Method for controlling patient stimulating effects in magnetic resonance imaging, corresponding computer program and computer-readable storage medium, and magnetic resonance imaging device

Abstract

Methods and systems for controlling patient stimulating effects in MR imaging. The methods and systems include calculating a first effective stimulus duration independently for each pulse flank of an MRI sequence individually and calculating a second effective stimulus duration for which a respective history of a changing gradient field during the sequence is taken into account. Dependent on an evaluation of both the first and second effective stimulus durations a threshold value for an allowable rate of change in the magnetic gradient field is then calculated. The respective MRI sequence is then evaluated against the calculated threshold value to determine whether or not the respective MRI sequence is safe to apply.

Claims

1. A method for controlling patient stimulating effects in magnetic resonance imaging due to a change in a changing magnetic gradient field, the method comprising: calculating, for an MRI sequence, a first effective stimulus duration independently for each monotonic gradient pulse flank of the MRI sequence individually; calculating, for the MRI sequence, a second effective stimulus duration for which at least along monotonic gradient pulse flanks of the MRI sequence a respective history of the changing magnetic gradient field during the MRI sequence up to a respective point in the MRI sequence is taken into account; and calculating, based on an evaluation of the first effective stimulus duration and the second effective stimulus duration, a threshold value for an allowable rate of change in the changing magnetic gradient field against which the MRI sequence is evaluated to determine whether or not the MRI sequence is safe to apply.

2. The method of claim 1, wherein the first effective stimulus duration is calculated by an algorithm that implements IEC 60601-2-33.

3. The method of claim 1, wherein the evaluation of the first and second effective stimulus durations comprises determining which one of the first and second effective stimulus durations is longer, and only the determined longer one of the first and second effective stimulus durations is used to calculate the threshold value.

4. The method of claim 1, wherein only a history of gradient activity inside a sliding time window of predetermined length extending back in time from a respective current point is used for calculating the second effective stimulus duration.

5. The method of claim 4, wherein the sliding time window includes a predetermined length between 3 ms and 300 ms.

6. The method of claim 4, wherein the sliding time window includes a predetermined length between 3 ms and 20 ms.

7. The method of claim 1, wherein calculating the second effective stimulus duration comprises using a weight function that assigns a lower weight to gradient activity that temporally precedes gradient activity that is temporally closer to the respective current gradient pulse flank or point in the sequence for which the second effective stimulus duration is calculated.

8. The method of claim 7, wherein the weight function comprises: assigning a constant non-zero weight to a first portion of a gradient activity history, and assigning to a second portion of the gradient activity history that temporally precedes the first portion weights according to a predetermined function that monotonically decreases in a direction back in time from the respective current point of the sequence.

9. The method of claim 8, wherein the predetermined function monotonically decreases to zero.

10. The method of claim 7, wherein the weight function at least for a portion of a gradient activity history assigns linearly decreasing weights according to a predetermined linear function that monotonically decreases going back in time.

11. The method of claim 7, wherein the weight function at least for a portion of a gradient activity history assigns exponentially decreasing weights according to a predetermined exponential function that exponentially decreases going back in time.

12. The method of claim 7, wherein the weight function assigns monotonically decreasing weights according to a predetermined step function with multiple steps that monotonically decrease going back in time.

13. The method of claim 1, wherein for calculating the second effective stimulation duration only absolute values of a time derivative of the gradient being greater than a predetermined threshold are taken into account.

14. The method of claim 1, wherein the respective threshold value is calculated for multiple spatial dimensions and a square root of a sum of the squares of ratios of a time derivative of the gradient activity for each spatial dimension to the corresponding threshold value is calculated to determine a relative stimulation probability for the sequence.

15. A non-transitory computer implemented storage medium that stores machine-readable instructions executable by at least one processor for controlling patient stimulating effects in magnetic resonance imaging due to a changing magnetic gradient field, the machine-readable instructions comprising: calculating, for an MRI sequence, a first effective stimulus duration independently for each monotonic gradient pulse flank of the MRI sequence individually; calculating, for the MRI sequence, a second effective stimulus duration for which at least along monotonic gradient pulse flanks of the MRI sequence a respective history of the changing magnetic gradient field during the MRI sequence up to a respective point in the MRI sequence is taken into account; and calculating, based on an evaluation of the first effective stimulus duration and the second effective stimulus duration, a threshold value for an allowable rate of change in the changing magnetic gradient field against which the MRI sequence is evaluated to determine whether or not the MRI sequence is safe to apply.

16. The non-transitory computer implemented storage medium of claim 15, wherein the evaluation of the first effective stimulus duration and the second effective stimulus duration comprises determining which one of the first and second effective stimulus durations is longer, and only the determined longer one of the first and second effective stimulus durations is then used to calculate the threshold value.

17. The non-transitory computer implemented storage medium of claim 15, wherein calculating the second effective stimulus duration comprises using a weight function that assigns a lower weight to gradient activity that temporally precedes gradient activity that is temporally closer to the respective current gradient pulse flank or point in the sequence for which the second effective stimulus duration is calculated.

18. A magnetic resonance imaging device comprising: an imaging system configured to acquire magnetic resonance data of a subject; a control unit configured to control the imaging system according to a predetermined imaging sequence; and a user interface over that the predetermined imaging sequence may be defined, wherein the control unit comprises a computer-readable storage medium and a processor configured to execute machine-readable instructions stored on the computer-readable storage medium, the machine-readable instructions comprising: calculating, for an MRI sequence, a first effective stimulus duration independently for each monotonic gradient pulse flank of the MRI sequence individually; calculating, for the MRI sequence, a second effective stimulus duration for which at least along monotonic gradient pulse flanks of the MRI sequence a respective history of the changing magnetic gradient field during the MRI sequence up to a respective point in the MRI sequence is taken into account; and calculating, based on an evaluation of the first effective stimulus duration and the second effective stimulus duration, a threshold value for an allowable rate of change in the changing magnetic gradient field against which the MRI sequence is evaluated to determine whether or not the MRI sequence is safe to apply.

19. The magnetic resonance imaging device of claim 18, wherein the evaluation of the first effective stimulus duration and the second effective stimulus duration comprises determining which one of the first and second effective stimulus durations is longer, and only the determined longer one of the first and second effective stimulus durations is then used to calculate the threshold value.

20. The magnetic resonance imaging device of claim 18, wherein calculating the second effective stimulus duration comprises using a weight function that assigns a lower weight to gradient activity that temporally precedes gradient activity that is temporally closer to the respective current gradient pulse flank or point in the sequence for which the second effective stimulus duration is calculated.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 depicts a magnetic resonance imaging device adapted to control a patient stimulation according to an embodiment.

(2) FIG. 2 depicts a flow chart for a method for controlling a patient stimulation in MRI imaging according to an embodiment.

DETAILED DESCRIPTION

(3) FIG. 1 depicts a magnetic resonance imaging device or MRI device 1 for short. The MRI device 1 may be used for imaging a patient 2, for example for acquiring MR data of the patient 2 and to generate a magnetic resonance image of the patient 2. The MRI device 1 includes an imaging system 3 that is here only indicated schematically. The imaging system 3 may for example include magnetic coils for generating a variable magnetic gradient field in a space occupied by the patient 2. The MRI device 1 further includes a control unit 4 for controlling the imaging system 3, for processing the MR data acquired by the imaging system three, and for generating a corresponding MR image of the patient 2. The control unit 4 may also be used for simulating an actual imaging process beforehand, for example, with the purpose of determining whether or not a proposed MRI sequence is safe for the patient 2 in terms of potentially stimulating effects, for example cardiac stimulation.

(4) The control unit 4 itself includes a processor 5 and connected thereto a memory 6. Stored on the memory 6 is a computer program that includes or encodes instructions that, when executed by the processor 5 or the control unit 4, respectively, cause the control unit 4 or the MRI device 1 as a whole to carry out the tasks and methods described herein. Corresponding results, such as a message, an input request, the generated MR image, and/or the like, may be output to a respective user by a schematically shown monitor 7 that may be part of the MRI device 1 or connected thereto.

(5) FIG. 2 depicts a flow chart 8 for a method for controlling a patient stimulation or patient stimulating effects in magnetic resonance imaging. The method may be carried out using the MRI device 1. Correspondingly, the mentioned computer program stored on the memory 6 may implement the method, i.e. the flowchart 8. The flowchart 8 includes multiple process steps S1 to S10 and multiple program path, only some of which are presently indicated as P1 to P7. The process steps and program path of the flowchart 8 may represent functions, routines, and/or instructions of the computer program stored on the memory 6.

(6) The problem of unwanted stimulations in MR imaging is known. For example, the IEC standard 60601-2-33 proposes limits for an electric field induced by gradient switching and/or for a maximum allowable rate of change of the magnetic field during a gradient switching based on an effective stimulus duration. Other approaches in dealing with patient stimulation include application of the so-called SAFE-model. Both of the approaches do, however, include their own problems and disadvantages.

(7) To remedy the problems and disadvantages, embodiments calculate and evaluate two different effective stimulus durations as will be described below.

(8) In process step S1 a respective MRI sequence to be used for imaging the patient 2 is selected or defined. This may be done manually by a respective user or automatically or semi-automatically by the MRI device 1, for example based on detected characteristics of the patient 2 and/or previous medical data of the patient 2.

(9) In process step S2 a first effective stimulus duration t.sub.s,eff,std is calculated independently for each monotonic pulse flank of the proposed MRI sequence individually. This may be done for all monotonic pulse flanks of the sequence in parallel or one after the other. It is also possible to implement an iterative execution of the flowchart 8 or the corresponding method. In each process step S2 the first effective stimulus duration t.sub.s,eff,std is only calculated for one of the monotonic pulse flanks of the proposed MRI sequence. The first effective stimulus duration t.sub.s,eff,std may be calculated according to the definitions and formulae put forth in the IEC standard 60601-2-33, in edition 3.2 of June 2015.

(10) In process step S3 a second effective stimulus duration t.sub.s,eff,mem is calculated for the proposed MRI sequence. In calculating the second effective stimulus duration a respective history of signal or gradient activity is taken into account at each point or pulse flank along the proposed MRI sequence, thus implementing a memory. A sliding time window that extends back in time from the respective pulse flank or a point of the MRI sequence is used to determine the gradient activity that is to be taken into account for the respective calculation of the second effective stimulus duration t.sub.s,eff,mem. For gradient activity, for example changes in the gradient field generated by the imaging system 3, during this sliding time window a respective duration of the gradient activity is multiplied with its sign and a time-dependent weight function W(t) and is then summed up for every point:

(11) $t_{s,\mathrm{eff},\mathrm{mem}}^{\mathrm{xyz}}\left(t_i\right)=\mathrm{dt}.Math.{.Math.}_{t_i-T_{\mathrm{win}}}^{t_i}W\left(t_i\right).Math.\mathrm{sign}\left(\frac{d{G}_{\mathrm{xyz}}}{\mathrm{dt}}\left(t_i\right)\right)$

(12) Instead of the discrete summation it is also possible to formulate the same calculation as an integral for continuous data.

(13) The weight function W(t) defines weights that are assigned to the gradient activity and implements or represents the mentioned memory or memory function. This is influenced by the predetermined length T.sub.win of the sliding time window as well as the type and parameters of the weight function W(t). The latter determines how gradient activity data is “forgotten” by the algorithm calculating the second effective stimulus duration t.sub.s,eff,mem. Some examples for the weight function W(t) are as follows:

(14) W(t) may have a constant value of 1 from t=0 to t=−T.sub.win.

(15) W(t) may have a constant value of 1 from t=0 to t=−T.sub.win/2 and decrease linearly to 0 from t=−T.sub.win/2 to t=−T.sub.win.

(16) W(t) may have a constant value of 1 from t=0 to t=−T.sub.win/5, a constant value of 0.8 from t=−T.sub.win/5 to t=−T.sub.win2/5, a constant value of 0.6 from t=−T.sub.win2/5 to t=−T.sub.win3/5, a constant value of 0.4 from t=−T.sub.win3/5 to t=−T.sub.win4/5, a constant value of 0.2 from t=−T.sub.win4/5 to t=−T.sub.win.

(17) W(t) decreases linearly from a value of 1 at t=0 to a value of 0 at t=−T.sub.win.

(18) W(t) decreases exponentially from a value of 1 at t=0 to t=−T.sub.win with the predetermined characteristic time constant τ and has a value of 0 for t<−T.sub.win.

(19) W(t) decreases exponentially from a value of 1 at t=0 to t=−infinity with the predetermined characteristic time constant τ.

(20) Only absolute values of the time differentiated gradient activity the dG.sub.xyz/dt(t.sub.i) that exceed a predetermined threshold thl may be taken into account in the summation or integration:

(21) $t_{s,\mathrm{eff},\mathrm{mem}}^{\mathrm{xyz}}\left(t_i\right)=\mathrm{dt}.Math.{.Math.}_{t_i-T_{\mathrm{win}}}^{t_i}W\left(t_i\right).Math.\mathrm{sign}\left({.Math.\frac{d{G}_{\mathrm{xyz}}}{\mathrm{dt}}\left(r_i\right).Math.}_{>\mathrm{thl}}\right)$

(22) In process step S4 both the first and second effective stimulus durations are evaluated to determine their maximum, i.e. the larger or longer one of the first and second effective stimulus durations as the effective stimulus duration t.sub.s,eff.

(23) In process step S5 a threshold value L.sub.ardiac for the maximum allowable rate of change in the magnetic gradient field is calculated based on the effective stimulus duration t.sub.s,eff:

(24) $L_{\mathrm{cardiac}}^{\mathrm{xyz}}\left(t_{s,\mathrm{eff}}^{\mathrm{xyz}}\right)=\frac{20}{1-\exp \left(-\frac{t_{s,\mathrm{eff}}^{\mathrm{xyz}}}{3}\right)}$

(25) The flowchart 8 then follows the program path P1 to process step S6. In process step S6 it is checked, if the proposed sequence at any point exceeds the calculated threshold value or limit L.sub.cardiac. If this is not the case, the flowchart 8 follows a program path P2. If, however, it is determined that the sequence in at least one point exceeds the calculated threshold value or limit L.sub.cardiac, the flowchart 8 follows a program path P3 to process step S10.

(26) In process step S10 a warning regarding the proposed sequence exceeding the calculated threshold value or limit L.sub.cardiac, is issued to a respective user, e.g. on the monitor 7. This may be the case if the calculations take place during a simulation run of the proposed MRI sequence. If the described calculations take place in real-time during an actual measurement or imaging process of the patient 2, then the imaging process may automatically be aborted in process step S10. A corresponding warning or information may also be displayed on the monitor 7 or be output in another way.

(27) The respective user or medical personnel may then adjust the proposed or used MRI sequence accordingly and the described process steps may be executed again, that is indicated here by the looping program path P7.

(28) If multiple gradients or gradient fields with different directions or axes are used or are proposed to be used according to the respective MRI sequence the flowchart 8 may also follow a program path P4 from the process step S5 to the process step S7. In the process step S7 the calculations for the different gradients are combined into a relative stimulation probability S.sub.cardiac,rel for the respective MRI sequence. In this case, calculating the first and second effective stimulus durations as well as the threshold value L.sub.cardiac in the process steps S2, S3 and S5 may include respective separate calculations for each gradient or gradient axis.

(29) In a process step S8 it is determined if S.sub.cardiac,rel<1 or if S.sub.cardiac,rel>1 is true. If S.sub.cardiac,rel is smaller than 1, then the respective MRI sequence may be considered safe and the flowchart 8 follows a program path P5. S.sub.cardiac,rel=1 provides that the probability to cause a cardiac stimulation is 2.Math.10.sup.−9 for the average patient 2.

(30) If following the program path P2 and/or the program path P5 the process step S9 is reached, then the proposed MRI sequence may be considered safe and thus may be applied or continued in the process step S9. This may also be indicated by a corresponding message that may be issued or output to the respective user through the monitor 7 or any other mechanisms.

(31) If in process step S8 it is determined that S.sub.cardiac,rel equal to or greater than 1, the flowchart 8 follows the program path P6 to the process step S10 where a corresponding warning may be output and/or the execution of the respective MRI sequence, for example, the corresponding imaging process, may automatically be aborted or shut down.

(32) The described examples show how an improved signal processing for controlling or monitoring cardiac stimulations in MR imaging may be achieved through the use of a memory function implemented through a weight function W(t) in combination with complying with the normative requirements for individual pulse flank evaluation.

(33) It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present disclosure. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent, and that such new combinations are to be understood as forming a part of the present specification.

(34) While the present disclosure has been described above by reference to various embodiments, it may be understood that many changes and modifications may be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.