Ascertaining a PSF for reconstructing image data from scan data recorded by means of a magnetic resonance system

Abstract

Techniques are disclosed for ascertaining a point spread function (PSF) for reconstructing image data from scan data recorded by means of a magnetic resonance system. The techniques include a comparison of values determined for a planned k-space trajectory for parameters characterizing the k-space trajectory with baseline values of the parameters characterizing the k-space trajectory deposited in a database for the magnetic resonance system, in each case together with an associated point spread function PSF to ascertain baseline values of the deposited baseline values that are as similar as possible to the values determined for the planned k-space trajectory for the parameters characterizing the k-space trajectory and, on the basis of this deposited PSF, a PSF to be used for a reconstruction of final image data is ascertained.

Claims

1. A method for ascertaining a point spread function (PSF) for reconstructing image data from scan data recorded using a magnetic resonance system, comprising: loading a k-space trajectory for a magnetic resonance measurement; iteratively comparing (i) valid values determined for the k-space trajectory using at least one parameter characterizing the k-space trajectory, with (ii) stored baseline values of parameters characterizing the k-space trajectory that are stored in a database to ascertain, from the stored baseline values, selected baseline values that minimally deviate from the values determined for the k-space trajectory; recording scan data using the k-space trajectory; iteratively reconstructing image data using the recorded scan data and a target PSF that is associated with the selected baseline values; and verifying reconstructed image data according to a quality criterion, wherein, when the quality criterion is not fulfilled, determining a further target PSF by iteratively repeating the act of reconstructing the image data, and wherein, when the quality criterion is fulfilled, using a most recent target PSF from the iterative comparing of the valid values to the stored baseline values for the reconstruction of final image data.

2. The method as claimed in claim 1, wherein, when the quality criterion is fulfilled, storing the most recent target PSF in the database with the selected baseline values as updated baseline values.

3. The method as claimed in claim 1, wherein, when no selected baseline values can be ascertained, the method further comprises: using an ideal PSF as the target PSF, the ideal PSF being calculated for the k-space trajectory for a first iteration of the act of iteratively reconstructing the image data.

4. The method as claimed in claim 1, wherein the at least one parameter characterizing the k-space trajectory includes at least one parameter that characterizes gradients to be switched to generate the k-space trajectory.

5. The method as claimed in claim 1, wherein the at least one parameter characterizing the k-space trajectory includes a maximum amplitude of at least one gradient to be switched to generate the k-space trajectory.

6. The method as claimed in claim 1, wherein the at least one parameter characterizing the k-space trajectory includes a rate of change of the amplitude of at least one gradient to be switched to generate the k-space trajectory.

7. The method as claimed in claim 1, wherein the at least one parameter characterizing the k-space trajectory includes an orientation of the k-space trajectory in physical k-space.

8. The method as claimed in claim 1, wherein the at least one parameter characterizing the k-space trajectory includes a shape of the k-space trajectory.

9. The method as claimed in claim 1, wherein the act of iteratively comparing the valid values with the stored baseline values comprises performing the iterative comparison in accordance with a predetermined sequence.

10. The method as claimed in claim 1, wherein the act of iteratively comparing the valid values with the stored baseline values comprises identifying a deviation between the valid values and the stored baseline values that is less than a predetermined threshold value.

11. The method as claimed in claim 1, wherein the k-space trajectory comprises a wave trajectory.

12. The method as claimed in claim 1, wherein the k-space trajectory comprises a spiral k-space trajectory.

13. The method as claimed in claim 1, wherein the k-space trajectory comprises a wave trajectory, and wherein the target PSF is determined for each direction in which modulated gradients are switched.

14. The method as claimed in claim 1, wherein the target PSF is stored in the database as a modulation transfer function (MTF).

15. The method as claimed in claim 1, wherein each respective one of the baseline values stored in the database has a corresponding time stamp identified with a time each respective baseline value was stored, wherein the database stores a plurality of PSFs, each respective one of the PSFs having a corresponding time stamp, and further comprising: removing, from the database, PSFs from among the plurality of PSFs that have a corresponding time stamp that exceeds a predetermined time interval from a current time.

16. A magnetic resonance system, comprising: a main magnet; and control circuitry configured to control the magnetic resonance system to ascertain a point spread function (PSF) for reconstructing image data from recorded scan data by: loading a k-space trajectory for a magnetic resonance measurement; iteratively comparing (i) valid values determined for the k-space trajectory using at least one parameter characterizing the k-space trajectory, with (ii) stored baseline values of parameters characterizing the k-space trajectory that are stored in a database to ascertain, from the stored baseline values, selected baseline values that minimally deviate from the values determined for the k-space trajectory; recording scan data using the k-space trajectory; iteratively reconstructing image data using the recorded scan data and a target PSF that is associated with the selected baseline values; and verifying reconstructed image data according to a quality criterion, wherein, when the quality criterion is not fulfilled, determining a further target PSF by iteratively repeating the act of reconstructing the image data, and wherein, when the quality criterion is fulfilled, using a most recent target PSF from the iterative comparing of the valid values to the stored baseline values for the reconstruction of final image data.

17. A non-transitory computer-readable medium having instructions stored thereon that, when executed by control circuitry of a magnetic resonance system, cause the magnetic resonance system to ascertain a point spread function (PSF) for reconstructing image data from recorded scan data by: loading a k-space trajectory for a magnetic resonance measurement; iteratively comparing (i) valid values determined for the k-space trajectory using at least one parameter characterizing the k-space trajectory, with (ii) stored baseline values of parameters characterizing the k-space trajectory that are stored in a database to ascertain, from the stored baseline values, selected baseline values that minimally deviate from the values determined for the k-space trajectory; recording scan data using the k-space trajectory; iteratively reconstructing image data using the recorded scan data and a target PSF that is associated with the selected baseline values; and verifying reconstructed image data according to a quality criterion, wherein, when the quality criterion is not fulfilled, determining a further target PSF by iteratively repeating the act of reconstructing the image data, and wherein, when the quality criterion is fulfilled, using a most recent target PSF from the iterative comparing of the valid values to the stored baseline values for the reconstruction of final image data.

Description

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

(1) Further advantages and details of the present disclosure are described from the exemplary embodiments described below and with reference to the drawings. The examples given do not constitute a restriction of the disclosure. In the drawings:

(2) FIG. 1 shows a schematic flowchart of a method according to one or more embodiments of the disclosure;

(3) FIG. 2 shows an exemplary helical k-space trajectory according to one or more embodiments of the disclosure;

(4) FIG. 3 shows a schematic depiction of part of a pulse sequence scheme for the acquisition of scan data which a k-space trajectory according to FIG. 2 can be achieved, according to one or more embodiments of the disclosure; and

(5) FIG. 4 shows a schematic depiction of a magnetic resonance system according to one or more embodiments of the disclosure.

DETAILED DESCRIPTION

(6) FIG. 1 shows a schematic flowchart of a method according to one or more embodiments of the disclosure for ascertaining a PSF for reconstructing image data from scan data recorded by means of a magnetic resonance system.

(7) In the method, a k-space trajectory kTr planned for a magnetic resonance measurement is loaded (block 101). Loading of planned k-space trajectories kTr can e.g. comprise loading the gradient fields Gx, Gy, Gz, which nominally have to be switched to obtain a desired k-space trajectory. A loaded k-space trajectory may have been calculated in advance in a known manner. A loaded k-space trajectory kTr is e.g. a k-space trajectory which may have imperfections leading to deviations of an actually executed k-space trajectory from the nominally planned k-space trajectory.

(8) Scan data MD may be recorded (block 107) using the loaded planned k-space trajectory kTr.

(9) As an example of a possible planned k-space trajectory, FIG. 2 shows an exemplary helical k-space trajectory W such as can be generated with a wave technique. The k-space trajectory depicted extends in a helical shape along a main direction HR (double arrow), here the k.sub.x direction, with in each case a magnitude and a frequency in directions perpendicular to the main direction and to each other such as specified by gradients switched in the k.sub.z and k.sub.y direction during the readout period.

(10) For further explanation, FIG. 3 shows a schematic depiction of part of a pulse sequence scheme for the acquisition of scan data with which a k-space trajectory W according to FIG. 2 can be achieved.

(11) The gradients used for position encoding of the signals scanned in a readout period Acq are generated as gradient fields on the orthogonal axes x, y, and z, for example corresponding to the physical axes of the magnetic resonance system, wherein the z direction mostly, but possibly also the x direction, is defined as pointing in the direction of the main magnetic field. The directions x, y, and z may be renamed without restriction of generality. The gradient fields G.sub.x, G.sub.y, G.sub.z are generated by means of three gradient coils which each respectively generate a field in the x, y, and z directions.

(12) To record scan data of a target region of an examination object, spins in the target region are excited in the usual manner by means of excitation Exc, and the echo signals generated by the excitation Exc are recorded as scan data in a readout period Acq. During the readout period Acq, for the spatial encoding readout gradients HG, WG.sub.y, WG.sub.z are switched that define the k-space trajectory, along which the scan data recorded in a readout period Acq is sampled. In the example depicted, a main readout gradient HG is switched in a main readout direction G.sub.x with a constant magnitude during the readout period Acq. At the same time as the main readout gradient HG, i.e., likewise during the readout period Acq, at least one further readout gradient WG.sub.y and/or WG.sub.z is switched, wherein further readout gradients have, for example sinusoidally-modulating magnitude, for example between a positive value of a maximum magnitude and the negative value of the maximum magnitude, each with a frequency f.sub.y=1/T.sub.y or f.sub.x=1/T.sub.z, and are switched in a direction G.sub.y or G.sub.z perpendicular to the main readout direction G.sub.x. Before and/or after the readout period Acq, further gradients, for example for a possibly desired dephasing or rephasing of the manipulated spins, may be switched in all the gradient axes G.sub.x, G.sub.y, G.sub.z.

(13) Continuing this example, the directions G.sub.y and G.sub.z are likewise perpendicular to one another. This results in a k-space trajectory W which extends in k-space in the direction k.sub.x corresponding to the main readout direction G.sub.x, and herein likewise modulates about the direction k.sub.x corresponding to the modulation of the switched further readout gradients WG.sub.y and WG.sub.x, as depicted in FIG. 2, for example. In this example, the modulation of the k-space trajectory about the direction k.sub.x corresponding to the main readout direction G.sub.x may also be wave-shaped (if only one further readout gradient WG.sub.y or WG.sub.z is switched) or, as depicted in FIG. 2, helical (if two further readout gradients WG.sub.y and WG.sub.z are switched).

(14) To record a set of scan data from a target region of an examination object from which image data of the target region can be reconstructed, a scheme depicted in FIG. 3 is repeated in the usual manner. For example, with different phase encoding by switching different gradients in the phase encoding direction (not depicted) between the excitation Exc and the readout period Acq, or also with different main readout directions G.sub.x, until a set of scan data sufficient for the reconstruction of the desired image data has been recorded.

(15) A further example of a planned k-space trajectory is a spiral k-space trajectory. A spiral k-space trajectory, which, for example, samples k-space in its k.sub.z-k.sub.y plane, may be achieved by a pulse sequence scheme, which, unlike the pulse sequence scheme depicted in FIG. 3, does not switch a gradient in the G.sub.x direction in the readout period. Instead of purely sinusoidal or cosinusoidal gradients WG.sub.y and WG.sub.z as in FIG. 3 in the G.sub.y or G.sub.z direction, over the course of time the pulse sequence scheme not only modulates the amplitudes of the gradients switched into these gradient directions sinusoidally and cosinusoidally, but causes them to increase beyond this, resulting in a spiral k-space trajectory.

(16) A value P valid for the planned k-space trajectory is in each case determined (block 103) from at least one parameter characterizing the planned k-space trajectory kTr.

(17) Parameters characterizing the k-space trajectory kTr may be e.g. parameters characterizing gradients to be switched for the generation of the k-space trajectory kTr. These are generally known, since the k-space trajectory kTr has usually been created on the basis of such parameters.

(18) For instance, parameters characterizing the k-space trajectory kTr may comprise at least one parameter from a group of parameters that may include: amplitude, e.g. maximum amplitude, of at least one gradient to be switched for the k-space trajectory kTr, rate of change, i.e., change over time, the amplitude of at least one gradient to be switched for the k-space trajectory kTr, orientation of the k-space trajectory in physical k-space (which results from the gradients to be switched), and basic shape, for example wave or spiral, of the k-space trajectory.

(19) The values P determined for the planned k-space trajectory for the parameters characterizing the k-space trajectory are compared with baseline values P.sub.B of the parameters characterizing the k-space trajectory kTr deposited in a database DB for the magnetic resonance system in each case, together with an associated point spread function PSF(P.sub.B). In this way, baseline values are ascertained of the deposited baseline values P.sub.B that are as similar as possible to the values P determined for the planned k-space trajectory for the parameters characterizing the k-space trajectory kTr and the associated point spread functions that are as similar as possible to the baseline values PSF(P.sub.B)=PSF.sub.i are ascertained (block 105). A similarity can, for example, be determined from the smallest possible (e.g. minimum) difference or a minimum deviation between the values P and the baseline values P.sub.B. The PSF PSF(P.sub.B) associated with baseline values P.sub.B deposited in the database DB may, for example, have been determined in advance on the magnetic resonance system or a magnetic resonance system of the same type, possibly by the manufacturer, for example by means of a method as described in the aforementioned article by Cauley et al.

(20) If a planned k-space trajectory kTr is a wave trajectory, a PSF(P.sub.B) can be deposited in the database for at least one direction in which gradients modulated for the wave trajectory are switched.

(21) If a planned k-space trajectory kTr is a wave trajectory, a PSF can be ascertained for each direction in which gradients modulated for the wave trajectory are switched. For example, in the case of a wave trajectory according to the example in FIGS. 2 and 3, values for parameters, e.g. the gradients to be switched in the G.sub.y and G.sub.z direction, may be determined separately from one another and, accordingly, PSFs may be ascertained separately for these directions and deposited in the database DB.

(22) When comparing the values P determined for the parameters with the baseline values P.sub.B to ascertain baseline values that are as similar as possible, a predetermined sequence in which the values of the parameters are compared may be observed. In this way, the search for the baseline values P.sub.B most similar to the determined values P can be made more efficient, and the result may be ascertained more quickly and with better quality. If, for example, a maximum amplitude and a rate of change of the switched gradients, and possibly an orientation of the k-space trajectory in physical k-space, are to be compared as parameters, the amplitude may be compared first and then, e.g. the rate of change, and then e.g. the orientation compared only with the baseline values with the most similar amplitudes.

(23) Additionally or alternatively, during the comparison of the values P determined for the parameters with the baseline values P.sub.B to ascertain baseline values that are as similar as possible, for at least one parameter, at least one threshold value may be specified by which a value of a baseline value may deviate at most from the determined value to be ascertained as a baseline value that is as similar as possible. Thus, a desired similarity may be enforced, and the results of the comparison may be positively influenced. For example, for the parameter of the orientation of the k-space trajectory, a deviation by a maximum angle, for example maximum 45°, may be specified.

(24) Image data BD.sub.i is reconstructed (block 109) on the basis of recorded scan data MD, and the PSF PSF.sub.i associated with the baseline values is ascertained as being as similar as possible. The reconstructed image data BD.sub.i is reconstructed in such a way that it allows conclusions to be drawn about the quality of the reconstruction. For this, it may be sufficient the reconstructed image data BD.sub.i was only reconstructed on the basis of scan data MD recorded in central k-space, and hence have a lower resolution.

(25) If no baseline values can be ascertained as being as similar as possible, a standard PSF or an ideal PSF calculated for the planned k-space trajectory may be used the first time block 109 is executed.

(26) Reconstructed image data BD.sub.i is verified according to a quality criterion Q (query 111). The quality criterion may, for example, be an evaluation of artifacts present in the reconstructed image data BD.sub.i, which, for example, either fall below a predetermined threshold value or correspond to an achievable minimum to fulfill the quality criterion Q. For instance, as in the aforementioned article by Cauley et al., an RMSE may be used to check the quality of the reconstructed image data BD.sub.i. The quality criterion Q may, e.g. if a minimum is sought, be deemed to be fulfilled if the last values ascertained for the image data BD.sub.i and describing the presence of artifacts, for example RMSE values, no longer differ from one another or only differ from one another by less than a defined (e.g. predetermined) threshold deviation. In this way, it is possible to find a sought-after PSF that is optimal for the reconstruction of the image data.

(27) If a planned k-space trajectory kTr is a wave trajectory, and a PSF(P.sub.B) was deposited in the database for at least one direction in which gradients modulated for the wave trajectory are switched, a sought-after (e.g. target) PSF.sub.i may also be ascertained on the basis of this deposited (e.g. stored) PSF(P.sub.B) for a direction in which gradients modulated for the wave trajectory are switched. A sought-after PSF.sub.i may, for example as in the aforementioned article by Polak et al., be broken down into components, for example according to the switched modulated gradients and possibly a tilt component. Thus, it is also conceivable to deposit only at least one component of a PSF for the baseline values P.sub.B in the database DB.

(28) Any missing components, for example a tilt component of a sought-after PSF.sub.i, may be determined in iterations of the check using the quality criterion Q.

(29) The conditions under which the quality criterion is fulfilled, such as, for example, threshold values or threshold deviations, may be adapted depending on baseline values P.sub.B already deposited in the database DB, for example to accelerate a convergence of the iterative search for the sought-after PSF.sub.i. For example, it is conceivable to tighten the conditions for fulfillment of the quality criterion Q, the more similar the baseline values P.sub.B determined from the deposited baseline values P.sub.B are to the values P, and/or the more baseline values P.sub.B already stored in the database.

(30) If the quality criterion Q is not fulfilled (query 111, n), the counter i is increased by one (i=i+1), and a further PSF (PSFi, i≠0) is determined (block 113), on the basis of which image data BD.sub.i is reconstructed again (block 109 is carried out again with a new PSF.sub.i). This process may continue until the quality criterion Q is fulfilled or until the counter i achieves a predetermined maximum value N (i≤N). To ascertain a further PSF.sub.i, i≠0, for example, one of the baseline values ascertained as being as similar as possible may be varied. Here, once again it is possible to proceed analogously to the method described in the aforementioned article by Cauley et al.

(31) If the quality criterion Q is fulfilled (query 111, y), the last-ascertained PSF PSFi can be used as the sought-after PSF.sub.i for the reconstruction of final image data BD on the basis of all scan data MD to be recorded (block 109, “if 111 y”). If the last reconstructed image data BD.sub.i has already been reconstructed on the basis of all scan data MD to be recorded, the last reconstructed image data BD.sub.i may be the final image data BD.

(32) Furthermore, if the quality criterion Q is fulfilled (query 111, y), the last determined PSF PSF.sub.i(P), together with the values P for parameters characterizing the planned k-space trajectory kTr, may be deposited in the database DB as new baseline values P.sub.B. In this way, the database DB is skillfully expanded by data that is relevant, because it is used in actual scans. If scans, the parameter values of which are not changed or are only slightly changed, are carried out more frequently on the magnetic resonance system with a k-space trajectory, this can considerably accelerate the iterative reconstruction of image data BD.sub.i.

(33) PSFs deposited in the database DB may be deposited as an associated modulation transfer function MTF. These may comprise (e.g. only) the frequency components of the PSF and therefore require less memory.

(34) It is furthermore conceivable that baseline values P.sub.B deposited in the database DB with an associated PSF PSF(P.sub.B) are given a time stamp when deposited, and baseline values P.sub.B with an associated PSF PSF(P.sub.B) with a time stamp that has a time interval from a current date that is greater than a specified value for a maximum deposit duration are removed from the database DB. Although the PSF for a given magnetic resonance system should not change, this is a way of ensuring that the baseline values PB and associated PSF PSF(P.sub.B) deposited in the database are always sufficiently up-to-date, so that any changes that may occur, for example when hardware components, such as e.g. the gradient unit are changed, do not have a negative impact on the method. In addition, in this way it can be achieved that the database DB is not excessively filled, which, inter alia, may prolong a comparison to determine similar baseline values.

(35) FIG. 4 is a schematic depiction of a magnetic resonance system 1 according to the one or more embodiments of the disclosure. The magnetic resonance system 1 comprises a magnet unit 3 (e.g. a magnet and/or associating drivers and hardware components) for generating the basic magnetic field, a gradient unit (e.g. gradient generation circuitry, which may include one or more processors, processing circuitry, hardware, software, executable instructions, or combinations of these) 5 for generating the gradient fields, a radio-frequency unit (e.g. RF generation circuitry, which may include one or more processors, processing circuitry, hardware, software, executable instructions, or combinations of these) 7 for irradiating and receiving radio-frequency signals, and a control facility (e.g. a computing device, controller, and/or control circuitry, which may include one or more processors, processing circuitry, hardware, software, executable instructions, or combinations of these) 9 embodied to carry out one or more methods according to the disclosure.

(36) FIG. 4 is a schematic depiction of partial units of the magnetic resonance system 1, and may include additional, or alternate components. For instance, the radio-frequency unit 7 may include a plurality of subunits, for example a plurality of coils, such as the schematically depicted coils 7.1 and 7.2, or more coils, which can be implemented to transmit RF signals and/or to receive the triggered RF signals.

(37) For an examination of an examination object U, for example a patient or a phantom, the object may be introduced into the scanning volume of the magnetic resonance system 1 on a bench L. The slice or slab Si represents an exemplary target volume of the examination object to be recorded from the echo signals and acquired as scan data.

(38) The control facility 9 is used to control the magnetic resonance system 1 and may for instance control the gradient unit 5 via a gradient controller (e.g. gradient control circuitry, which may include one or more processors, processing circuitry, hardware, software, executable instructions, or combinations of these) 5′ and the radio-frequency unit 7 by means of a radio-frequency transmission/reception control system (e.g. RF transmission/reception circuitry, which may include one or more processors, processing circuitry, hardware, software, executable instructions, or combinations of these) 7′. In this connection, the radio-frequency unit 7 may comprise a plurality of channels on which signals can be sent or received.

(39) Together with its radio-frequency transmission/reception control system 7′, the radio-frequency unit 7 is responsible for generating and irradiating (transmitting) an alternating radio-frequency field for manipulating the spins in a region to be manipulated (for example in slices S to be scanned) of the examination object U. Herein, if possible, the mid-frequency of the alternating radio-frequency field, also referred to as the B1 field, is generally ideally set close to the resonance frequency of the spins to be manipulated. Deviations of the mid-frequency from the resonance frequency are referred to as off-resonance. To generate the B1 field, controlled currents are applied to the RF coils in the radio-frequency unit 7 via the radio-frequency transmission/reception control system 7′.

(40) Furthermore, the control facility 9 comprises a PSF-determining-unit 15 (e.g. PSF-determining circuitry, which may include one or more processors, processing circuitry, hardware, software, executable instructions, or combinations of these), with which PSFs according to the disclosure can be determined for conversion by the gradient controller 5′. The control facility 9 is implemented to execute one or more methods according to the disclosure.

(41) A computing unit 13 (e.g. a computing device, controller, and/or control circuitry, which may include one or more processors, processing circuitry, hardware, software, executable instructions, or combinations of these) comprised by the control facility 9 is embodied to execute all the computing operations required for the necessary scans and determinations. Interim results and results required for this purpose or ascertained in this connection can be stored in a memory unit S of the control facility 9. Continuing this example, the units depicted should not necessarily be understood as being physically separate units, but may represent a subdivision into coherent units but which can also be implemented, for example, in fewer units or even only one single physical unit.

(42) An input/output facility E/A of the magnetic resonance system 1 can, for example, be used by a user to route control commands to the magnetic resonance system 1 and/or to display results of the control facility 9 such as, for example, image data.

(43) A method described herein can also be present in the form of a computer program product (e.g. a non-transitory computer-readable medium), which comprises a program and implements the described method on a control facility 9 when it is executed on the control facility 9. Likewise, an electronically readable data carrier 26 (e.g. a non-transitory computer-readable medium) may be provided with electronically readable control information stored thereon, which comprises at least one above-described computer program product and is implemented to execute the described method when the data carrier 26 is used in a control facility 9 of the magnetic resonance system 1.