Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: cat_with_no_eyes on 28/11/2010 18:41:57
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Hydrogen which has bound with fat will of course have a shorter relaxation time compared to it being bound with tissue. But how are these relaxation times actually detected?
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Cat_with_no_eyes:
It is apparent that there is nobody, ready to hand, who can answer your question. Why don't you do a little research and bring the answer back here so everybody can learn.
Steve
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Steve fish,
It depends what you mean by "ready to hand".
The answer is that they measure the line width of the transition. Shorter relaxation times have broader bandwidth.
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Bored chemist:
You will need to expand your answer quite a bit before most of the people who read here will be able to learn something. Please do a little teaching.
Steve
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Bored chemist:
You will need to expand your answer quite a bit before most of the people who read here will be able to learn something. Please do a little teaching.
Steve
Thank you for the replies, SteveFish - any help from anybody is always better than having no help at all so please let all share their knowledge.
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Googling NMR and line width will get more information on this than you could shake a stick at.
Notwithstanding Steve's suggestion, I don't plan to repeat it here, not least because most people probably don't care.
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Apparently the original poster, CWNE, cares. So do I. I learned this many moons ago when I was teaching neuroscience from MRIs but my memory is dim and I would also like a refresh from somebody, close to hand, that is up to date on the technique. Steve
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Here's what I recall from learning about MRIs in physics class:
Water and other molecules basically act on some level like tiny magnets. The MRI has two basic components that effect these molecules: a strong magnet and a radio frequency signal. The strong magnet causes all the molecular magnets to line up in the same direction. Then the radio frequency signal is applied and it overpowers the strong magnet and causes the molecular magnets to flip direction. When the radio signal is turned off, the molecular magnets will align themselves back with the strong magnet direction. This process releases some energy as photons, which gets detected by the MRI machine.
Here's what I learned from Wikipedia, assuming it's correct:
Relaxation time is the time it takes for the molecular magnets to all align with the strong magnetic field again. Because the molecules in different tissues behave differently, the times vary between different types of tissue, so you can tell the difference between water and fat by the time it takes for the molecules to realign with the strong magnet.
Hope that helps.
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Thanks JP, much appreciated. I try to answer questions in the areas in which I am knowledgeable. Steve
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Let me explain a little more about Nuclear Magnetic Resonance Imaging.
Nuclear Magnetic Resonance is associated with the difference between two energy hyperfine energy levels created by the presence of the magnetic field and the relative orientation of the spins between the electron and the proton in a hydrogen atom. The magnetic resonance frequency depends on the value of the magnetic field that the specific atom is in. The stronger the field is the higher is the frequency.
An even static magnetic field creates a fixed bias value and this sets the range of radio frequency used for the measurements. An adjustable gradient field creates a regular and known departure from this value over the sample and a pulsed field disturbs this causing the material to "Ring" with its characteristic RF frequency each part of the sample has slightly different frequencies because of the gradient and the fading of the ringing gives information on the binding state of the hydrogen atoms. Because of the extremely weak coupling in this process these frequencies are extremely narrow band (High Q) and can be measures very precisely
This RF signal is analysed by taking the Fourier transform of the received signal to sort out all the individual frequencies from the material. THe values of the frequencies from each part of the sample are known from the bias and gradient fields so each frequency received can be located to particular areas of the sample (usually in the form of lines or planes in the sample)
This process is repeated many times with different field gradients and structures across the sample allowing a 3 dimensional image of the binding state of the atoms in the sample to be created by processing the line and plane image information into a set of points (a bit like an X ray tomogram) One way of looking at this process is described as "filtered back projection" in which you take a set of line information across a mathematical image of the sample weighted with a "window function" (similar to a fourier transform) and add them all up to produce the image.
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Apparently the original poster, CWNE, cares. So do I. I learned this many moons ago when I was teaching neuroscience from MRIs but my memory is dim and I would also like a refresh from somebody, close to hand, that is up to date on the technique. Steve
I work in a trace analysis lab and NMR just isn't sensitive enough (pity) so the last time I did any NMR was as a student about 20 years ago.
That means I can point out how the relaxation times are related to the line widths (which won't have changed in 20 years) but I can't add much more to the topic. On the other hand, searching for NMR and line width will get you lots of stuff that's a good deal more up-to-date than anything I could add.