Talk:Magnetic resonance imaging

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[edit] Reading an MRI signal

The article contains the text:

"As the high-energy nuclei relax and realign, they emit energy which is recorded to provide information about their environment."

I believe that this is a popular misconception about how MRI machines work. What the MRI machine actually measures is the bulk magnetization in the X-Y plane, NOT a radio signal that is emitted by the relaxing protons. Protons actually emit virtually no electromagnetic radiation as they undergo T-1 relaxation; the energy that is lost when they realign along the Z-axis is transferred into the environment as thermal motion. That's why T-1 relaxation is sometimes called "spin-lattice relaxation" - the protons are transferring their excess energy into the lattice as heat via dipolar interactions, paramagnetic interactions, etc. So, it does not seem accurate to me to describe the MRI machine as recording “the energy that is emitted when the nuclei relax.” -July 20, 2005

Hoping that some contributors may notice, I would like to propose one addition here (being not that bold for the moment):

The increasing number of MRI scans ordered has become a significant cost factor for healthcare. Even when a CAT examination would be able to answer the same questions, MRI scans are ordered where available.

Opinions whether this statement is NPOV and may be added? Pjacobi 15:43, 19 Jul 2004 (UTC)

  • It may be worth rephrasing, but seems valid to add. It may also be worth noting that unlike CAT, MRI studies don't involve radioactive contrast materials. --Improv 14:30, 21 Feb 2005 (UTC)

Where I work, I feel MRI is in fact underutilized. The radiation dose and contrast load of enhanced CT are serious things. As mentioned above, MRI has neither of these risks, as far as we know, and in many cases -- not all -- provides superior diagnostic information. It is very true that cost and time (MRI is much more time intensive than CT, both in preparation and acquisition) are real factors, but this will only improve in the near future, as it has in CT. I would choose MRI over CT for my family any time I could. xiggelee 02:04, 1 Mar 2005 (UTC)

"Such open bore magnets are often lower field magnets, typically in the 0.2 Tesla range, which decreases their sensitivity but also decreases the Radio Frequency power potentially absorbed by the patient during a protracted operation."

Is this true? I don't think the field strength of the large coil necessarily has anything to do with the intensity of the RF. - Omegatron 03:38, Feb 14, 2005 (UTC)

The frequency of the RF field is proportional to the main field of the magnet: lower field => lower RF freq. => lower SAR. Also the induced currents into the conducting instruments (such as needles) decrease with lowering of the frequency.

AFAIK, the only outcome of increased deposition of RF energy in a patient is increased heat deposition, and it is on a miniscule order of magnitude -- not enough heat to feel. (Any heat felt during the scan would most likely be from the equipment, not the RF pulse.) Importantly, the signal-to-noise ratio is wildly better on the stronger field magnets (usually 1.5 T). The open bore magnets yield lower quality images. xiggelee 02:04, 1 Mar 2005 (UTC)

Yes, the whole body SAR is typically not a factor at lower fields (excluding very small patients, such as babies). But the localized SAR, for example, at the tip of an antenna-like needle, can easily exceed the SAR limits - Evahala

SAR is definetly a factor at high field strength (3T and above) which is becoming more and more common now days. I am working with an MRA sequence (at 4T) that could run in 13min but takes about 30 min to spread out the power deposition from the RF energy. Also higher field strengths have much more strict limitations on what sort of implanted devices you can have in the magnet. Almost anything goes at 0.2T or even 1.5T, but you get up to 4T and only permanent dental work is allowed (somewhat due to the higher field, mostly due to the increase RF deposition). - SBarnes

[edit] MRI and depression

Might it be appropriate to mention the anecdotal evidence that MRI can alliviate depression? jScott 06:12, 2005 Feb 21 (UTC)

  • I've never heard anything about this, but I'll mention it to my subjects :) Seriously though, unless it's at all a well known belief or happens to be true (I know of no studies on the topic, but I'll do a litsearch when I get to work), I doubt it's worth mentioning. It seems extremely unlikely to me that MRI could affect people that way given the physics involved, although it's possible that the novel experience of being in an enclosed space for the length of a study (or for a medical scan), or being around researchers/doctors, may make people feel better. Even if MRI were, on some off chance, actually capable of affecting things, it would be prohibitively expensive to do frequent treatment with it -- where I work, our scanner is typically backed up for weeks in advance, and costs around $700/hr to run. --Improv 14:30, 21 Feb 2005 (UTC)
  • You may be referring to repetitive transcranial magnetic stimulation (rTMS). I don't know a lot about it, but while it does involve magnetism applied to the brain, it is otherwise a completely different technique than MRI. rTMS produces a transient high-strength rapidly-changing magnetic field in the brain, and it is a treatment alternative to electroconvulsive therapy (ECT). In MRI, the magnetic field is only the background: the magnet aligns the nuclei, an RF pulse knocks them out of alignment, and as they go back, you watch what happens and gather information. xiggelee 02:04, 1 Mar 2005 (UTC)
  • It's actually a study from McLean Hospital in the US by Rohan et al. in 2004 that found bipolar patients reporting, in anecdotal fashion, an improvement in their depression following a MR spectroscopic imaging scan. Follow-up experiments indicated that only specific imaging parameters lead to the behavioural effect (that is, their MRSI sequence did, but another sequence, I believe it was PEPSI didn't). Am J Psychiatry. 2004 Jan;161(1):93-8. See Bioelectromagnetics Potatophysics 09:15, 8 November 2005 (UTC)

[edit] Spins of atomic nuclei

"When the object to be imaged is placed in a powerful, uniform magnetic field, the spins of the atomic nuclei ..." I may be rusty on NMR, but isn't it the spin of the electrons, and not the nuclei? --jag123 06:22, 1 Mar 2005 (UTC)

Nuclear Magnetic Resonance —jScott 23:47, 2005 Mar 17 (UTC)

It's the nuclei (*Nuclear* Magnetic Resonance). ESR (Electron Spin Resonance) uses electron spin. However, electrons do change the magnetic environment of the nuceli, which gives rise to tissue differences, etc...

[edit] Encoding of position information, and T1 and T2 evaluation

I've read through the article a couple of times, and the "technique" section could use clarification. Specific questions are:

  • How are the T1 and T2 times measured, signal-wise? As a precessing proton is a quantum system, they shouldn't emit RF while precessing, but instead emit only when changing state. What are these state transitions, and what do emission spectra look like? Or are _absorption_ spectra what's measured? Some of this is covered in NMR spectroscopy, but what's measured here seems to be something different.
  • It seems odd that RF of the frequency of ground-state resonance would bump the precessing nuclei into much-higher-energy states; I'd expect the energy applied to have to be the difference between source and destination states. There's some aspect of this I'm not seeing, which it would be handy to fold into the description.
  • I'm having a great deal of trouble seeing how detailed spatial information is extracted in more than one dimension. Applying a field gradient in one direction would give you information by having resonant frequency change with position on that axis, but applying such fields sequentially gives you far less information than would be needed to unambiguously reconstruct the scanned object. I also don't see how you'd do phase encoding, or how frequency encoding wouldn't interfere with the coding resulting from the first applied field, or how you'd get 3D spatial frequency information to work with. Clarification of this (possibly including diagrams) would be very handy.

Thanks. --Christopher Thomas 17:34, 23 Jun 2005 (UTC)

Update - I see how you can get intensities for all of the spatial frequencies by varying the strength of the three gradient fields at once and checking different linear combinations of them, but you'd need to take a minimum of several hundred samples to do this, which means your subject would probably have moved enough during that time to muck up the results. Still waiting with interest for the actual answer.--Christopher Thomas 04:12, 24 Jun 2005 (UTC)

Quantum mechanically speaking, the system is a classic two-state system (see Feynman Lectures on Physics Vol 3). In the presence of a background magnetic field, the proton exists in two states of definite energy ie two stationary states (call it spin parallel and spin anti-parallel). In thermal equilibrium, slightly more protons are in the lower energy state. The presence of an oscillating polarized magnetic field at Larmor frequency (with energy corresponding to the energy gap between states) causes transitions between the states. If the field has amplitude B, the proton goes from one state to the other in a time π / γB where γ is the gyromagnetic ratio. This corresponds to a "180 degree flip". At other times, the proton is in a superposition of both states. If the oscillating field is then switched off, in the presence of the background field, this superposition of two states will evolve a phase change which is identical to "precession". A quantum mechanical description of how this precessing system couples with a radio receiver to give a signal is beyond me.

Empirically, the system can be described as a magnetization vector that obeys the Bloch equations.

Spatial encoding: Usually only one slice of protons is excited by applying an oscillating B field with a narrow range of frequencies simultaneously with a "slice-select" gradient. Encoding is thus reduced to two dimensions. It is possible to obtain a one dimensional distribution (projection) of proton density over the entire slice with one excitation, by applying a "read" gradient (perpendicular to the slice-select gradient) during signal reception. The projection is then derived from the signal by a Fourier transform. The projection will be perpendicular to the direction of the read gradient. By rotating the direction of the read gradient following different excitations (typically ~128 or 256), projections at different angles are obtained, from which the 2d proton distribution in the slice can be calculated using the inverse Radon transform, similar to computed tomography. In practice, either filtered back-projection (a less computationally intensive form of the inverse Radon transform algorithm), or regridding, is used instead. Alternatively, it is possible to keep the read gradient during signal reception in the same direction, but to turn on, briefly (switch on after excitation but switch off before signal reception), a gradient in the direction perpendicular to both the read and slice-select gradients. By changing the strength of this "phase-encoding" gradient before every signal, projections at different phase encodings are obtained, from which the 2d proton distribution can be calculated using another Fourier transform in the phase-encoding direction. Note that in imaging it is important to change the strength rather than the time the phase encoding is left on: this keeps the echo time constant and is the basis of the classic "spin warp" technique (see Edelstein WA et al "Spin warp NMR imaging and applications to human whole-body imaging" Physics in Medicine and Biology 25(4):751-6 1980) - August 17, 2005

Thanks for the response. It cleared up a lot of confusion. --Christopher Thomas 06:11, 5 November 2005 (UTC)

T1 and T2 contrast: MRI machines don't measure the T1 and T2 times directly. Instead the T1/T2 properties are used to modify contrast already present. The T1 time represents the time constant for relaxation of the spins following excitation. T2 represents the time constant for disappearance of the bulk transverse magnetization (this is much quicker than T1 and is due to loss of phase coherence of the individual nuclei).

Consider the trivial case where the transverse magnetization is measured immediately following application of the exciting RF pulse. The intensity of the signal would depend only on the number of protons in the region of interest (hence a proton density image). Now consider, what occurs if there is a delay between measurement of the excitation and measurement of the signal - in this case, the signal will have faded from the previous scenario. However, the degree of fading is related to the T2 of the individual tissues - tissues with short T2 will fade more than tissues with long T2. An image obtained under these conditions is said to be T2-weighted because it now contains information about the T2, but is not in itself a measure of the T2. The degree of T2-weighting is controllable by changing the delay ('echo time' or TE).

Information on the T1 can be obtained if T1 relaxation is incomplete. Tissues with a long T1 time will recover longitudinal magnetisation slowly, hence there will be less LM with which to generate transverse magnetisation when the next pulse is applied. Long repetition times (TR) mean almost complete LM recovery, in all tissues, and hence very little influence of T1 on the signal. Short TR produces a large influence of T1 on the signal.

In practice, it isn't possible to separate Proton density, T1 and T2 effects completely - but it is possible to weight the image in one direction or other.

Similarly, in practice, the signal fades faster than T2, because of local magnetic field inhomogeneities and other effects. (A non-uniform field will cause non-uniform precession within a region, and hence rapid loss of phase coherence). This is called T2*. A technique called 'spin echo' can be used to recover the true T2 - the simple description is that the direction of precession is reversed mid-way between excitation and measurement - hence protons with a higher Larmor frequency, which had developed a phase lead, are flipped into a lagging phase at the mid point, before coming back into phase at the time of measurement. ChumpusRex 00:28, 7 February 2006 (UTC)

[edit] More on Phase Encoding

The Radon transform is not used in MRI. That would involve convoluting the data from projections through the patient. Typical single and multi slice techniques use a 2 Dimensional Fourier Transform for reconstruction (2dft). Christopher's observation about needing numerous samples is correct. That is the 128 to 256 excitations mentioned in the previous paragraph. Obviously, it has been possible to fit them into usable techniques covering a time for which most patients can remain still. However, these techniques cannot be used for things like imaging the intestines, which move on their own. Echo planar imaging is very fast and solves the motion problem. Maybe I'll look it up sometime and send in some info on it.

Some specifics on phase encoding: After each excitation, the signal is read with the frequency encoding gradient on and always at the same level. Thus, the correspondance of frequency across the fequency encoded direction remains constant. However, prior to the readout, an additional phase encoding gradient is turned on and then off. The amount of time that it remains on is different for each excitation. Usually it builds to a maximum in one direction, then goes to zero and builds again in the opposite direction. During the time that the gradient is on, the frequencies vary as a function of position in the third dimension. When the gradient is turned off, the frequencies return to a constant (the slice selection frequency). However, in the interim, the signals have acquired a phase difference along the PE (phase encoded) direction. Now, if you reorder these acquisitions so that the phase encoding increases monotonicly (spelling?) and examine a single frequency for each excitation, you will have a one dimensional signal which can be analysed with another fourier transform. The result of the transform gives the relative strength for the selected frequency encoding position as a function of the position in the phase encoding direction. This is what you need in order to sort out the final dimension.

Earlier in the discussion, someone cited the danger of injected radioactive substances in CT. That's in Nuclear Medicine (PET, SPECT, Bone Scans, Blood Scans, etc.). The radiation in CT comes from the X-Ray tube. The contrast agents in CT simply absorb more X-rays than normal tissue and therefore show up on the image.

About depression: At one time I got a whole lot of MRI during software development and (particularly) during testing. I don't know if I was less depressed during that period (I take Effexor now), but I know I did get more sleep because I always fell asleep in the scanner.

Bulk Magnetization: The first note on this discussion page talks about RF emissions and the measurement of bulk magnetization. The receiver coil on an MRI scanner is definately an RF antenna. It is not a magnetometer. It receives an RF signal at the Larmor frequency (actually the spread of frequencies dictated by the frequency encoding gradient). I can't think of any source for this RF other than the relaxing photons. Certainly the signal strength is low. Lots of people thought that usable diagnostic images could never be produced via MRI for this reason.

The T1 characteristics of this signal are tied to the properties of the local tissue. This indicates that the vast majority of the RF is absorbed locally and transformed to heat energy in the tissue. The receiver coil provides another energy absorption option for the nuclei to emit to. If the receiver could be made immensely more effective, then it would also dominate the T1 property,and at the least remove tissue T1 sensitivity from the image.

A great deal of mystery is due to the position of MRI at the cusp of classical and quantum physics. Most systems for which the quantum equations are reasonably simple cannot be analysed using classical physics without producing gross errors (the electronic shell structure of the atom is a good example). Conversely, most systems which can be correctly analysed using classical mechanics become absurdly complex when quantum mechanics is applied (did you ever try to compute the allowed energy levels for a bowling ball?). MRI can be approached from either direction. Therefore, most texts use both tools to illuminate the subject. Unfortunately, this require great precision in the definition and use of terms and it is sometimes lacking. I suspect that this is the cause of some of these misperceptions.

You can find a lot of this information in Bushberg's "Essential Physics of Medical Imaging." There is also an excellent (!!!) MRI online tutorial at http://www.cis.rit.edu/htbooks/mri/inside.htm, complete with lots of animated spin vectors.

P.S. I just created an account and this is my first interaction with Wikipedia, so please be ruthless in chopping out whatever doesn't belong. I won't be offended. My name is comming out like an ip address on the preview, so I am Ed Solem (edsolem) and my address is els@plexar.com. 67.23.4.116 16:29, 13 August 2005 (UTC)

Thanks for the additional clarification. It was very useful. --Christopher Thomas 06:12, 5 November 2005 (UTC)

[edit] Nobel Prize Controversy

Removed the words "Several MRI experts and university professors were quoted in the ads expressing their disappointment and dismay at the exclusion." from the article. In the MR community, a much larger number of scientists and MRI experts expressed no such disappointment (or rather, supported it), and without mentioning them, this seems to violate NPOV as an appeal to authority. Perhaps we should consider shortening this section and letting people read about in the Damadian article, rather than replicate it here?

  • I added back in the text to the ad. I think that should be a link here at least, as it was a notable event. There could/should be a whole other article regarding the Damadian controversy. Semiconscious (talk · home) 18:37, 11 September 2005 (UTC)
  • Someone's made significant edits to this section, and it reads quite poorly. Does someone want to volunteer to clean this up?
  • There is far more about Damadian in the Nobel Prize section than either Lauterbur or Mansfield. There was much more noise than controversy, and many in the MR community feel the decision was sensible. Does anyone mind if I tidy up? Full disclosure - I work in Mansfields centre in Notts.

[edit] Animated image

Good though it is, it's not wise to have a rapidly- changing, animated image; they're problematic for some people with epilepsy (and other conditions); and of course such people may be particularly attracted to an article such as this. It's also contrary to the [[W3C] web accessibility standards, which we should strive to meet. Accordingly, I'm replacing it with a link. Andy Mabbett 11:33, 7 October 2005 (UTC)

I think the illustration is far more useful than any supposed problems it may cause. I suggest that you ask on the Village pump and seek to build a more general concensus that animations should be removed from Wikipedia, before making a decision like this. In the mean time I will restore the illustration. -- Solipsist 16:19, 10 October 2005 (UTC)
What mroe evidence do you need, than the WAI guidelines: 7.1 and 7.3 expecially? Note also my point about the likely audience for this subject Andy Mabbett 16:29, 10 October 2005 (UTC)
I suspect you are misunderstanding the W3C guidelines. Individual pixels flickering are not likely to cause problems of any sort. The guidelines are about flashing the whole screen on large blocks of text. -- Solipsist 16:37, 10 October 2005 (UTC)
Which part of avoid movement in pages do you think I am misunderstanding? Andy Mabbett 17:25, 10 October 2005 (UTC)

"These guidelines are a specification developed by the W3C, an international, vendor-neutral industry consortium, and have been developed under W3C process. W3C is not a legislative body and the Web Content Accessibility Guidelines specification is not a regulation. The guidelines may be informally or formally adopted by different kinds of organizations to clarify what level of accessibility that organization requires for particular Web sites. If you would like to learn more about specific laws or policies in different countries which have bearing on accessibility requirements for Web sites, some of these are available in the policy references section of the WAI site. Please contact the relevant legal authority for more details on obligations and/or enforcement."

The specific guideline in question here is 7.3, as it is fairly explicit that 7.1 does not apply for a slow-moving, small image.

7.3 is rated as "priority 2", defined as "A Web content developer should [note: emphasis theirs] satisfy this checkpoint. Otherwise, one or more groups will find it difficult to access information in the document. Satisfying this checkpoint will remove significant barriers to accessing Web documents." The animated gif image on this page does not inhibit one from accessing the page; that is, its inclusion does not prohibit one from reading the page in any browser.

If you feel there are certain disabled populations who will suffer accessibility to this page because of its inclusion, you should justify that and include your reasoning Andy. Given this is a priority 2 guideline and that its inclusion does not limit accessibility, I feel that the utility and merit of the image supersede any potential dangers or accessibility deficits the image may cause. semiconscious (talk · home) 18:20, 10 October 2005 (UTC)

  • I concur with Semiconscious on this. Unless you can demonstrate that either Wikipedia has made a commitment to obey w3 recommendations or that there is other policy on animated images, I don't think you've demonstrated a good enough reason to have the image removed. --Improv 22:26, 11 October 2005 (UTC)
*User has had problems in the past, and just needs some attention. Please see the (RFC) filed against user, of which he has yet to answer; just ignore him, as he never creates any article, or has loaded any pictures, etc. Quote: "Which part of avoid movement in pages do you think I am misunderstanding"? Just look at his user page (user:Pigsonthewing). Scott 17:36, 17 October 2005 (UTC)
he never creates any article A bare-faced lie by User:Scottfisher. Andy Mabbett 19:23, 17 October 2005 (UTC)
  • I don't even know why I'm bothering because it has nothing to do with the point at hand, but I find myself commenting never the less. Andy Mabbett seems to have contributed to many articles on Wikipedia. However—in the strictest sense at the time of this writing—Scott is correct: in reviewing his user contributions Andy has not created any articles. Can we please move on though and refrain from taking this... this whatever it is... any further? semiconscious (talk · home) 22:28, 17 October 2005 (UTC)
Scott is correct: in reviewing his user contributions Andy has not created any articles.: Poppycock. I look forward to your appology. Andy Mabbett 22:46, 17 October 2005 (UTC)
  • I apologize for saying you have contributed many articles to Wikipedia. :) Just kidding. It would seem I was hasty in my previous declaration and my skills at using Wikipedia have not fully developed. You have indeed made many unique, new page contributions, so I apologize. Best. semiconscious (talk · home) 01:08, 18 October 2005 (UTC)

[edit] Source of the animated image

There is a more basic problem with the animated image. It is not from an MRI scan. The animated image is a cross-sectional series from a CT scan. There is no way to make bone light up like that using an MRI machine. I think it should be removed. —The preceding unsigned comment was added by HankD (talk • contribs) on 16:24, 1 March 2006.

It's a T1 weighted axial series from an MRI. I don't think there's any doubt about that. CT can't show the difference between grey and white matter as well as that. The reason it looks wrong is because it's a negative of the conventional way of displaying images. Thus, the bone, which produces almost no MR signal shows up white on this image (by convention no signal is shown as black - an exception is 'phase sensitive' inversion recovery imaging, where a 'negative' signal is usually shown as black and 'positive' as white).ChumpusRex 18:45, 1 March 2006 (UTC)
It's my head. It was scanned at UC Berkeley on their Varian 4T MRI. It's still a bad animation and should be removed though. :) Semiconscious • talk 06:19, 22 March 2006 (UTC)

[edit] RFC request

Comment. I am responding to the RFC request on Wikipedia:Requests for comment/Maths, natural science, and technology. I think the animated image should stay. According to WAI, the "Web Accessibility Initiative (WAI) is an effort to improve the accessibility of the World Wide Web (WWW or Web), especially, but not only, for people with disabilities." In my opinon, the "not only" means we should consider all of those who may benefit from the image, not simply those who might. And in my understanding of epilepsy, it is extremely unlikely that this type of graphic would trigger a seizure in the very small percentage of epileptics who might be predisposed to photosensitive epilepsy anyway. Edwardian 05:46, 18 October 2005 (UTC)

  • I am also responding to the RFC. As it is, the image does not seem to add anything to the article. Where is it discussed? How is having an animation more effective than having still pics? I would say remove. Physchim62 07:51, 28 October 2005 (UTC)
  • I am responding to the RFC. Cross-sectional imaging is often only meaningful when slides are seen in a continuum. Radiologists see things with their scroll button that clinicians don't see when looking at printed films. I think an animated image is an excellent example of the uses of MRI and makes it a lot more tangible. JFW | T@lk 13:07, 31 October 2005 (UTC)
  • Responding to the RFC. People with very severe photosensitive epilepsy are likely to use a browser that allows to disable animation, or a separate application that does the same. However, the best of both worlds would be allowing the reader to control the animation. Unfortunately it seems that MediaWiki can't do this. Maybe have a static placeholder on this page with a link to the animated version? Aapo Laitinen 18:38, 4 November 2005 (UTC)


I consider the animated image rather irritating, quite independent of the WAI aspect. I was wondering why it is still there, as I remember that it has been replaced with a still image and link to the animated version. Ahhhh, that was on de:! HAve a look at [1]. I propose switching to this solution. --Pjacobi 23:37, 5 November 2005 (UTC)

  • Responding to the RFC: I don't think the image is large enough or fast-moving enough to pose a serious risk to people with photosensitive epilepsy. However, I think it should be buried a bit deeper in the article along with a relevant discussion of using MRI to take multiple cross-sections. When I first saw the RFC regarding a moving image on a MRI page, I expected to see a moving diagram of a precessing/dephasing set of spins. Also, the caption is "fMRI scan" and this doesn't appear to be a very good representation of an fMRI -- I'm much more used to seeing those with the false colour overlay indicating the regions of differential activation seen with the BOLD effect. Perhaps the image appearing at the bottom with the Nobel Prize discussion should head up the article. Potatophysics 10:27, 8 November 2005 (UTC)
In other words, swap the current two image around and improve the captions. This sounds like a good idea. And whilst I was at it I found a clearer fMRI image on Commons. -- Solipsist 11:06, 8 November 2005 (UTC)

[edit] MRI and tatoo

In one of the rerun episode of House on Fox, Dr. House told his patient (a tough guy from the death row) in the MRI that it would be very painful for him because of the metallic tatoo ink on his body. The show even dramatized the tough guy squirming in pain as the tatoo on the patient's body turning red hot during the MRI. How much of such dramatization was true? Kowloonese 20:12, 30 December 2005 (UTC)

If you have _conducting_ ink, you can get induction currents set up by the RF probe pulses, but my understanding is that that kind of ink is rare, if it's used at all. Normal inks won't do anything (there was an amusing "myth busters" episode where they did their best to put an MRI-sensitive tattoo on pig skin). Some inks contain magnetic particles that might be affected by the strong DC magnetic field, but I doubt this effect would be significant in the concentrations used (the MB team saw their tupperware tub full of ultra-magnetic dye move, but you're wearing about a hundred thousand times less ink than that). For purposes of the article, medical citations would be needed either way, of course (neither of our TV examples would count). --Christopher Thomas 22:06, 30 December 2005 (UTC)
When I scan people for my experiments, tattoos are considered potentially disqualifying because many inks used do cause burns, and while many others don't, it's better to be safe than sorry when you're scheduling time for something which costs about $700/hour to run. According to the MRI techs at the scanner, they've seen people who have had their tattoos burn them in the scanner. I realise that this still is hearsay, but it's a little bit less hearsayish on topics like these. --Improv 23:51, 30 December 2005 (UTC)


Hmm...what sort of power output do you get from the RF transmitter in an MRI? It seems unlikely to me that there would be enough power to heat something up like that.
  • Here is a sample safety procedure guideline that isn't too dissimilar to the training information I received before I was permitted to scan. This site also has more info on the RF pulse levels -- the MR Techs do calculations based on weight and expected heating before the scan begins. It is important to note that when the subject is in the scanner for long periods of time (my experiments, with the structural parts added in, run nearly 2 hours), moderate heading could add up. --Improv 07:32, 11 February 2006 (UTC)
  • The RF output for a typical medical MR scanner can be significant. In medical devices, transmitters with peak outputs of 15-35 kW are typical. In most cases the duty cycle is very low. However, certain rapid imaging techniques (e.g. fast spin echo) can require average output powers of over 1 kW. Current guidelines recommend that the SAR be limited (e.g. to 4 W/kg over the whole body averaged over 15 minutes). One challenge that many researchers/manufacturers are facing is how to accelerate image acquisition, or improve image quality, while still meeting these guidelines. This is particularly the case with the tendency for manufacturers to offer incresingly stronger magnets (SAR is approx proportional to B02) as a way to improve image quality. ChumpusRex 22:08, 15 February 2006 (UTC)

[edit] Nomenclature

While the sentence about the cost of MRI equipment is important and valid, I think it is quite out of place under a subheading of nomenclature. Also, I don't think this sentence needs to start with "unfortunately." Perhaps this sentence should be brought up to the first paragraph? Faeanna 04:00, 8 January 2006

[edit] What can be inferred from this technique?

Correct me if I'm wrong, but I don't see any discussion in the article about what sort of information researchers may be able to/have discovered by using this technique. Without wanting to sound melodramatic, can this technique be used to read your thoughts, to some extent? I've been asked - and am inclined - to take part in an experiment using one of these things, so I'm kind of curious about it. --Kick the cat 12:33, 4 February 2006 (UTC)

No, MRI gives strictly anatomical information. fMRI may light up certain areas used in certain kinds of cognitive activity. As it involves no radiation, it is generally considered to be harmless (unless you've got metal clips somewhere). JFW | T@lk 05:23, 5 February 2006 (UTC)
Read thoughts? That one's new to me! :)--Zereshk 05:27, 30 March 2006 (UTC)

[edit] Nuclear technology template

Magnetic resonance is not a nuclear technology in the sense that nuclei are only used as sensors and are not modified as in radioactivity of in nuclear fission. No harmful high-energy radiation is involved in this technique, and this is precisely the reason why the word "nuclear" is avoided in the name of the technique in the context of its application in medicine (as opposed to the NMR, a term used in physics). Andreas 14:48, 19 March 2006 (UTC)

[edit] Damadian

Does the GE litigation mentioned in the Damadian part of the article belong to "1997" or "1979"?--Zereshk 05:25, 30 March 2006 (UTC)

[edit] So when was it invented?

The nobel price contoversy section hints at 1974, but thats not for a magnetic resonance imager. When was the design as we use today invented? Thought it should definatly be included both in the article and in here - Jak (talk) 16:52, 10 July 2006 (UTC)

Herman Carr invented the gradient technique used today and actually produced one-dimensional images in the 1950's. The key event was the discovery about 1970 of tissue relaxation times differences by first Freeman Cope with deuterium oxide and then Raymond Damadian with regular water. This meant "contrast", if a way could be found to effectively localize the signal. Lauderbur and Mansfield figured out how to do this, using a development of Carr's original technique. Sesquiculus1 00:45, 22 August 2006 (UTC)

Vested interest, but no-one ever seems to mention Professor John Mallard (plus Jim Hutchison, Bill Edelstein and Margaret Foster) - key figures in both MRI and PET development. They developed the spin-warp technology, imaged a mouse in 1974 and produced the first clinically useful images in 1980... don't want to add anything as yet as wanted a worldwide perspective first.

Hutchison JMS, Mallard JR, Goll CC. In-vivo imaging of body structures using proton resonance. Proceedings. 18th Ampère Congress. Magnetic resonance and related phenomena. Nottingham 9-14 September 1974. Amsterdam, Oxford: North-Holland Publishing Company. 283-284.

Edelstein WA, Hutchison JMS, Johnson G, Redpath TW. Spin-warp NMR imaging and applications to human whole-body imaging. Phys Med Biol 1980;25:751-756.

Mallard JR. Magnetic resonance imaging—the Aberdeen perspective on developments in the early years. Phys Med Biol 2006;51:R45-R60 PMJ 11:50, 06 November 2006 (GMT)


On the intro section, there is mention of the first MRI patient. Could someone post a date for that first patient scan?

[edit] Is MRI safe?

I'm currently preparing to a degree in diagnostic radiography and this article has been very useful. Thanks to all who worked on it.

One thing I would query though is the article's claim that MRI is harmless (See Application section). Everything I have read says that it is believed to be safe but that this has never been proven.

I will not edit the article as there are obviously much better qualified people than me to do so but if MRI has been proven to be safe then there should be a reference and if it has not then this sentence needs to be amended.GordyB 15:53, 24 July 2006 (UTC)

[edit] Claustrophobia

People who are claustrophobic (whether they realize they are or not) often cannot undergo MRI scans without sedation (valium or similar). I think this should definitely be mentioned since it is a crucial part of any pre-MRI questionnaire.

If you look up claustrophobia on Wikipedia you'll see some good statistics (with citation) about MRI claustrophobia.

[edit] MRI scanners?

There is plenty of discussion of the physics involved but I couldn't find any description of what an MRI scanner consists of or why it is so expensive to acquire and maintain. In the safety section there is the first and only mention of superconducting magnets and cryogens. A picture of a scanner and a diagram of its important components would be nice. Pretzelpaws 02:24, 28 July 2006 (UTC)

I have added a picture of a MRI scanner. --WS 21:00, 22 November 2006 (UTC)

[edit] WOW!

Has anyone ever seen this before?!! [2] I had no idea that real time several Hz MRI images could even possibly be taken like this. I was completely shocked when I saw this video the first time to the point that I thought it might be fake. But it is apparently real! [3]. Can someone work this into the article? --Deglr6328 07:00, 19 August 2006 (UTC)

[edit] Effects of titanium

I removed this addition because it had no source and didn't make sense to me:

However, There is a 50/50 chance that the space where the titanium is will cause a blank space in the MRI. While this poses NO risk to the patient it deems the MRI useless in that part of the body.

Why in the world would there be a "50/50 chance"? —Keenan Pepper 00:09, 23 August 2006 (UTC)

[edit] Recent edits

A recent edit [4] removed large amounts of text, and left a stray t in the middle. Someone who knows the subject needs to review.

Also why does Lauterbur become Lauderbur ? -- Beardo 23:14, 10 October 2006 (UTC)

[edit] MRI measures and intelligence

The article Francis Galton claims there was a 0.4 correlation between intelligence and MRI measures. Is that true? If yes, please add a reference. Further the article claims this had proven Daltons hypothesis that head size was an reliable indicator of intelligence. Is there empirical support for this hypothesis? I don't think so but don't have any references. Thanks in advance, Falk Lieder 14:09, 23 October 2006 (UTC)

Yeah, that Dalton guy was one crazy mother. There was a whole movement back in his day in which many anthropologists and neuroscientists were trying to figure out exactly which traits meant you were intelligent, be it your race, the shape of your features, the nature of the bumps on your head and where they were, and, of course, the size of your head. What is meant, exactly, by a "0.4" correlation? Is this a value of beta in some kind of linear regression? That's not a very strong correlation. I believe I read that, upon Dalton's autopsy, it was found that his brain was actually quite small... I highly recommend that anyone interested or intruiged by this read Steven Jay Gould's book The Mismeasure of Man. 74.245.67.106 22:01, 16 March 2007 (UTC)

[edit] EU EMF Directive

The original text was a little partial here. Maybe a little too political also. I've edited it to better reflect both sides of a very complex issue.

Fixed Phil 08:49, 30 October 2006 (UTC)

[edit] CT versus MRI

A recent addition, (Also MRI can generate cross-sections that CT cannot, such as diagonal slices through the body.) The concept of MRI providing better 2D and 3D reconstructions in comparison to CT is debateable, however, CT and MR are two different modalities, useful for imaging different anatomical and physiological properties and processes.

Reconstruction algorithms for both CT and MR images can provide images with almost equal useability to a surgeon. In some cases fusion images (blended CT and MRI images), along with Nuclear Medicine scans provide accurate simulation maps for radiotherapy treatment. One modality is not necessarily better or worse, just different.

--Read-write-services 01:01, 3 November 2006 (UTC)

This section (CT vs MRI) ends with the phrase "farts and legs". As a layman who knows nothing about this topic, I have a vision of a polar graph with a starfish shaped contour on it. But the reference is not at all clear. Can someone explain this a bit better. Thanks.

Ccalvin 20:59, 26 January 2007 (UTC)

Hi there, it is vandalism-a sad fact with this medium-well spotted, but I think it has been rectified since, regards,--Read-write-services 23:07, 28 January 2007 (UTC)

[edit] Magnetic field strength and resolution?

What are the relationships between:

  1. magnetic field strength & spatial resolution and
  2. magnetic field strength & temporal resolution?

I think the above questions would be interesting to examine in the article. The article proclaims that MRI scanners with up to 20 Tesla fields are in use in research settings. What does 20 Teslas get you? What is the compromise that is made when using a weaker field-- is it just less cost? If the issue is more complex than just cost, it would be interesting to understand the pros and cons of a stronger field. Nephron  T|C 21:04, 25 November 2006 (UTC)

The most important impact of field strength is signal to noise ratio (SNR). Higher static field strengths result in a larger population of spins aligning parallel to the field, thus the net magnetization is greater. Net magnetization is approximately proportional to B02; thus doubling the field strength quadruples the signal strength.
Temporal resolution and spatial resolution are both related to the SNR. By recording the signal more quickly, you receive less information and hence have a poorer SNR. Similarly, by using smaller pixels you divide your signal more finely - the result of which is poorer SNR. SNR is important because it determines the minimum contrast that is visible in the image. The benefit of higher field strength is therefore the ability to increase spatial resolution or decrease scan time without degrading image noise.
Spectroscopy is a technique which can provide information as to the chemical composition of a tissue, and is an important tool in diagnosis of brain diseases - particularly tumors. Spectroscopy gains particular benefit from high field strengths, not just because it samples only a very small volume of tissue (and hence has intrinsically poor SNR) but also because the chemical shift effect (on which spectroscopy depends) is proportional to the field strength.
Increasing field is not without problems:
- Higher field means a higher Larmor frequency. A higher RF frequency means greater RF absorption in tissue causing greater tissue heating and poorer penetration into the body. At field strengths above 1.5 T, RF heating effects are a serious problem - and get worse proportional to B02. This is a particular problem for metal implants, which suffer significant heating - in general, few implants are regarded as safe above 1.5 T.
- Chemical shift and magnetic susceptibility effects are more evident at high fields. This is further exacerbated by the fact that spin echo techniques (which are tolerant to chemical shift and susceptibility effects) are limited by RF heating, and need to be replaced by gradient echo techniques (which are highly sensitive to these effects).
- Relaxation times are prolonged at high field - because scan times and contrast are related to the tissue relaxation times, the scan time must either be prolonged to compensate or image contrast degraded (particularly T1 contrast).
- Increased risk of side effects - exposure to fields above 3 T have been reported to cause nausea and dizziness. At higher strengths Lenz law and magnetohydrodynamic effects on the cardiovascular system are potentially significant - causing marked distortion of the ECG waveform and raising blood pressure (due to MHD effects causing an apparent increase in blood viscosity).
- Considerably increased cost of the magnet; both installation and maintenance costs.
- There are limits to the field strength at which ancillary equipment (e.g. patient monitoring devices, or remote injector devices) can operate.
In practice, general medical scanners are usually 1.5 T or less - the drawbacks of higher fields tend to outweigh the benefits. However, 3 T scanners are used for specialist work (e.g. brain imaging/spectroscopy, cardiac imaging) and for research.
Even higher field machines (e.g. 4 - 7 T or even 9.4 T) are not used for clinical work (nor are they approved as safe for clinical work) and are purely research tools. Machines using fields higher than 9.4 T are reserved for laboratory small animal or specimen use (partly due to the impracticality of making human sized high-field magnets, but also due to safety concerns).ChumpusRex 02:09, 26 November 2006 (UTC)
Wow. Very detailed answer-- that well surpassed my expectations. Sounds like you're very knowledgable about the subject; I'd bet you have a degree or two in physics or engineering. Any how, it will take me a while to digest what you've written. Also, I'm unfortunately terribly busy with school... but after things look better on that front, perhaps we can somehow integrate your nuggets of wisdom into the article in language that is relatively easy to digest for joe average and makes the article even better (if someone doesn't beat us to it). Nephron  T|C 22:44, 1 December 2006 (UTC)

[edit] More basic, non-technical info

This article needs much more background. For example: why are MRIs round? In a machine where is the magnet? Where do the radio waves originate and where do they go? Other simple things. Kevlar67 05:50, 3 January 2007 (UTC)


  • Hi there,

NOT all MRI machines are round, some are "C" shaped, some are donut shaped while the usual older style technology has a cylindrical bore for the patient to lay inside. The typical cylindrical machine uses helium cooled, superconductive wire that forms the intense magnetic feild-it is an electromagnet that one it is ramped up (energised with current), the cables are removed and due to superconducxtivity, the magnet remains energised (provided that the helium level is maintaned). The Magnet is the whole unit that you typically see in a picture of the machine. The RF (radio frequency energy) is sent in via the transmitter located outside the magnet room itself. The transmitter is connected to the machine via low loss coaxial cable. The RF energy is 'coupled' to the body using the coil surrounding the patient's head or other region of interest, or may be transmitted from the Body coil located just inside the internal skin of the machine's bore.

I hope this helps-don't know if we need such simplistic explanations though on the actual article? any comments? if so I would be delighted to create an introduction to MRI systems for the article if there is interest! cheers!--Read-write-services 09:34, 4 January 2007 (UTC)

[edit] Agree on the more basic info!

K-space? What the heck is that? Has anyone noticed that thi spage only makes sense if you're a physicist? Can't someone explain MRI in terms that a regular person can understand? You know - without the math...

[edit] Split page ?

It may be time to split the page into multiple pages, which can then address issues relevant to different communities (laymen, patients, physcicists, doctors etc) more effectively. This will also address issues raised by comments above on "more basic info" and "construction of MR machines". What do others think ?Abecedare 18:28, 6 January 2007 (UTC)

  • Agree article should be split. That said, WP (WP:NOT) isn't a manual for experts; so, I disagree with the idea of splitting the article to address different communities.
Here is a proposal for a split:
I think it should be like renal failure or dialysis -- with "main" article links, e.g. {{main|Safety of MRI}}. Nephron  T|C 18:52, 6 January 2007 (UTC)
I concur. I think it may be useful to create the red-linked articles listed by Nephron, and maybe others such as Cardiac MRI (which covers cine imaging, currently unaddressed). Then we can move some of the details to these articles and to others like Diffusion MRI, Diffusion tensor imaging etc.
A useful guideline to follow is WP:Summary style. Abecedare 19:08, 6 January 2007 (UTC)
I agree. The main article is becoming very large and daunting. As previous authos, I don't think that different articles for different audiences are appropriate. While some of the technical details are somewhat arcane, this is perhaps best addressed by a short paragraph, for the layman, in the main article linking to a more in-depth article.
I've also just added a section on scanner design/construction. Again, this is probably something that could have its own article. ChumpusRex 23:42, 6 January 2007 (UTC)


[edit] Requested move

Magnetic resonance imaging → Magnetic Resonance Imaging — The obvious title of this article seems like it should be Magnetic Resonance Imaging because that's how it's used in the article, but I think a few people might disagree so I'm suggesting it before moving. Vicarious 05:28, 18 January 2007 (UTC)

[edit] Survey

Add  # '''Support'''  or  # '''Oppose'''  on a new line in the appropriate section followed by a brief explanation, then sign your opinion using ~~~~. Please remember that this survey is not a vote, and please provide an explanation for your recommendation.

[edit] Survey - in support of the move

  1. Support I am sure that MRI is an abbreviation of Magnetic Resonance Imaging, therefore it would be suitable to change it to MRI. Personally, I'd change the heading to read Magnetic Resonance Imaging or MRI, when using the abbreviation in the body text. --Read-write-services 00:22, 23 January 2007 (UTC)

[edit] Survey - in opposition to the move

  1. Oppose. I think it is alright the way it is. Also, I think it is as per naming convention see Wikipedia:Naming_conventions_(capitalization) and WP:NAME. Nephron  T|C 07:33, 18 January 2007 (UTC)
  2. Oppose move. Current name is as per naming convention + system anyway redirects from "Magnetic Resonance Imaging". Also see: Computed tomography. Abecedare 08:03, 18 January 2007 (UTC)

[edit] Discussion

Add any additional comments:

I'm not sure I understand either of your arguments. The naming conventions page says only capitalize if it's a proper noun, which this clearly is. If it's not a proper noun than someone should go through the article and make it lower case in all the many places it's currently upper. As for listing another article that is currently also incorrect, I think the only proper reply is to list a random article that is correct, e.g. Kyoto Protocol. Vicarious 09:14, 18 January 2007 (UTC)

  • "Magnetic resonance imaging" is not a proper noun but rather than argue that issue, I would request you to check the usage in (not the title alone) standard MRI textbooks such as "Magnetic resonance imaging" by Haacke et al; or journals such as "Magnetic resonance in medicine" or "IEEE Transactions on medical imaging" or "Radiology"; or in other encyclopedias such as "Britannica" [5] to confirm that MRI is almost universally spelled out as "magnetic resonance imaging" in academic usage except possibly when it is used for the first time in an article/book and the abbreviation is introduced. The same principle holds for other medical imaging technologies including CT, PET and SPECT, as well as other relevant terms including RF, SNR, GRAPPA, SENSE, FOV, CBF, MRA, NMR etc (the exceptions being terms such as DFT or FFT in whose expansion Fourier alone is capitalized as it is a proper noun). So yes, someone should go through the article and convert the terminology to lowercase. Hope this clears the confusion. Regards. Abecedare 10:27, 18 January 2007 (UTC)
    • Now that's some reasoning I can get behind. Vicarious 23:12, 18 January 2007 (UTC)

[edit] Principle

Was thinking of perhaps layifying some of the language a bit:

Medical MRI most frequently relies on the relaxation properties of excited hydrogen nuclei in water and lipids.

Does the MRI work only with hydrogen atoms? Does it work only with hydrogen atoms in water and lipids? Are there any hydrogen atoms in a human (or other things commonly scanned) which are not contained in water or lipids? (i.e., could this be changed to state simply it relies on relaxation properties of hydrogen nuclei (or perhaps any nuclei with net non-zero spin)?

It can work on any nuclei with a net nuclear spin (see multinuclear imaging in the main article). However, hydrogen is a major constituent of the body, and the scanner requires dedicated RF hardware tuned to the resonance frequency of each nuclear isotope to be imaged. While all Hydrogen atoms will be excited by the scanner, only 'free' molecules produce a useful signal - where there is a tight lattice (e.g. bones, tendons and ligaments), the MR signal decays so quickly that it has faded before the scanner can become ready to detect it. (This is used to great effect in diagnosis, because injury to tendons/ligaments allows free fluid into the tissue causing a dramatic change in signal characteristics).

When the object to be imaged is placed in a powerful, uniform magnetic field, the spins of atomic nuclei with a resulting non-zero spin have to arrange in a particular manner with the applied magnetic field according to quantum mechanics. Hydrogen atoms (= protons) have a simple spin 1/2 and therefore align either parallel or antiparallel to the magnetic field.

My understanding of spin is a little rusty; "with resulting non-zero spin" - what does the word 'resulting' mean here? The spin is created as a result of applying the magentic field? Any given proton may obtain either spin 0, 1/2 or -1/2? Is the direction of alignment (parallel/antiparallel) arbitrary or a result of some other property of the proton which will precisely determine the direction (+ or - spin)?

Within the nuclei protons (and neutrons), in the same shell, pair off with opposing spins. E.g. Helium-4 contains 2 paired protons (1 spin +1/2 and 1 spin -1/2) and 2 paired neutrons with no net nuclear spin; but Helium-3 contains an unpaired neutron so has a net nuclear spin of 1/2.
The alignment of an individual proton with the magnetic field can be either parallel or anti-parallel. The result is a dynamic equilibrium with some protons aligning parallel, and the rest anti-parallel, with a constant exchange between the 2 states. However, because nuclei have lower energy when aligned parallel than anti-parallel, the equilibrium is not symmetrical, with a very slight excess of nuclei aligned in the parallel direction.

Common magnetic field strengths range from 0.3 to 3 T, although field strengths as high as 9.4 T are used in research scanners [6] and research instruments for animals or only small test tubes range as high as 20 T. Commercial suppliers are investing in 7 T platforms. For comparison, the Earth's magnetic field averages around 50 μT, less than 1/100,000 times the field strength of a typical MRI.

Is this paragraph appropriate/needed in the principal section?

I think it provides a bit of perspective. Magnets in typical MR scanners are many times stronger than magnets that most people typically handle, not to mention very much larger. It is their magnetic property that is the most recognised 'feature' of the scanners.

The spin polarization determines the basic MRI signal strength. For protons, it refers to the population difference of the two energy states that are associated with the parallel and antiparallel alignment of the proton spins in the magnetic field and governed Boltzmann's statistics.

Is this saying that spin polarization is the integral difference in protons in one state versus the other? So, if I have 500,001 protons that are parallel and 500,000 protons that are antiparallel then my spin polarization is 1? What is Boltzmann's statistics and what does that have to do with the calculation of spin polarization? What is the cause for the descrepancy; is it arbitrary or is it a result of some fundemental property of protons?

Spin polarization is more usually called net magnetization. This is perhaps a better term. It essentially represents the residual magnetic alignment after those protons aligned parallel and anti-parallel have cancelled. In your example above, it would be 1 spin per million in the parallel direction. The Boltzman distribution is a mathematical description of the equilibrium that forms when particles can occupy different energy levels (see above).

In a 1.5 T magnetic field (at room temperature) this difference refers to only about one in a million nuclei since the thermal energy far exceeds the energy difference between the parallel and antiparallel states. Yet the vast quantity of nuclei in a small volume sum to produce a detectable change in field.

The thermal energy causes protons to switch between parallel and antiparallel? (which seems to argue that the state is arbitrary) If the selection of parallel and antiparallel is arbitrary than each "small volume" would statistically cancel out each other "small volume" (since that would not be helpful; seems to argue that the selection isnt arbitrary and must favor either parallel or antiparallel statistically. If so, why?) '...detectable change in magnetic field...'?

It is the thermal energy that allows the protons to change state, hence allows an equilibrium to form. Because of a very slight preponderance in the parallel direction, then the net magnetization doesn't completely cancel - there is a tiny residual. However, the sheer number of protons is so high, that even this tiny residual is sufficient for detection.

Most basic explanations of MRI will say that the nuclei align parallel or anti-parallel with the static magnetic field though, because of quantum mechanical reasons, the individual nuclei are actually set off at an angle from the direction of the static magnetic field. The bulk collection of nuclei can be partitioned into a set whose sum spin are aligned parallel and a set whose sum spin are anti-parallel.

This is describing the heisenberg effect? Each proton could never be precisely in a specific alignment, but simply has a probablity cloud of alignments which is centered along the magnetic field? If thats the case then there is a certain probablity that a given proton is precisely aligned with the magnetic field and the probablity cloud is statistically parallel with the magnetic field? Can be partitioned theoretically or is partitioned in practice during the scan?

My knowledge of QM is limited, and I don't think I've seen this issue discussed at any length in general MR texts. The spins are already partitioned parallel and anti-parallel by the magnetic field. The point made here is that regarding all the spins as being aligned either perfectly parallel or anti-parallel is adequate to predict the aggregate behavior (which is all that can be detected), even though the true direction of an individual spin is uncertain.

The magnetic dipole moment of the nuclei then precesses around the axial field. While the proportion is nearly equal, slightly more are oriented at the low energy angle. The frequency with which the dipole moments precess is called the Larmor frequency.

Ok, starting to get really lost... Is this saying that similar to -- electron orbitals around a nuclei--, the proton's axis has multiple quantized "orbitals" which effect the probablity cloud of its angle of incidence with the magnetic field? Ok, all my previous theories start to breakdown when adding that the proton precesses at a specific rate. And the angle of incidence with the magnetic field is a function of energy level. Perhaps the higher level orbitals are donuts centered around the magnetic field which allows for a specific frequency, but then it seems that the 0 level orbital should still have no frequency (and should still include the magnetic axis itself). ...the proportion is nearly equal... Proportion of 0 level to 1 level? Why would it be nearly equal? Why are the protons in the level 1 state at all? Aren't there states above level 1? Larmor frequency is specific to the level 1 state or all states precess at the same frequency?

Each spin has a magnetic dipole moment. This can be imagined as a bar magnet aligned with the spin. The bar magnet rotates around the axis of the spin at the precession rate (Larmor frequency) of the spin. To further simplify matters, it is only necessary to imagine a single bar magnet for the whole net magnetization of a region.
Under equilibrium conditions the spins are aligned with the external magnetic field - this means that the net magnetization is aligned with the magnetic field. Because the net magnetization rotates along its own axis, this rotation does not result in any net change in the magnetic field produced by the net magnetization.

The tissue is then briefly exposed to pulses of electromagnetic energy (RF pulses) in a plane perpendicular to the magnetic field, causing some of the magnetically aligned hydrogen nuclei to assume a temporary non-aligned high-energy state. Or in other words, the steady-state equilibrium established in the static magnetic field becomes perturbed and the population difference of the two energy levels is altered. The frequency of the pulses is governed by the Larmor equation to match the required energy difference between the two spin states.

Is the EM applied from a single direction or radially? The EM waves hit the protons and move them from level 0 to level 1? How are the states measured; photons created when the proton relaxes and released in a random direction as opposed to the original direction of the source beam? What does that information tell one about the material being scanned? What is really being measured; proportion of hyrdrogen atoms at every given location? Different regions are detected by measuring intensity of photons detected bouncing randomly? I have no idea how far off from reality I am at this point, but if that's the case how does measurement work in 3D?

The EM only needs to be applied perpendicularly. Essentially, the RF energy allows some protons to be moved from the low energy level, into the high energy level. It would be possible to apply a measured amount of energy and equalise the parallel/anti-parallel proportions discussed earlier (saturation) or even apply more and reverse the net magnetization.
However, the actual effect is more subtle. The EM pulse results in a change in the angle of precession of the individual spins. Recall that the net magnetization is rotating about its axis (but the symmetrical nature of the dipole magnetic field means that this rotation has no net effect). However, the change in spin precession angle causes the net magnetization to tilt on its axis of rotation. EM pulses are constructed in such a way as to rotate or 'flip' the net magnetization by a specified angle.
Commonly a '90 degree' pulse may be used. This tilts net magnetization so that it is perpendicular to the static magnetic field, yet the rotation axis continues to be aligned with the field. This results in a rotating magnetic dipole - imagine a bar magnet attached to a rotor shaft as in a simple permanent magnet alternator. Just as in an alternator, this rotating magnetic field is induces an AC voltage in wire coils placed around it. It is this AC voltage that forms the basis of the MRI signal, and is what is detected by the scanner.
Other flip angles are possible and are used for special purposes. 180 degree pulses completely reverse the direction of the net magnetization and on their own result in no detectable signal - but can be used as part of more complex sequences of pulses to improve performance. Low angle pulses can also be used, but produce a weaker signal (because only the transverse component of the rotating magnetization is detectable).
Up to now we haven't discussed localisation of the spins at all. Localisation is more complex - but the fundamental concept is that while the spins have been flipped transversely, the magnetic field of the scanner is changed so that the field changes across the scanner (a gradient). The Larmor frequency is related to the magnetic field strength that the spin experiences - and it is the Larmor frequency that determines the frequency of the EM pulse that will flip the spins, and also the frequency of the signal that the spins will subsequently produce. By applying a magnetic field gradient across the scanner, the Larmor frequency of spins will vary across the scanner. It is therefore possible selectively to flip spins within regions of the scanner and analyse the received signal to receive positional information.

Anyway, I'm sure I'll have more questions and if this goes well I'd like to try and tackle the k-space section also. Aepryus 07:33, 23 January 2007 (UTC)

It might be useful to split this section off next - I'll probably do it when i have a bit more time. In the meantime, if you think you can understand it, why not have a go at simplifying the text? ChumpusRex 20:11, 25 January 2007 (UTC)

[edit] Noises

While the article makes it clear that an MRI machine produces the rumbling/growling noises because of the rapid expansion/contraction of the magnets, it doesn't explain something that I've wondered for years: When the machine pauses between the various stages of a scan and the rumbling/growling stops, there is the single, occasional, very loud "clack" sound, like a piece of wood being struck by a hammer, or perhaps a really *really* loud slap stick. Since the magnets are essentially idle (or so I understand) during this time, what then makes this sound? Vanessaezekowitz 17:19, 15 March 2007 (UTC)

You are most probably referring to the "RF scaling" of the scanner, this is to test the duration and energy level of the RF pulse to achieve the transition to 90 degrees away from the static feild. i.e. to provide the maximum received energy in the antenna, the magnetic lines of force must cross the coil perpendicularly. the ultimate aim is to set up the RF system to receive the best possible signal and (to a lesser degree) to minimise patient RF exposure.--Read-write-services 00:44, 16 March 2007 (UTC)

[edit] Contrast Agent Safety

With respect to, I believe, gadolinium contrast agents, someone wrote the following (which was unreferenced and, in my opnion, is off tone and unlikely to be true):

However, one should be careful with the anaphylactoids. It has to be pumped in to the bloodstream, usually through a vein in the arm, the same place as where you take bloodsamples. If it is pumped into a muscle, it will have enormous consequences, and an amputation may be needed.[citation needed]

Seriously. How enormous, exactly, will the consequences be? As enormous as a house, or more like a mountain? And what class of substances are the "anaphylactoids"? If it's "pumped" into the extravascular space, it'd make its way into the lymphatic system and very soon make its way back into the vasculature, soon to be removed by the kidneys. Cajolingwilhelm 22:14, 16 March 2007 (UTC)

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