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Round table discussion and comments
Magnetic Remnance Imaging. Vol. 9, pp. 747-748, Fnnwd in the U.S.A. All rights reserved.
0730-725x831 53.M) + .oo Copyright 0 1991 F’qamc+~ Rs plc
1991
l Porous Media and Biomedical NMR
ROUND TABLE DISCUSSION AND COMMENTS PETER MANSFIELD Department
of Physics,
University
of Nottingham,
Nottingham
NG7 ZRD, England
lar experiment was concerned with looking at the rate at which water leaves the cell. One can study diffusion problems at this high resolution. Here is a Tl map at high resolution which is fairly bland. There are little patches here and there but it is surprisingly uniform. Another area common to both biology and porous media is concerned with signal reception and RF coil structures. There may be applications of half or split resonator arrays in the study of porous media as well as in biological systems. Here is a half cavity resonant structure placed in close proximity to a conducting screen. This is an image of a water phantom which is placed in the split-resonator without screen and fills the half cage. There is quite a considerable variation of signal intensity, and that reflects the variation in the field as you go through the sample. With the screen, the field homogeneity is much improved. The results can also be presented as RF field contour maps. These show numerically the phantom result that I showed first of all, namely, the half structure without screen. The RF field variations go from 90% down to 30%, that is to say, a 60% variation. But when you put the screen on the RF field variation is reduced from 90% down to 60%, giving a 30% variation. A lot of these variations come about not because of the presence of the screen, but because the cage itself is not made perfectly. Here is a biological image through the leg of a volunteer obtained with such an array. The point about this design is that there is an easy access since the half coil is demountable. The specimen is placed on the screen plate and the cover placed over the object. It has obvious applications in biological and medical imaging. There are variations of this because you may use not only a circular array but also just a conducting strip which is activated on one side close to a flat plate. There
In this discussion the intention is to relate our experiences concerning biomedical applications and connections with work on mineral systems and porous media. I want to mention very briefly two subjects under this heading. The first is NMR microscopy, because I think that there are a number of important applications which could transcend the biological applications, for example, applications in porous media for which many new NMR techniques have been specially developed. This slide shows a part of the system for a 500 MHz NMR microscope which we have constructed at Nottingham. On the right hand side there is a screened gradient set, so that we can apply very large gradients to the specimen within the close confines of the magnet, without inducing transient effects. Here is a result obtained at 500 MHz and it is a 7.5 pm resolution image of a section of a pelurgonium plant stem; it has relevance to the topic discussed here because this plant stem has actually been immersed in water in order to fill up the spaces between the cells. But we have not managed to do it completely and you can see here the cell structure. Within the plant stem itself you can see a dark region corresponding to the vascular bundles which conduct the nutrient in the plant. One of the problems we found was that at resolutions of up to 4.5 pm the air spaces give very considerable susceptibility effects in the image, and in some cases can wipe out the image completely. Susceptibility effects are also important when considering applications at these resolutions for the general study of porous media. Rocks, for example, have very large susceptibility differences with water and as a result of that one should take special precautions in order to produce an image. Here is a 10 pm image of onion cells, and the resolution here is sufficiently large to see rather clearly the cell structure itself. In this particular case the cells have been plasmolised in a sucrose solution and the particu747
748
Magnetic
Resonance
Imaging 0 Volume 9, Number 5, 1991
are a number of variants of this idea which allow easy access to the sample. In spectroscopy, for example, the sample can be placed on the screen plate and the second half of the structure placed very close to it. This may allow one to wind very small coils with consequent signal-noise improvements. Question: How long did it take to make the microscopy ages?
im-
Answer: It typically takes about 10 to 15 min, but it obviously depends on the resolution; on a 128* pixel image it may take 10 min, on a 256* pixel image it takes roughly four times or about 40 min. Question: I could not really understand your coil result. Do you say that it has a penetration effect or are you saying that it is basically the distribution of Bl inside the coil? Answer: It is the distribution
of Bl inside the coil.
Reply (a):: I do not think you made the ing to have to disagree with you get a much more uniform field. frequencies. I do not see why a any difference.
coil correctly, I am gohere. I think you should We have done it at high conducting plane makes
Reply (b): It makes a big difference, because you are symmetrising the conditions of the electro-magnetic wave at the surface of the conducting plane. If you have a half cage, then the variation in the B field may be calculated so that you obtain a very large gradient in B 1 field. If you put the plate there, you symmetrise the system. Reply (a): Does it make a difference because the wave length makes a difference and you are getting close to the coil size at 500 MHz? Reply (b): The leg and phantom results shown were obtained at 22 MHz. The object was large. It was about 10 or 15 cm in diameter at 22 MHz. Reply (a): So you made the coil wrongly, because I can show you other results in which we give a perfectly uniform field at 64 MHz with much larger samples.
Reply (b): I said that with a half resonator you do not get a uniform field, but you can make it more uniform by satisfying the boundary conditions. Reply (a): So you agree that with a full resonator you can get a uniform field. The only other comment that I have on this is that for your application you can actually cut the cage anyway and take out a part, then put it back together and get a field which is much more uniform than if you had used a plate. Reply (b): I think you are missing the point. The point is that in biological systems it is often much more convenient to use something that literally just places over the object. Here is the plate and here is the object, and you just place the coil on the plate enclosing the object. Question: I would like to come back to the microscopy experiment with the vascular tissues that appeared very dark. As I understand it, the vascular bundles themselves are probably cylindrical regions. So for the water inside there the symmetry should be as for the uniform field. I see that other people have seen this effect, but can you make a comment on this please? Answer: That is true for an isolated cylinder. Put the problem in biological systems it is that if you have a cell packed with spaces between and you have a filled tubule, this behaves like a dipole. That induces a susceptibility mismatch. The adjacent part of the object that is in the dipole field experiences the field gradient of its neighbour. If you do not take the precautions, as we have tried to do here and in some other experiments, of susceptibility matching, you will not get much of an image. Question: One of the problems of NMR microscopy is the third dimension. I was wondering whether you have ever tried any fast imaging with a sort of microslicing? Answer: The best slice selection at the moment is about 50 Frn. We did use selection, but it was not essential, and it did not make any difference in the case of onion cells, since they were typically 40 km thick.
0730-725x831 53.M) + .oo Copyright 0 1991 F’qamc+~ Rs plc
1991
l Porous Media and Biomedical NMR
ROUND TABLE DISCUSSION AND COMMENTS PETER MANSFIELD Department
of Physics,
University
of Nottingham,
Nottingham
NG7 ZRD, England
lar experiment was concerned with looking at the rate at which water leaves the cell. One can study diffusion problems at this high resolution. Here is a Tl map at high resolution which is fairly bland. There are little patches here and there but it is surprisingly uniform. Another area common to both biology and porous media is concerned with signal reception and RF coil structures. There may be applications of half or split resonator arrays in the study of porous media as well as in biological systems. Here is a half cavity resonant structure placed in close proximity to a conducting screen. This is an image of a water phantom which is placed in the split-resonator without screen and fills the half cage. There is quite a considerable variation of signal intensity, and that reflects the variation in the field as you go through the sample. With the screen, the field homogeneity is much improved. The results can also be presented as RF field contour maps. These show numerically the phantom result that I showed first of all, namely, the half structure without screen. The RF field variations go from 90% down to 30%, that is to say, a 60% variation. But when you put the screen on the RF field variation is reduced from 90% down to 60%, giving a 30% variation. A lot of these variations come about not because of the presence of the screen, but because the cage itself is not made perfectly. Here is a biological image through the leg of a volunteer obtained with such an array. The point about this design is that there is an easy access since the half coil is demountable. The specimen is placed on the screen plate and the cover placed over the object. It has obvious applications in biological and medical imaging. There are variations of this because you may use not only a circular array but also just a conducting strip which is activated on one side close to a flat plate. There
In this discussion the intention is to relate our experiences concerning biomedical applications and connections with work on mineral systems and porous media. I want to mention very briefly two subjects under this heading. The first is NMR microscopy, because I think that there are a number of important applications which could transcend the biological applications, for example, applications in porous media for which many new NMR techniques have been specially developed. This slide shows a part of the system for a 500 MHz NMR microscope which we have constructed at Nottingham. On the right hand side there is a screened gradient set, so that we can apply very large gradients to the specimen within the close confines of the magnet, without inducing transient effects. Here is a result obtained at 500 MHz and it is a 7.5 pm resolution image of a section of a pelurgonium plant stem; it has relevance to the topic discussed here because this plant stem has actually been immersed in water in order to fill up the spaces between the cells. But we have not managed to do it completely and you can see here the cell structure. Within the plant stem itself you can see a dark region corresponding to the vascular bundles which conduct the nutrient in the plant. One of the problems we found was that at resolutions of up to 4.5 pm the air spaces give very considerable susceptibility effects in the image, and in some cases can wipe out the image completely. Susceptibility effects are also important when considering applications at these resolutions for the general study of porous media. Rocks, for example, have very large susceptibility differences with water and as a result of that one should take special precautions in order to produce an image. Here is a 10 pm image of onion cells, and the resolution here is sufficiently large to see rather clearly the cell structure itself. In this particular case the cells have been plasmolised in a sucrose solution and the particu747
748
Magnetic
Resonance
Imaging 0 Volume 9, Number 5, 1991
are a number of variants of this idea which allow easy access to the sample. In spectroscopy, for example, the sample can be placed on the screen plate and the second half of the structure placed very close to it. This may allow one to wind very small coils with consequent signal-noise improvements. Question: How long did it take to make the microscopy ages?
im-
Answer: It typically takes about 10 to 15 min, but it obviously depends on the resolution; on a 128* pixel image it may take 10 min, on a 256* pixel image it takes roughly four times or about 40 min. Question: I could not really understand your coil result. Do you say that it has a penetration effect or are you saying that it is basically the distribution of Bl inside the coil? Answer: It is the distribution
of Bl inside the coil.
Reply (a):: I do not think you made the ing to have to disagree with you get a much more uniform field. frequencies. I do not see why a any difference.
coil correctly, I am gohere. I think you should We have done it at high conducting plane makes
Reply (b): It makes a big difference, because you are symmetrising the conditions of the electro-magnetic wave at the surface of the conducting plane. If you have a half cage, then the variation in the B field may be calculated so that you obtain a very large gradient in B 1 field. If you put the plate there, you symmetrise the system. Reply (a): Does it make a difference because the wave length makes a difference and you are getting close to the coil size at 500 MHz? Reply (b): The leg and phantom results shown were obtained at 22 MHz. The object was large. It was about 10 or 15 cm in diameter at 22 MHz. Reply (a): So you made the coil wrongly, because I can show you other results in which we give a perfectly uniform field at 64 MHz with much larger samples.
Reply (b): I said that with a half resonator you do not get a uniform field, but you can make it more uniform by satisfying the boundary conditions. Reply (a): So you agree that with a full resonator you can get a uniform field. The only other comment that I have on this is that for your application you can actually cut the cage anyway and take out a part, then put it back together and get a field which is much more uniform than if you had used a plate. Reply (b): I think you are missing the point. The point is that in biological systems it is often much more convenient to use something that literally just places over the object. Here is the plate and here is the object, and you just place the coil on the plate enclosing the object. Question: I would like to come back to the microscopy experiment with the vascular tissues that appeared very dark. As I understand it, the vascular bundles themselves are probably cylindrical regions. So for the water inside there the symmetry should be as for the uniform field. I see that other people have seen this effect, but can you make a comment on this please? Answer: That is true for an isolated cylinder. Put the problem in biological systems it is that if you have a cell packed with spaces between and you have a filled tubule, this behaves like a dipole. That induces a susceptibility mismatch. The adjacent part of the object that is in the dipole field experiences the field gradient of its neighbour. If you do not take the precautions, as we have tried to do here and in some other experiments, of susceptibility matching, you will not get much of an image. Question: One of the problems of NMR microscopy is the third dimension. I was wondering whether you have ever tried any fast imaging with a sort of microslicing? Answer: The best slice selection at the moment is about 50 Frn. We did use selection, but it was not essential, and it did not make any difference in the case of onion cells, since they were typically 40 km thick.