NMRI electromagnetic principles

[stevmg]

In nMRIs the strong magnetic field plus the radio wave aligns protons.

1). Which protons? Just the proton ions (actually H3O+) from water; or protons in all molecules that contain hydrogen atoms such as water, fats, carbohydrates, proteins?

2). The relaxation phase does what? As the protons go back to equilibrium there is an emission of radio waves?

3). Isn't that a form of γ radiation? Doesn't it have inherent risks?

HELP!

 

[Evo]

What has your research into this shown you" Please post the appropriate studies from acceptable journals that you have read that have caused your concern.

 

[Astronuc]

In nMRIs the strong magnetic field plus the radio wave aligns protons.

1). Which protons? Just the proton ions (actually H3O+) from water; or protons in all molecules that contain hydrogen atoms such as water, fats, carbohydrates, proteins?

2). The relaxation phase does what? As the protons go back to equilibrium there is an emission of radio waves?

3). Isn't that a form of γ radiation? Doesn't it have inherent risks?

HELP!

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

In the presence of the static magnetic field which produces a small amount of spin polarization, a radio frequency signal of the proper frequency can induce a transition between spin states. This "spin flip" places some of the spins in their higher energy state. If the radio frequency signal is then switched off, the relaxation of the spins back to the lower state produces a measurable amount of RF signal at the resonant frequency associated with the spin flip. This process is called Nuclear Magnetic Resonance (NMR).

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

Since the MRI uses proton NMR, it images the concentration of protons. Many of those protons are the protons in water, so MRI is particularly well suited for the imaging of soft tissue, like the brain, eyes, and other soft tissue structures . . .

and

Once excited by the RF signal, the hydrogens will tend to return to their lower state in a process called "relaxation" and will re-emit RF radiation at their Larmor frequency.

Radio-frequency is not gamma (γ) radiation, not even X-ray, and in fact is lower energy than visible or infrared.

http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/larmor.html (I recommend some calculations of Larmor frequencies)
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/spinpol.html

From - http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr1.htm
"Note that this electromagnetic radiation falls in the radio and television broadcast spectrum. Nmr spectroscopy is therefore the energetically mildest probe used to examine the structure of molecules."

For protons, read the section 2. Proton NMR Spectroscopy, and note the differences among protons in different compounds.

Another very basic tutorial - http://www.cis.rit.edu/htbooks/nmr/inside.htm

http://www.brynmawr.edu/chemistry/Chem/mnerzsto/The_Basics_Nuclear_Magnetic_Resonance%20_Spectroscopy_2.htm

http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/nmr1.htm

 

[stevmg]

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

and
Radio-frequency is not gamma (γ) radiation, not even X-ray, and in fact is lower energy than visible or infrared.

http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/larmor.html (I recommend some calculations of Larmor frequencies)
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/spinpol.html

From - http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr1.htm
"Note that this electromagnetic radiation falls in the radio and television broadcast spectrum. Nmr spectroscopy is therefore the energetically mildest probe used to examine the structure of molecules."

For protons, read the section 2. Proton NMR Spectroscopy, and note the differences among protons in different compounds.

Another very basic tutorial - http://www.cis.rit.edu/htbooks/nmr/inside.htm

http://www.brynmawr.edu/chemistry/Chem/mnerzsto/The_Basics_Nuclear_Magnetic_Resonance%20_Spectroscopy_2.htm

http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/nmr1.htm

 

[stevmg]

To Astronuc - The question was does the nMRI technique as used in medicine align all hydrogen atom protons such as those in water, fats, carbohydrates, proteins or just the water ones? I cannot decipher the answer to that from the extensive discussions these websites presented - too much information.

 

[jerromyjon]

When a rotating field gradient is used, linear positioning information is collected along a number of different directions. That information can be combined to produce a two-dimensional map of the proton densities. The proton NMR signals are quite sensitive to differences in proton content that are characteristic of different kinds of tissue. Even though the spatial resolution of MRI is not as great as a conventional x-ray film, its contrast resolution is much better for tissue. Rapid scanning and computer reconstruction give well-resolved images of organs.

 

[Astronuc]

To Astronuc - The question was does the nMRI technique as used in medicine align all hydrogen atom protons such as those in water, fats, carbohydrates, proteins or just the water ones? I cannot decipher the answer to that from the extensive discussions these websites presented - too much information.

According to the literature one can excite water or fat, and ostensibly any organic compound by selecting the proper frequency.

This article mentions water (bound and unbound) and fat.
http://www.revisemri.com/blog/2010/fat-suppression/

Another article describes NMR/MRI of sugar by exciting protons in the glucose hydroxyl groups as opposed to water.
"GlucoCEST works by measuring glucose uptake through the chemical exchange of protons between glucose hydroxyl groups and water. By selectively saturating the magnetization of protons in the hydroxyl groups, using radiofrequency pulses, this exchange of protons causes a reduction in the MR signal from water."
http://medicalphysicsweb.org/cws/article/research/54007

So the population of protons, which are to be excited, depends on the radiofrequency.

 

[stevmg]

According to the literature one can excite water or fat, and ostensibly any organic compound by selecting the proper frequency.

This article mentions water (bound and unbound) and fat.
http://www.revisemri.com/blog/2010/fat-suppression/

Another article describes NMR/MRI of sugar by exciting protons in the glucose hydroxyl groups as opposed to water.
"GlucoCEST works by measuring glucose uptake through the chemical exchange of protons between glucose hydroxyl groups and water. By selectively saturating the magnetization of protons in the hydroxyl groups, using radiofrequency pulses, this exchange of protons causes a reduction in the MR signal from water."
http://medicalphysicsweb.org/cws/article/research/54007

So the population of protons, which are to be excited, depends on the radiofrequency.

Good answer. Explains what I need to know. I take it that medical MRI uses radio frequencies for water and fat, true?

 

[Astronuc]

Good answer. Explains what I need to know. I take it that medical MRI uses radio frequencies for water and fat, true?

Correct.

Edit/update: Perhaps a useful document
http://www.mri-physics.net/bin/mri-physics-en-rev1.3.pdf

 

[stevmg]

Correct.

Thank you. Do you know how many doctors and healthcare workers do NOT understand this basic concept? All they know is it is something to do with protons but they don't know what and where protons are. I didn't know that regular analog X-ray machines have better resolutions than MRIs.

Stephen M. Garramine, MD

 

[e.bar.goum]

Thank you. Do you know how many doctors and healthcare workers do NOT understand this basic concept? All they know is it is something to do with protons but they don't know what and where protons are. I didn't know that regular analog X-ray machines have better resolutions than MRIs. I didn't know that regular analog X-ray machines have better resolutions than MRIs.

The resolution of an MRI depends on the intensity of the field you use. The higher the magnetic field, the higher the resolution. Typical MRIs operate at around 1.5 T, but higher field MRIs are gaining in popularity, and in research settings they have gone as high as 9T for people, and 21 T for animals. In addition, whilst proton MRI's are the norm, you can use any nucleus that has a net nuclear spin. helium-3, lithium-7, carbon-13, fluorine-19, oxygen-17, sodium-23, phosphorus-31 and xenon-129 have all been developed. These are for different diagnostic situations.

 

[stevmg]

The resolution of an MRI depends on the intensity of the field you use. The higher the magnetic field, the higher the resolution. Typical MRIs operate at around 1.5 T, but higher field MRIs are gaining in popularity, and in research settings they have gone as high as 9T for people, and 21 T for animals. In addition, whilst proton MRI's are the norm, you can use any nucleus that has a net nuclear spin. helium-3, lithium-7, carbon-13, fluorine-19, oxygen-17, sodium-23, phosphorus-31 and xenon-129 have all been developed. These are for different diagnostic situations.

Seems like the phosphorus-31 would be handy for bone MRIs
Still, straight X-rays give better resolution. Is the CT scan still better?

Where would I obtain a list of nuclei with net spin? Also, what is the percentage of these isotopes in human tissue and is it sufficient to detect in clinical situations?

And, most doctors don't have a clue what you are talking about.

Time to have a little yellow book, "MRIs for Idiots" with all this explained, and it can be quite easily.

 

[e.bar.goum]

Depends on what you mean by "better".

Given that CT scans expose patients to (sometimes very) large radiation doses, if you can use an MRI you should. CT scans are also sensitive to different things to MRIs. And stuff like fMRI can't really be done with CT.

(Also I don't know how these new 3+ T magnets stack up against CT)

 

[stevmg]

Depends on what you mean by "better".

Given that CT scans expose patients to (sometimes very) large radiation doses, if you can use an MRI you should. CT scans are also sensitive to different things to MRIs. And stuff like fMRI can't really be done with CT.

(Also I don't know how these new 3+ T magnets stack up against CT)

Reich, from University of Wisconsin, has on line an excellent summary of nuclei with spin and their relative abundance. I can't make this IPAD copy the link. Sorry.

 

[e.bar.goum]

That's ok, I have nndc! :) http://www.nndc.bnl.gov/