Method for obtaining an operating parameter, storage medium, and magnetic resonance apparatus

Abstract

A storage medium, a magnetic resonance apparatus, and a method for obtaining an operating parameter of a magnetic resonance apparatus are disclosed herein. The method includes generating of at least one echo train, wherein the generation of an echo train includes: setting a given set of parameters; applying at least one radio frequency excitation pulse; and applying a dephasing gradient in readout direction; and reading out the echo train having at least two echo signals, wherein a readout gradient is applied while reading out the echo signals. The method further includes acquiring at least two echo signals, wherein the set of parameters differs in at least one parameter being used for different echo signals; processing the echo signals line by line to projections; and obtaining the operating parameter using the projections.

Claims

1. A method for obtaining an operating parameter of a magnetic resonance apparatus, the method comprising: generating at least one echo train, wherein the generation comprises: setting a given set of parameters; applying at least one radio frequency excitation pulse; applying a dephasing gradient in a readout direction; reading out the at least one echo train, each echo train of the at least one echo train comprising echo signals; and applying a readout gradient while reading out the echo signals, wherein the readout gradient has a sinusoidal form having at least one arc, acquiring at least two echo signals, wherein the set of parameters differs in at least one parameter being used for different echo signals; processing the echo signals line by line to projections; and obtaining the operating parameter using the projections, wherein the operating parameter is a correction factor of a gradient moment of the readout gradient.

2. A magnetic resonance apparatus comprising: a magnetic resonance (MR) data acquisition scanner comprising a radio-frequency transmitter, a radio-frequency (RF) receiver, and a gradient coil arrangement; a memory in which parameter sets are configured to be stored; and a computer having access to the memory and configured to read the parameter sets from the memory, wherein the computer is configured to: generate at least one echo train, which comprises: setting a given set of parameters; applying at least one radio frequency excitation pulse; applying a dephasing gradient in a readout direction; reading out the at least one echo train, each echo train of the at least one echo train comprising echo signals; and applying a readout gradient while reading out the echo signals, wherein the readout gradient has a sinusoidal form having at least one arc; acquire at least two echo signals, wherein the set of parameters differs in at least one parameter being used for different echo signals; process the echo signals line by line to projections; and obtain an operating parameter using the projections, wherein the operating parameter is a correction factor of a gradient moment of the readout gradient.

3. A method for obtaining an optimal correction factor of a magnetic resonance apparatus, the method comprising: acquiring a plurality of echo trains in a plurality of different segments of k-space in a readout direction, each echo train of the plurality of echo trains having applied a respective dephasing gradient, wherein each echo train of the plurality of echo trains comprises a plurality of echo signals acquired using a plurality of correction factors; combining echo signals from each echo train of the plurality of echo trains acquired with a same correction factor to provide a plurality of combined k-space lines; processing the combined echo signals into one-dimensional projections via Fourier transformation; determining amplitudes of edges of each one-dimensional projection of the one-dimensional projections; and determining a correction factor that creates a least amplitude of the determined amplitudes of edges.

4. The method of claim 1, wherein gradient moments or extreme values of the readout gradient have varying values.

5. The method of claim 1, wherein gradient moments or extreme values of the readout gradient have increasing values.

6. The method of claim 1, wherein gradient moments or extreme values of the readout gradient have decreasing values.

7. The method of claim 1, wherein the echo signals acquired with a same gradient moment are combined to a combined k-space line.

8. The method of claim 1, wherein the echo signals of the at least one echo train are acquired without phase encoding.

9. The method of claim 1, wherein the echo signals cover only a segment of a k-space in the readout direction.

10. The method of claim 1, wherein the at least one echo train comprises a plurality of echo trains, and wherein each echo train of the plurality of echo trains has a plurality of echo signals.

11. The method of claim 1, wherein the at least one echo train comprises a plurality of echo trains, and wherein every echo train of the plurality of echo trains has a same number of echo signals.

12. The method of claim 1, comprising a plurality of correction factors for multiplication with the readout gradient, the plurality of correction factors being a modified parameter of the set of parameters.

13. The method of claim 1, wherein a gradient moment of the dephasing gradient is a modified parameter of the set of parameters.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further details of the disclosure are provided with reference to the figures.

(2) Parts that correspond to one another are labeled with the same reference characters in all figures.

(3) FIG. 1 depicts an embodiment of a magnetic resonance apparatus.

(4) FIG. 2 depicts a prior art sequence diagram of a RESOLVE sequence.

(5) FIG. 3 depicts a prior art k-space acquisition scheme of a RESOLVE sequence.

(6) FIG. 4 depicts an example of a sequence diagram of a measurement sequence.

(7) FIG. 5 depicts an example of a k-space acquisition scheme of the sequence of FIG. 4.

(8) FIG. 6 depicts an example of a first combined k-space line and projection.

(9) FIG. 7 depicts an example of a second combined k-space line and projection.

(10) FIG. 8 depicts an example of a flowchart of a method of adjusting an operating parameter.

DETAILED DESCRIPTION

(11) FIG. 1 depicts a magnetic resonance apparatus 1. The magnetic resonance apparatus 1 has a scanner 2. A transmit coil arrangement 3 is part of the scanner 2. The transmit coil arrangement 3 may be configured as a body coil, and thus includes a single coil.

(12) Furthermore, the magnetic resonance apparatus 1 has a reception coil arrangement 4. The reception coil arrangement 4 is a coil array with coils 5, 6, 7, and 8. To enable the coils 5, 6, 7, and 8 to be distinguished more easily, the transmit coil arrangement 3 is shown by a dashed outline.

(13) A control computer 9 controls the operation of the magnetic resonance apparatus 1.

(14) The magnetic resonance apparatus 1 also has a non-transitory data storage medium 10 as part of the control computer 9 or independent thereof, on which computer code for carrying out magnetic resonance measurements is stored.

(15) The coil array 4 is used only to read out the measurement signal which may be an echo signal. The coils 5, 6, 7, and 8 of the coil array 4 read out the measurement signal at the same time. Instead of the coil array 4, an individual coil may also be used as the detection coil for individual embodiments.

(16) Further components of the magnetic resonance apparatus 1, such as gradient coils and a patient bed, are not shown for clarity.

(17) FIG. 2 depicts a sequence diagram 11 of a RESOLVE sequence which is known, for example, from Porter and Heidemann, High Resolution Diffusion-Weighted Imaging Using Readout-Segmented Echo-Planar Imaging, Parallel Imaging and a Two-Dimensional Navigator-Based Reacquisition, MRM, 62:468-475, 2009.

(18) A diffusion preparation section includes an excitation pulse 12 and a refocusing pulse 13. Slice selection gradients 14 and 15 are applied at the same time to select a defined slice in a patient. It is known to use an additional slice rephrasing gradient 16 to compensate the dephasing fraction of the slice selection gradient 14. The diffusion encoding gradients 17, 18 and 19 before the refocusing pulse 13 and the respective gradients 20, 21 and 22 are also basically known.

(19) The excitation pulse 12 and the gradients 14 and 16 are part of an excitation phase 23 of the RESOLVE sequence. The following evolution phase 24 lasts to the end of the diffusion gradients 20, 21 and 22.

(20) After that the readout phase 25 starts. A dephasing gradient 26 having different gradient moments by varying its strength puts the beginning of the readout in the readout direction to a desired position in k-space. This is shown in the following figure.

(21) A sinusoidal readout gradient 27 has a plurality of arcs 30, 31, 32, 33, 34, 35, 36, and 37. Every arc 30, 31, 32, 33, 34, 35, 36, and 37 encodes one partial line in a readout direction in the k-space.

(22) The phase encoding gradients 38 shift the encoding for one step in phase encoding direction. Therefore, the phase encoding gradients 38 are called blips or gradient blips.

(23) An initial phase encoding gradient 39 puts, similar to the dephasing gradient 26, the beginning of the readout in the phase encoding direction to a desired position in k-space.

(24) In the readout phase 25, all echo signals 40 of one so called segment may be acquired. All signal echoes of an excitation cycle generate an echo train 41. At the end of the readout phase the encoding is put back to the starting point by applying a gradient 42 which has the same gradient moment as the dephasing gradient 26 but the opposite sign.

(25) After the readout phase 25 a navigator phase 43 follows. The respective gradients 44, 45, 46, and 47 operate as described with regard to the readout phase 25. The echo signals 48 are generated using a refocusing pulse 49 and a slice selection gradient 50.

(26) FIG. 3 depicts a k-space acquisition scheme used by the sequence 11. An axis 51 denotes the k(x) direction of the k-space 52 and an axis 53 the k(y) direction. The k(x) direction is also called readout direction and the k(y) direction phase encoding direction.

(27) After the preparation of the signal for example by diffusion weighting the gradients 26 and 39 put the encoding to the first starting point 54. The partial line 55 is acquired while the arc 30 is applied, the partial line 56 at the same time as the arc 31 applied. The shift in phase encoding direction is achieved by one of the blips 38.

(28) The additional partial lines 57, 58, 59, 60, 61, and 62 are created in the same way. The partial lines 55 to 62 or echo signals 40 constitute an echo train 41.

(29) The partial lines 55 to 62 cover a segment 63 of the k-space 52 which is separated in k(x) direction.

(30) Applying the sequence 11 by using a dephasing gradient 26 having a different gradient moment allows the acquisition of the echo signals of one the segments 64, 65, 66, or 67 of the k-space 52.

(31) If an echo train 41 has all echo signals of a segment 63 to 67 of the k-space 52, a number of excitation cycles is needed that is the number of segments the k-space 52 has.

(32) If an echo train 41 has only a fraction of the echo signals of a k-space segment, the excitation cycle has to be repeated more often. Then, the k-space 52 was divided in readout direction and phase encoding direction.

(33) The trajectories 68 and 69 of two adjacent segments, e.g., the parts 63 and 64, have a gap for the sake of clarity. In reality, the echo signals of a k-space line cover the k-space 52 totally without gaps.

(34) Images are reconstructed using the echo signals of all excitation cycles having the same position in phase encoding direction as one k-space line.

(35) FIG. 4 depicts a sequence diagram 71 of an adjustment sequence. Parts that correspond to FIG. 2 are labeled with the same reference characters.

(36) Diffusion gradients 17, 18, 19, 20, 21, and 22 as well as phase encoding gradients 38 are not applied. Then, the central k lines in phase encoding direction are acquired.

(37) To fasten the adjustment measurement, the arcs or gradient moments 30 to 37 of FIG. 2 are multiplied with a correction factor a.sub.+, a.sub.−, b.sub.+, b.sub.−, c.sub.+, c.sub.−, d.sub.+, and d.sub.−, respectively. That means that the gradient moment of arc 30 is multiplied with the correction factor a.sub.+, the gradient moment of arc 31 with the correction factor a.sub.−, and so on. Every gradient moment of an arc is multiplied with one of the correction factors. If the positive extreme value was set to 1, the correction factor may set a range from 1.03 to 0.99. Hence a.sub.+ may be 1.05, a.sub.− 1.04, b.sub.+ 1.03, b.sub.− 1.02, c.sub.+ 1.01, c.sub.− 1.00, d.sub.+ 0.99 and d.sub.−0.98. Of course, there may be more echoes in an echo train and a respective number of correction factors. There may be more than forty echo signals 40 in an echo train 41 and a respective number of correction factors. The range may be from 0.98 to 1.04 and independent of the number of echo signals. The arcs or gradient moments have new character signs 72, 73, 74, 75, 76, 77, 78 and 79 to show the difference to FIG. 2. The gradient moments 72 to 79 show a larger decrease than that the correction factors cause in reality to make the decrease visible.

(38) The arcs or gradient moments 72, 73, 74, 75, 76, 77, 78, and 79 show a larger decrease than that the correction factors cause in reality to make the decrease visible.

(39) One of the partial lines, e.g., the partial line 55, is acquired n times, n being the number of echoes of the echo train 41, by executing one excitation cycle.

(40) Instead of decreasing gradient moments increasing gradient moments may be used as well for the arcs of the sinusoidal readout gradient.

(41) The increase or decrease of the correction factors is advantageously linear, as shown by the numbers above.

(42) At least two of the segments 63 to 67 may be acquired, but only one partial line of them, respectively. Instead of a phase encoding a correction factor variation is applied between different echo signals 40.

(43) In one excitation cycle, one echo train 41 is acquired. The position of its echo signals 40 are shown in FIG. 5. There are eight echo signals, which originally are at the same position in k-space 52. Only the orientation may differ. They are shifted in phase encoding direction to make them all visible.

(44) Then all acquired partial lines 59, 81, 82, 83, and 84 being acquired using the same correction factor are combined to a combined k-space line 85. This is shown in FIG. 6. If eight correction factors are used, eight combined k-space lines 85 are achieved. The more correction factors are applied the more combined k-space lines are received.

(45) These combined k-space lines 85 are processed line by line. They may be Fourier transformed in one dimension. A zerofilling may be performed. Keeping the example of eight combined k-space lines one gets eight projections.

(46) The optimal correction factor may be found by an automatic examination of the projections. For example, an edge detection algorithm may be executed. The least number/amplitude of edges shows the best correction factor.

(47) To avoid bias an interpolation may be used.

(48) If the sequence 71 of FIG. 4 is executed several times, several combined k-space lines are acquired.

(49) Let assume a number of five repetitions. The first repetition is executed using a first gradient moment of the dephasing gradient 26 as a first operating parameter. The correction factors a.sub.+, a.sub.−, b.sub.+, b.sub.−, c.sub.+, c.sub.−, d.sub.+, and d.sub.− are applied changing the gradient moment of the readout gradient being a second operating parameter being changed.

(50) In the second repetition, a second gradient moment of the dephasing gradient 26 as the first changed operating parameter is used. Then eight echo signals 40 of a second segment are acquired.

(51) This is repeated with a third, fourth, and fifth gradient moment of the dephasing gradient 26. Using them echo signals in the segments 65, 66 and 67 are measured.

(52) These echo signals are combined as follows.

(53) The first echo signals 40 of every excitation cycle, which have been measured using the correction factor a.sub.+, are combined to a combined k-space line 86. This is shown in FIG. 6. Partial line 87 comes from the first repetition, partial line 88 from the second repetition, and so on.

(54) A second combined k-space line 92 is shown in FIG. 7. There the second echoes of the five repetitions acquired using the correction factor a.sub.− are combined.

(55) The partial lines 93, 94, 95, 96, and 97 form the combined k-space line 92. They are measured as described.

(56) This combination may be performed for every echo signal 40 in an echo train 41.

(57) Every combined k-space line is then Fourier transformed to a projection 98 and 99, respectively.

(58) A combination of projections 87, 88, 89, 90, 91 into a combined k-space line 86 may involve a gridding operation, e.g., an accounting for the non-linear k-space acquisition during the sinusoidal gradient lobes.

(59) Afterwards, all projections 98 and 99 are examined with an algorithm, for example, an edge detection. The number of detected edges may be plotted against the correction factor. The correction factor stands for a gradient moment. Using the number of edges, a minimal number may be found using a regression analysis or a different known way of analysis.

(60) The procedure to achieve an optimal correction factor is shown in the flow chart of FIG. 8, where all described acts are combined.

(61) In act S1-1, a first echo train 41 is acquired using the sequence 71 and having set a first dephasing gradient 26. The echo train 41 has e.g. eight echo signals which are acquired using the correction factors a.sub.+, a.sub.−, b.sub.+, b.sub.−, c.sub.+, c.sub.−, d.sub.+, and d.sub.−. All echo signals are located at the same position in k-space phase encoding direction and are in a first of the segments 62 to 67. That says that the gradient moment of the sinusoidal readout gradient being an operating parameter is changed within the excitation cycle.

(62) In act S1-2, a second echo train 41 is acquired using the sequence 71 and having set a second dephasing gradient 26. The echo train 41 again has eight echo signals. All echo signals are located at the same position in k-space and are in a second of the segments 62 to 67.

(63) Accordingly, acts S1-3 to S1-n are performed using further gradient moments for the dephasing gradient 26 to cover some or all of the further segments 62 to 67. In the case of five segments, acts S1-1 to S1-5 would be performed to cover all segments.

(64) The gradient moment of the dephasing gradient is a second operating parameter which is changed between two excitation cycles. This change is not with regard to an optimization but due to the segmentation of the k-space.

(65) In act S2, combined k-space lines 85, 86, and 92 are created by combining all first echo signals of the acquired echo trains to a first combined k-space line, all the second echo signals to a second combined k-space line, and so on. This presumes that the first echo signals are acquired with the correction factor a.sub.+, the second ones with the correction factor a.sub.−. The main thing combining the echo signals is not the position of the echo but the correction factor.

(66) In act S3, the combined k-space lines 85, 86 and 92 are Fourier transformed in one dimension to projections 98 and 99.

(67) In act S4, an edge detection algorithm determines the amplitude of the edges of every projection 98 and 99.

(68) In act S5, the gradient moment and correction factor, respectively, creating the least amplitude of the amplitudes of the edges is determined.

(69) This method may be performed just before a RESOLVE measurement taking only about a few seconds. Then the images have less artifacts.

(70) Although the disclosure has been illustrated and described in detail using the exemplary embodiments, the disclosure is not limited by the disclosed examples, and a person skilled in the art may derive other variations therefrom without departing from the scope of protection of the disclosure. 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.

(71) 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.