Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/445—MR involving a non-standard magnetic field B0, e.g. of low magnitude as in the earth's magnetic field or in nanoTesla spectroscopy, comprising a polarizing magnetic field for pre-polarisation, B0 with a temporal variation of its magnitude or direction such as field cycling of B0 or rotation of the direction of B0, or spatially inhomogeneous B0 like in fringe-field MR or in stray-field imaging
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/381—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using electromagnets

Definitions

• the subject of the present invention is a low-field, dynamically polarized nuclear magnetic resonance (NMR) machine.
• the NMR machines concerned are more particularly NMR imaging machines which can be used in particular in the medical field.
• the object of the machine of the invention is, by improving the signal to noise ratio of the detected signal, to make the images obtained sharper. Furthermore, it can contribute significantly to reducing the cost of such machines by simplifying their homogeneity correction coils.
• an NMR machine essentially comprises a magnet, or a coil playing the same role, for subjecting a body to be examined to an intense and permanent orienting magnetic field. Subjected to this influence the body is then electromagnetically excited by a high frequency electromagnetic wave. At the end of the excitation, an electromagnetic wave of de-excitation emitted by the body is measured and which provides information on the intimate nature of this body. It is known that the amplitude of the detectable electromagnetic signal in such machines is of the Bo 2 type. In this expression (_ is the magnetic susceptibility of the body to be examined, Bo being the intensity of the orienting field of the machine.
• the magnetization given to the particles of a body to be analyzed is important. This magnetization is linked to the polarizing force of this field. This is all the stronger the higher the field.
• the spins of the particles start to resonate at a frequency proportional, it also, to the orienting field Bo-
• the orienting field therefore plays, in addition to its role of polarizing field , a second role: a role of resonance field.
• a role of resonance field As much the use of an intense orienting field is useful as regards polarization, magnetization, since it increases the amplitude of the detectable signal, as much the use of a high field at the time of resonance is delicate .
• the resonance field is of the order of 1 Tesla, and if the particles examined are particles of hydrogen (present in water, and therefore in large quantities in human bodies), the resonance frequency of the spins is around 40 MHz. An inhomogeneity of 1 millionth of the value of the resonance field therefore causes a resonance deviation of about 40 Hz. This means that after a duration of 12.5 ms of the contributions to the global NMR signal made by particles neighboring, but subjected to resonance fields different from each other by 1 millionth, find themselves in phase opposition. It immediately follows that the global electromagnetic signal, the only detectable one, is then canceled.
• the phase dispersion of the different contributions to the NMR signal of the signals emitted by the different particles can be combated by the use of two techniques.
• spin echo In the latter, with an additional electromagnetic excitation pulse, after the application of the excitation pulse, a reflection of the phase dispersion is caused so that the NMR signal is reborn after a double period from that which separates the excitation pulse from the reflection pulse (called the echo pulse).
• This technique has a drawback in that it condemns the use of spin echo pulses.
• the object of the invention is to remedy these drawbacks by proposing a generation of different NMR machines in which, essentially, the polarizing and resonance effects of the orienting field are dissociated.
• the particles are subjected to a strong polarizing field, for example of 1 Tesla, then this field is canceled.
• the magnetization imparted to magnetic moments will therefore tend to dampen.
• the cancellation of the polarizing field will be faster than the decrease in the magnetization of the particles of the body.
• resonance can be much weaker but on the other hand much more homogeneous. Because the polarizing field is strong, the usable magnetization is strong despite its decrease. The detectable signal is therefore strong.
• the resonance frequency of the NMR signal associated with the excitations is low, and the noise is also low. Furthermore, since the frequency of the resonance signal is low, the influence of the inhomogeneities is, for experiment times of the same order as previously, proportionally much lower. Indeed if instead of a Tesla one chooses a resonance field of 100 Gauss, the resonance frequency will be divided by 100, and the duration at the end of which the signals coming from regions of space, where reign inhomogeneities of the millionth, will only come to oppose their contributions after a period of 100 times greater. So we keep the same signal-to-noise ratio with given polarization but in addition we gain with regard to the need to reduce the inhomogeneities of the resonance field. Furthermore, a high inhomogeneity of the polarizing field can be tolerated, even if it is worth, for example of the order of 3 dB because it only sounds as a slightly annoying weighting of the detected signal.
• the invention therefore relates to an NMR machine comprising
• ⁇ means for subjecting a body to be analyzed to an intense magnetic field of polarization
• ⁇ means for exciting said body with an electromagnetic wave
• - means for measuring an electromagnetic wave re-emitted in response by the body characterized in that it also comprises - means for creating a resonance field significantly weaker than the polarization field
• the cancellation of the polarizing magnetic field will occur even before the electromagnetic excitation. Since the homogeneity of the intense polarization field does not have to be perfect, we choose to simplify the construction a coil production of this polarization field which does not have much self, and which is consequently not very homogeneous. On the other hand, the homogeneity of the resonance field will be much higher than that of the polarizing field. Furthermore, given that the resonance field is weak, and that the frequency of the electromagnetic resonance wave is also low, it becomes necessary to use antennas whose resistance provided in parallel with that of the body is low. Preferably the electromagnetic signal excitation and detection antennas will be of the superconductive type. Preferably, the invention is implemented in an NMR imaging machine. With this imaging machine, series of excitation-measurement sequences of the electromagnetic wave are implemented in the body, and preferably the polarization field will be restored periodically.
• FIG. 1 an NMR machine according to the invention
• FIG. 2a to 2f temporal diagrams of signals used in an imaging sequence with the machine of the invention
• FIG. 3 a diagram representative of the evolution of the magnetization during the switching of the polarization field
• FIG. 4 an improved generator for supplying a coil for producing a magnetic polarization field according to the invention
• FIG. 1 shows an NMR machine comprising a set of coils 1 and 2 supplied by a generator 3 to subject a body 4 to be analyzed to an intense magnetic field of polarization B2.
• the field B 2 is oriented along Y.
• the machine also includes an antenna 5 supplied by a generator 6 to excite the body 4 with an electromagnetic wave.
• the reception circuit 7 also performs processing so as to represent on a display device 9 an image of a section of the body 4.
• the NMR imaging experiment undertaken must include a series of excitation-measurement sequences during which additional magnetic field pulses are applied to the body 4 by a set of gradient coils 10 supplied by a generator 11 of gradient coils.
• the definition of the NMR machine described so far is conventional.
• the generator 3 is designed to be cut.
• Another set of coils 12 to 15, supplied by a generator 16, is provided for subjecting the body 4 to a magnetic field of resonance Bo clearly weaker than the field B2.
• the induction Bo will be maintained permanently without disadvantage.
• the field Bo produced by the coils 12 to 15 is, in an examination area 17 opposite the antenna 5, relatively much more homogeneous than the field B 2 -
• the intensity of the switched B2 field is of the order of 2 Tesla when this field is present, and of course zero when it closes.
• the Bo field is of the order of 100 Gauss.
• all of the equipment of the machine operates under the command of a sequencer 18 which organizes the functional and temporal dependence of all these organs. To cause the periodic control of the generator 3, it suffices to connect this generator, functionally, to the sequencer 18.
• coils are chosen with the fewest turns possible. These coils thus have the least possible self so as not to oppose the birth of this polarization induction field as well as its cutting.
• the field B 2 can also preferably exert an influence only in the imaging volume 17. Under these conditions the coils 1 and 2 are small.
• the field B2 In order to ensure the greatest nullity of the field B2, it is oriented in such a way that, in the image area 17, - it is perpendicular to Bo »Besides that this contributes at the time of its cancellation to make it negligible this solution makes it easy to place the coils 1 and 2 in the tunnel of the NMR machine, so that the field B 2 is perpendicular to the elongated direction of the body 4.
• the field Bo in this case is longitudinal.
• FIG. 2 represents the time diagrams of signals usable in the invention.
• FIG. 2a shows that the resonance field Bo is preferably maintained permanently.
• the polarization field B2 is established during a period 20, it persists for a period 21, and is cut off during a period 22. It remains cut for a period 23.
• a growth time 100 ms for the establishment of a nominal B2 field at 2 Teslas.
• the switched powers are of the order of 200 JcVA for plates 21 of the order of 2 Teslas in 100 ms. These powers are multiplied by 3 to 4 if the diameter of the useful area 17 increases to 30 cm.
• FIGS 2c to 2f show such an imaging sequence with a 2DFT type imaging method with spin echoes.
• a radio frequency electromagnetic excitation 25 is applied to the body 4 by the antenna 5 in the presence of a pulse 26 of a cutting selection gradient along the axis Z.
• a pulse 27 of a phase coding gradient oriented along the Y axis is applied.
• the particularity of this phase coding excitation is that, from one sequence to another of the series of sequences serving to make the image, the value of this pulse 27 varies.
• a radiofrequency electromagnetic pulse 28 called spin echo 28 is applied in the presence of a reselection pulse 29 of the same cut with the cut coding gradient. G 2 .
• the resurgent NMR signal 30 is measured, in the presence of a read pulse 31 on a reading gradient G x .
• the cut selection pulse 26 comprises a consecutive pulse 32 for cutting recoding, and pulse 31 is itself preceded by a read precoding pulse 33.
• the duration 23 is of the order of 100 ms, it can even be envisaged to indulge during this duration 23 in several successive sequences similar to those of FIGS. 2c to 2f. For example, if such a sequence lasts on the order of 25 ms, it can be repeated four times. In this case, however, one chooses to carry out an image in another cut, with another cut selection pulse than the pulse 26. It is interesting to note that during this period 23 one can ultimately engage in all the methods excitation and apply all the imaging methods already known from the prior art.
• Figure 3 shows that the magnetization M, that the magnetic moments of the particles, keep their values appreciably during the breaking of the polarization field B 2 • Only their orientations change.
• this change in orientation, this vector rotation of M is only possible if the rotation of the vector B resulting from the algebraic composition of B2 and Bo, is small compared to the resonance frequency, the precession frequency, magnetic moments.
• This precession frequency is itself proportional to B on the one hand and to Y on the other hand, being the gyromagnetic ratio associated with the particles examined (with hydrogen particles in medical imaging). It is therefore ultimately the angular velocity of B is lower much to B.
• Now there ⁇ > is even limited her bottom by B 0- Indeed when B2 is no B becomes the same Ybo.
• a power supply comprising a switching circuit 35 (FIG. 4) capable of producing + V or + V volts during durations 34 of period 22, or 38 of period 20, or a voltage 0 volts during periods 21 and 23.
• This circuit 35 is a floating circuit. It subjects the inductors 1 and 2 to a constant voltage. These inductors are preferably superconductive coils. These inductors 1 and 2 therefore allow a linearly increasing current to pass when. they are subject to constant tension. They therefore produce a linearly increasing field.
• Circuit 35 comprises two double bridges of transistors in series.
• a first double bridge 40 comprises the bridges 41 and 42.
• the bridge 41 comprises the transistors 43 and 46.
• the bridge 42 comprises the transistors 44 and 45.
• the second double bridge 47 comprises the bridges 48 and 49.
• the bridge 48 comprises the transistors 50 and 53.
• the bridge 49 includes the transistors 51 and 52. All the transistors are provided with a reverse destocking diode.
• the two double bridges are mounted in series, in their midpoint, each with a battery, respectively 54 and 55. Depending on the direction of current conduction in the bridges, the voltages delivered by these batteries add up or neutralize each other.
• an adder 56 is placed in series with the double bridges and the coils 1 and 2, an adder 56 is placed. This adder produces a voltage step that can be used to create the small ramps during periods 39 and 37.
• FIG. 5 shows orders 0 ⁇ , 02, O3 and O4 applicable on bridges 41, 42, 48, 49 respectively.
• the ADD signal is the signal produced, for example by the sequencer 18, and which is added in the adder 56 to the voltage 0 Volts produced by the batteries.
• the signal ADD is applied while the conduction of bridges 42 (O2) and 48 (O 3 ) neutralizes the two voltages equal V supplied by each of the batteries 54 and 55.
• the ADD signal is cut, the bridge 42 (O2) is blocked while the bridge 41 (0) is conductive, the bridge 48 (0 3 ) remains conductive. Under these conditions a voltage +2 V (typically 320 volts) is applied to the coils 1 and 2.
• the bridges 41 and 42 are again inverted. Also a constant current continues to flow in the coils 1 and 2.
• the bridges 48 (0 3 ) and 49 (04) are reversed.
• a -2V voltage is provided: the current in the coils 1 and 2 decreases.
• the bridges 48 and 49 are again reversed (the battery voltages are neutralized) while an ADD signal of reverse polarity is applied via the adder 56.
• the current in the coils 1 and 2 drops to zero.
• the circuit 35 is in the same configuration as during period 21 (but during this period no more current flows in the coils 1 and 2). If, as is possible, one chooses to orient B 2 as Bo the magnetization problems no longer arise. It is nevertheless necessary to take into account then the coupling between the coils 1 and 2 and 12 to 15.
• the Bo field of resonance cannot be as weak as one would like. Indeed the gradients cause a variation of the magnetic field to which the particles are subjected during the experiment. If for a gradient of a given force, we obtain a variation
• BQ is at least equal to twice B.
• this proposition is more easily fulfilled if with a low resonance field Bo we also use weak gradients.
• the fact of using too powerful gradients would no longer make it possible to consider as negligible the component, which is not oriented longitudinally with this resonant field of the magnetic fields of the gradients.
• the impedance of the antenna 5 is no longer negligible compared to the impedance presented by the body of a patient, the impedance of this antenna must be relatively reduced.
• an antenna of superconductive type In this case, this antenna is coupled to a cooled amplifying head, of the type used in space communications.
• the duration of the periodic field establishment cycle B 2 is preferably chosen to be equal to a repetition time Tr itself determined for a particular imaging method. For example if we want to make images in spin-spin relaxation time (T2), it is interesting to choose repetition times Tr of the order of 1 or 2 seconds. If one wants to make images in spin-lattice relaxation time (Ti), it is preferable to choose, depending on the nature of the materials to be examined, repetition times Tr of the order of 200 to 400 ms. These are entirely compatible with the indications given so far.
• the average magnetization of the magnetic moments is not equal to that which they would have if the plates 21 were maintained permanently. Rather, it is equal to or less than what they would have for half that value.

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• Physics & Mathematics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Spectroscopy & Molecular Physics (AREA)
• High Energy & Nuclear Physics (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)

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• 1988
  • 1988-09-30 FR FR8812836A patent/FR2621392A1/fr active Pending
• 1989
  • 1989-09-22 WO PCT/FR1989/000485 patent/WO1990003583A1/fr not_active Application Discontinuation
  • 1989-09-22 JP JP1510491A patent/JPH0620437B2/ja not_active Expired - Fee Related
  • 1989-09-22 EP EP89911254A patent/EP0445125A1/de not_active Withdrawn
  • 1989-09-22 US US07/671,696 patent/US5208533A/en not_active Expired - Fee Related

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