DE10313052B3 - Method and device for acquiring data for magnetic resonance imaging with reduced T1 contrast - Google Patents

Abstract

To reduce unwanted T1 contrast in the detected signals of a magnetic resonance imaging sequence, non-uniformly spaced inversions of the longitudinal magnetization M¶z¶ are implemented in a manner that leads to the establishment of a state in which magnetic resonance signals that occur between the inversions or generated after a sequence of such inversions have a reduced T1 dependency. This state is established by controlling the time intervals (TP, TA) of the M¶z¶ inversions, which are inserted as individual RF intermediate actions in the course of generating a magnetic resonance signal sequence and / or are generated as a result of inversions before a signal sequence ,

Description

The The present invention relates to the spatially resolved investigation of objects using magnetic resonance (MR) and specifically affects one Process for acquiring data for displaying an image, which the spatial Contrast of the magnetic resonance behavior of an object within a chosen Local area shows, according to the generic term of claim 1 or 26. The invention further relates a device for performing of the procedure.

• [1] J.Henning et al. "RARE imaging: a fast imaging method for clinical MR", Magn Reson Med, 3:823-833, 1986.
• [2] D.G.Norris et al. "On the application of ultra-fast RARE experiments", Magn Reson Med 27:142-164, 1992.
• [3] Haase A. "Snapshot FLASH MRI. Applications to T1, T2, and chemical-shift imaging", Magn Reson Med 13:77-89 1990.
• [4] J.Henning et al. "Hyperechoes", Magn Reson Med 46:6-12, 2001.
• [5] Patentschrift DE 100 35 319 C2 (05.12.2002).
• [6] J.Hennig et al. Optimization of signal behavior in the transition to driven equilibrium in steady-state free precession sequences. Magn Reson Med 48:801-809, 2002
• [7] Offenlegungsschrift DE 101 12 704 A1 (02.10.2002).
• [8] A.M.Andrew, P.Styles. "Spin Echo Entrapped Perfusion Image (SEEPAGE). A Nonsubtraction Method for Direct Imaging of Perfusion", Magn Reson Med 43:701-704, 2000.
• [9] W.T.Dixon et al. "Multiple Inversion Recovery Reduces Static Tissue Signals in Angiograms", Magn Reson Med 18:257-268, 1991.
• [10] A.Oppelt et al. "FISP – a new fast MRI sequence", Electromedia (Engl.Ed.) 54:15-18, 1986.
• [11] Offenlegungsschrift DE 44 27 497 A1 (15.02.1996).
• [12] Teng-Yi Huang et al. "Are TrueFISP Images T2/T1-Weighted?" Magn Reson Med 48:684-688, 2002.
• [13] K.Scheffler, J.Hennig. "Is TrueFISP a gradient-echo or a spin-echo sequence?" Magn Reson Med 49:395-397, 2003.
• [14] K.Scheffler et al. "Magnetization Preparation During the Steady State: Fat-Saturated 3D TrueFISP", Magn Res Med 45:1075-1080, 2001.
• [15] Patentschrift EP 0 191 431 B1 (02.01.1989).
• [16] P.Mansfield et al. "Biological and Medical Imaging by NMR", J. Magn. Reson. 29:355-373, 1978.
• [17] V.S.Deshpande et al. "Reduction of transient signal oscillations in true-FISP using a linear flip angle series magnetization preparation", Magn Reson Med 49:151:157, 2003.

In the description below, corresponding references in the generally accessible literature are given when stating and appreciating the prior art:

• [1] J. Henning et al. "RARE imaging: a fast imaging method for clinical MR", Magn Reson Med, 3: 823-833, 1986.
• [2] DGNorris et al. "On the application of ultra-fast RARE experiments", Magn Reson Med 27: 142-164, 1992.
• [3] Haase A. "Snapshot FLASH MRI. Applications to T1, T2, and chemical-shift imaging", Magn Reson Med 13: 77-89 1990.
• [4] J. Henning et al. "Hyperechoes," Magn Reson Med 46: 6-12, 2001.
• [5] Patent specification DE 100 35 319 C2 (05.12.2002).
• [6] J. Hennig et al. Optimization of signal behavior in the transition to driven equilibrium in steady-state free precession sequences. Magn Reson Med 48: 801-809, 2002
• [7] Disclosure DE 101 12 704 A1 (02.10.2002).
• [8] AMAndrew, P. Styles. "Spin Echo Entrapped Perfusion Image (SEEPAGE). A Nonsubtraction Method for Direct Imaging of Perfusion," Magn Reson Med 43: 701-704, 2000.
• [9] WTDixon et al. "Multiple Inversion Recovery Reduces Static Tissue Signals in Angiograms", Magn Reson Med 18: 257-268, 1991.
• [10] A.Oppelt et al. "FISP - a new fast MRI sequence", Electromedia (Engl.Ed.) 54: 15-18, 1986.
• [11] Disclosure DE 44 27 497 A1 (15.02.1996).
• [12] Teng-Yi Huang et al. "Are TrueFISP Images T2 / T1-Weighted?" Magn Reson Med 48: 684-688, 2002.
• [13] K.Scheffler, J.Hennig. "Is TrueFISP a gradient-echo or a spin-echo sequence?" Magn Reson Med 49: 395-397, 2003.
• [14] K.Scheffler et al. "Magnetization Preparation During the Steady State: Fat-Saturated 3D TrueFISP", Magn Res Med 45: 1075-1080, 2001.
• [15] Patent specification EP 0 191 431 B1 (02.01.1989).
• [16] P. Mansfield et al. "Biological and Medical Imaging by NMR", J. Magn. Reson. 29: 355-373, 1978.
• [17] VSDeshpande et al. "Reduction of transient signal oscillations in true-FISP using a linear flip angle series magnetization preparation", Magn Reson Med 49: 151: 157, 2003.

This Locations are in the description by specifying the above Reference numbers in square brackets [].

technical background

In the usual MR imaging methods, the object area to be examined, ie the "sample", is arranged in a stationary magnetic field B ₀ , the direction of which is usually referred to as the z direction, while the other two orthogonal directions of the three-dimensional space are referred to as x and y-direction. The sample is then exposed to a sequence of at least one exciting high-frequency electromagnetic pulse (RF pulse) of selected frequency and pulses from magnetic field gradients in different spatial directions, such that a magnetic resonance signal or a sequence of several magnetic resonance signals appear as a result of the RF excitation and are coded as location-coded MR signals are detected and provide information about the characteristics of the sample.

The stationary magnetic field aligns the spins of the sample longitudinally to the B ₀ field lines, whereby a directional quantization results parallel and antiparallel to the z direction. The two states "parallel" and "antiparallel" correspond to different energy levels, which, if they are not occupied with the same frequency, lead to a net difference in the sample, which results in macroscopic magnetization parallel or antiparallel to the z direction, which is called "Equilibrium" is called. In the case of the type of spin that is usually addressed in MR imaging (mostly proton spins), the equilibrium magnetization M _{0 is} parallel to the z direction; but there are also spin types in which M _{0 is} antiparallel to z direction. The term "B ₀ -parallel" (or "B ₀ -antiparallel") used in the following is therefore to be understood generally as "parallel (or antiparallel) to the direction of the net magnetization that results in equilibrium".

Excitation of the sample by RF energy (with the Larmor frequency f ₀ characteristic of the relevant field strength H _{0 of} the B ₀ field) by means of an RF coil changes the population difference in such a way that the positive amount of the magnetization vector M _z decreases, the extent of this reduction depends on the energy content of the RF pulse applied to the coil. With increasing energy content of the HF pulse, M _z finally drops to zero ("saturation state", ie the same occupation of the energy levels), and then increases in the opposite direction until M _z = −M ₀ (reversal of the original occupation ratio). After the end of the RF pulse, M _z gradually returns to its original equilibrium state M _z = M ₀ , in accordance with an exponential function with a sample-specific time constant T1, which is referred to as the "spin-lattice relaxation time".

After the RF pulse has ended, a temporary RF radiation from the sample can be measured in the RF coil. This radiation appears as if there were a transverse magnetization vector M _xy in the xy plane perpendicular to the z direction, which rotates about the z axis with the Larmor frequency f _0. This M _xy vector is represented as under the influence of RF excitation, the previously effective M _z vector M _{z (0) would} have been tilted by a flip angle "α", the cosine of which is the ratio of the new M _z vector M _{z (1)} to the previous M _z - Vector M _{z is (0)} .

From the time of the termination of the RF pulse, the further fate of the longitudinal magnetization M _z and the transverse magnetization M _xy follows different laws. While M _z strives _for its equilibrium amplitude M _z = (+) M ₀ with the time factor T1, M _xy is subject to various influences that determine the decay of the measurable RF signal:
With perfect (purely theoretical) homogeneity of the stationary magnetic field within the sample, the transverse magnetization M _{xy decays} with a time constant T2 (spin-spin relaxation time), which depends on the molecular dynamics of the sample. T2 is usually smaller than T1; in the case of living biological tissue ("in vivo"), T2 can be in the range from 40 ms to well over 1000 ms, while T1 can be more than 2500 ms.

The presence of field inhomogeneities in the static magnetic field or the sample causes the M _xy vector to be increasingly broken down into elementary components, the phases of which increasingly deviate from the original vector phase. This "defocusing" allows the resulting M _xy vector and thus the RF radiation that can be felt by the RF coil (induction signal in the coil) to decay faster than T2. For this "free induction drop" (FID) there is an effective relaxation time T2 * <T2.

A subsequent application of an RF pulse with α = 180 reverses the direction of the current M _z vector (−M _z ) and, on the other hand, causes a mirroring of the instantaneous phases of the defocused M _xy components. Since the direction of the phase migration of the individual components of the M _xy vector remains the same, this vector is temporarily "refocused", ie the T2 * defocus is gradually released and then started again. This leads to the well-known "spin echo" in the RF coil, the amplitude of which depends solely on T2.

When a magnetic field gradient is applied after HF excitation has taken place (because of the additional inhomogeneity of the field caused by this), additional defocusing occurs, so that the signal felt by the HF coil drops even faster than with T2 *. When the magnetic field gradient is switched, the phase migration of the individual components of the M _xy vector caused by the previous gradient reverses its direction, so that the gradient-related defocusing is "refocused" in this way. The original measurement signal returns, with an amplitude reduced according to T2 *. This return is referred to as "gradient echo", the amplitude of which thus contains information about T2 *.

The Time period from excitation of transverse magnetization to Appearance of an echo is referred to as an "echo time" TE. In addition to the spin echoes and the gradient echoes there are other types of echoes, such as the so-called "stimulated echoes", which follow a Result from at least three RF pulses with flip angles not equal to 180 ° and their amplitude is information about the ratio T1 / T2 contains.

For the spatial resolution of the MR signals, the fact is used that the resonance frequency of the Spins and thus both the necessary frequency of a stimulating RF pulse and the frequency of the measurable MR signals is determined by the local magnetic field strength. For spatial resolution, a so-called "read gradient" is therefore impressed in a selected spatial direction in all imaging methods during the signal detection in order to assign different frequencies in the signal to different locations along this direction (frequency coding). The different frequencies and thus the contributions of different locations can be separated by a Fourier transformation. This enables a spatial resolution in the spatial direction in question, which is also referred to as the "reading direction" (abbreviated: R direction).

to spatial resolution in a second spatial direction, which is preferably orthogonal to the reading direction is, is usually temporarily before the appearance of the signal to be detected a gradient is imprinted in this direction, which causes the in of the sample excited vibrations (spins) along the relevant Dephasing spatial direction. By gradually changing the time integral of this "phase coding gradient" from echo to echo changes the phase of the Echo signal contribution from a location to echo. The signal contributions of the various locations along this direction, which is also called the "phase coding direction" (abbreviated: P direction) can be called by a Fourier transformation with respect to the serial number of the echo are separated. Because frequency and phase each separately dependent from the position along two spatial coordinates (R and P direction) are, can reconstruct a two-dimensional image of the object.

A Location selection in a third spatial direction, which is preferably orthogonal to the plane of the R and P directions and also as "layer direction" (abbreviated: S direction) is denoted by creating a gradient in it Direction during of the exciting slice-selective RF pulse. This "layer gradient" creates a layer in the object for selected the suggestion.

The most common MR imaging methods work with the combined described above Frequency and phase coding. For the representation e.g. of a two-dimensional N-line image successive N echoes are generated, each with a different phase coding, and each echo of this N-echo sequence is transmitted in the same way frequency-coded the reading gradient and detected as an MR signal. The samples of the detected signals become two-dimensional Matrix formed from data, the so-called k-space, each row or "line" of a different frequency-coded Echo is assigned and contains samples of the relevant echo. The Row direction is also referred to as the frequency axis of k-space. The orthogonal axis of the k-space is the phase coordinate scaled, i.e. the position of a row along this axis is determined by the integral of the phase coding gradient. The so organized The data matrix then becomes a two-dimensional Fourier transform (2D-FT) to get the pixel values of the image.

Also others less common MR imaging (projection reconstruction imaging, spiral Imaging) scan k-space, with the strict separation between Phase coding and reading gradient direction canceled in these methods is. Basically, such procedures involve step-by-step turning the reading gradient to make projections in different spatial directions to obtain. In some of these methods, the k-space becomes non-equidistant and scanned in non-right-angled trajectories. Therefore, for this Other image reconstruction methods can also be used. There are also special forms in which the MR signal is immediately after the RF excitation appearing direct FID signal detected and for image reconstruction is read out.

The total sequence of MR signals necessary for recording an N-line image is usually through a chain of MR sub-experiments generated, hereinafter also called "partial sequences". Mostly sits down the entire sequence is made up of a sequence of identical partial sequences (Partial sequence repetition); it can but also different sub-sequences follow one another.

problem

In addition to the density of the spins that can be influenced by the RF pulses, the characteristics of a sample that can be analyzed by magnetic resonance also include the various relaxation time constants of the spin magnetization, including the spin-lattice relaxation time T1 and the spin-spin relaxation time T2 and the effective spin-spin relaxation time T2 *. Macroscopic flow phenomena in the sample as well as diffusion processes are also features that can be made visible by MR imaging. By specifically selecting the type of MR sequence and the amplitude and time relationships of RF and gradient pulses within the sequence, the strength of the generated MR signals or echoes can be achieved depends in particular on certain, selected characteristics of the sample. Such a feature-related "weighting" can produce an image, the contrast of which reflects the spatial distribution of the feature in question.

On The important parameter in imaging diagnostics is the transverse one Relaxation time T2. T2 contrast can provide information about tumors, strokes, infarcts and other pathological changes in biological tissue and is therefore extremely important for clinical MR imaging. In many cases achieving one if possible pure T2 contrast without annoying overlay a T1 contrast is particularly desirable.

State of technology

An almost pure T2 contrast can be obtained by means of a spin echo imaging experiment. On the one hand, a longer echo time TE is required to generate a desired T2 weighting, on the other hand, the repetition time must be selected to be very long (ideally TR ≥ 5T1 _max ) in order to return the magnetization after the acquisition of a k-space line wait for equilibrium and thus avoid an influence of T1 on the image contrast. The recording of T2-weighted spin echo images is therefore associated with very long total measurement times, which limits their clinical applicability.

A shorter measuring time is possible with a multiple spin echo sequence, which also under the acronym RARE (or turbo spin echo, fast spin echo) is known, cf. [1]. Here, after a first stimulating RF pulse successively applied many refocusing pulses at regular intervals, to by periodically refocusing the excited transverse Magnetization to generate successive echoes. So after one echo, several echoes or k-space lines can be acquired, as a result of which the total measurement shorten accordingly leaves.

at Use a 90 ° excitation pulse and 180 ° refocusing pulses The amplitude curve of the signal ideally follows the T2 relaxation. Generate refocusing pulses with a flip angle of less than 180 ° always a longitudinal magnetization that only occurs at a later stage the sequence is brought into the transverse plane. This leads to contributions from "higher order echoes" which are analogous can be understood to the stimulated echo, which by at least three RF pulses are generated. Consequently, the waveform a complicated function of the echo time TE, the flip angle, of T1 and T2, cf. [2].

In in practice there is a T1 influence due to RARE experiments never completely avoid the inevitable inaccuracies of the 180 ° pulses. With "layer-selective" 180 ° pulses Such inaccuracies result from the fact that the "pulse profile", which is the amplitude and the phase of the impulse over represents the frequency, never has the ideal rectangular contour, in order to have all spins over the To influence layer thickness evenly. Thus, some areas of the layer thickness experience a different than the nominal flip angle of 180 ° and so contribute to the unwanted T1-dependent Component in transverse magnetization at.

The rapid sequence of 180 ° pulses in RARE sequences is associated with a high output of high-frequency power to the sample. This leads to the heating of the sample and can be a limiting factor, especially for MR devices with high B ₀ field strengths. The problem can be avoided by using RARE sequences with refocusing pulses of small flip angles, but then no T2 image contrast is achieved.

On Another approach to T2-weighted imaging is application a suitable preparation sequence in which the T2 contrast is followed, followed by a different sequence, which the actual imaging involves what it is normally is a sequence that generates gradient echoes, cf. e.g. [3]. This method is practically only suitable for special cases such as cardiac images, image contrast between blood vessels and the heart tissue, because the one set by the preparation T2 contrast becomes weaker as the sequence progresses. The Influence of T1 relaxation, which depends on the type of the actual imaging sequence so with increasing sequence an increasing relative weight, so that said Method is useless for many clinical applications where pure T2 weighting is required.

Finally, an option for the generation of T2-weighted MR images is provided by the "hypecho" principle recently presented, cf. [4] and [5]. It can be shown that if you have a pattern of RF pulse sen with arbitrary flip angles and phases, followed by a 180 ° pulse, and then repeating the same pattern in reverse order, including a single echo with T2 weighting appears, the so-called hypechoic. Such sequences may possibly be used to recover T2 weight after a long series of pulses, but then only the particular hypechoices have this pure weight, while other echoes of the sequence are both T1-weighted and T2-weighted, which is undesirable ,

A also in the youngest Time-developed technique that offers the possibility of T2 weighting The TIDE approach ("Transition Into Driven Equilibrium "), cf. [6] and [7]. By using a sequence of RF pulses alternating Phase with the flip angle series 90 °, 180 ° followed from n pulses with gradually reduced flip angle to one certain value, an oscillation-free transition to steady state can be achieved. Clearly, you start with a pulse train RARE sequence to then continuously transition to a "FISP" sequence as is known from [10]. If n is chosen large, a T2 contrast can be obtained be generated.

The The object of the invention is an advantageous one for MR imaging Specify technique to minimize the influence of T1 relaxation on the image contrast, so that the Contrast of another selected Feature in the examining object, preferably the T2 contrast, preferably is expressed purely. This object is achieved according to the invention that specified in claim 1 or that in claim 26 Procedure solved.

Accordingly, the invention is implemented in a method for acquiring data for the display of an image which shows the spatial contrast of the magnetic resonance behavior of an object within a selected location area, the object being arranged in a stationary longitudinal magnetic field B ₀ and a sequence of RF Pulses and pulses of magnetic field gradients are exposed in different spatial directions, so that a sequence of magnetic resonance signals is generated from excited transverse magnetization, which are read out by scanning under the action of one magnetic field gradient and are used as a data set for filling the k-space.

According to one first approach according to the invention this method is characterized in that the process of generation the signal sequence above at least part of the entire sequence in consecutive blocks is divided by unevenly spaced RF interferences, which in each case the direction of the current invert longitudinal magnetization. The relation between the temporal lengths Two immediately adjacent blocks are shown over the course of the sequence controlled so that the Influence of Spin-lattice relaxation effects on the course of the signals are minimized becomes or at least less than with a uniform distance throughout the HF interactions.

The RF interferences are preferably dimensioned so that they magnetization present at the end of the previous block in convert a new start state into which the longitudinal magnetization at least approximately same amount and the reverse direction as at the end of the previous Block. That is why the specific design of the HF intermediate effects depend on the type of generation of the magnetic resonance signals of the blocks.

If the signal sequence is generated by repetition of partial sequences with flip angles of the same amount over the course of several successive blocks, then the RF intermediate effects are preferably dimensioned such that they convert the prevailing magnetization state into an "equivalent" new start state in which the longitudinal magnetization along the z-axis points in the opposite direction and the transverse magnetization has the same amount as at the end of the previous block. Thus, the generation of the signal sequence for imaging can be continued without serious transition problems due to improper jumps in the M _z and M _xy amounts and in the magnetization phase. Said equivalent new starting state can be obtained from the previous state at least approximately by means of a simple symmetry operation such as point mirroring, axis mirroring or mirroring on a plane.

The principle according to the invention means a fundamental departure from the previously used concepts for minimizing the T1 contrast. The aim of all previous approaches, after excitation of the transverse magnetization M _{xy, was} to prevent portions of M _xy in the z direction and back after T1 relaxation by successive RF pulses during the subsequent generation of the MR signals be guided into the xy plane. With the invention, however, such a return is deliberately permitted. However, the corresponding T1-dependent "gain" in longitudinal magnetization by T1 relaxation is compensated for during appropriately dimensioned intervals in which, due to the B ₀ -antiparallel orientation of the magnetization, there is a T1-dependent decrease in the amount (ie the absolute value) of M _z ,

This results in a "steady state" in the sense that every inverting RF intermediate action encounters a longitudinal magnetization M _z , the amplitude of which has a finite value between zero and the equilibrium state, but this value on average, even for spins different T1 relaxation time, shows the same course.

The measure according to the invention reduced So the T1 contrast in all MR signals in the intervals between the inverting RF effects even if new transverse magnetization occurs within the intervals is generated by RF excitation pulses.

A Reduction in T1 contrast can also be achieved if one the mentioned creates a steady state by preparation, i.e. without simultaneous generation and acquisition of echoes. A corresponding one Procedure is a second way to solve the task, it is according to the invention characterized that before the generation of the signal sequence and / or during interruption of the generation the signal sequence is a series of inversions of the longitudinal Part of the magnetization implemented in uneven time intervals is, the time intervals over the Sequence of inversions can be controlled so that a steady Sets state in which the amounts of longitudinal magnetization for spins different spin lattice relaxation time T1 all the same, non-zero intermediate value between the inversions cross.

Ultimately, both approaches are based on the same basic idea: using unevenly spaced inversions of the longitudinal magnetization M _z in a manner that leads to the creation of a state in which MR signals generated between the inversions or after a sequence of such inversions reduce the number Have T1 dependency. Each of the two approaches on its own can be sufficient to achieve a noticeable reduction in the T1 contrast. However, it is also within the scope of the invention to combine the two approaches.

From [8] it is known per se to bring about a steady state in the longitudinal magnetization by successive RF pulses, which in each case reverse the M _z vector, which corresponds to the zero level on average or at least very close to zero. This takes place by means of uniformly spaced 180 ° pulses in a preparation sequence, without an intentional transverse magnetization being present between the RF pulses and echoes for imaging being obtained. The stationary longitudinal magnetization settles to zero level, while magnetization, which flows into the 180 ° impulse after this preparation, is not affected. The actual imaging sequence for obtaining a perfusion-weighted MR image only begins after this state has been reached.

Out [9] it is known per se, unevenly spaced HF (inversion) pulses to apply. Here, however, these serve as pure T1 preparation, with the the zero crossings for different T1 times during the subsequent free relaxation process coincide in time. By a subsequent image acquisition at this fixed point in time can be targeted the signal from compartments with certain T1 relaxation times repressed become. The required distances between the pulses and the image acquisition are easy calculate and are in the order of magnitude of the T1 times.

With the technique described in the two references [8] and [9] is the signal from certain tissue by utilizing the T1 relaxation suppressed, for reinforcement of the T1 contrast or to achieve a T1 dependency to emphasize. This is practically the opposite of what happens with the present invention is sought and achieved, and can thus the procedure according to the invention do not suggest.

Special Refinements of methods according to the invention are each in dependent claims characterized. A device operating according to the principle of the invention for MR imaging is characterized in claim 30.

The Concept of the invention and particular embodiments are as follows based on drawings explained.

1 shows, as a result of a simulation, the time course of the amplitude and the amount of the longitudinal magnetization for spins of different T1 relaxation times when applying a sequence of equally spaced 180 ° pulses.

2 to 4 show, as results of simulations, the temporal course of the amplitude and the amount of longitudinal magnetization for different cases of unequal spacing of the 180 ° pulses.

5 outlines the behavior of longitudinal magnetization in a sequence of unevenly spaced 180 ° pulses.

6 describes the ideal ratio TP / TA for the preservation of a certain longitudinal magnetization M _m .

7 shows an imaging sequence working according to the classic "TrueFISP" method.

8th shows, as a result of a simulation, the time courses of the MR signal amplitudes for spins of different T1 relaxation times and different T2 relaxation times in the classic TrueFISP sequence 7 ,

9 shows schematically a TrueFISP imaging sequence modified according to the invention.

10 shows embodiments of an RF intermediate action by means of a single RF pulse.

11 shows an embodiment of a multi-part RF intermediate _{action in} order to be able to carry out the inversion of the M _z at a moment of transverse magnetization M _xy .

12 shows, as a result of a simulation, the time profiles of the MR signal amplitudes for spins of different T1 relaxation times and different T2 relaxation times in the sequence according to the invention 11 ,

13 shows MR images taken from the same object area using (a) a conventional TrueFISP sequence, (b) a TrueFISP sequence designed according to the invention and (c) a conventional RARE sequence.

14 shows in a similar representation as 8th , also as a result of a simulation, the amplitude profiles of the MR signals in the event that the distance ratio of the RF intermediate effects is changed in the course of the sequence.

15 shows in a similar representation as 8th , also as a result of a simulation, the amplitude profiles of the MR signals in the event that the distance ratio of the RF intermediate effects is continuously changed in the course of the sequence.

16 shows in a similar representation as 8th , also as a result of a simulation, the amplitude curves of the MR signals for the combination of a classic TrueFISP sequence 7 with a TrueFISP sequence designed according to the invention 11 ,

17 shows schematically the structure of a device for performing methods according to the invention.

Concept of invention

In the following, the concept of the invention is first clearly explained using graphs obtained by simulation. Simple 180 ° pulses are initially assumed to be inverting RF effects, and the temporal course of the longitudinal magnetization M _z during a sequence of such 180 ° pulses is considered, ie without imaging pulses or signal acquisition. The intervals of successive pulses are alternating between TP and TA, where TP is the interval after an inversion , which converts M _z in the B ₀ parallel direction, and TA is in each case the interval after an inversion, which converts M _z in the B ₀ antiparallel direction, and where TA TP TP. From simulations it follows that after a sufficiently long sequence of this kind, a steady state is established.

The 1 to 4 show the temporal course of the longitudinal magnetization in this steady state for different ratios TP / TA, that is to say after a long sequence of correspondingly spaced pulses. TP + TA = 48 ms applies to all diagrams, ie the distance between a 180 ° pulse and the next but one is 48 ms.

In the diagrams above 1 to 4 the temporal course of the amplitude M _{z of} the longitudinal magnetization is shown in each case, the corresponding amount | M _z | The behavior for magnetization of only two different T1 relaxation times is shown: for an extremely short T1 time of 200 ms and an extremely long T1 time of 3000 ms. For T1 times between these two values, there are temporal courses that lie between the two graphs shown.

The scaling of the ordinates is normalized to the value 1 for M _z = M ₀ , where M _{0 is} the amplitude of the magnetization vector resulting in the equilibrium state under the influence of the stationary magnetic field B ₀ .

In 1 the behavior of M _z for TP / TA = 1 (or for TP = TA = 24 ms) is shown. After an inversion pulse (time t = 0), M _{z is} antiparallel to the main magnetic field B ₀ and begins to relax. The absolute value of M _z rises faster for spins that have a small T1 (high relaxation rate) than for spins that have a larger T1 (low relaxation rate). The course of M _z has a zero crossing for all T1 values, which is in good approximation (TA, TP each << T1) in the middle of two inversions. Immediately before the next inversion pulse (t = 24 ms), there is a pattern of different positive M _z amounts for different types of spins. For all T1 values, an oscillation of M _z around the zero value results, which is more pronounced with a shorter T1.

In 2 the steady state achieved after many pulses is shown for the case TP / TA = 2, ie for TP = 32 ms and TA = 16 ms. Again, from t = 0, the absolute value of M _z for spins with a short T1 increases faster than for spins with a long T1. The following 180 ° pulse at t = 36 ms inverts M _z from positive to negative. The relaxation that follows leads to a reduction in the M _z amounts, likewise at different speeds for different types of spins, but overall faster than before, because the exponential function which describes the spin-lattice relaxation is steeper the further M _z / M ₀ is removed from the value 1. Accordingly, the general rule is that the rate of increase of the M _z amounts when relaxing from the positive (i.e. B ₀ -parallel) orientation is always lower than the rate of decrease from the negative (i.e. B ₀ -antiparallel) orientation. As a result of the relaxation, the M _z amount is reduced, but without exceeding the zero value. The magnetization is thus parallel to the main magnetic field B ₀ for the time interval TP and anti-parallel for the time interval TA. After many pulses, a steady state is reached in which the decrease in the amount of M _z during TA corresponds exactly to the increase in M _z during TP. The amount of M _z oscillates for all T1 values by a value M _m = 0.33, here also with a larger amplitude if T1 is shorter.

In 3 and 4 the steady state is shown for the case TP / TA = 3 or the case TP / TA = 5. There are analogous time profiles of M _z , with oscillation around the values M _m = 0.5 for TP / TA = 3 (TP = 36 ms, TA = 12 ms) or M _m = 0.67 for TP / TA = 5 (TP = 40 ms, TA = 8 ms).

The simulations show that Mm depends on the ratio TA / TP, but is largely independent of T1. The value of M _m approximately corresponds to the amount of M _z in the middle of the interval TA (hereinafter referred to as M _a ) and can thus be calculated analytically. The development of M _z from the middle of TA (time a) to the middle of the next interval TA (time f) is in 5 represented and can be described as follows:
a: Starting point M _a
a ⇒ b: relaxation: M _b = 1- (1-M _a ) exp (–TA / (2T1))
b ⇒ c: inversion: M _c = –M _b
c ⇒ d: relaxation: M _d = 1- (1-M _c ) exp (–TP / (T1))
d ⇒ e: inversion: M _e = –M _d
e ⇒ f: relaxation: M _f = 1- (1-M _e ) exp (–TA / (2T1))

The condition for the steady state is fulfilled if M _f = M _a = M _m . From successive insertion of the resulting equation for M _m has the solution

For TA << T1 and TP << T1, the approximation exp (–TA / T1) 1-TA / T1 applies, analogously for TP. This turns M _m into:

Further approximation for TA, TP << T1 follows:

This equation agrees very well with the results previously obtained from simulations and shows that M _{m is,} to a very good approximation, independent of T1, ie for a very wide range of T1 values the longitudinal magnetization oscillates around the same final value in the steady state.

Conversely, the following applies: a certain longitudinal magnetization M _z can be preserved, so to speak, by means of a sequence of unevenly spaced inversion pulses, regardless of T1. If a certain M _z is present, there is an optimal TP / TA with which the direct influence of T1 can be eliminated. In 6 this optimal TP / TA is plotted _{as a} function of M _z .

The described property of unequally spaced RF pulses exploited according to the invention to magnetization in the course of an imaging sequence to a temporal one To force course of independent is from T1. Among other things, this allows the generation of a contrast behavior, which is only determined by other properties of the sample, e.g. through the T2 relaxation time.

preferred embodiment

As first embodiment is the application of the concept of the invention to a fast MR imaging sequence described, which is known from [10] and is now referred to with the acronym "TrueFISP". The following are first described the principle and properties of this imaging method. For simplification, there is no so-called off-resonance effects received the TrueFISP signal behavior can strongly influence.

The basic scheme of the known TrueFISP sequence is in 7 shown and consists of N-time execution of similar partial experiments or "partial sequences". In the presence of a slice gradient S, each partial sequence begins with a slice-selective RF excitation pulse for a flip angle, the magnitude of which | α _R | is usually less than 180 °. After applying a phase gradient P and reversing the polarity of an applied reading gradient R, an echo appears, which is detected as an MR signal and written into a line of k-space determined by the amplitude of the phase gradient. Then the gradients are switched again to ensure that the time integral of all gradients at the end of the partial sequence is zero again.

The subsequent partial sequences run in the same way, with a repetition time TR, however, the sign (ie the direction or the phase) of the flip angle α _{R is} alternated from partial experiment to partial experiment. After a sufficient number of TR intervals, a steady state results, in which the resulting magnetization vector flips between the angular positions + α _R / 2 and −α _R / 2 with each excitation in the course of the sequence. Because of | α _R / 2 | The longitudinal magnetization Mz always remains in the B ₀ parallel (positive) orientation at <90 °.

mit E1 = exp(–TR/T1) und E2 = exp(–TR/T2).The steady state can in principle be maintained continuously by continuing the sequence. This implies that the "loss" of Trans Versal magnetization is just compensated for by the "gain" in longitudinal magnetization due to T1 relaxation. The TrueFISP steady state signal can be described by

with E1 = exp (–TR / T1) and E2 = exp (–TR / T2).

For TR << T1, T2 the signal is approximately determined by the ratio of T2 to T1, according to the following proportionality:

Reaching steady state can take many TR intervals; The time course of the magnetization is characterized by strong oscillations, which generally makes interference-free imaging impossible. An improvement can be achieved by a preparation with an RF pulse of the flip angle α _R / 2, which is placed in front of the actual imaging sequence at a distance of TR / 2 and which approximates the magnetization vector to the "cone" for the steady state to be achieved. Condition (cf. [11]). In this way, strong oscillations of the transverse magnetization can be largely avoided and the signal acquisition can begin immediately after the preparation. The signal curve from the first echo to steady state is primarily determined by T1, T2 and by the flip angle α _{R used} . The contrast of an MR image is mainly determined by the signal relationships of the middle k-space lines. If these phase steps are acquired during the transition to the steady-state state, the picture shows a mixed contrast that deviates from the steady-state contrast, cf. [12].

In 8th this is illustrated by means of a diagram whose curves show the decrease of these amplitudes for different T1 values (between 500 ms and 2500 ms) and for different T2 values (75 ms, 125 ms, 175 ms, 500 ms and 1000 ms) in the course show the sequence. A repetition time of TR = 5 ms and a flip angle of α _R = 110 ° were assumed as measurement parameters of the sequence.

The scale of the ordinate is normalized to the maximum possible value of the echo peak amplitude, ie to M ₀ . The T1 influence can be seen in the fact that for each T2 value there is not a single curve but a family of curves that is relatively broad. The fanning reflects the different T1 values and thus also the T1 influence in the contrast of an MR image that is reconstructed from k-space. After a sufficient number of pulses, the MR signal reaches the steady state value, which can be seen in Eq. 4 or Eq. 5 can be described.

In 9 a TrueFISP sequence modified according to the invention is shown schematically. It consists of successive "blocks" with a different number of True FISP part-sequences.

It begins with an α _R / 2 preparation and a first block of length TP, consisting of K _P -periodic execution of the in 7 shown TrueFISP partial sequence with alternating RF pulses of the flip angle ± α _R (where K _{P can} also be 1). This block is followed by an inverting HF intermediate action I _A , by means of which the longitudinal magnetization is brought into the B ₀ -antiparallel (negative) orientation. So that the imaging in the subsequent block of length TA can be continued directly with the regular excitation flip angle ± α _R (so to speak in the "inverted steady state", ie without strong oscillations occurring, which was also referred to as the "equivalent state" at the beginning) the magnetization vector is transferred by the HF intermediate action in a direction in which it encloses an angle of 180 ° + α _R / 2 or 180 ° −α _R / 2 with the positive z-axis.

The K _A repetitions of the second block following this renewed starting state are again carried out with the regular flip angles α _R alternating signs, such that the resulting magnetization vector with each excitation in the course of the sequence between the angular positions 180 ° + α _R / 2 and 180 ° –Α _R / 2 is folded back and forth. The subsequent RF intermediate action I _P transfers the magnetization vector again in the direction parallel to B ₀ with a positive M _z component _such that it encloses an angle of + α _R / 2 or −α _R / 2 with the z axis. The starting state is thus renewed again and the imaging sequence can be continued directly.

In this first exemplary embodiment, consider the simple case that K _{A is} smaller than KP and that further blocks with K _P repetitions with positive M _z and blocks with K _A repetitions with negative M _z alternate until all lines of the k-space with the detected MR echoes are filled and from this the MR image can be reconstructed.

The said inverting RF action between the blocks should ensure, that itself as the starting state for each block an "equivalent State " i.e. that itself the "inverted Steady state "im does not have to settle the following block, but from the previous one Orientation is immediately converted into the inverted orientation.

Simple designs of suitable RF interferences in the form of a single RF pulse (or an equivalent combination of several RF pulses) are shown in 10 shown. Here the direct transfer is sketched with a 180 ° HF pulse, with phase positions of the pulse that prevent rotation about the x-axis ( 10a ) or around the y-axis ( 10b ) of the rotating coordinate system.

In order to maintain a property of the TrueFISP sequence, the refocusing of off-resonant spins (cf. [13]), a time at a distance from TR after the last α _R pulse is advantageous for the 180 ° pulse. Alternatively, it is only possible to add an additional α pulse as aftertreatment at this time, with a 180 ° pulse around the time interval TR / 2 afterwards, ie at the actual echo time. In this case, the sequence in the next block is preferably continued after the time TR / 2. With this timing diagram, it is also possible to use an inverting RF pulse with the flip angle (180 ° –α _R ) ( 10c ). In all cases, attention must be paid to the correct phase of the first α _R pulse in the following block.

In order to avoid the occurrence of undesired spin echoes, it can be advantageous in practice to carry out the inversion of the magnetization at a time when there is no transverse magnetization. As in 11 shown, this can be achieved by dividing the RF intermediate action into several successive RF actions: (i) a first RF action as post-treatment at the end of the previous block, consisting of a preferably slice-selective RF pulse with a flip angle of ± α _R and a preferably slice-selective RF pulse with a flip angle of ± α _R / 2 at a distance of TR / 2; (ii) a second RF action in the form of a 180 ° pulse that causes the actual inversion; (iii) a third RF exposure as pretreatment at the beginning of the next block, consisting of a preferably slice-selective RF pulse with a flip angle of ± α _R / 2.

With the first RF exposure, that is to say for the aftertreatment of the previous block, the pattern of the signs is to be selected depending on the sign of the α _R excitation pulse of the last partial sequence of the previous block; it can therefore differ from block to block, which also depends on whether the number K _P or K _{A of} the partial experiments in the previous block is even or odd.

In order to dephasize remaining transverse residual magnetization, a so-called "spoiler" gradient can additionally be applied immediately before and / or after the M _z inversion. In addition to the layer direction S, the reading direction R may also be appropriate. In order to avoid the refocusing of unwanted spin echoes by the 180 ° pulse, an asymmetrical design is advantageous, ie the same spoiler gradient should not be switched in the same spatial direction before and after the 180 ° pulse. A suitable gradient circuit is in 11 for the example of a TrueFISP sequence modified according to the invention.

For the third RF action, ie the pretreatment for the following block, the sign of the α _R / 2 pulse is theoretically irrelevant; with M _xy = 0, in principle any new phase can be selected; however, these then determine the phases of the α _R pulses within the following block.

Instead of that in the embodiment 11 breakdown of the intermediate RF effects used, other breakdowns can also be used in order to achieve the smoothest possible time course of the MR signal. It can also be advantageous to start successive blocks with a flip angle that is smaller than α _R and gradually increases to the value α _R. If, generally speaking, the last RF pulse of the previous block has the flip angle α ₁ and the first RF pulse of the subsequent block has the flip angle α ₂ , then the RF intermediate action and a spoiler gradient can, for example, according to one of the following time schemes be switched:
-Α _1/2 - spoiler - 180 ° - spoiler - + α _2/2
or 180 ° ± α _1/2 - spoiler - + α _2/2
or -α _1/2 - spoiler - 180 ° ± α _2/2.

The Spoiler gradients therefore meet states in which the magnetization vector shows essentially parallel or antiparallel to the z-direction.

It should be mentioned at this point that the insertion of special HF effects between blocks of TrueFISP part-sequences is known per se, these intermediate effects consisting of a backflipping α _R / 2 pulse, a preparation block (for fat saturation) and a subsequent α _R / 2 preparation impulse exist, cf. [14]. In this state of the art, the RF interferences are expressly "periodically"; Subdivision into blocks of different lengths is not suggested by this. The HF effects described in [14] serve expressly to eliminate fat signals; the inversion of the magnetization in order to pick up signal in individual blocks with a negative M _z is not suggested.

By a suitable choice of the ratio K _P / K _A , the duration TP of the intervals with B ₀ parallel M _z magnetization in relation to the duration TA of the intervals with B ₀ parallel M _z magnetization can be set such that the desired effect is achieved results in a decrease in the T1 contrast. The diagram of the 12 illustrates the result of a simulation for the case TP = 60 ms and TA = 30 ms (i.e. for TP / TA = 2). In 12 is in a similar representation as in 8th plots the decrease in the echo peak amplitudes for different T1 values and for different T2 values over the course of the overall sequence, plotted over time t. Both the parameters of the TrueFISP blocks and the assumed T1 and T2 times correspond to those 8th (TR = 5 ms, α _R = 110 °, T1 = 500 ms to 2500 ms, T2 = 75 ms, 125 ms, 175 ms, 500 ms, 1000 ms).

A comparison of both diagrams ( 8th and 12 ) shows that the fanned family of curves has become bundles of zigzag lines in which each reversal point marks the time of an M _z inversion in the sequence. In the early stage of the sequence, where the echoes are still relatively strong, the zigzag lines of each bundle oscillate closely and practically without a significant fanning out around a common curve line. The signal curve accordingly becomes independent of T1, ie the T1 contrast is minimized in the desired manner during this stage, so that practically a pure T2 contrast results over a long time phase of the sequence.

The Signal drop in time is slower at a flip angle <180 ° as an exponential decay with the time constant T2, so that until Reaching the final level more T2-weighted Acquisitions possible than with a RARE sequence and thus a very short total measuring time can be realized.

In the later stage of the sequence (ie with a decreasing amount of M _z ), the chosen scheme with TP / TA = 2 proves to be not optimal for the elimination of T1 effects. Their influence is noticeable as an increasing diversification of the signal contributions with different T1 times, and thus theoretically as a certain T1 influence on the image contrast. However, this is less critical for the image contrast if the echoes of the stage with T2 contrast are written in the central and thus contrast-determining region of the k-space.

13 shows in the left image an MR slice image of the human brain obtained with a classic true FISP sequence at steady state (TR = 6.46 ms, flip angle α _R = 50 °, field-of-view (FOV) 23 cm, Matrix 256 × 256, acquisition time 1.7s). The image contrast corresponds to the well-known TrueFISP contrast, which is given by Eq. 4 or Eq. 5 and where brain fluid (CSF) with strong signal intensity appears, white and gray brain mass with almost the same lower intensity. The middle picture shows a picture of the same object area, which was recorded using a TrueFISP sequence modified according to the invention (TR = 6.46 ms, flip angle α _R = 50 °, FOV 23 cm, matrix 234 × 256). With a P-block length of TP = 91 ms and an A-block length of TA = 33 ms (i.e. with TP / TA ≈ 3), the total acquisition time was also 1.7 s. The central lines of k-space were acquired about 295 ms after the start of the sequence, which results in a pronounced T2 contrast in which CSF (T2> 1000 ms) appears very bright, and gray mass (T2 ≈ 85 to 110 ms) shows a slightly higher intensity than white mass (T2 ≈ 70 to 90 ms). In comparison, the image on the right shows a recording obtained with a fast RARE sequence (turbo spin echo, fast spin echo) (mean TE = 99 ms, TR = 3000 ms, FOV 23 cm, matrix 256 × 256, acquisition time ≈ 1 min ), The image largely shows a typical T2 contrast, but subcutaneous fat appears artificially too bright due to its extremely short T1 time.

In contrast to the classic TrueFISP recording with a con contrast, a T2 contrast is present in the image according to the invention. Compared to the RARE recording, the recording according to the invention, in addition to the shorter measuring time, may have the advantage that the fat signal is not represented with an artificially increased intensity.

In addition to the described simple time schedule with alternating periods of length TP and TA, other more variable schedules are possible and advantageous. The suppression of the T1 contrast becomes even better if measures are taken to prevent the in 8th to reduce the visible fanning out of the zigzag lines even in the later stage of the sequence. This is achieved by reducing the TP / TA ratio at a later stage. The 14 shows in a similar representation as 8th and 12 the case chosen as an example that in the early stage of the sequence the same TP / TA ratio (TP / TA = 2) for about 360 ms as in the case of 12 is used (TP = 60 ms, TA = 30 ms), while a TP / TA ratio of 1.33 is used for the rest of the sequence (TP = 40 ms, TA = 30 ms). One recognizes in 14 clearly that the T1 dependence of the MR signals is also reduced for the later weak echoes.

This can be understood from the preliminary considerations given above: At a certain time of the measurement, a certain ratio TP / TA is optimal, which according to Eq. 3 can be approximately adapted to the M _z magnetization present at this time. However, this is different and should ideally only depend on T2, since T2 contrast is desired. Thus, a TP / TA ratio can actually only be optimal for a certain T2 component; however, the simulations show that TP / TA values can be found which are sufficiently good for a very wide range of T1 and T2 values. If, in practice, T1 and T2 values in a certain range are of particular interest, the time schedule can be individually optimized for these parameters. It will be optimal to gradually reduce the TP / TA ratio over several stages in the course of the overall sequence, preferably from block pair to block pair. If, for example, a signal separation or contrast generation is desired due to small T2 differences for short T2 values, for example between gray and white brain mass, the time schedule should be adapted to the temporal course of the M _z values to be expected for these short T2 times become.

An example of this is in 15 shown: TA remains constant (TA = 30 ms) and TP gradually decreases from the initial 60 ms to 30 ms. As a result, the families of curves for longer T2 values (T2 = 500 ms or T2 = 1000 ms) are forced less effectively onto the same time profile, but components with different short T2 values are effectively separated and the zigzag profiles are each less pronounced.

Under certain circumstances, a somewhat better initial signal strength and thus an improvement in the signal-to-noise ratio (SNR) can be achieved if one omits the M _z inversions in the first stage of the sequence, for example the classic TrueFISP method (or a other classical sequence), and only introduces the M _z inversions according to the invention at a later stage. The 16 shows in a similar representation as 12 the case chosen as an example that after about 300 ms of a classic TrueFISP sequence a TrueFISP sequence modified according to the invention is started with TP = 40 ms and TA = 30 ms. However, the SNR gain comes at the expense of T1 suppression in the early echoes. An associated increase in the T1 influence on the image contrast can be countered by postponing the acquisition of the central k-space area.

The at times zigzag-shaped The course of the MR signal can cause problems with linear phase coding Bring image artifacts. That can be segmented by a suitable one arranged phase coding order can be counteracted. simulations show that the Zigzag course is less pronounced, the shorter TA and TP selected become. In practice, however, the RF intermediate action does not have one negligible finite length. TA and TP must in a meaningful relationship chosen to this in order to achieve a high temporal efficiency of the measurement.

Other embodiments

The invention is of course not limited to the examples specifically described above. The principle of M _z inversion at uneven intervals TP and TA according to the invention can also be used in conjunction with other sequence types, that is to say other types of echo generation than TrueFISP echo generation. The optimal time course of the TP / TA ratio depends on the nature of the partial sequences between the M _z inversions and the type of echo generation, and is also dependent on the excitation flip angles before or between the M _z inversions; it can be predicted theoretically on the basis of the Bloch equations by calculation or simulation or experimentally by targeted ver search series can be determined.

In general, the method according to the invention introduces a new, previously unrecognized setting parameter for minimizing undesired T1 contrast, namely the temporal course of the relationships between the duration TP of the M _z relaxation from the B ₀ parallel orientation and the duration TA of the M _z relaxation from the B ₀ antiparallel orientation. With the stipulation of non-uniform distances TP and TA according to the invention and the set goal of reducing the T1 contrast, the designer of MR imaging sequences has a sufficient requirement to use the ratio value TP / TA in a suitable manner to achieve this goal within the scope of his ability sized. A wide variety of sequences can thus be modified according to the invention in order to minimize an undesired T1 contrast and to keep the remaining contrast, which is typical of the sequence in question, as pure as possible. In general, the principles and variants described above in connection with the TrueFISP sequence type for the dimensioning of the TP / TA ratio and for the formation of the HF interferences can be applied in the same or similar manner in many other sequence types.

• – schnelle Gradientenecho-Methoden mit kleinen Flipwinkeln (z.B. FLASH, vgl. [15]);
• – "Single-Shot"-Sequenzen mit vielen Gradientenechos aus jeweils einer HF-Anregung (z.B. EPI, vgl. [16];
• – schnelle Spinecho-Methoden, auch mit Refokussierungsimpuls-Flipwinkeln < 180°.

As examples of further possible uses of the invention, representative of many other, in particular the following, sequence types for MR imaging are mentioned:

• - fast gradient echo methods with small flip angles (eg FLASH, cf. [15]);
• - "Single-shot" sequences with many gradient echoes from one RF excitation each (eg EPI, cf. [16];
• - Fast spin echo methods, also with refocusing pulse flip angles <180 °.

Applicable are also methods in which the signal sequence is used for imaging Sampling of the direct FID signal without echo generation and under Location coding is obtained with a rotating reading gradient (Projection Reconstruction Imaging). Also the mix of sequence types (e.g. sequence A during TA and sequence B during TP) is possible likewise the acquisition according to the so-called Half Fourier method, at which detected echo signals only in one half of the k-space become. Naturally is the inventive method also applicable in connection with sequences for three-dimensional Imaging.

The intervals TP and TA between the RF intermediate effects can also be measured by inserting waiting times after individual partial sequences or at the beginning or end of an entire block. In this case, the number K _P and K _{A of} the partial sequences per block can remain the same. Especially with fast gradient echo sequences and when using the TrueFISP sequence, it is also possible to insert the inverting HF intermediate effects after each individual partial sequence, in each case after a different waiting time, in order to set the desired TP / TA ratio.

To optimize the T1 reduction, it can help to dimension the excitation flip angles differently during the B ₀ anti-parallel blocks than during the B ₀ parallel blocks, to change them in the course of the overall sequence, and / or to change them continuously within the individual blocks to vary. Optimized flip angle sequences can further smooth the zigzag curve of the signal or further improve the mutual approximation of the signal curve for components with different T1. For some sequence types that consist of repeated partial sequences, it may also be advantageous to gradually increase or decrease the amount of flip angle from partial sequence to partial sequence starting from an initial value, for example during the first partial sequences of the entire sequence or each block, as from [17] known per se. In such cases, when dimensioning the RF intermediate action to bring about the new start state, it must be taken into account that the first RF pulse of the following block has a different flip angle than the last first RF pulse of the previous block.

A additional Option is the connection with concepts like the one mentioned above TIDE concept [cf. [6] and [7]) with which by a sequence of pulses with suitable different flip angles a smooth transition in a steady state can be achieved. This approach also offers the possibility to obtain T2-weighted MR signals.

Furthermore, it is a promising possibility to use the present concept in connection with a preparation, for example inversion, saturation, fat suppression, generation of diffusion contrast or generation of magnetization transfer contrast (MTC, provides information about the interaction of free spins with macromolecules). Particular mention should also be made of a T2 pretreatment, for example by means of a CPMG pulse sequence (sequence of 180 ° pulses according to Carr-Purcell-Meiboom-Gill) in order to generate initial T2 contrast and to maintain it according to the invention by suitably spaced HF intermediate effects , In principle, the use of preparation modules is also within the HF-Zwi influences such as additional fat saturation in analogy to [14].

As already mentioned above, a reduction in the T1 contrast can also be achieved if the principle of the invention is used in the context of a preparation, that is to say without simultaneous generation and acquisition of echoes. With such a preparation, which can be implemented before the generation of the echo sequence and / or during the interruption of the generation of the echo sequence, the time intervals of the M _z inversions are controlled in such a way that a steady state occurs, as described in 2 to 4 is shown.

In this state, the amounts of longitudinal magnetization for spins of different spin-lattice relaxation times T1 all cross the same nonzero intermediate value | M _m | between the inversions, as shown by the 2 to 5 has been described. In a subsequent MR imaging sequence, this state can gradually disappear, but it remains noticeable for a certain amount of time in order to reduce or even compensate for the T1 dependence of the signal sequence generated in this sequence if the sequence is sufficiently short , The actual imaging sequence after the preparation can be of any type and therefore does not necessarily have to be divided into blocks by further, unevenly spaced inverting HF intermediate actions.

In the preparation according to the invention, the time intervals TP of an inversion that converts the longitudinal magnetization in the B ₀ -parallel direction to the subsequent inversion should be larger on average than the time intervals TA of each one that converts the longitudinal magnetization in the B ₀ -antiparallel direction Inversion up to the subsequent inversion. The ratio TP / TA is preferably kept constant over the sequence of the inversions, the larger time intervals TP preferably being equal to one another and the smaller time intervals TA being identical to one another.

Device according to the invention

Methods according to the invention can be carried out with an MR device, as is shown in a highly simplified block diagram in FIG 17 is shown. The MR device shown contains a measuring station 10 , a power supply part 20 , a control section 30 and a computer 40 , The measuring station 10 contains a magnet 11 for generating a constant, homogeneous ("longitudinal") B ₀ field, furthermore three sets of coils 12 . 13 . 14 Which are arranged spatially and wound such that they can generate magnetic fields corresponding to the B ₀ field three gradients G _x, G _y and G _z stamp, of which usually a (G _z) parallel to the B ₀ field and the other both (G _x and G _y ) are directed perpendicular to the B ₀ field and perpendicular to each other. By combined simultaneous excitation of selected examples of the coil sets 12 . 13 . 14 resulting magnetic field gradients can be generated with selectable strength and in any spatial direction.

In the measuring station 10 is also a coil arrangement 15 provided to which RF pulses can be supplied, including the RF pulses required for excitation and for refocusing transverse magnetization. This RF coil 15 can also be used to receive the MR signals emitted by the object. The power supply part 20 contains a power supply unit 21 for the B ₀ magnet 11 , a power supply unit 22 for the gradient coils 12 . 13 . 14 and a high frequency generator 23 which is the RF pulses for the coil 15 supplies. The control section 30 controls the duration and amplitude of the currents supplied to the respective gradient coils as well as the duration, amplitude, frequency, phase and envelope of the RF pulses. The control section 30 also controls the time window to that of the RF coil 15 received MR signals to the computer in selected time periods and with a selected sampling frequency 40 read out how it with the gate circuit shown 50 is symbolized. The read samples are digitized and stored in a memory which forms the k-space. The A / D converter required for digitization and the memory can be in the computer 40 be integrated.

MR systems of this type are known and commercially available, so that a further explanation of apparatus details is unnecessary. In order to carry out a method according to the invention, the object to be examined is placed in the B ₀ and gradient coils 11 . 12 . 13 . 14 brought surrounding space, and the RF coil 15 is arranged so that the object area to be examined lies in its area of influence. Then after a selected program that is in the control section 30 has been fed in, the HF generator 23 who have favourited Gradient Coil Supply Unit 22 and the readout circuit 50 controlled to apply the required RF pulses and gradient pulses for selected imaging sequences and to read out the appearing echo signals. The computer 40 extracts the pixel matrix for image reproduction from the data then written into the k-space memory by means of mathematical transformations.

The control section 30 for dimensioning the time, frequency and amplitude parameters for the RF pulses, the gradients and the echo readout is in each case designed or programmed so that the data acquisition processes take place in accordance with the respectively selected embodiment of a method according to the invention. The control section 30 can of course also, in whole or in part, on the computer 40 be integrated.

Claims (30)

Method for acquiring data for the display of an image, which shows the spatial contrast of the magnetic resonance behavior of an object within a selected location area, the object being arranged in a stationary longitudinal magnetic field B ₀ and a sequence of RF pulses and pulses of magnetic field gradients in different Exposure to spatial directions is such that a sequence of magnetic resonance signals is generated from excited transverse magnetization, which are read out by scanning under the action of one magnetic field gradient at a time and used as a data set for filling at least part of the k-space, characterized in that the process the generation of the signal sequence over at least part of the entire sequence is divided into successive blocks by unevenly spaced HF intermediate actions, which invert the direction of the current longitudinal magnetization The ratio between the temporal lengths (TP, TA) of two immediately adjacent blocks is controlled over the course of the sequence so that the influence of spin-lattice relaxation effects on the course of the signals is minimized or at least less than in consistently uniform distance of the HF intermediate effects. Method according to Claim 1, characterized in that the length (TP) of those blocks which are each introduced by an HF intermediate action which converts the longitudinal magnetization in the B ₀ parallel direction is at least in parts of the block sequence greater than the length (TA ) of the immediately preceding or following block, which is initiated by an RF intermediate action which converts the longitudinal magnetization into the B ₀ -antiparallel direction. Method according to one of claims 1 or 2, characterized in that that this relationship (TP / TA) between the temporal lengths (TP, TA) two each immediately adjacent blocks is constant in the course of the block sequence. Method according to one of claims 1 or 2, characterized in that that this relationship between the temporal lengths (TP, TA) each of two immediately adjacent blocks in the course the block sequence changed step by step becomes. Method according to one of claims 1 or 2, characterized in that that this relationship between the temporal lengths (TP, TA) each of two immediately adjacent blocks in the course the block sequence changed periodically becomes. Method according to one of Claims 1 to 5, characterized in that the length (TA) of the blocks which are each introduced by an HF intermediate action which converts the longitudinal magnetization into the B ₀ parallel direction, the course of the block sequence is constant. Method according to one of claims 1 to 6, characterized in that that a Subdivision of the process of generating the signal sequence in different long blocks only over part of the total sequence takes place. Method according to one of claims 1 to 7, characterized in that that the Generation or acquisition of signals during selected blocks of the Block sequence is suspended. Method according to one of claims 1 to 7, characterized in that that the RF interferences each at the end of the previous one Block existing magnetization in a new start state, in which the longitudinal magnetization at least approximately same amount and the reverse direction as at the end of the previous Blockes and from which the generation of the signal sequence for further To fill of k-space can be continued without noticeable transition problems through jumps in the amounts and occur in the phase of transverse magnetization. A method according to claim 9, characterized in that the signal sequence over the course of several successive blocks is generated by repetition of partial sequences. Method according to Claim 10, characterized in that each of the RF intermediate effects taking place between the blocks corresponds overall to a 180 ° rotation of the magnetization state about an axis orthogonal to the B ₀ direction or a reflection of the magnetization state with respect to the plane orthogonal to the B ₀ direction , A method according to claim 10, characterized in that the Intermediate RF effects each include: one yourself first RF exposure following the end of the previous block, which brings transverse magnetization to zero without the to invert longitudinal magnetization; a subsequent second RF exposure for a flip angle of at least approximately 180 °; a subsequent third RF exposure, which is the transverse magnetization for the new Establishes the initial state. Method according to Claim 12, characterized in that the phase of the RF pulses used in the repeated partial sequences is alternated from partial sequence to partial sequence and in that in the intermediate RF actions the first RF action from an RF pulse for a flip angle of the amount | α _1/2 | or from a combination of an RF pulse for a flip angle of the amount | α ₁ | and an RF pulse for a flip angle of the amount | α _1/2 | is _1, where α the flip angle of the last RF pulse of the previous block, and that in the RF-intermediate actions, the third RF effect of an RF pulse for a flip angle of the amount | α _2/2 | exists, where α _{2 is} the flip angle of the first RF pulse of the subsequent block. A method according to claim 12 or 13, characterized in that immediately at the beginning of the second RF exposure or at the end of it a spoiler magnetic field gradient is applied for dephasing transverse magnetization, such that the Prevents generation of spin echoes due to the RF interference or is reduced. A method according to claim 12 or 13, characterized in that both immediately at the beginning of the second RF exposure as well as at the latter End a spoiler magnetic field gradient for dephasing transverse Magnetization is applied, the two spoiler magnetic field gradients be laid out in different spatial directions or an uneven one Have time integral such that the Prevents generation of spin echoes due to the RF interference or is reduced. A method according to claim 10, characterized in that the RF interferences consist of two successive RF actions, the first of which has a flip angle through which the transverse Magnetization at least approximately is brought to zero, and the second has a flip angle, due to the longitudinal and transverse magnetization accordingly the new starting state, and that immediately after the first RF exposure a spoiler magnetic field gradient for dephasing remaining transverse magnetization is applied, That the phase which is used in the repetierten partial sequences of RF pulses of sub-sequence to sub-sequence is alternated, and that in each RF intermediate action of the flip angle of the first RF exposure equal to 180 ° ± α _1/2 method according to claim 16, characterized and the flip angle of the second RF exposure is equal to -α _2/2 , where α _{1 is} the flip angle of the last RF pulse of the previous block and α _{2 is} the flip angle of the first RF pulse of the subsequent block. A method according to claim 16, characterized in that the phase which is used in the repetierten partial sequences of RF pulses from subsequence to subsequence alternates and in that in each RF intermediate action of the flip angle of the first RF exposure equal to -α _half and the flip angle the second RF action is equal to 180 ° ± α _2/2 , where α _{1 is} the flip angle of the last RF pulse of the previous block and α _{2 is} the flip angle of the first RF pulse of the subsequent block. Method according to one of claims 10 to 18, characterized in that that the Magnetic resonance signals in the repeated partial sequences as gradient echoes be generated. Method according to one of claims 10 to 19, characterized in that that the Magnetic resonance signals in the repeated partial sequences as spin echoes are generated by each excited from a transverse Magnetization of several successive spin echoes through repetitive RF refocusing pulses are caused. Method according to one of claims 10 to 20, characterized in that the length (TP, TA) of the blocks is set by selecting the number (K _P , K _A ) of the partial sequences within the respective blocks. Method according to one of claims 10 to 21, characterized in that that repeated Partial sequences within one or more blocks and / or in different blocks are of different lengths. Method according to one of claims 10 to 22, characterized in that that the Excitation of transverse magnetization in repeated partial sequences within one or more blocks and / or in different blocks done with different flip angles. Method according to one of claims 1 to 23, characterized in that that before the generation of the signal sequence and / or at the beginning of selected blocks preparation sequence added is for the weighting of the subsequent signals for induction or reinforcement a desired one Contrast that is different from the T1 contrast. Method according to one of claims 1 to 24, characterized in that that the Generation of magnetic resonance signals within a block or in different blocks by partial sequences of different types. Method for acquiring data for the display of an image, which shows the spatial contrast of the magnetic resonance behavior of an object within a selected location area, the object being arranged in a stationary longitudinal magnetic field B ₀ and a sequence of RF pulses and pulses of magnetic field gradients in different Chen spatial directions is exposed, so that a sequence of magnetic resonance signals is generated from excited transverse magnetization, which are read out by scanning under the action of a magnetic field gradient and used as a data set for filling at least part of the k-space, characterized in that before the generation of the signal sequence and / or during interruption of the generation of the signal sequence, a sequence of inversions of the longitudinal portion of the magnetization is implemented at non-uniform time intervals (TP, TA), the time intervals over the The sequence of the inversions can be controlled in such a way that a steady state arises in which the amounts (| M _z |) of the longitudinal magnetization for spins of different spin-lattice relaxation times T1 all have the same non-zero intermediate value (| M _m |) cross between the inversions. A method according to claim 26, characterized in that the time intervals (TP) of an inversion which converts the longitudinal magnetization in the B ₀ -parallel direction to the subsequent inversion are larger on average than the time intervals (TA) of one each the longitudinal magnetization Inverting in the B ₀ -antiparallel direction until the subsequent inversion. A method according to claim 27, characterized in that this relationship (TP / TA) of the larger time intervals to the smaller intervals over the Consequence of the inversions is constant. Method according to claim 27 or 28, characterized in that that about the Consequence of the inversions the longer time intervals (TP) are the same and the smaller time intervals (TA) are the same. Device for acquiring data according to the method according to one of Claims 1 to 29 with the following devices: a device ( 11 . 21 ) for generating a stationary longitudinal magnetic field B ₀ in the object area to be examined, a device ( 15 . 23 ) for generating RF pulses that act on the object area, devices ( 12 . 13 . 14 ) for generating magnetic field gradients in different spatial directions, a device ( 40 . 50 ) to read out the magnetic resonance signals emanating from the object area and to write them into k-space, and a control device ( 30 ) for controlling the device generating the RF pulses and the devices generating the magnetic field gradient and the readout device.