DE102013005612A1 - Method for operating a magnetic resonance tomograph - Google Patents

Abstract

The invention relates to a method for operating a magnetic resonance tomograph for spatially resolved spin resonance measurement on an object, in particular on a living object, which is arranged in a static magnetic field B0, which results in an alignment of the spins of the object and a longitudinal net magnetization Mz along the direction Z of the static Magnetic field results and a transverse magnetization component Mxy is generated or changed, in particular by generating a spin flip from the Z direction by a desired flip angle, in particular by 90 degrees by means of at least one high-frequency excitation pulse (1) in spin Resonance, with a sequence of high-frequency pulses (4, 5, 7) and switched gradient magnetic fields (2, 3, 6) spin echo signals from at least one desired volume element of the object are measured, the at least one volume element by several at least temporarily the homogeneous Magnetf Eld B0 superimposed gradient magnetic fields is determined, the sequence of high-frequency pulses comprising a first (4) and a second high-frequency rephasing pulse (5), in particular a first (4) and second high-frequency rephasing pulse (5) directly after generation of a transverse magnetization component by means of a high-frequency excitation pulse (1), wherein from the second high-frequency rephasing pulse (5) both a spin echo and a stimulated echo are generated, in addition to a gradient magnetic field (6) in the frequency coding direction, which during the measured value acquisition for Detection of a line of the raw data space is switched, an additional gradient magnetic field (8) is switched in the frequency coding direction, which is generated by the high-frequency rephasing pulses (5, 7) newly generated echo signals or echo signal components, in particular those that do not refocus the original one Spin echoes after the first high frequency rephasing simpuls (4) and / or the original stimulated echo after the second high-frequency rephasing pulse (5) can be traced, from which the raw data space or raw data line acquired during the next measurement recording are shifted out.

Description

The invention relates to a method for operating a magnetic resonance tomograph for spatially resolved spin resonance measurement on an object, in particular on a living object, which is arranged in a static magnetic field B0, whereby an orientation of the spins of the object and a longitudinal net magnetization Mz along the direction Z of the static Magnetic field results and wherein a transverse magnetization component Mxy is generated or changed, in particular by generating a spin flip from the Z direction by a desired flip angle, in particular by 90 degrees by means of at least one high frequency excitation pulse in spin resonance, wherein With a sequence of high-frequency pulses and switched gradient magnetic fields, spin-echo signals from at least one desired volume element of the object are detected by measurement, wherein the at least one volume element is superimposed by a plurality of degrees at least temporarily superimposed on the homogeneous magnetic field B0 ientenmagnetfelder is determined and the sequence of high-frequency pulses comprises a first and a second high-frequency Rephasierungsimpuls, in particular a first and second high-frequency Rephasierungsimpuls directly after generating a transverse magnetization component by means of a high-frequency excitation pulse, starting from the second high-frequency Rephasierungsimpuls both a spin echo as well as a stimulated echo is generated.

Magnetic resonance imaging is a cross-sectional imaging technique that can image organs or parts of the body to be examined without the use of X-rays or radioactive substances.

For example, anatomical images that claim to be very detailed are usually generated by means of a fast spin-echo technique (English: FSE, fast-spin echo). In this case, a train of several high-frequency Rephasierungsimpulsn is applied after an excitation pulse. As a result, the magnetization initially excited is multiply refocused and stored in the raw data space (k-space) with the signal decaying by means of repeated spatial coding (technical: phase coding). FSE sequences can be used both as single-shot and as segmented images. The high-frequency rephasing pulses are ideally formed here as so-called 180-degree pulses and assumed as such in the theoretical consideration.

By means of the appropriate choice of sequence parameters, here mainly the echo time TE, an FSE sequence can either generate T ₂ weighted (magnetization relaxation transverse time) or proton density (PD) weighted images or combined with any other known contrast preparation module of the MRI. _T2-weighted FSE sequences play a major role in the detection of acute infarction tissue in the brain or heart muscle.

A modified FSE sequence - z. For example, in the implementation of displaced UFLARE, the magnetization imposes a sensitivity to microscopic field inhomogeneities (T ₂ ^* weighting) by means of an additionally inserted evolution time τ. These field inhomogeneities occur at boundaries of different tissues (susceptibility jumps) or may be physiological - e.g. B. Increased iron concentration caused by hemoglobin in the blood, or also by changes in tissue morphology due to scarring, results in an irreversible loss of the phase relationship of the magnetization. Thus, this sequence offers the opportunity to study the oxygen content of blood or the iron content of tissue or the detection of heart attacks. T ₂ ^* weighted imaging has a wide range of applications: dynamic imaging after contrast administration, functional MRI or T ₂ ^* cardiac muscle mapping.

In most applications, per image a certain previously modulated contrast - such. B. PD, T ₂ - or T ₂ ^* weighting - generated. If, as in many cases, one is interested in several different MRI contrasts, the corresponding images are generated sequentially - sequentially - in different image recordings - also called sequences. This takes a long time and can lead to misregistration of image layers.

The mapping of certain tissue parameters such as T ₂ or T ₂ ^* is of great interest to MRI. A parameter map is calculated from a series of measurements in which the respective parameter (eg T ₂ or T ₂ ^* ) is successively changed. The individual measurements have a high spatial resolution, so that the parameter maps are very accurate. This allows conclusions to be drawn about the microstructure of the examined tissue.

A disadvantage of this technique is that it is very time consuming due to the number of measurements and their resolution. In many cases, several parameters are mapped one after the other and thus the time factor plays an even bigger role.

Therefore, an acceleration of parameter mapping is of great interest for clinical use.

In addition, the problem of layer misregistration often occurs when mapping is performed sequentially. As a result, maps of different parameters are not comparable.

The object of the invention is thus to provide a method for operating a magnetic resonance tomograph, by means of which within a sequence and thus a sequence of switched high-frequency pulses and gradient magnetic fields to record in the raw data recorded multiple contrast types, in particular T2 * -weighted data and another contrast, for example T2-weighted data or a weighting according to the proton density. In accordance with the invention overlays of successively generated echoes are to be avoided.

This object is achieved in that, in addition to a gradient magnetic field in the frequency coding, which is switched during the acquisition of data to detect a row of raw data space, an additional gradient magnetic field is switched in Frequenzkodierrichtung, which by the high frequency rephasing pulses newly generated echo signals or echo components in particular, the magnetization components not originating from a refocusing of the magnetization components originally produced after the second high-frequency rephasing pulse are expelled, from which the raw data space or the raw data row acquired during a next measured value recording are moved out.

The subject of the invention can be used, for example, in conjunction with the fast spin-echo technique (English: FSE, fast-spin echo), according to which follow the two aforementioned high-frequency rephasing pulses further equidistant high-frequency rephasing pulses.

The signal generated by FSE techniques is usually a superposition of spin echoes and stimulated echoes, which are also referred to below as SE and STE, respectively.

The theory of the FSE technique describes the application of multiple 180-degree high-frequency rephasing pulses after a 90-degree excitation to multiply reverse dephasing the magnetization component previously tilted in the transverse plane to produce multiple echo signals. Practically, however, pure 180-degree high-frequency rephasing pulses can not be realized, so that a high-frequency rephasing pulses usually always tilt back a part of the transverse magnetization from the transverse plane in the longitudinal Z direction.

Further, the theory describes the generation of stimulated echoes upon excitation of transverse magnetization by two consecutive 90 degree RF pulses, the first of which flips one component of the precessing transverse magnetization back in the Z direction and leaves another component in the transverse plane. The second pulse tilts the Z-component with the same effective direction as before again into the transverse plane and generates a so-called stimulated echo. However, even pure 90-degree pulses can not be generated in the metrology practice.

Thus, a high frequency rephasing pulse, often referred to simply as an ALPHA pulse, may be mathematically considered to be a linear combination of a 180 degree component, a 90 degree component, and a 0 degree component, and thus always generates one when applied to transverse magnetization Subcomponent which is rotated in the transverse plane by 180 degrees and a subcomponent of the magnetization, which is tilted back in the Z direction. In the FSE technique, therefore, there occur practically superposed, at least after the second high-frequency rephasing pulse SE and STE.

The excitation pulse tilts the magnetization initially stored in the longitudinal axis into the transverse plane. By means of the first high-frequency rephasing pulse, the magnetization is divided. One part remains in the transverse plane and already generates an SE, another part is brought back into the longitudinal axis and stored there until the next high-frequency rephasing pulse acts. This brings the stored portion of the magnetization back into the transverse plane, where it forms a STE.

At the same time, the re-refocusing of the first SE again produces an SE superimposed on the STE. Depending on the number of intervals between the high-frequency rephasing pulses (inter-echo intervals), which the echo-forming magnetization was already in the transverse plane, one speaks of odd (E _u ) and even (E _g ) echoes. As SEs and STEs overlap, even odd and even echoes (interference) mix and contribute to the resulting image.

In the present method, a gradient magnetic field in the frequency encoding direction switched between the first and second high frequency rephasing pulses and a gradient magnetic field in the frequency coding direction connected before the first high frequency rephasing pulse first causes the pulse to be detected from the second inter-echo interval at the time of the measured value acquisition a separation of the spin echo and the stimulated echo in a detected line of the raw data space takes place.

The invention makes use of the fact that different phase shifts act on the SE and the STE by the mentioned gradient magnetic fields in the frequency coding direction before and after the first high-frequency rephasing pulse, which transfer to the following measured value captures, since in the meantime one of the high-frequency rephasing pulses has a magnetization component in FIG the Z direction is tilted back and thus remains unaffected by the gradient in the frequency coding direction after the first high-frequency rephasing pulse, which thus acts only on the transverse component. In the case of later switched gradients in the frequency coding direction, this results in a separation of the two types of echo within a row of the raw data space, which is determined by the phase encoding gradients connected in each case during the recording of the measured value of the echoes.

A phase shift which deviates from 50% of the phase shift, in particular corresponds to 25% of the phase shift which is caused by the gradient magnetic field connected between the first and second high-frequency rephasing pulses in the frequency-encoding direction, is preferably effected with the gradient magnetic field switched in the frequency coding direction before the first high-frequency rephasing pulse ,

In this case, the setting of the 25% effect is particularly preferred since, as a result, the data portions of both types of echo are uniformly distributed around the center of the line within a row of the data space, which offers advantages in the later evaluation and Fourier transformation. On the basis of the 25% effect of the phase shift of the first frequency-encoding gradient (after the excitation pulse), the separation effected can also be explained clearly:
Thus, on the magnetization prepared transversally with the excitation pulse (with initial phase zero), the frequency encoding gradient first, which causes 0.25 of the phase shift of the later Frequenzkodiergradienten. This magnetization is folded back partly by the first high frequency rephasing pulse in the Z direction. This part retains its 0.25-fold phase shift. The transversal remaining portion is phase-shifted by 180 degrees by the same pulse, thus changing its phase to -0.25. By the Frequenzkodiergradienten after the first high frequency rephasing pulse thus a line of the raw data space is detected, in which the generated spin echo is formed at the position 0.25, since there the prepared phase of -0.25 has converged to zero and generates the echo. However, the same frequency-coding gradient then effects its effect on this magnetization component by a further phase shift of 0.75 and thus at the 0.75 position with respect to the abscissa of the raw data space. The next spin echo of this magnetization would therefore occur at the position 0.75 of a row of the raw data space, since this phase is then prepared by the 180-degree effect of the following high-frequency rephasing pulse before data acquisition to a phase of -0.75.

The second high frequency rephasing pulse retrieves the magnetization previously folded in the Z direction, but with the same effective direction as before, thus also producing an effective 180 degree phase shift, so that this retrieved magnetization is at -0.25 with respect to the abscissa of the raw data space is prepared with a Frequenzkodiergradienten in the measured value acquisition then the STE at the position 0.25 of the selected raw data space line is formed and as previously mentioned the SE at the position 0.75. Accordingly, the measurement data of both types of echo are centered around the center of the raw data line, i. H. at the positions 0.25 and 0.75 of the raw data space with respect to the abscissa of the room.

According to the invention it can now be provided to detect different weights or contrasts with the two types of echo.

For this, an embodiment may provide that the spin echoes detect a T2 * weighting.

According to the invention, provision may be made in an evolution time between the first and second high-frequency rephasing pulses for a transverse magnetization component T2 * -related dephasing processes after the first high-frequency rephasing pulse, wherein a magnetization component of the first high-frequency rephasing pulse in the longitudinal direction thereof remains untouched, wherein the absolute time between the first and second high frequency rephasing pulses is prolonged by the evolution time beyond the inter-echo interval present between subsequent equidistant high frequency rephasing pulses, in particular 180 degree high frequency rephasing pulses.

It can thus be carried out series of measurements in which different evolution times are set from zero.

The clear separation of the two types of echoes produced in accordance with the invention is exploited in order to obtain a different weighting and thus a different one in a suitable preparation experiment to generate different image contrast. The insertion of an evolution time tau in the first inter-echo interval leads to a T ₂ ^* weighting of the SE magnetization.

Since the portion of the magnetization which will form the STE is stored in the longitudinal axis during this time, it remains unaffected by the T ₂ ^* weighting, and accordingly can have a different weighting, e.g., T ₂ ^* weighting of the SE magnetization, e.g. B. imprinted a T ₂ weighting or proton density weight. Thus, the proposed approach allows independent contrasting of the SE and STE magnetizations.

According to the invention, only the repeated refocussing of the first jointly occurring pair of SE and STE is used for signal generation, i. H. the pair of second spin echo and first stimulated echo. Accordingly, the spin echo generated after the first high-frequency rephasing pulse can be rejected by measurement.

As essential and independent of the type of spin echo and stimulated echo weighting chosen, it is considered in the invention that stimulated echoes arising from later high frequency rephasing pulses be shifted out of the acquisition window by additional gradients along the reading direction. Thus, it is possible to acquire pure SE and pure STE.

According to the invention, it is provided for this purpose that, in addition to one or each gradient magnetic field in the frequency coding direction, which is switched during the measured value detection for detecting a line of the raw data space, in particular from (including) that gradient magnetic field in frequency encoding direction which is switched after the second high frequency rephasing pulse, an additional gradient magnetic field is switched in the frequency coding direction, which is due to the high frequency rephasing pulses newly generated echo signals or echo components, in particular those not caused by a refocusing of the magnetization components originally generated after the second and / or first high frequency rephasing pulse, from the a raw data space or the acquired raw data space line acquired.

A gradient magnetic field in the frequency encoding direction, which is switched during the acquisition of the measured value to detect one row of the raw data space, is essentially produced during the period of an echo interval, ie the time between two high-frequency rephasing pulses, in particular 180-degree pulses. A supplementary switching of said additional gradient magnetic field preferably means that both gradient magnetic fields adjoin one another in terms of time, in particular not overlapping in time. The adjoining can preferably be done without a time interval. Switching of the additional gradient magnetic field also occurs within an echo interval between two high frequency rephasing pulses.

It can be provided that successive additional gradient magnetic fields are switched alternately before and after the gradient magnetic field in frequency encoding for detecting a row of raw data space and thereby cancel two consecutive additional gradient magnetic fields in their effect on the remaining after a high frequency rephasing pulse magnetization component.

In connection with the evaluation and a Fourier transformation to be carried out, the invention can furthermore provide that partial data sets are formed before each evaluation of the measured values recorded in the raw data space, which comprise both types of echo, each comprising only one of the two echo types.

Both partial data sets can then be processed separately to produce image representations.

During the processing of the raw data, it may further be provided that, in the case of some of the lines of the raw data space, in particular of every second intererecho interval, the data portions are interchanged on both sides of the middle of the line. Since the separation of the echo types in successive echoes and the position of the echo types is interchanged in successive Interechointervallen, can be made so that one of the echo types z. B. always left of the middle of the line and another type is always right of the middle of the line. Sub-datasets can then be generated in a simple manner by dividing the raw data space in the middle.

Unless a uniform positioning of the data of both types of echo around a respective middle of the line has taken place with the selection of the first two frequency encoding gradients described above, a further development can additionally provide that an educated partial data set is newly centered in the raw data space prior to the evaluation.

An embodiment will be described with reference to the drawings.

shows in practical application an RF excitation pulse 1 with a 90 degree effect, thus ideally folding the existing original longitudinal magnetization into the transverse XY plane, as known in the art.

It becomes a gradient magnetic field thereafter 2 in preparation for the frequency coding and thus switched in the reading direction of a raw data line, which according to the lower diagram in the the phase of transverse magnetization changes. Here is the gradient magnetic field 2 in its effect, for example, to 0.25 the effect of the subsequent gradient magnetic field 3 selected.

This is followed in chronological order by a first high-frequency rephasing pulse 4 which folds back a part of the magnetization in the Z direction and whose phase position according to the dashed line remains in time. By contrast, a transversal component is shifted by 180 degrees to negative phase position as visualized by the solid line.

Between the first high-frequency rephasing pulse 4 and the second high frequency rephasing pulse 5 becomes a gradient magnetic field 3 in the reading direction, ie switched in the frequency encoding, during the action of a first metrologically discarded spin echo is generated and the phase position of the spin echo-generating magnetization changes to the positive of, for example, here -0.25 to 0.75. Through this gradient magnetic field 3 the magnetization stored in Z direction (dashed) remains unaffected.

By the second high-frequency rephasing pulse 5 Both the magnetization from the Z-direction is tilted back into the transverse plane and shifted phase by 180 degrees, so also generated within the transverse plane a 180 degree phase shift of the remaining there magnetization component.

At the following gradient magnetic field 6 For the acquisition of a raw data line, both a stimulated echo at position 0.25 and a spin echo at position 0.75 of the raw data line are now generated, as illustrated by the separated zero crossings of the phase positions.

To take account of a T2 * weighting, an evolution time tau is inserted here between the first and second high-frequency rephasing pulses, which is the absolute time interval between the two high-frequency rephasing pulses 4 and 5 extended beyond the inter-echo interval, ie the time interval between the later high-frequency rephasing pulses 7 ,

Because every high-frequency rephasing pulse 5 , respectively. 7 In turn, due to imperfections generated in turn magnetization components in the Z direction, which lead to echoes that do not go back to a refocusing of magnetizations of the initially generated SE and STE, it is provided here according to the invention, in each case at least one additional gradient magnetic fields 8th in frequency encoding direction before or after a respective gradient magnetic field 6 to switch, with such an additional gradient magnetic field 8th the parts folded back from the Z direction in the transverse direction are shifted in the phase position such that they do not occur in a recorded line of the raw data space. Successive gradient magnetic fields 8th are switched so that they cancel each other out with respect to their effect on magnetization remaining in the atversal plane.

further shows that the SE and STE change their respective positions from one inter-echo interval to the next. Therefore, before the reconstruction of the stored raw data, resorting of the k-space is performed as it is shows. This ensures that only SEs are in the right half and only STEs in the left half of the acquisition window or vice versa. Since even and odd echoes are mixed with each other and these can differ in their phase, preferably a phase correction can be made. Both k-space parts can form partial datasets, which can then be Fourier-transformed separately, resulting in two images. The image which is reconstructed from SEs is T ₂ ^* weighted, and the other is T ₂ or proton density weighted.

Since the proposed approach produces two images from one FSE sequence, this solution can be termed a 2-in-1 FSE technique.

The described preparation module 90 ° -α-τ-α, which only weights a selected part of the magnetization T ₂ ^* , is not limited to the combination with the FSE technique. It can also be used in conjunction with other imaging techniques such as echo planar imaging (EPI), gradient and spin-echo (GRASE), or gradient echo techniques as a preparation experiment.

It is also conceptually feasible to use non-Cartesian phase encoding, such as the implementation of a radial k-space trajectory. In addition, the initial excitation pulse may be replaced by other spin preparation modules that generate transverse magnetization.

Conceptually, it is also possible to introduce a different preparation for the SE fraction instead of the T ₂ ^* weighting by the evolution time τ. In particular, the use of diffusion gradients in the first inter-echo interval is of great interest. The proposed approach is not limited to imaging but also applies to NMR and MR based spectroscopy, including spectroscopic imaging techniques.

As a further embodiment, the invention may be used to simultaneously generate a proton density weighted (PD) and a T ₂ ^* weighted image of an object, e.g. As the brain can be used.

This is z. B. an evolution time τ = 15 ms inserted in the first Interechointervall. shows two resulting consecutive raw data lines and illustrates the different weights of the SEs and STEs as well as changing their respective positions.

After re-sorting all k-space lines (see ), both k-space halves are reconstructed separately. A Fourier transformation of both parts produces one image each.

The acquisition of a high-resolution layer of the brain using the FSE technique takes about 5 minutes, so taking a PD and a T ₂ ^* weighted image with the standard techniques takes a total of 10 min. With the invention, the time of data acquisition is reduced to 5 min.

The use of the proposed technique is not limited to imaging the brain, but can be extended to other target regions. Among other things, the imaging of the heart is of great interest.

Claims (9)

A method for operating a magnetic resonance tomograph for spatially resolved spin resonance measurement on an object, in particular on a living object, which is arranged in a static magnetic field B0, resulting in an alignment of the spins of the object and a longitudinal net magnetization Mz along the direction Z of the static magnetic field and a transverse magnetization component Mxy is generated or changed, in particular by generating a spin flip from the Z direction by a desired flip angle, in particular by 90 degrees, by means of at least one radio-frequency excitation pulse ( 1 ) in spin resonance, wherein a sequence of high-frequency pulses ( 4 . 5 . 7 ) and switched gradient magnetic fields ( 2 . 3 . 6 ), Wherein the at least one volume element is determined by a plurality of gradient magnetic fields superposed at least temporarily on the homogeneous magnetic field B0, the sequence of high-frequency pulses comprising a first ( 4 ) and a second high-frequency rephasing pulse ( 5 ), in particular a first ( 4 ) and second radio frequency rephasing pulse ( 5 ) directly after generation of a transverse magnetization component by means of a high-frequency excitation pulse ( 1 ), starting from the second high-frequency rephasing pulse ( 5 ) generates both a spin echo and a stimulated echo, characterized in that in addition to a gradient magnetic field ( 6 ) in Frequenzkodierrichtung, which is switched during the Meßwerterfassung for detecting a line of the raw data space, an additional gradient magnetic field ( 8th ) is switched in Frequenzkodierrichtung, which by the high-frequency rephasing pulses ( 5 . 7 ) newly generated echo signals or echo signal components, in particular those not based on a refocusing of the original spin echo after the first high-frequency rephasing pulse ( 4 ) and / or the original stimulated echo after the second high frequency rephasing pulse ( 5 ), are removed from the recorded in a next measured value raw data space or raw data line. Method according to claim 1, characterized in that successive additional gradient magnetic fields ( 8th ) alternately before and after the gradient magnetic field ( 6 ) in frequency encoding direction for detecting a row of the raw data space, in particular wherein two successive additional gradient magnetic fields ( 8th ) in their effect on that after a high-frequency rephasing pulse ( 5 . 7 ) cancel the remaining magnetization component transversally. Method according to claim 1 or 2, characterized in that in an evolution time (tau) between the first ( 4 ) and second radio frequency rephasing pulse ( 5 ) to one after the first high frequency rephasing pulse ( 4 Transverse magnetization component T2 * -related relaxation processes act and one after the first high-frequency rephasing pulse ( 4 ) in the longitudinal direction magnetization component thereof remains unaffected, in particular wherein the absolute time between the first ( 4 ) and second radio frequency rephasing pulse ( 5 ) is prolonged by the evolution time beyond the duration of an echo interval between successive equidistant radio frequency rephasing pulses ( 5 . 7 ) is present. Method according to one of the preceding claims, characterized in that the high-frequency rephasing pulses ( 7 ), which after the first ( 4 ) and second radio frequency rephasing pulse ( 5 ) can be set as 180 degree pulses. Method according to one of the preceding claims, characterized in that the first ( 4 ) and second high frequency rephasing pulses ( 5 ) are chosen identically, in particular each cause a magnetization rotation by less than 180 degrees and preferably not equal to 90 degrees. Method according to one of the preceding claims, characterized in that prior to an evaluation of the measured values stored in the raw data space, which comprise both types of echo, partial data sets are formed which each comprise only one of the two echo types. Method according to claim 6, characterized in that the raw data space at the ordinate is halved to form the partial data sets and either before halfing some of the lines of the raw data space interchange of data on both sides of the middle of the line or after halving some of the lines of the partial data sets between the Be exchanged partial datasets. A method according to claim 6 or 7, characterized in that a respective formed partial data set is re-centered in the raw data space before the evaluation. Method according to one of the preceding claims, characterized in that each pair of spin echo and stimulated echo at successive echo intervals is encoded by different selection of the phase encode gradient in another row of the raw data space, in particular wherein the raw data space starting from the ordinate origin alternately to both sides of the ordinate origin and is filled continuously to the edge of the raw data space.

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