MRI is a state-of-the-art diagnostic instrument, its story shows how gradient modulated magnetic fields can be applied to improve the lives of patients.

In simple terms the MRI detects the concentration of protons within the body, most of which make up water molecules. Using magnetic field gradients and radio frequency the MRI is able to produce a two dimensional image which roughly correlates to the concentration of water. Hence bone and cavities show up as black and fat very light. Different tissue types respond differently and this is how a radiologist is able to make sense of the MRI films.

Some 30 years after their initial discovery, Paul Lauterbur and Sir Peter Mansfield won the 2003 Nobel Prize in medicine for resolving how to use magnetic field gradients to generate two dimensional images. Like the period before the discovery of the MRI, most practitioners and observers of magnetic therapy are stuck in the old North and South Pole paradigm, but the science has moved on. The common bipolar magnet generates a uniform magnetic field with negligible field gradient or variation. Bringing North and South poles together such as in the Quadrapolar array, generates steep field gradients which produce rapid changes in field strength.

This can be difficult to conceptualise as magnetic fields are invisible and as a vector quantity have both magnitude and direction i.e. are positive or negative. One way to observe the flow of magnetic field lines is by sprinkling iron filings over different magnet arrangements. Field gradients are strongest at either pole of a magnet and this is where the greatest force is exerted on iron filings, other magnets or charged particles. That's why when you sprinkle iron filings on a bar magnet most of them are attracted to the ends. Where the field lines are straight, the gradient is running parallel with the field lines as can be seen with the bipolar magnet (see Image A below).

Image B shows the original design by Dr Holcomb of four bipolar magnets arranged in a quadrapolar array. Notice the sharp changes in direction of the iron filings, these follow the field gradient and the directional changes between the poles where they align horizontally show that the gradient at that point is running perpendicular to the field lines. This is vitally important as the cell studies on the biological effects of magnetic fields show that the maximally effective region is where the gradient is predominantly perpendicular to the local field vector.

Image C shows iron filings sprinkled over the new Q magnet design with a quadrapolar array within one magnetic body. You may notice a more accentuated change of direction and a longer horizontal alignment in the flow of the iron filings. This is because there is no "wasted space" between the poles that exist with the original design and hence there is a greater interaction between the poles.

Finally Image D is the new ProH hexapole magnet where you can see the iron filings form around six poles. This shows that there are more active regions in the hexapole model since there are six gradient zones running horizontally instead of four.

Image A - Bipolar Magnet
Image B - Quadrapolar Array
Schematic representation of quadrapole
schematic representation of Quadrapole
Image C - Quadrapolar Magnet
Image D - Hexapolar Magnet
Schematic representation of hexapole
schematic representation of hexapole

Dr Holcomb's ground breaking research on gradient producing static magnetic fields will one day be recognised as every bit as significant as the discovery of the MRI. His research can be very simplistically summarised as...

(Magnet Strength + Magnet Size) X Field Gradient = Therapeutic Effect

Where Tesla (or Gauss) is the strength of the magnet (which along with the size, determines the depth of penetration) and Gradient (running perpendicular to the local field vector) is the change in the field strength over distance. The therapeutic effect is pain relief, reduced inflammation and normalising local circulation.

See the following links for more detailed infromation on MRI...

http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/mri.html

Wikipedia entry

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