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]
  • G01R33/446—Multifrequency selective RF pulses, e.g. multinuclear acquisition mode
• • 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
• • 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]
  • G01R33/46—NMR spectroscopy
  • G01R33/4616—NMR spectroscopy using specific RF pulses or specific modulation schemes, e.g. stochastic excitation, adiabatic RF pulses, composite pulses, binomial pulses, Shinnar-le-Roux pulses, spectrally selective pulses not being used for spatial selection

Definitions

• the invention relates to a method for generating a spectrum of nuclear magnetic resonance signals by irradiating a sequence of n RF pulses onto a sample which is located in a homogeneous static magnetic field, the envelope of the nuclear magnetic resonance signal response in the frequency domain being approximately a rectangular function.
• HF pulses the envelope of which is provided by simple analytical functions, e.g. Gaussian, Sine or Hermite functions can be described.
• the object of the present invention is therefore to present a method according to the preamble of claim 1, in which a sequence of n RF pulses with as few as possible
• Adaptation parameters a nuclear magnetic resonance spectrum with rectangular characteristics in the frequency domain is generated.
• n RF pulses of the sequence are amplitude-modulated and their respective amplitude distributions follow approximately the shape of bell curves, and in advance the amplitude distribution of the entire pulse sequence in a suitable optimization procedure under the criterion of minimizing the deviation of the envelope the magnetic resonance signal response in the frequency domain has been determined by a rectangular function by varying 3 ⁇ n parameters Wk max , tk max , ak, where wk max is the relative amplitude of the kth pulse of the sequence at the position tk max of its extremum and ak the Mean width of the kth pulse.
• the pulse sequence consists of a superimposition of well-known, easily reproducible RF pulses, the number n of which is limited to a maximum of ten, which is sufficient in any case for the envelope of the signal response to optimally approximate a rectangular function.
• the total number of free parameters to be checked does not exceed 3 n.
• the sequence of bell-shaped RF pulses according to the invention is optimized in advance in a manner known per se by varying the parameters Wk max , tk max and ak which characterize the bell curves and which are changed until the envelope of the magnetic resonance signal response in the frequency domain deviates from one Rectangular function is minimal.
• the parameters could also be determined by more or less qualified guessing, by trial and error or by a simple iteration carried out "by hand".
• the deviation of the envelope of the nuclear magnetic resonance signal response in the frequency domain from a rectangular function is particularly simple and convenient by using a numerical fit program on an automatic data processing system.
• the deviation of the envelope of the nuclear magnetic resonance signal response in the frequency domain from a rectangular function is minimized by optimizing a predetermined error function which describes the deviation of the envelope from a rectangular function.
• the optimization can be canceled if the error function falls below a specified limit.
• the bell-shaped RF pulses of the sequence can generally also be slightly asymmetrical.
• the envelope of the inversion signal response comes particularly close to a rectangular function if the parameters W k max , t k max / t P and ⁇ t k 1/2 / t p each have a corresponding value from the interval
• the method according to the invention is particularly well suited for the in-phase excitation of transverse magnetization.
• a particularly good result is achieved if the parameters W k max , t k max / t p and ⁇ t k 1/2 / t p each have a corresponding value from the interval
• the use of the method according to the invention is particularly advantageous as a component of imaging methods, in particular in NMR tomography, in multidimensional NMR spectroscopy and in particular for volume-selective NMR spectroscopy.
• the sequence of n RF pulses according to the invention can be part of a NOESY pulse sequence or part of a COZY pulse sequence.
• the invention also relates to an NMR spectrometer according to the preamble of claim 18, the memory of which contains a data record for generating a sequence of RF pulses according to the characterizing part of claim 1.
• a spectrometer can be developed in such a way that the above-described configurations of the method according to the invention can be carried out on it.
• each individual pulse has a width proportional to the flip angle
• Fig. 2 shows a comparison of the Mz responses of inversion pulses, the envelope
• FIG. 3 shows the Mz responses to an inversion pulse cascade according to the invention, starting from a start function (a) and after various optimization cycles
• the Fourier transform of an ideal pulse cascade outside the relevant bandwidth must be as close to 0 as possible. Abrupt variations in amplitude should therefore be avoided because they lead to erratic Fourier transforms. Therefore, the pulse shapes of the individual pulses in the cascade were chosen as Gaussian functions in contrast to the otherwise usual rectangular envelope. In addition, the shape of the Fourier transform of a pulse is not influenced by its amplitude, but only by its width.
• the Gaussian pulses in the frequency domain will therefore only be assigned to frequency-dependent phases that reflect in the cascade. It can be seen from this that the output sequence for a Gaussian cascade should have individual pulses with the same widths a k and amplitudes ⁇ k max which vary according to the desired nominal flip angles. In this way, one can expect the excitation to drop as quickly as that of a single Gaussian pulse.
• FIG. 1 shows the Fourier transforms together with numerical simulations of the ⁇ M xy > responses to two Gaussian pulse cascades, the individual pulses of which each have a nominal on-resonance flip angle of + 45 °, -90 ° and + 135 °. If the amplitudes are constant and the widths are proportional to the flip angles, it is obvious that the tails of the excitation profiles very slowly go to zero. In contrast, a cascade of Gaussian pulses with the same pulse widths but amplitudes proportional to the flip angles leads to a response that decays much more quickly. Note also that the Fourier transform does not directly predict on-resonance behavior.
• amplitude modulation in contrast to the modulation of the pulse duration, can easily be extended to pulse cascades with any phase shift, only pulse trains with constant phase should be considered at the moment, whereby phase reversal (e.g. from x to -x) should be allowed.
• the impulses were optimized on the computer using a modified simplex procedure, whereby the deviation from a rectangular target function was minimized. Note that the Fourier conditions mentioned above are sufficient for the desired off-resonance behavior, but are in no way necessary. It may well be that cascades with unequal pulse widths ak also lead to benign response signals.
• the sequence consists of a simple preparation cycle followed by an ordinary inversion pulse.
• G ⁇ 270 ° - x ⁇ G ⁇ 270 ° x ⁇ cycle has no net effect in the vicinity of the resonance, but rather prepares the off-resonance magnetization for the subsequent action of the 180 ° pulse to produce a rectangular one Inversion profile.
• all three Gaussian pulses with the same width but with amplitudes proportional to the flip angle were chosen.
• the error functions are defined as
• the subscripts inv and exe refer to functions that are used to optimize inversion and excitation pulses.
• the parameters ah which are the error functionals according to G1. Define (6) to (8) can be changed from cycle to cycle during the optimization and are given in Tab. 1 and 2 together with the values m and m 'and the parameters for each individual pulse in the cascade, the latter parameters in the form of the position t k max of the kth pulse in units of the time duration t p of the sequence, the relative amplitude W k max and the line width Atk 1/2 are also shown in units of t p . Note that either c or h in G1. [8] is zero.
• Optimization cycle consists of 150 iterations of a simplex minimization of the error function. 3a to d are also Shown waveforms that arise after each cycle, so that it is possible to visualize the gradual optimization of the pulse.
• the last inversion pulse, called G 3 has the parameters given in Table 1 and is quite similar to the initial value at cycle 0. To obtain the present result, only three cycles of the optimization procedure were required, which took approximately 15 minutes of cpu time on a VAX 8550 composite at 14 mips.
• the G 3 cascade can not only be used for inversion, but also as a refocusing pulse. Although the impulse was not optimized for this purpose, it still behaves favorably in this regard. This property is used in the following for the construction of excitation pulses.
• Another goal is to find a pulse that can convert the longitudinal magnetization into transverse magnetization with the least possible phase dispersion in a selected frequency interval. Outside the selected interval, the initial state should not be disturbed as much as possible.
• cascade is to be indicated by a subscript that represents the phase of the first individual pulse.
• the total on-resonance flip angle of the sequence in G1. [9] is therefore + 90 °.
• the course of the optimization is shown in Tab. 2. Since excitation is inherently a more difficult problem, the optimization converged slower and took 11 cycles and around 90 minutes of cpu time.
• Fig. 4 shows the responses to the start pulse (a) and to the end pulse (b) as generated by the optimization.
• G 4 the maximum phase deviation of the G 4 pulse in this window is less than 5 °.
• FIGS. 1 to 3 and the two left boxes in FIG. 4 show the envelope of the Gaussian pulse cascade used in each case.
• the 90 ° pulse being an ordinary hard non-selective pulse.
• the position of the proton resonance of benzene (doped with Cr (acac) 3 ) was changed by incrementally incrementing the transmitter frequency.
• the agreement with the theory is excellent.
• the actual experimental result is shown in the upper image, while the lower image in FIG. 5a shows the simulation of this experiment.
• the RF inhomogeneity has only a slight effect on the resulting profiles, which is also predicted by the theory.
• FIG. 6 shows the simulated behavior (a) of a single 90 ° Gaussian pulse (b) of a 270 ° Gaussian pulse and (c) the G 4 excitation cascade. It can be seen that the cascade represents a considerable improvement both for the profile itself and for the phase properties of the impulse response.
• the method according to the invention in particular the use of the G 3 inversion cascade and the G 4 excitation cascade, can also be used as part of imaging methods in NMR tomography. Since the pulse sequences according to the invention can be easily generated experimentally, their use in multidimensional NMR spectroscopy, in particular in volume-selective NMR spectroscopy, is also advantageous.
• the sequence of n RF pulses according to the invention can in particular also be part of a pulse sequence for correlated NMR spectroscopy (COZY) or a NOESY pulse sequence.

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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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• 1989
  • 1989-12-08 DE DE3940633A patent/DE3940633A1/de active Granted
• 1990
  • 1990-10-06 US US07/852,235 patent/US5285159A/en not_active Expired - Lifetime
  • 1990-10-06 WO PCT/EP1990/001681 patent/WO1991009322A1/de active IP Right Grant
  • 1990-10-06 JP JP2513615A patent/JPH0726924B2/ja not_active Expired - Fee Related
  • 1990-10-06 EP EP90914701A patent/EP0502850B1/de not_active Expired - Lifetime
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