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--------------------- #1504 - Nuclear Magnetic Resonance
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- Nuclear magnetic resonance has nothing to do with radioactivity. Nuclear refers to the nucleus of atoms which are made up of charged particles called “protons.” Protons spin on a rotational axis. They carry a positive charge of 1.9*10^-19 Coulombs, the same as the negative charge of an electron. A spinning electric charge creates a magnetic moment.
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- We do not usually detect this magnetic moment because all the protons are lined up in random directions. However, if the atoms are placed in an external magnetic field they can be made to all line up in the same direction. Using this phenomenon allows medicine to create Magnetic Resonance Imaging, an MRI, of the tissues in the body. Medicine drops the “Nuclear” in the names because patients would not stick their head in a machine that was called “ nuclear”.
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- The external magnetic field aligns all nuclei’s magnetic moments to point in the same direction. The north poles of the spinning protons have an axis of rotation in parallel with the magnetic field lines of the external field. The next step is to get the rotating axes to tilt and precess around this true north axis. The rotating protons also have angular momentum and this can cause a wobble like a spinning top with the rotational axis making a circle around the true vertical.
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- To inner act with this precession we use a radio wave at right angles to the magnetic field lines. The electro-magnetic wave has a magnetic field that aligns perpendicular to the precessing magnetic moment causing it to tilt even further than its natural precession when the wave frequency is exactly the same as the precession frequency. When this happens it is called frequency resonance. Energy is absorbed in the angular momentum of the rotating proton. When the radio wave is turned off the proton returns to its original precession angle and emits that fixed amount of energy corresponding to the resonant frequency. This is what is detected as a unique signal in the frequency spectrum representing the particular atom that is doing the precessing. The resonant frequency is unique for every element.
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- The precession angle is a function of the strength of the magnetic field. The resonance frequency is 42 Megahertz per Tesla of magnetic strength. An MRI machines uses a very strong magnetic field, say 5 Tesla. So , the resonant frequency is 210 Megahertz for the hydrogen that it is detecting to form the image. Thus, that is the reason a radio frequency orthogonal to the magnetic field is the what is chosen.
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- The MRI technology uses hydrogen as the atom nucleus to be detected. The radio frequencies used in other application varies from 60 to 1,000 Megahertz. These are the familiar range of frequencies used in VHF and UHF television broadcasting. Each atomic structure has a unique resonant precession frequency and detecting it can identify the element or the molecule involved.
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- Atomic elements that have an odd number of protons, or an odd number of neutrons (called isotopes) have natural magnetic moments of precession that are easier to detect. If the atomic nucleus has an even number of protons or neutrons its spin is zero and balanced lacking a magnetic moment.
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- Our bodies are mostly water ( H2O) so looking at the tissues in the body works well using the resonant frequency of hydrogen. Carbon 13 is also used with its 6 protons and 13 neutrons. But, Nuclear Magnetic Resonance has many different applications other than MRI, imaging. There are many isotopes that can be detected in any number of elements and molecules found in poisons, toxins, environmental pollution, explosives, etc. Here is a list of some of the elements having natural magnetic resonances:
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--------------------- hydrogen 1
--------------------- carbon 13
--------------------- hydrogen 2
--------------------- lithium 6
--------------------- boron 10
--------------------- boron 11
--------------------- nitrogen 14
--------------------- nitrogen 15
--------------------- oxygen 17
--------------------- fluorine 19
--------------------- sodium 29
--------------------- silicon 29
--------------------- lead 31
--------------------- cadmium 113
--------------------- xenon 129
--------------------- platinum 195
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- A significant application for NMR is having a non-intrusive way of identifying these elements and their molecules. To get to 3-dimensional imaging we need a few more tricks.
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- A constant uniform magnetic field works well in detection in 1-dimension to simply identify that a target element is there. However, to locate the target in 2-dimensions we need to very the magnetic field intensity over space and time. Then, when the resonant frequency is emitted matching the correct magnetic field intensity we also have its location in space. This gradient in the magnetic field allows us to create a 2-dimensional image throughout the target. The image represents a “slice” of detected hydrogen protons. Where there is more water the image is white, less water in fat is gray, still less water in muscle is dark gray, bone with little water and is black.
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- To get a 3-dimensional image we simply move the target slightly and get another “slice”. After passing through the entire target we put the slices together and create a 3-dimensional image of the inside of the body. Medical experts can study the MRI image and complete a diagnosis. The non-intrusive image is very similar to an X-ray image, but much safer for the patient.
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- NMR also works in chemistry to determine the structure of molecules. For example, NMR was used to discover the structure of the “ buckyball”, a carbon 60 atoms structured to form a sphere. In biology it is used for non-destructive study of proteins, including DNA.
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- NMR is used in oil drilling, the magnetic resonance detection is lowered into the drill hole to discover oil and natural gas. Once the oil is in the refinery NMR is used again in process control to produce the oil products.
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- Someday NMR may be used in quantum computing
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- Lasers can be used to align magnetic moments in place of strong external magnetic fields. NRM technology is on the verge of many new discoveries. An announcement will be made shortly, stay tuned.
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- 2 learn more Wikipedia has excellent animations and videos that explain nuclear magnetic resonance.
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- Google Alex Pine, University of Berkeley, California for several links to the state of the art in NMR. I attended his lecture at Sonoma State on October 22, 2012.
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- The Earth has a magnetic field that can be used in NMR with sensitive enough detectors. The Earth is also a precessing magnetic axis with its north pole pointed exactly at the star Polaris, The North Star. The planet is not a balanced sphere and it is influenced by gravitational fields from the Sun and the Moon. The magnetic axis precesses in a giant circle. In a few thousand years the north pole axis will be pointing at the star Vega. It takes 26,000 years for precession to complete one cycle back to Polaris.
( See Review # 622 to learn more about Polaris.)
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