MRI question for our Heavy Weights

[Tsu]

Our MRI literature states that 98% of the hydrogen atoms in our body are lined up in the same direction while we aere in the magnet. The other 2% are in counter-allignment to the field.

Why is this so?

 

[Tsu]

c'MON PFer's! I promised my tech. buddy an answer! I told him you guys know EVERYTHING! Don't let me down now!

 

[imabug]

Hmm, I seem to recall from my studyings that the difference between the number of protons aligning parallel and anti-parallel to the $$B_0$$ field is actually very small (a few percent at best), but sufficient enough to result in a net magnetization vector along $$B_0$$. But all my texts are in my office, which is where I am not at the moment. I'll have to wait until after the weekend to find out the answer unless someone else comes along with it first.

Had something to do with the spin states of the protons or something like that. My brain is a little fuzzy on MRI things.

 

[Tsu]

Thanks, imabug. What 'studyings' were those?

I'm pretty fuzzy on MR, too. Not to mention that fact that, whenever I'm in the scan room, I have this really INTENSE desire to walk NORTH!

 

[MR Guy]

This has bugged me enough to answer my own question. imabug is right from what I have found on 3 different papers on the internet. What I am reading indicates that there is a nearly equal distribution of magnetic moments parallel and anti-parallel to the static magnetic field. One paper indicates that at room temperature and at standard clinical MRI magnetic field strengths that out of every 1 million H protons there is only one more parallel than there are anti-parallel. There is actually a very small energy state difference between parallel and anti-parallel. Thermal energy is the primary difference between these two energy states. Also, it probably goes without saying that as the magnetic field strength increases, so does the ratio of parallel to anti-parallel spins. (I guess I should do more reading before I ask questions in the future )

(Thanks to Tsunami for posting my initial question)

 

[Tsu]

This has bugged me enough to answer my own question. imabug is right from what I have found on 3 different papers on the internet. What I am reading indicates that there is a nearly equal distribution of magnetic moments parallel and anti-parallel to the static magnetic field. One paper indicates that at room temperature and at standard clinical MRI magnetic field strengths that out of every 1 million H protons there is only one more parallel than there are anti-parallel. There is actually a very small energy state difference between parallel and anti-parallel. Thermal energy is the primary difference between these two energy states. Also, it probably goes without saying that as the magnetic field strength increases, so does the ratio of parallel to anti-parallel spins. (I guess I should do more reading before I ask questions in the future )

(Thanks to Tsunami for posting my initial question)

No problem, MR Guy. But it was an interesting question! I wanted an answer and I wanted it fast! Most of the time if I have a question these guys are all over it, like - NOW! They're GREAT! But now, because I'm so fuzzy on the principles of MR, I'm going to have to go find more info about magnetic moments and a few other things to even start to understand your answer!

 

[Tsu]

http://en.wikipedia.org/wiki/Magnetic_moment

Excuse me. I feel a seizure coming on...

 

[imabug]

Thanks, imabug. What 'studyings' were those?

those would be my studyings to become a medical physicist and pass the board certification exam. i learned enough about MRI to pass my exams, but my brain gets all tied up in knots once you get to the imaging sequences. Now I leave the MRI stuff to the guys who know it better than I do and focus on x-ray and nuclear medicine.

 

[GENIERE]

In the late 1970’s I attended a fairly intense 3-day seminar on MRI theory, but never had any hands on experience. I can’t answer the percentage question but just a little of the basics:

There is a strong, static magnetic field that aligns (tilts) the atoms in one direction.

Surrounding the patient is a coil wherein (I believe) the windings are spaced closely on one end and gradually the spacing is increased. A DC current is passed through the coil that causes a gradient in the produced magnetic field. This field then interacts with the stronger static field to alter the alignment (tilt), thus the tilt varies from head to toe, front to back, and side to side so to speak. This is done to produce a spatial reference.

Finally an RF signal is introduced at a specific frequency and causes the tilt to flip. When flipped the atoms produce a RF signal that contains the spatial characteristics of the flipped atom, I think by frequecy modulation of the RF signal, but its been too many years. That data is used to generate an artificial image.

That’s about all I remember and it may be erroneous.

...

 

[Tsu]

imabug - Oh, that's right! I asked you that in the CT/Mammogram thread. I'm old. I forget stuff... Do you work in a hospital setting? Are you employed by a hospital? Is a medical physicist the same as a radiation physicist? I wouldn't think so, but I'm not sure where the line would be drawn... Am I asking too many questions?

Geneire - Yes, that's my basic understanding of it as well. Those RF signals are then assigned a value-specific shade of gray to produce the image. But I'm still not following why 2% of our H atoms flip in counter-allignment to the magnetic field. (Am I even asking this right?)

MR Guy - T2 would have been my choice for your name...

 

[pervect]

http://en.wikipedia.org/wiki/Magnetic_moment

Excuse me. I feel a seizure coming on...

How do you feel about electric dipole moments?

http://en.wikipedia.org/wiki/Electric_dipole_moment

It's the same idea, but in the electric case you can describe it simply as the vector distance between two opposite and equal charges times the charge.

If we had magnetic charges, you could do the same thing with the magnetic moment, but we don't. It's basically the same concept, though, you have a north pole and a south pole, and it will tend to "twist" (generate a torque) in the presence of a magnetic field, just as the electric dipole will "twist" in an electric field. For the electric dipole, the forces on the charges are easily calculated by columb's law.

 

[Tsu]

I must admit I wasn't aware that the coil windings were spaced farther apart on one end. What is the purpose of this? How does that affect the image? (God, I HATE this stuff that I just love! Why am I even ASKING this?? I'm not going to understand the dang answer anyway... )

No. Wait. I refuse to not understand <gets drill, plugs it in, sound of drilling in background>

there is a nearly equal distribution of magnetic moments parallel and anti-parallel to the static magnetic field. One paper indicates that at room temperature and at standard clinical MRI magnetic field strengths that out of every 1 million H protons there is only one more parallel than there are anti-parallel. There is actually a very small energy state difference between parallel and anti-parallel. Thermal energy is the primary difference between these two energy states. Also, it probably goes without saying that as the magnetic field strength increases, so does the ratio of parallel to anti-parallel spins

OK. I think I'm beginning to get it now...
Sort of...

 

[imabug]

imabug - Oh, that's right! I asked you that in the CT/Mammogram thread. I'm old. I forget stuff... Do you work in a hospital setting? Are you employed by a hospital? Is a medical physicist the same as a radiation physicist? I wouldn't think so, but I'm not sure where the line would be drawn... Am I asking too many questions?

Yes, I work in a hospital setting as a clinical diagnostic physicist. Radiation physicist is a term that I usually see referring to medical physicists that work in radiation therapy, but it is also a pretty generic term. I don't do radiation therapy though.

 

[Tsu]

How do you feel about electric dipole moments?

Don't ask.

http://en.wikipedia.org/wiki/Electric_dipole_moment

It's the same idea, but in the electric case you can describe it simply as the vector distance between two opposite and equal charges times the charge.

If we had magnetic charges, you could do the same thing with the magnetic moment, but we don't. It's basically the same concept, though, you have a north pole and a south pole, and it will tend to "twist" (generate a torque) in the presence of a magnetic field, just as the electric dipole will "twist" in an electric field. For the electric dipole, the forces on the charges are easily calculated by columb's law.

Oooooookkaaaaaayyyyyy. Well. It looks like seizure time again.

You guys just LOVE to torture me, don't you? I really do appreciate your help, though. The more I read... well, the more I read! That's always good...

So... this:

vector distance between two opposite and equal charges times the charge

really means something to you guys?

Oh. OK. Can it be said differently? So that a NON rocket scientist can understand it?

 

[Tsu]

Yes, I work in a hospital setting as a clinical diagnostic physicist. Radiation physicist is a term that I usually see referring to medical physicists that work in radiation therapy, but it is also a pretty generic term. I don't do radiation therapy though.

Thanks for the info! Do you enjoy your work? Is it very challanging? Do you work very closely with Technologists? You mentioned NM - do you get to work with PET scanners?

 

[imabug]

I must admit I wasn't aware that the coil windings were spaced farther apart on one end. What is the purpose of this? How does that affect the image? (God, I HATE this stuff that I just love!

the coils are responsible for producing the gradient fields (secondary magnetic fields that change linearly along one direction) that allow you to spatially localize the RF signal emitted as the excited protons relax to their ground state.
When the protons relax, the RF they emit is given by the Larmor equation which looks something like
$$f = \gamma (B_0 + B_g(x))$$
where $$\gamma$$ is the gyromagnetic ratio for hydrogen (the value of which escapes me at the moment) and $$B_g$$ is the magnitude of the gradient field at position x.
Thus the frequency of the RF emitted will tell you where in space it's being emitted from. There are 3 gradients for each axis: slice, phase and frequency encoding.

Somewhere on my bookshelf I have a very good book that describes MRI in terms radiologists understand. Should be very suitable for technologists too, although it might not go into enough detail for techs. I'll find out what it's called when I get back to work on Monday.

Thanks for the info! Do you enjoy your work? Is it very challanging? Do you work very closely with Technologists? You mentioned NM - do you get to work with PET scanners?

In fact I usually work fairly closely with the technologists. They're my primary source of information about what's happening with the equipment and how well it's working. They tell me when they think a piece of equipment isn't operating correctly and I'll come by with my test equipment to check it out to tell the Biomed folks what to fix. I also do annual inspections required by state and federal regs.

I also provide physics support to our NM department The hospital I work at is in the process of purchasing the first of 2 PET/CT scanners, so I will be getting more involved in PET in the near future.

I enjoy the job a great deal, although it does get pretty routine sometimes. But there's always something interesting going on to get involved in.

 

[humanino]

Hello Tsu !
I am a bit late, but I can at least answer one question :

Can it be said differently? So that a NON rocket scientist can understand it?

No this is impossible, sorry.

Wait ! I lied
There is not much mystery in the sentence you quote. Except that it might be purposedly obscure...

The electric dipole moment is a vector. This is merely an arrow : it has a length and a direction. The direction of the vector goes from negative to positive charge. The length of the vector measure the strength : if you double the charges, the length of the vector doubles too. But the length of the vector is also proportional to the distance between the charges.

Maybe it is not less obscure at all

 

[pervect]

So... this:

really means something to you guys?

Oh. OK. Can it be said differently? So that a NON rocket scientist can understand it?

/me scratches his head...

Well, you've probably seen the expression F = ma before. And hopefully somene has told you that F is a vector, because it has a magnitude and direction.

Now, if you also happen to know that a is also a vector, because it has magnitude and direction, and that m isn't a vector, but is a plain old number, you should see, via this example, that it is possible to take vectors, and multiply them by numbers, and that the result of this is another vector.

So that's exactly what we are doing with the electric dipole moment.

Maybe the concept of a vector seems hard, I don't know. The distance between two points (charges, in this case), interpreted as a vector, has got a starting point, and an ending point. The starting point is located in this case on the negative charge, the ending point on the positive charge.

We've already specified that the charges are of opposite sign and equal in value, so one must be positive, the other has to be negative.

The vector has a length, the distance between the two charges, and a direction (north, south, east, west, northeast, etc.)

This is the distance vector. It's about as simple as a vector can get.

We've already seen we can mulitply vectors by numbers. If you haven't had this in your coursework yet, or if you selept through that part of your classes I suppose you just have to Take It On Faith. When you multiply a vector by a number, you keep the direction of the vector unchanged, but you multiply it's length by said number.

The other thing you can do with vectors is add them together, but we don't need to do that for this example.

All we need to do is to take the distance vector, and multiply it by the value of the charge.

 

[Tsu]

the coils are responsible for producing the gradient fields (secondary magnetic fields that change linearly along one direction) that allow you to spatially localize the RF signal emitted as the excited protons relax to their ground state.
When the protons relax, the RF they emit is given by the Larmor equation which looks something like
$$f = \gamma (B_0 + B_g(x))$$
where $$\gamma$$ is the gyromagnetic ratio for hydrogen (the value of which escapes me at the moment) and $$B_g$$ is the magnitude of the gradient field at position x.
Thus the frequency of the RF emitted will tell you where in space it's being emitted from. There are 3 gradients for each axis: slice, phase and frequency encoding.

Good grief, imabug. Am I just an ignoramous or is there something in this that explains the spacing of the coils? I'm so sorry, but you are talking a foreign language to me.

Somewhere on my bookshelf I have a very good book that describes MRI in terms radiologists understand. Should be very suitable for technologists too, although it might not go into enough detail for techs. I'll find out what it's called when I get back to work on Monday.

That would be great. We may have it work or possibly MR Guy has it and would let me borrow it. I do hope it's not quite as technical as what you wrote or I'm just going to have to go back to school for nothing but physics! Frankly, I don't love it 'THAT much! At least not near as much as I love my patients. I'll just have leave that end of it to you and I'll take care of patients. But I do like to have a pretty good idea of just how our images are generated. I have very good understanding of CT and Xray but MR - not so much. I started doing MR a few years back but it was about the most boring thing I'd ever done so I dropped it like a hot potato. There is just not enough actual patient contact - which is what I do best and thrive on.

In fact I usually work fairly closely with the technologists. They're my primary source of information about what's happening with the equipment and how well it's working. They tell me when they think a piece of equipment isn't operating correctly and I'll come by with my test equipment to check it out to tell the Biomed folks what to fix. I also do annual inspections required by state and federal regs.

God bless you guys for designing and maintaining my machines so I can help my patients, but I wouldn't trade jobs with you for all the world! It's MUCH easier to deal with blood and guts, trauma and death than:
$$f = \gamma (B_0 + B_g(x))$$
where $$\gamma$$ is the gyromagnetic ratio for hydrogen (the value of which escapes me at the moment) and $$B_g$$ is the magnitude of the gradient field at position x....

BTW, what do you think of a Lead CT Tech. who instructs his techs to NOT provide shielding for their patients, saying it's not enough radiation to worry about?

I also provide physics support to our NM department The hospital I work at is in the process of purchasing the first of 2 PET/CT scanners, so I will be getting more involved in PET in the near future.

I enjoy the job a great deal, although it does get pretty routine sometimes. But there's always something interesting going on to get involved in.

Now THAT will be cool! I'd LOVE to get into PET scanning. Not much chance of that for me where I live, though...

 

[Tsu]

Pervect and humanino -

Thanks for your responses. I'll have to get back to this tomorrow. I have to go get ready to go out to dinner with Santa!

'Till tomorrow, then...

 

[imabug]

Good grief, imabug. Am I just an ignoramous or is there something in this that explains the spacing of the coils? I'm so sorry, but you are talking a foreign language to me.

I wouldn't worry too much about it. Coil design is a whole sub-field in MRI research and for most people (myself included) an arcane magic voodoo.

That would be great. We may have it work or possibly MR Guy has it and would let me borrow it. I do hope it's not quite as technical as what you wrote or I'm just going to have to go back to school for nothing but physics!

It was written for radiologists, so it's not very technical at all.

BTW, what do you think of a Lead CT Tech. who instructs his techs to NOT provide shielding for their patients, saying it's not enough radiation to worry about?

Most people will tell you that the majority of dose to non-imaged areas of the patient is from internally scattered radiation, so the actual benefit of additional lead shielding is dubious. I seem to recall one study from a while back that demonstrated a small reduction in patient exposure to non-imaged areas. I say if it makes you (the tech) and the patient feel better, then go ahead and lay on an apron. No harm in doing it, and maybe a little bit of benefit.

Now THAT will be cool! I'd LOVE to get into PET scanning. Not much chance of that for me where I live, though...

Demand for PET techs especially with CT experience is huge now. I'd say if you really are interested, look into cross-training in nuclear medicine. Might have to go back to school for a year or two though. And there is lots of patient interaction in NM/PET.

 

[imabug]

An MRI Reading List

This is some of what I have on my bookshelf on MRI. Definitely not a comprehensive list...just the books that I find useful for my purposes.

Magnetic Resonance Imaging: Principles, Methods and Techniques - Perry Sprawls
This is a very good introductory book on MRI. Perry Sprawls is a very well respected medical physicist and is well known for his educational efforts. Not a very big book, but with lots of simple easy to understand images.

MRI: Basic Principles and Applications - Mark Brown, Richard Semelka
Another small introductory book on MRI. A tiny bit more technical than Sprawls but still with plenty of easy to understand pictures. I used this to learn MRI for my board exams.

NMR Imaging in Biomedicine - P Mansfield, PG Morris
By far the most technical book on MRI that I have. Still a very thin book but packed with math and physics about MRI.

NMR in Biomedicine: The Physical Basis - E Fukushima (ed)
This is actually a collection of seminal papers involved in the development of NMR. It's more about MR and less about imaging, and with a lot of older papers. Still very interesting read for those interested in how NMR was developed. Very technical book.

 

[Integral]

Pervect and humanino -

Thanks for your responses. I'll have to get back to this tomorrow. I have to go get ready to go out to dinner with Santa!

'Till tomorrow, then...

HO! HO! HO!

It was good!

And a good time was had by all!

 

[Tsu]

Indeed it was! I'm already Jonesin' for more of that Thai food!

Imabug - thanks for the book titles. We have the one by Brown and Semelka in our MR library. I'll check with MR Guy and see if he has the first one. Regarding the CT scatter - my radiation physicist told me that there is still considerable scatter at the foot of the table, so I will continue to shield my patients as I would want to be shielded. He also told me that if the door between the scan and control rooms is left open, we, as operators, receive some of the scatter as well! This is a bit disturbing to those of us in the scan room - especially since our supervisor is particularly fond of leaving the door open most of the time. Thanks for the tip about coil design. I'm feeling MUCH better now...

OK. Pervect and humanino - Now (oh, GOD! ) I know that force = mass x acceleration. I also know that a vector has both magnitude and direction. But I also know quite a lot about MEN, and, just like physics equations, even though I 'know' them, it doesn't mean that I UNDERSTAND them! I'm pretty sure you understand what I'm saying here. I mean really - do you UNDERSTAND women?

So I guess I'll just read my little books and reread the things you guys have written and pray that I glean a little more understanding of the physics of my toys. I must say, though, that it certainly is much more fun to learn to manipulate the machines to obtain the proper images than it is to learn how the machines MAKE the images... and my patients are always such a kick in the pants...

Thanks for all your help, guys! :!)

 

[imabug]

Regarding the CT scatter - my radiation physicist told me that there is still considerable scatter at the foot of the table, so I will continue to shield my patients as I would want to be shielded. He also told me that if the door between the scan and control rooms is left open, we, as operators, receive some of the scatter as well! This is a bit disturbing to those of us in the scan room - especially since our supervisor is particularly fond of leaving the door open most of the time.

If the door is right next to your control area, then I could see a small amount of scatter reaching the operators. But this will be very minimal and probably very low energy by the time it reaches you through multiple scatters. If it concerns you, head over to your nuclear medicine department (or your physicist) and borrow one of their ionization chambers (not the geiger counter though) and set it up near the control booth. Exposure levels should not go much higher than the microR or microSv range (if it goes over background at all). Your physicist will be the one to give you a definitive amount on how much scatter you get exposed to though.

So I guess I'll just read my little books and reread the things you guys have written and pray that I glean a little more understanding of the physics of my toys. I must say, though, that it certainly is much more fun to learn to manipulate the machines to obtain the proper images than it is to learn how the machines MAKE the images

One of the things I always try to emphasize to the radiology residents that I teach is that knowing how to read images isn't enough, but they also need to know how the modality works in order to get the best use of it. Anybody can be taught how to read and diagnose films or how to use a piece of equiment. But the really good radiologists (and technologists!) know how and why their equipment works so they can use it to maximum efficiency, tweak parameters to get the best image quality, know the limitations of their modality and identify problems and artifacts in the images.

I encourage you to keep a good relationship with your resident physicist and ask him lots of questions. we like that kind of thing :)

 

[motai]

Now THAT will be cool! I'd LOVE to get into PET scanning. Not much chance of that for me where I live, though...

Hmm... PET makes use of short-lived radioisotopes, so there has got to be some sort of PET facility somewhere in your area. I visited one a while back when I was up in North Carolina, they are all over the place.

 

[Tsu]

If the door is right next to your control area, then I could see a small amount of scatter reaching the operators. But this will be very minimal and probably very low energy by the time it reaches you through multiple scatters. If it concerns you, head over to your nuclear medicine department (or your physicist) and borrow one of their ionization chambers (not the geiger counter though) and set it up near the control booth. Exposure levels should not go much higher than the microR or microSv range (if it goes over background at all). Your physicist will be the one to give you a definitive amount on how much scatter you get exposed to though.

He said that it was very low at the door, but at the same time, strongly encouraged us to keep it closed. I just make sure I don't work much with the guy who leaves it open all the time but I feel badly for the other techs who get caught walking into the room while he is scanning. Their first reaction is an immediate U-turn!

One of the things I always try to emphasize to the radiology residents that I teach is that knowing how to read images isn't enough, but they also need to know how the modality works in order to get the best use of it. Anybody can be taught how to read and diagnose films or how to use a piece of equiment. But the really good radiologists (and technologists!) know how and why their equipment works so they can use it to maximum efficiency, tweak parameters to get the best image quality, know the limitations of their modality and identify problems and artifacts in the images.

That's pretty much where I live. I've always made sure I spent a lot of time with the service guys and learned as much as possible about the scanners innards. There were lots of times when the scanner went down, but I knew how to fix it! Sometimes I'd have to call service, but they could usually just talk me through fixing it over the phone. My radiologists just LOVED me for that! I found that doing this really did help in learning ALL of the scanner applications and capabilities.

I encourage you to keep a good relationship with your resident physicist and ask him lots of questions. we like that kind of thing :)

I always do! We like that kind of thing, too.

 

[Tsu]

motai - Our PET scanners in Oregon are basically traveling minstrel shows. They are doing what CT and MR did in their infancy and putting the scanners on giant trucks and covering numerous hospitals all over the state. I don't even think they have a permanent PET scanner up in Portland. The place where I was working in Portland (a VERY large health system) had just set up a mobile PET pads at their hospitals. Same goes with the hospital where I now work. I had a blast doing mobile CT back in the '80's but I'm way too old to be a traveling tech again. I now live on a little farm with numerous critters that takes up WAY more time and energy than an apartment with no critters. I have made up my own version of PET SCANNING! LIVE ON A FARM!

 

[pervect]

To reuse an old, bad, joke in a new context - given carefully controlled experimental conditions, when conducting precisely controlled and well thought out scientific experiments on women, one finds that they do what they damn well please :-)

As far as MRI goes, you've gotten me curious about the topic, so I've been looking into it. There are some rather subtle things going on. The magnetic dipole moment, though, is a farily easy part of the situation to understand. (The author ducks an imagined glare from Tsunmai). No, really, that part isn't that hard.

What you have are a bunch of bar magnets, in essence. And you put them in a magnetic field. Just like electric dipoles, they tend to align with the field. So, obviously, when you turn on the magnet in an MRI machine, all the protons just line up to be parallel with the field, right?

Ummm - ooops, no that's not right. OK, well, we ignored temperature, and these protons are all moving around and being jostled, so only SOME of them line up. Right?

<read read> Hmmm, it says here that the precess.

Oh yeah, that's right, the protons are all like little gyroscopes too. OK, I can see why the might precess if they weren't lined up - it's the same as any other gyroscope, when you put a torque on it, it precesses. Going back to our bar magnets, that's just what the magnetic field does, it trys to "twist" them back into position.

The author visualizes one of the toy gyroscopes he played with as a kid. If it's spinning rapidly, it balances on its point nicely. As the spin winds down, the gyroscope starts to wobble, or precess. The tip of the gyroscope moves in a circle. This can be explained quite nicely mathematically. You usually get a full treatment in an introductory graduate physics course, like Goldstein's "Classical Mechanics". The author imagines Tsunami's probable reaction to graduate level physics, and decides that it would be a good idea to skip a rigorous treatment of gyroscopes. This also saves him from having to re-read the textbook . l guess all we really need to know is that it's torque that makes a gyroscope precess. And we can see that the magnetic field provides a torque on the magnetic dipoles. So that part all makes sense.

What's a little difficult to see, though, is what controls the angle of precession, or why it has to happen at all. It's fairly easy to imagine the behavior of a bunch of classical bar magnets, a little trickier to imagine the behavior of a bunch of rotating gyroscopic bar magnets. But now we have quantum-mechanical rotating bar magnets...

As far as the energy, goes, though, it's easy enough to see that the total energy will be the dot product of the magnetic field and the vector representing the magnetic dipole. . This just means that when the two vectors point in the same direction, one multiplies their lengths, making the total energy equal to the field strength times the dipole strength when the dipole is perfectly aligned.

The energy varies smoothly as one changes the angle, droping to zero when the dipole is oriented perpendicularly to the field. Here the energy is zero, but the torque is a maximum. When one anti-aligns, the energy is the lowest, being the negative of the energy in the first case.

The rule for dot products can be written as $$\vec{a} \cdot \vec{b} = |a||b| cos(\theta)$$, where $$\theta$$ is the angle between the vectors, which is 0 when they point in the same direction, and 180 when they point in opposite directions.

The point of calculating the energy is to know what the Lamour frequency will be. This again comes from quantum mechanics, it's the famous relation between the energy of a photon , and it's frequency.

But we don't actually have to get bogged down in these details of how the Lamour frequency is actually calculated to get some idea of how the system works. All we need to know is that the Lamour frequency depends on the magnetic field strength. Thus varying this strength varies the frequency, and allows one to distinguish position. It's not quite clear yet how one determines all of x,y, and z via this mechanism though (it would seem like there are too many variables to encode with one magnetic field strength).

 

[imabug]

What's a little difficult to see, though, is what controls the angle of precession, or why it has to happen at all. It's fairly easy to imagine the behavior of a bunch of classical bar magnets, a little trickier to imagine the behavior of a bunch of rotating gyroscopic bar magnets. But now we have quantum-mechanical rotating bar magnets...

The precession angle (or more commonly called flip angle) is determined by how long you apply the RF pulse to the body.

It's not quite clear yet how one determines all of x,y, and z via this mechanism though (it would seem like there are too many variables to encode with one magnetic field strength).

There are two sets of coils used in MR systems. The gradient coils (there are three, one for each axis) which establish the gradients used for spatial localization, and the transmit/receive coils which are placed near or on the body to transmit the imaging pulse sequences and listen for the emitted RF signals as the protons realign with $$B_0$$. Apply a Fourier transform to the signal, and you will get a whole bunch of frequencies which you can have the computer reconstruct into an image since you know what gradients you've applied.

MR image reconstruction algorithms are yet a whole other arcane MRI sub-field that I avoid when I can.

 

[Tsu]

OMG! OH... OH...! OMG!

*Tsunami dancing all over the place, on the couch, on table, on the piano bench, on the piano...*

I followed and understood everything you guys said! Even pervect's equation! WOOHOOOOOO! I'm a PHYSICIST!

imabug - I argree with you on the image recon algroithms. It's the same in CT and CR. But its imperative that we know what each and every algorithm does, so that we can apply them in the proper situations. I was able to show my Rad a basilar skull fx with associated small subdural hematoma (that he had missed) by knowing how to use the algorithms and manipulate the data on my new scanner. That same week, I demonstrated a tiny area of bleeding (secondary to trauma) in the brain of a young man. It changed his course of treatment.

 

[motai]

motai - Our PET scanners in Oregon are basically traveling minstrel shows. They are doing what CT and MR did in their infancy and putting the scanners on giant trucks and covering numerous hospitals all over the state. I don't even think they have a permanent PET scanner up in Portland. The place where I was working in Portland (a VERY large health system) had just set up a mobile PET pads at their hospitals. Same goes with the hospital where I now work. I had a blast doing mobile CT back in the '80's but I'm way too old to be a traveling tech again. I now live on a little farm with numerous critters that takes up WAY more time and energy than an apartment with no critters. I have made up my own version of PET SCANNING! LIVE ON A FARM!

*pulls out PETnet pamphlet and looks at a map*... there is a PETnet facility in Portland, and the only other one in that geographical region is in Seattle. There are a few facilities in California, almost none in the box states (w/exception to Denver), some scattered through the mid-west, and lots in the Eastern half of the United States. Their center of operations is in Knoxville and their customer service provider is in Orlando.

http://www.petnetpharmaceutical.com

That is... unless the PETnet facility is your farm w/critters...

 

[pervect]

The precession angle (or more commonly called flip angle) is determined by how long you apply the RF pulse to the body.

OK, there's something I must be fundamentally confused about here, it's been too long since I've used my quantum mechanics.

They way I recall quantum spin, the spin state could be completely described as a superposition of |up> and |down> spins. Pick an axis, any axis, and the total spin can be described as a superpositon of up & down spins along that axis. The Stern-Gerlach experiment is the cannonical example - a beam of silver ions that splits into two parts when passing through a spatially varying magnetic field.

So I always imagined the spins as either being aligned or anti-aligned, i.e. either |up> or |down>, but the descriptions I see on the WWW are of precession.

 

[Tsu]

*pulls out PETnet pamphlet and looks at a map*... there is a PETnet facility in Portland, and the only other one in that geographical region is in Seattle. There are a few facilities in California, almost none in the box states (w/exception to Denver), some scattered through the mid-west, and lots in the Eastern half of the United States. Their center of operations is in Knoxville and their customer service provider is in Orlando.

http://www.petnetpharmaceutical.com

That is... unless the PETnet facility is your farm w/critters...

It's just that getting into PET will require a lot of travel and being away from home. I just don't want to do that anymore. I liked it a lot when I did it years ago, but now I'm a homebody! I like to hang with my critters and dig in my gardens.

 

[Tsu]

I think this may answer your question. I'm copying out of a MR registry study manual.

"When the protons' net magnetization is aligned with the main magnetic field, there is no measurable signal and we have no information to make an image or to determine anything unique about the appearance of normal and abnormal tissue. To learn about the tissue, we must disturb the proton's equilibrium and move their net magnetization out of alignment with the main magnetic field. (This is done with the RF signals) Once we stop the applied distubance, the magnetization will move back, or relax, toward its equilibrium position through two distinct relaxation processes. It is through the relaxation processes that the tissues can be distinguished in an image. The precessional motion of the proton's magnetization is very important because it occurs at a very specific rate or frequency. This is expressed by the Larmor Equation." (which all of you already know about, right? )

Please let me know if this cleared up your confusion or just caused more. (I'm famous for the latter... )