# MRI question for our Heavy Weights

1. Dec 3, 2004

### 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?

2. Dec 3, 2004

### 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!!

Last edited: Dec 4, 2004
3. Dec 3, 2004

### 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.

4. Dec 3, 2004

### 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!!! :rofl: :rofl: :rofl:

5. Dec 4, 2004

### 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)

Last edited: Dec 4, 2004
6. Dec 4, 2004

### Tsu

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! :surprised 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! :yuck:

7. Dec 4, 2004

8. Dec 4, 2004

### imabug

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.

9. Dec 4, 2004

### 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.

...

10. Dec 4, 2004

### 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 physist 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...

11. Dec 4, 2004

### pervect

Staff Emeritus
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.

12. Dec 4, 2004

### 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... :yuck:)

No. Wait. I refuse to not understand <gets drill, plugs it in, sound of drilling in background>
OK. I think I'm beginning to get it now... :surprised
Sort of...

13. Dec 4, 2004

### imabug

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.

14. Dec 4, 2004

### Tsu

Don't ask. :grumpy:

Oooooookkaaaaaayyyyyy. Well. It looks like seizure time again.

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

So... this:
really means something to you guys? :rofl: :rofl:

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

15. Dec 4, 2004

### Tsu

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?

16. Dec 4, 2004

### imabug

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.

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.

17. Dec 4, 2004

### humanino

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

18. Dec 4, 2004

### pervect

Staff Emeritus
/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 :tongue: 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.

19. Dec 4, 2004

### Tsu

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. :surprised :rofl:
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!! :yuck: Frankly, I don't love it 'THAT much! :surprised 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.

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.
... :rofl: :rofl: :rofl:

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?

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

20. Dec 4, 2004

### 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! :surprised

'Till tomorrow, then...

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