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]
  • G01R33/48—NMR imaging systems
  • G01R33/58—Calibration of imaging systems, e.g. using test probes, Phantoms; Calibration objects or fiducial markers such as active or passive RF coils surrounding an MR active material
  • G01R33/583—Calibration of signal excitation or detection systems, e.g. for optimal RF excitation power or frequency
• • 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]
  • G01R33/48—NMR imaging systems
  • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
  • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
  • G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
  • G01R33/56518—Correction of image distortions, e.g. due to magnetic field inhomogeneities due to eddy currents, e.g. caused by switching of the gradient magnetic field

Definitions

• the present invention relates to a method for measuring, for a given period of time, the effects of eddy currents.
• This process is mainly intended to be used in medicine in nuclear magnetic resonance (NMR) experiments. It is used to measure the effects resulting from the application of magnetic field gradient pulses, by means of magnetic field gradient coils, in an NMR imaging machine.
• Nuclear magnetic resonance imaging is known. In imaging experiments according to such methods, a body is placed, of which one wishes to give images of interior parts, in a region where an intense homogeneous magnetic field is created by a magnet Bo- Under the effect of this intense field the magnetic moments of the particles of the body are oriented in the direction of the homogeneous magnetic field.
• NMR resonance excitation signal
• an NMR machine includes the magnet to produce the homogeneous field, an antenna to apply radio frequency electromagnetic excitation, and the actual gradient coils.
• the magnet is of the superconductive type, it further comprises a screen capable of absorbing the magnetic energy created, in the event that, by accident, the phenomenon of superconductivity which makes it possible to maintain the field would fail (by default magnet cooling system, for example).
• one of the peculiarities of the measured de-excitation NMR signal is that it is rapidly evanescent.
• This evanescence is essentially linked to the lack of homogeneity of the main magnetic field of the machine.
• the magnetic moments return to their initial orientation by precessing at a frequency which is a function of the intensity of the main magnetic field.
• magnetic moments in phase at the origin can quickly find themselves in phase opposition with respect to each other. Due to their difference in precession frequency these phases shift. So that the NMR signals produced by particles located in the different regions of the body tend to neutralize after a certain time. Under these conditions, their measurement no longer gives a result.
• the phenomenon of the reflection of the phase dispersion evolves, after the rebirth, like a phase dispersion proper. It is then possible during each acquisition-measurement sequence to reiterate the reflection action so as to give rise to a NMR signal of second echo, and so on of third echo, " and even of fourth echo or more.
• the evolution of the NMR signal between the first echo and the fourth echo can be very revealing of the evolution of the spin-spin relaxation time, also called T 2 , of the particles under examination.
• T 2 the evolution of the spin-spin relaxation time
• the fourth echo signal is more difficult to measure because it is much weaker than the first echo signal, its physical significance is important because it makes it possible to accurately measure the desired relaxation time T 2 .
• the fourth echo signal is to be measured, it is important to have neutralized the effects of the eddy currents until the time of this. measurement of this fourth echo signal.
• the measurement of the fourth echo NMR signal is carried out well after the excitation itself. With echo times of the order of 60 milliseconds, the fourth echo is measured after a duration substantially equal to 240 milliseconds after the birth of the NMR signal. It is therefore important to know during all this time, the effects of eddy currents. This measurement is complex because after such a period the effects of such eddy currents are weak: their measurement is not very precise. However, their effects are very disruptive.
• the principle of these measurements consists in placing an NMR probe in a suitable place of the machines , to electromagnetically excite the material capable of magnetic resonance in this probe, to subject it to a pulse of characteristic field gradient, to measure the evolution of the resonance signal over time, and to compare this evolution to an expected theoretical evolution corresponding to a perfect field gradient at the place where the probe is located.
• the field gradient to be evaluated acts in the probe as an inhomogeneity of the main magnetic field: it makes the resonance signal of the probe evanescent. This evanescence occurs even if, as it should be, the resonant material of the sound ⁇ at a long relaxation time T2 (longer than the duration of the sequence with four echoes).
• a gradient echo phenomenon is caused by using a gradient pulse to be evaluated, the direction of which alternates regularly during the measurement, a certain number of times, however, this does not counter the effect of 1 * inhomogeneity of the field Bo which always tends to phase the magnets.
• a susceDtible material of resonance magnetic but whose spin-spin relaxation time is very short is taken from the order (5 milliseconds) of the evanescent time of the NMR signal, evanescence due to 1 • inhomogeneity created by the gradient to be measured.
• the duration of this evanescent time is due to the size of the probe (of the order of 2 milli ⁇ meters) and to the alteration of the precession frequency over such a distance for a magnetic field gradient of given value (0.25 Gauss per centimeter).
• this given value is an intermediate value with respect to the nominal nominal value of a real gradient to be implemented.
• the successive measurements are guaranteed to be independent of one another.
• a repetition of the electromagnetic excitation of the probe is then carried out and consecutive measurements of the NMR signal of de-excitation which 'it re-emits. We re-energize as often, and at all times in the sequence, as necessary.
• the gradient pulse is applied, except for the presence of an excitation pulse in the probe, this gradient pulse is cut (we are then in the presence of the drag of this impulse: which interests us), we regularly excite the probe during this drag, and we measure each time a de-excitation signal.
• the subject of the invention is therefore a method for measuring, for a period of time, the effects of the eddy currents, these effects resulting from the application of a magnetic field gradient pulse, by means of a gradient coil.
• magnetic field in a machine to perform NMR experiments comprising the following steps:
• the probe is subjected to a gradient pulse
• the probe is excited by means of radio frequency electromagnetic excitation
• FIGS. 3a and 3b the frequency diagram of the free precession signal originating in the probe because of the size of this probe, its position, and the importance of the field gradient.
• FIG. 1 schematically shows an NMR machine for implementing the method according to the invention.
• This machine comprises, symbolically represented by a coil 1, a magnet for producing a homogeneous and intense magnetic field Bo in an examination area 2 where a body to be placed is supposed to be placed.
• the machine further comprises an antenna, for example of the radiating bar type ' 3 to 6, for inducing in zone 2 at the time of the examination of the electro-agneic radio-frequency excitation pulses produced by a generator 7.
• the machine further comprises gradient coils symbolized by the devices 8 and 9 and supplied by a gradient pulse generator 10.
• a reception circuit 11 makes it possible to receive the NMR signal for de-excitation emitted by the particles of the body at the end of the excitation.
• a display device 12 makes it possible to show images of the sections of the body under examination in the machine after processing of the received NMR signals. All of these means operate under the command of a sequencer 13 which organizes the application of the excitation pulses, field gradient pulses, and the measurement of the NMR signal.
• a probe 14 is used, consisting essentially of a small amount 15 of a material susceptible to magnetic resonance and an associated antenna 16.
• the probe is placed in the examination zone 2 of the machine.
• the antenna 16 is used to take the NMR signal emitted by the material 15 under the effect of an excitation applied by the antenna 3 to 6.
• the excitation can also be applied by the antenna 16 by controlling the operation of this antenna by the sequencer, and by interposing, in its connection to the generator 7 and to the receiver 11, a duplexer. In normal operation, the receiver 11 can moreover be connected by a duplexer (not shown) to the bar antenna 3 to 6.
• FIGS. 2a to 2c respectively show the RF radio frequency excitation signals, the gradient signals G, and the NMR signals received S.
• the experimentation carried out in the invention essentially comprises radio frequency excitations such as 17 to 19 giving rise, in each case, to free precession signals respectively 20 to 22.
• the gradient pulse of which it is sought to evaluate the effects of the eddy currents is, for example, pulse 23.
• the constant value of this impulse at its apex is located substantially half of the value of a nominal nominal quantity GN of a field gradient usable with this machine. In this way the study of the gradient pulse is made in the linear variation range of this pulse.
• the gradient coil studied is a gradient coil producing a gradient along Z ( Figure 1).
• Figure 2b shows the descent time 24 of the gradient pulse 23, and the drag 25 of the magnetic field which remains and which corresponds to the effects of the eddy currents produced by the gradient pulse 23, after the cut of this pulse.
• the duration of this trail is long, its decay time constants are great.
• the measurement of the value of this trail must be made over a long duration TM of the same order as the sequences of 'longest excitation that the machine can allow to implement.
• NMR signal from the probe is brief: it is around 5 milliseconds. This is particularly visible by examining in FIGS. 2c the responses 20 to 22. It is therefore conceivable that the phase dispersion due to the presence of the gradient pulse 23, or of its drag 25 which it is sought to measure, is such that at the end of the duration M no NMR signal will no longer be measurable. It is for this reason that, in the state of the art, on the one hand, an echo phenomenon was used, to revive the NMR signals of the probe at the end of a duration double the duration which separates 1 excitation of the probe of the instant when one causes the echo, and that on the other hand one chose a spin-spin 2 long relaxation time. In this way, at the time of the end of the duration TM this NMR signal, reborn under the effect of an echo, was still measurable. In practice, the state of the art probes contained pure water.
• a material is chosen having a low spin-spin relaxation time T 2 . This is penalizing since there is then only a short time to measure the NMR signal.
• the material of the probe can be re-excited relatively frequently without giving rise, in this material, to phenomena analogous to a phenomenon of SSFP (Steady State Free Precession in Anglo-Saxon literature) type between the various excitations. These SSFP type phenomena, which are such that there would then be a rejection of the NMR signal due to an excitation in the NMR signal of a subsequent excitation, would then falsify the measurement results.
• SSFP Steady State Free Precession in Anglo-Saxon literature
• this magnetization is made to tilt at a small angle, for example 30 degrees. So that we can consider that at the end of the duration Ti the longitudinal magnetization has been completely restored. This would not be the case if the tilting had been maximum, if the tilting of orientation had been 90 degrees. As a result, the NMR signals 20 to 22 measured at the end of each of these excitations are completely comparable with one another. The other great advantage is then to be able to have a measurement of the NMR signals on any dates during the TM duée without having to make any concessions on the shape of the pulse 23.
• FIG. 2b shows moreover that the excitation 17 is applied after the fall 24 of the gradient pulse 23 to be evaluated.
• FIG. 3a shows the spectrum of the NMR signal sampled by the antenna 16. If the gradient were not present, and if there was no eddy current effect, the probe would resonate at a frequency fo depending on the intensity of the BQ field and the giromagnetic ratio r of the probe material.
• FIG. 3b shows, in correspondence with FIG. 3a, the probe 15 placed at the distance L from the center 26.
• the NMR signal produced by the magnetic moment of the particles located at the center 28 of the probe 15 is different from the signal produced by the magnetic moments of the particles located at the ends 29 and 30 respectively of this probe according to the direction of the gradient Z applied.
• the spectrum of the signal corresponding to these different contributions has a width ⁇ f. This width ⁇ f is linked to the dimension d by the same relationship as that which links (fi - fo) to L.
• the use of an autonomous probe that is to say having its own transmitting and receiving antenna and its own receiving circuit can also prove to be particularly advantageous for the implementation of the method of invention.
• the method of the invention allows, at the output of the device 31 for searching the center frequency of the spectrum of the resonance NMR signal of the probe, to give the value fi.
• This value is directly representative of the effect of the eddy currents at the time of their measurement.
• TM time of a period
• we know the history of the drag of the effects of these currents it is then possible to use this information delivered by the device 31 to introduce it into a processing device 32 which develops a correction quantity. This correction quantity can itself be applied to the inputs of the gradient generator 10.
• the value of the gradient pulses is regulated without having to carry out theoretical modeling in order to deduce them.
• the results of these measures' could not be transposed directly to the pulses' gradient fields effectively implemented in the sequences without a theoretical analysis. This theoretical analysis precluded the implementation of a reaction against simpl "acting in real time on adjusting the power of the gradient coils.
• this probe When the probe is used to allow regulators tion of the value of the gradient pulses , this probe can be left in the machine, even when an imaging experiment is undertaken on a patient, provided that the antenna is slightly shielded, because of the association of the antenna 16 at material 15, the gain at the location of material 15 is such that a light shielding is sufficient.
• field gradient pulses will be applied during the experiment itself at any time, and they will also be cut at any time.
• the shape of the gradient pulses applied is preferably the same as the shape of the pulses actually used in the imaging experiments.
• Known techniques of system identification then make it possible to find the transfer function linking the control of the gradients and the response, measured by fi, of the eddy currents.

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• Physics & Mathematics (AREA)
• High Energy & Nuclear Physics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Health & Medical Sciences (AREA)
• General Health & Medical Sciences (AREA)
• Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
• Radiology & Medical Imaging (AREA)
• Engineering & Computer Science (AREA)
• Signal Processing (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)

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• 1988
  • 1988-03-18 FR FR8803582A patent/FR2628839B1/fr not_active Expired - Lifetime
• 1989
  • 1989-03-20 JP JP1503454A patent/JPH03500016A/ja active Granted
  • 1989-03-20 US US07/576,452 patent/US5126672A/en not_active Expired - Lifetime
  • 1989-03-20 WO PCT/FR1989/000122 patent/WO1989008852A1/fr not_active Application Discontinuation
  • 1989-03-20 EP EP89903754A patent/EP0407422A1/fr not_active Withdrawn

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