Method and apparatus for triggering magnetic resonance recordings with object movements

Abstract

High-quality magnetic resonance (MR) recordings are triggered with movements of an object, for example the heartbeat. In a method and apparatus for obtaining raw data reconstruction for an MR image, a spin-echo-based sequence is executed that includes applying a static magnetic field and applying a magnetization pulse train. A movement of the object to be imaged is detected and a target contrast for two tissue types of the object is prespecified. The repetition time of the pulse train is set in dependence on the movement of the object to be imaged, and the flip angle is set such that prespecified target contrast for the two tissue types is obtained at the set repetition time.

Claims

1. A method for obtaining raw data for reconstructing a magnetic resonance (MR) image, comprising: operating an MR data acquisition scanner to apply a static magnetic field in a positive z-direction in the scanner, which produces magnetization in the positive z- direction in an object situated in the scanner; operating said MR data acquisition scanner so as to execute a spin-echo-based MR data acquisition sequence including applying an excitation pulse that tilts said magnetization by a predetermined angle; in said spin-echo-based sequence, applying a refocusing pulse; operating said MR data acquisition scanner to additionally apply an RF pulse at a time of an echo caused by said excitation and refocusing pulses, which deflects said magnetization in a negative z-direction by a flip angle; detecting a movement of said object using a motion detector; providing a computer with a predetermined target contrast for two tissue types in said object; in said computer, setting a repetition time of said excitation pulse in said spin- echo-based sequence based on the detected movement of the object; and in said computer, setting the flip angle in said spin-echo-based sequence so that said predetermined target contrast for said two tissue types is obtained at the repetition time that has been set, wherein the flip angle is dynamically adapted based on the detected movement of the object.

2. A method as claimed in claim 1 comprising calculating a contrast for a predetermined model repetition time, using specific T1 and T2 values respectively for said two tissue types, the calculated contrast being provided as the predetermined target contrast.

3. A method as claimed in claim 1 wherein the detected movement of the object is a respiration-produced movement of the object.

4. A method as claimed in claim 1 wherein the detected movement of the object is a heartbeat-produced movement of the object.

5. A method as claimed in claim 4 wherein the heartbeat-produced movement is a heartbeat obtained by an electrocardiogram (ECG), or by detecting a pulse of said object.

6. A method as claimed in claim 4 wherein the repetition time is determined based on a mean time between two consecutive heartbeats, in a plurality of heartbeat intervals.

7. A method as claimed in claim 1 comprising specifying, as said two tissue types of the object, white brain tissue and gray brain tissue, and wherein said contrast is a gray-white contrast.

8. A method as claimed in claim 1 comprising, after said excitation pulse, preceding said refocusing pulse with at least one further refocusing pulse.

9. A method as claimed in claim 1 comprising setting said predetermined angle by which said magnetization is tilted by said excitation pulse to be within a range of 45° to 135°.

10. A method as claimed in claim 1 comprising tilting said magnetization by 180° or less with said refocusing pulse.

11. A method as claimed in claim 1 comprising, after said excitation pulse, preceding said refocusing pulse with at least one further refocusing pulse, and tilting said magnetization by 180° or less with said refocusing pulse or said at least one further refocusing pulse.

12. A method as claimed in claim 1 wherein the repetition time of said excitation pulse in said spin-echo-based sequence is set based on a cycle time of the detected movement of the object.

13. A magnetic resonance (MR) apparatus comprising: an MR data acquisition scanner; a computer configured to operate said MR data acquisition scanner: so as to apply a static magnetic field in a positive z-direction in the scanner, which produces magnetization in the positive z-direction in an object situated in the scanner; so as to execute a spin-echo-based MR data acquisition sequence including applying an excitation pulse that tilts said magnetization by a predetermined angle; in said spin-echo-based sequence so as to apply a refocusing pulse; and so as to additionally apply an RF pulse at a time of an echo caused by said excitation and refocusing pulses, which deflects said magnetization in a negative z-direction by a flip angle; and a motion detector configured to detect a movement of said object and provide a signal to said computer that represents the detected movement, wherein: the computer is provided with a predetermined target contrast for two tissue types in said object; and the computer is further configured to: set a repetition time of said excitation pulse in said spin-echo-based sequence dependent on the detected movement of the object; and set the flip angle in said spin-echo-based sequence so that said predetermined target contrast for said two tissue types is obtained at the repetition time that has been set, wherein the flip angle is dynamically adapted based on the detected movement of the object.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a pulse sequence according to the present invention with a settable flip angle.

(2) FIG. 2 is a flowchart of the method according to the invention.

(3) FIG. 3 is a block diagram of an MR apparatus according to the invention.

(4) FIG. 4 is a diagram of the flip angle versus the repetition time TR for maximum gray-white contrast.

(5) FIG. 5 is a diagram of the gray-white contrast versus the repetition time TR, wherein the flip angle is changed such that the contrast remains constant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) By way of example, brain tissue is to be examined in an MR system. However, it is also possible to examine other tissue types. Spin-echo-based, T1-weighted imaging is selected, for example. The following examples relate to turbo-spin-echo techniques, but they can also be implemented analogously with simple spin-echo techniques.

(7) According to FIG. 1, a spin ensemble of an atom nucleus, and hence the magnetization 1 of an affected object, is first aligned in the z-direction. To this end, a static magnetic field is applied to the object in the z-direction (longitudinal direction). At a point in time t0, a (90°) excitation pulse α1 is applied to the object. As a result, the magnetization is tilted by a predetermined (settable) angle, which can be 90°, or also more or less than this. Hence, at least one component of the corresponding magnetization vector is in the x-y plane. This results in magnetization 2 tilted by 90°. This tilted magnetization 2 is now perpendicular to the direction of the static magnetic field. The individual magnetization vectors of the tilted magnetization 2 now diverge clockwise and counterclockwise about the z-axis. Herein, the individual magnetization vectors have different speeds. The magnetization vectors are dephased.

(8) In order to now achieve rephasing or refocusing, one or more refocusing pulses α2, α3, . . . , αn are applied. The effect of such refocusing is depicted in FIG. 1 for the refocusing pulse α3 specifically for an individual magnetization vector 3. Here, this is tilted by 180° for example about the y-axis, as a result of which the magnetization vector 4 tilted by 180° is obtained in the x-y plane. The tilting can also take place with a different angle, but then a corresponding component is also obtained in the x-y plane. This tilted or refocused component then returns to the starting position, here the y-axis corresponding to the tilted magnetization 2 after the (90°) excitation pulse. All other spins in the x-y plane are also tilted by 180° or the corresponding angle and also return at their speed to the starting position. Hence, the refocusing pulse causes synchronization of the individual magnetization vectors since at a point in time t2 these meet again in said starting position and herein generate the so-called spin echo. Therefore, the point in time t2 corresponds to the echo time. The echo time TE represents a time interval between the time t0 of the (90°) excitation pulse α1 and the echo time t2. The point in time t1 of the refocusing pulse α2 lies exactly between the two point in times t0 and t2, since, after the refocusing, the magnetization vectors again require exactly the same time for rephasing as for dephasing.

(9) In the present example, the further refocusing pulses α3 and an occur at the point in times t3 and tn. The number of refocusing pulses can be selected as required. However, it must be at least one.

(10) The last echo of the TSE echo train occurs with a time interval of TE/2 after the time tn, i.e. at the point in time tn+1. Now, according to the invention, at exactly this point in time, an RF pulse αn+1 is applied which deflects the remaining transverse magnetization 5, which has decreased slightly compared to the tilted magnetization 2 due to losses, toward the negative z-axis as a result of which flipped magnetization 6 is obtained. The flip angle φ between the remaining transverse magnetization 5 and the flipped magnetization 6 can be set by the RF pulse αn+1, which can also be referred to as the “after-train pulse”

(11) Therefore, the “after-train pulse” can achieve contrast modification. This is in particular particularly useful if the repetition time is based on movements of the object or patient. Without the “after-train pulse”, the contrast of the MR images (for example gray-white contrast between gray and white brain tissue) would substantially only change due to the movement or repetition time. With the “after-train pulse”, the contrast can also be changed by the flip angle ϕ.

(12) FIG. 2 shows a corresponding method sequence in which the repetition time is triggered by a movement of the object and a specific target contrast is to be achieved. In a first method step S1, a static magnetic field is applied to the object in the positive z-direction so that magnetization in the positive z-direction occurs in the object to be imaged. In addition, to obtain a measuring signal for an MR image for spin-echo-based imaging, a pulse train is applied according to step S2. Herein, first, a (90°) excitation pulse is applied as a result of which the magnetization of the nuclei in the object is tilted by a predetermined angle, for example 90 degrees. After a predetermined time, a refocusing pulse is applied. At the same time after the refocusing pulse, an RF pulse is applied at the point in time of an echo caused by the pulses, as a result of which the magnetization is deflected in the negative z-direction by a predetermined flip angle.

(13) Simultaneously with or in close temporal relation to the application of the pulse train in step S2, there is a detection of a movement of the object to be imaged in step S3. Although, in FIG. 2, the step S3 is shown as after step S2, this step can also take place before step S2 or simultaneously with step S2. In this step S3, for example the movement of the object or a part of the object due to the heartbeat, respiration, eye movement etc. is identified by suitable sensors.

(14) In a further step S4, a target contrast for two tissue types of the object is prespecified. Although in the example in FIG. 2, this step S4 takes place after step S3, it can also take place at the beginning of the method or before or after the above steps S1 to S3 or simultaneously therewith. On the prespecification of the target contrast, a corresponding value can, for example, be read from a table and used for the method. Alternatively, however, the target contrast can be prespecified by calculating it in advance for a specific tissue pair in a specific way.

(15) In a subsequent step S5, a repetition time TR of the (90°) excitation pulse is set in dependence on the movement of the object to be imaged. Therefore, the repetition time is, for example, triggered with the movement of the object to be imaged or with synchronized therewith.

(16) In a subsequent step S6, the flip angle is set such that the prespecified target contrast from step S4 is obtained for the two tissue types at the set repetition time. The setting of the flip angle can be performed once or tracked dynamically. If, for example, the repetition time changes constantly, for example with changes to the patient's pulse rate, the flip angle can be correspondingly tracked dynamically so that the contrast can be retained. Finally, the set flip angle is used again to apply the pulse train including the “after-train pulse” in step S2.

(17) FIG. 3 is a block diagram of an MR apparatus 6. The MR apparatus 6 has an MR data acquisition scanner 7 and a motion detector 8. Optionally, the scanner 7 and the motion detector 8 can be integrated in a common housing. The scanner 7 performs the actual MR examination. It has a first magnetization arrangement 9 and a second magnetization arrangement 10. The first magnetization arrangement 9 applies the static magnetic field and the second magnetization arrangement 10 applies the pulse train.

(18) The motion detector 8 is, for example, an ECG device or a pulsimeter. However, the motion detector 8 can also be any other types of sensor for detecting a movement of an object or partial object, such as respiration, eye movement, etc. A signal from the motion detector 8 is provided to the second magnetization arrangement 10 in order to trigger the pulse train or set the repetition time. In dependence on this, the second magnetization arrangement 10 optionally automatically sets the flip angle for a predetermined contrast of a tissue pair.

(19) The optimum flip angle φ for each TR that results in maximum contrast is shown in FIG. 3 by the curve 11. It rises steadily after about 650 ms from the flip angle equal to 0° and, after about 1700 ms, reaches the flip angle 90°, which is optimum here. While, therefore, the gray-white contrast for increasing TR greater than 1100 ms conventionally decreases (see FIG. 5, curve 12), with an increasing “after-train” flip angle φ, the gray-white contrast can be further increased.

(20) For example, to calculate the optimum “after-train” flip angle, it is for example possible to calculate the signals for the TSE sequence of the tissue of interest or the tissue pair of interest by means of known algorithms, for example for all integer flip angles from 0 to 90°. Such an algorithm can be a so-called “phase-graph” algorithm or another algorithm for solving the Bloch equation. The signal amplitudes for the different tissues can be used to calculate the contrast and identify the flip angle φ resulting in the maximum contrast.

(21) In many cases, it is not important always to achieve the maximum contrast. Rather, it may also be desirable to keep a contrast constant when the repetition time TR changes. For example, the repetition time may be extended to reduce the stress on the patient or reduce energy consumption, while at the same time the contrast is to be retained. In such a case, the above-described calculation of the optimum “after-train” flip angle can be used to determine the flip angle for a specific TR that generates a contrast that is as similar as possible to the contrast that is generated at another TR (and possibly another flip angle or flip angle 0).

(22) To this end, FIG. 5 shows the gray-white contrast, which represents a measuring signal to be obtained, over the repetition time TR of about 500 to 2000 ms. For purposes of comparison, a curve 12 for a conventional TSE sequence without an “after-train pulse” is depicted. Over the repetition time, as described above, the contrast K, first increases and then falls off again. The curve 13 shows the gray-white contrast for a TSE-sequence with an “after-train pulse”, wherein the contrast with different flip angles φ can be kept substantially constant over specific range of TR (here about 900 to 2000 ms). Therefore, the flip angle φ will always be selected in dependence on the TR such that a target contrast Kz is achieved.

(23) The prespecification of the contrast can thus be in order to keep the contrast constant at a specific value, for example target contrast Kz, independently of the heartbeat. In this case, the flip angle is then adapted to the repetition time TR synchronized with the heartbeat such that, for example, the gray-white contrast remains constant. According to the invention, therefore, the contrast modification can be used by means of the flip angle, for example, to enable ECG-triggered recordings without thereby the T1-contrast being (exclusively) defined by the time between two heartbeats. A concrete method sequence can be summarized once again as follows:

(24) 1. A prespecified TR is used to calculate the contrast between two tissue types (for example gray and white brain tissue) for the corresponding frequency, for example on the basis of the tissue-specific T1 and T2 values. This defines the target contrast.

(25) 2. An ECG or pulse measurement is used to determine the mean duration between two heartbeats. This duration defines the TR* actually implemented in a triggered recording.

(26) 3. The flip angle of the “after-train pulse” is determined such that, at the mean interval between two heartbeats TR*, the contrast between the two predetermined tissue types corresponds to that achieved at the set TR without an additional “after-train pulse”.

(27) 4. A feedback loop can be used to adapt the flip angle gradually if the TR* changes over the duration of the measurement. Herein, excessively large jumps in the flip angle should be avoided in order not to disturb the T1 steady state. Tracked or rolled averaging over a plurality of heartbeat intervals is recommended.

(28) Advantageously, therefore, the invention, enables T1-weighted TSE recordings to be obtained, wherein the T1 contrast (between two predefined tissue types) is prespecified independently of the patient's heartbeat by a selectable target TR.

(29) Hence, according to the invention, it is possible to provide an MR device with a first magnetization arrangement used to apply the static magnetic field and a second magnetization arrangement embodied to apply the SE or TSE sequence including the “after-train pulse” with the specific flip angle triggered with the measuring arrangement. This specific flip angle is set in accordance with the above specifications.

(30) Although modifications and changes may be suggested by those skilled in the art, it is the intention of the Applicant to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of the Applicant's contribution to the art.