How does 90 degree pulse translate alignment to phase

[EeePC1005PR]

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Not sure if this is the place to ask. This is not for exam, just one of those persistent lingering queries which no body seems to answer simply for me:

In MR, we read FID (free induction decay) from transverse magnetization. FID is pretty much just linear intensity fluctuation of the transverse magnetization, which we listen to.

We flip T1 with 90 degree pulse, which turns that T1 into transverse magnetization, which we can then listen to.

I am cool with all of above.

My question is this: in effect we've turned information about "proton alignment" (vertical magnetization) into "proton precession phase" (transverse magnetization), when we apply the 90 degree pulse.

Alignment and phase are two separate things. How does applying a RF wave expresses former in terms of latter?

Hope my question makes sense. Thanks a lot. If you know the correct answer, simple explanations please. Imagine explaining it to final year high school physics student, plus a little bit deeper (just a little bit), that's about the right level for me 😛

 

[shark2000]

Not sure if this is the place to ask. This is not for exam, just one of those persistent lingering queries which no body seems to answer simply for me:

In MR, we read FID (free induction decay) from transverse magnetization. FID is pretty much just linear intensity fluctuation of the transverse magnetization, which we listen to.

We flip T1 with 90 degree pulse, which turns that T1 into transverse magnetization, which we can then listen to.

I am cool with all of above.

My question is this: in effect we've turned information about "proton alignment" (vertical magnetization) into "proton precession phase" (transverse magnetization), when we apply the 90 degree pulse.

Alignment and phase are two separate things. How does applying a RF wave expresses former in terms of latter?

Hope my question makes sense. Thanks a lot. If you know the correct answer, simple explanations please. Imagine explaining it to final year high school physics student, plus a little bit deeper (just a little bit), that's about the right level for me 😛

I really don't know what are you asking. From your description, I wonder if you have understood the basics. Your statement appears off the track.


Sorry.

 

[EeePC1005PR]

I really don't know what are you asking. From your description, I wonder if you have understood the basics. Your statement appears off the track. Sorry.

Don't be sorry.

I only expected people who understand the question to answer.

 

[cowme]

how does one "listen" to transverse magnetization? Is this at all bath salts related? If so, gimme

 

[shark2000]

how does one "listen" to transverse magnetization? Is this at all bath salts related? If so, gimme

4 +
Though he does not care. He is waiting for someone to understand it. The rest of it is just some vocabulary many not used appropriately, which he has read somewhere.

 

[DO bound]

Your pre-question statements are correct, but as pointed out above your question doesnt exactly hold water (precessing protons and all).

I would suggest checking out eanatomy, http://www.imaios.com, (pretty awesome radiology anatomy website). They specifically have a great "e-course" (3rd tap at top of home page) dedicated to MRI learning on a pretty basic level, though it does get pretty in depth as you progress. The tutorial has great pictures, animations, flash videos, that will give you a great base knowledge of MRI. This also goes for earlier rad residents who are yet to have taken physics (I know this years physics kick is past). You will have to register for this site and I believe the free registration will include this MR tutorial. Good stuff, check it out.

 

[EeePC1005PR]

Take it easy einsteins.

My question stands as is. It is a simple question put plainly. If one knows the topic well enough, he/she can answer in simple terms.

 

[HamOnWholeWheat]

By applying a 90deg pulse, you take the proton alignment out of the vector of the main magnet (B0). Normally, the magnet is keeping all of the protons in the same direction as the main field, and thus constantly corrects for little interactions between protons. When you blast them out of the plane of the magnet, this correction no longer applies, and interactions between protons predominate, allowing spin spin interactions to dephase the spins relative to each other. After a very short time, they're sufficiently dephased as to null out the signal, which you correctly stated is best received at 90 deg to the B0.

So its not that alignment and phase are two different things, but rather two different effects predominate whether you're aligned parallel or perpendicular to the "aligning force" of B0.

Its easy to get stuck on terminology, especially in MRI physics.

 

[tenau12]

The answer to your question lies in one word. Environment!

It's due to the different environment protons are in that we get something useful to look at. Otherwise everything will be just a same color blob! Hope that helps

 

[mrmandrake]

The longitudinal and transverse processes are totally independent of each other so one is not "turned" into the other.

The longitudinal magnetization is deflected into the transverse plane via the RF pulse and recovers and in the process gives off RF waves. This is T1.

I'm not sure what you mean by "proton precession phase" but the protons always precess at the Larmor frequency which depends on the strength of the magnetic field.

"Alignment and phase" are indeed two separate things and "phase" typically refers to T2 processes. I don't think the former is expressed in terms of the latter.

I can try to give a better answer if you clarify your question.

My question is this: in effect we've turned information about "proton alignment" (vertical magnetization) into "proton precession phase" (transverse magnetization), when we apply the 90 degree pulse.

Alignment and phase are two separate things. How does applying a RF wave expresses former in terms of latter?

 

[HamOnWholeWheat]

The longitudinal and transverse processes are totally independent of each other so one is not "turned" into the other.

The longitudinal magnetization is deflected into the transverse plane via the RF pulse and recovers and in the process gives off RF waves. This is T1.

I'm not sure what you mean by "proton precession phase" but the protons always precess at the Larmor frequency which depends on the strength of the magnetic field.

"Alignment and phase" are indeed two separate things and "phase" typically refers to T2 processes. I don't think the former is expressed in terms of the latter.

I can try to give a better answer if you clarify your question.

"I'm not sure what you mean by "proton precession phase" but the protons always precess at the Larmor frequency which depends on the strength of the magnetic field."

Agreed, but you're talking about frequency, not phase. All the protons can precess at the larmour frequency yet be 100% in phase or 100% out of phase, or somewhere in between. I don't want to speak for the OP, but I think when he said proton precession phase, he was referring to dephasing due to spin spin interactions in the transverse plane. The interactions will cause the precession of ions to dephase while aligned in the transverse plane, but not measurably in the plane of B0 (as they're constantly re-aligned by the essentially static field of B0). I tried to explain that in my last post, but apparently failed. 🙂

The OP's question does make sense, its just the terminology he/she is using is a little mixed up (easy to do).

 

[EeePC1005PR]

Thanks alot HamOnWholeWheat and mrmandrake.

You have cleared my question. Much appreciate it.

 

[Browser]

All this posturing reminds me of the first year of med school..

 

[samsoniter]

Okay, not completely unrelated question (BTW eanatomy has a great MRI review)...and obviously I am a rookie but I hope I can accurately convey my question.
In terms of magnetization, longitudinal vs transverse, I cannot grasp why, given the same TR in 2 tissues with different T1s the transverse component would have higher amplitude in the tissue with MORE longitudinal relaxation in that given TR.
To rephrase: tissue A has a a shorter T1 and has completely relaxed (longitudinal vector only) after this TR, tissue B has a longer T1 and has not completely relaxed- it still has a transverse component to the vector. But then when you hit tissue A and B with a 180 degree pulse (bear in mind tissue A has completely recovered and tissue B still has some transverse component to the vector) WHY does the fully relaxed tissue A have a higher transverse amplitude?
My logic is the tissue B with a longer T1 (thus given this TR has a tranverse component and is not completely relaxed to longitudinal vector only -where as tissue B has fully relaxed) would have a HIGHER transverse component after the 180 degree pulse because its vector is not fully longitudinal....ie it retains some of the transverse amplitude prior to the 180 pulse which would give it a higher transverse amplitude.
TO rephrase yet again: why does a 180 pulse result in a higher transverse amplitude in a fully relaxed (i.e. shorter T1) proton than an incompletely relaxed (longer T1) proton?

Perhaps the 180 pulse is more "efficient" at imparting transverse vectors in fully longitudinally relaxed
protons?


SINCERE apologies for lack of proper terminology and/or confusing manner of posing this question, if possible please use analogy/2nd grade level verbiage. THANKS!

 

[samsoniter]

perhaps I have answered my own (kind of ridiculous) question.....the shorter T1 tissue has a higher signal at the given TR, thus the starting point for the curve for T2 (at TE) is higher....?

 

[HamOnWholeWheat]

perhaps I have answered my own (kind of ridiculous) question.....the shorter T1 tissue has a higher signal at the given TR, thus the starting point for the curve for T2 (at TE) is higher....?

Good question, but I think I see what's tripping you up. I'll explain it with this scenario which assumes we're talking about a standard spin echo sequence, of a single 90 degree excitation, followed by a single 180 degree refocusing pulse per TR. When you generate a signal in MRI, its from flipping spins from the longitudinal plane into some off-longitudinal plane, thus all of your potential transverse magnetization is directly proportional to the amount of longitudinal magnetization you have to work with. Most commonly, its a 90 degree flip, so that ALL of your transverse magnetization comes from the amount of longitudinal magnetization which recovered.

Lets say in your example you have two tissues with short (A), and long (B) T1 values. You apply a 90 degree pulse to flip them into the transverse plane, you apply a 180 degree refocusing pulse to generate an echo, and then wait a period of time, TR, for them to recover in the longitudinal plane. Tissue A recovers completely, but B does not, as it has a longer T1 (T1 > TR). At this point, A is fully recovered in the longitudinal plane, and B is some fraction recovered in the longitudinal plane. You've explained this scenario in your prior post.

On the next TR, there is less longitudinal magnetization availble to flip into the transverse plane, and thus the signal you get will be smaller. B may have residual useful transverse magnetization, or it may have completely dephased. This property (T2) is independent of the T1 effect, but I understand why you're equating the two. Picturing the diagram, if the longitudinal magnetization hasn't recovered, there MUST be some transverse component by necessity (i.e. the longitudinal arrow is precessing about Z, and until it completely restores, you have to have a small transverse component, right?). The problem with thinking about this transverse component this way is that it assumes that this residual transverse component is relevant to T1 signal generation, when its not. Suppose the subequent 90 degree pulse was applied, and this small residual transverse component in B was flipped and added to the longitudunal component. Is it coherent? Meaning, are all the electrons processing on the same phase? Or are they all scattered and out of phase? They're all out of phase, because there has been nothing to completely rephase them since the last TR, and thus generate not signal. The only way to restore useful coherence after a TR in a spin echo sequence, is to use the longitudinal magnetization induced by B0, which is why the shorter T1 generates a stronger signal: it is fully recovered and coherent.

I hate to muddy the waters, but you kept referring to subsequent 180 degree pulses, rather than 90-180 degree pulses. There are sequences which use a series of 180 degree pulses without a repeat excitation, and there are called echo-train, or turbo spin echo, or a million other names. These aren't the best way to learn the physics early on, as they can be quite confusing without a firm grasp of the fundamentals.

Hopefully this made sense.

Hammy

 

[samsoniter]

Dear Hammy, not only have you made my stomach roar as it has been a long and lunch-less day, but you have done a great job of explaining my erroneous logic- I WAS equating the transverse component with T1...though the waters are still muddy, it makes much more sense now that I realize T1 and transverse have nothing to do with each other. I will continue to read up on the fundamentals, perhaps with a ham sam in hand.
THANKS!