Method and computer for producing a pulse sequence for controlling a magnetic resonance imaging apparatus

Abstract

In a method and magnetic resonance apparatus for generating a B.sub.0 map of a region of interest, a magnetic resonance data set containing a number of image data sets is obtained and provided in a computer, wherein the image data sets are recorded using at least two measurement sequences and the mutually corresponding pixels of the image data sets each represent a time-dependent signal evolution. A B.sub.0 map of the region of interest is generated by the computer from the image data sets, wherein the B.sub.0 value of a pixel of the B.sub.0 map is determined from the associated signal evolution.

Claims

1. A method for generating a B.sub.0 map of a region of interest in an magnetic resonance (MR) scanner comprising a basic field magnet that generates a static, basic magnetic field B.sub.0, said method comprising: providing a computer with an MR data set comprising a plurality of image data sets obtained with said MR scanner by executing at least two measurement sequences, each of said MR data sets comprising pixels, with mutually corresponding pixels of the respective image data sets, each representing a time-dependent signal evolution; in said computer, generating said B.sub.0 map of said region of interest from said image data sets with pixels in said B.sub.0 map each having a value determined from the signal evolution of the mutually corresponding pixels in said image data sets that correspond to the respective pixels in the B.sub.0 map; in said computer, comparing said signal evolutions with simulated signal evolutions in order to determine said B.sub.0 value, or to determine said B.sub.0 value and at least one other parameter value; and in said computer, simulating said signal evolutions in said computer only in a reduced B.sub.0 value range, wherein said B.sub.0 value range is limited to values of (1/T.sub.r)/2 to +(1/T.sub.r)/2, wherein T.sub.r is a repetition time for acquiring said image data sets.

2. A method as claimed in claim 1 comprising, in said computer, deriving a susceptibility map from said B.sub.0 map.

3. A method as claimed in claim 1 comprising deriving a susceptibility map from said B.sub.0 map by eliminating low-frequency field changes from said B.sub.0 map.

4. A method as claimed in claim 1 comprising deriving B.sub.1 values, for a B.sub.1 field in said MR scanner, from said signal evolutions.

5. A method as claimed in claim 1 comprising acquiring said magnetic resonance image data sets in at least two sections, with image data sets being recorded using a TrueFISP measurement sequence and acquiring image data sets in another section using a FLASH measurement sequence.

6. A method as claimed in claim 5 comprising, in at least one of said at least two sections, acquiring said image data sets using a FISP measurement sequence.

7. A method as claimed in claim 1 comprising, upon completing said comparison with said simulated signal evolutions, determining reference B.sub.0 values by extending the comparison to B.sub.0 values outside of said reduced range.

8. A method as claimed in claim 7 comprising calculating an extension B.sub.0 map in said computer, and comparing said extension B.sub.0 map to said B.sub.0 map in order to determine phase information pixel-by-pixel, and thereby transforming the B.sub.0 values to a B.sub.0 value range that contains values outside of said reduced B.sub.0 value range.

9. A method as claimed in claim 1 comprising determining T.sub.1 value and T.sub.2 value from said signal evolutions.

10. A magnetic resonance (MR) apparatus comprising: an MR data acquisition scanner comprising a basic field magnet that generates a basic magnetic field B.sub.0; a computer provided with an MR data set comprising a plurality of image data sets obtained with said MR scanner by executing at least two measurement sequences, each of said MR data sets comprising pixels, with mutually corresponding pixels of the respective image data sets, each representing a time-dependent signal evolution; said computer being configured to generate said B.sub.0 map of said region of interest from said image data sets with pixels in said B.sub.0 map each having a value determined from the signal evolution of the mutually corresponding pixels in said image data sets that correspond to the respective pixels in the B.sub.0 map; said computer being configured to compare said signal evolutions with simulated signal evolutions in order to determine said B.sub.0 value, or to determine said B.sub.0 value and at least one other parameter value; and said computer being configured to simulate said signal evolutions in said computer only in a reduced B.sub.0 value range, wherein said B.sub.0 value range is limited to values of (1/T.sub.r)/2 to +(1/T.sub.r)/2, wherein T.sub.r is a repetition time for acquiring said image data sets.

11. A non-transitory, computer-readable data storage medium encoded with programming instructions, said storage medium being loaded into a computer of a magnetic resonance (MR) apparatus comprising an MR data acquisition scanner that has a basic field magnet that produces a basic magnetic field B.sub.0, said programming instructions causing said computer to: receive an MR data set comprising a plurality of image data sets obtained with said MR scanner by executing at least two measurement sequences, each of said MR data sets comprising pixels, with mutually corresponding pixels of the respective image data sets, each representing a time-dependent signal evolution; generate a B.sub.0 map of said region of interest from said image data sets with pixels in said B.sub.0 map each having a value determined from the signal evolution of the mutually corresponding pixels in said image data sets that correspond to the respective pixels in the B.sub.0 map; compare said signal evolutions with simulated signal evolutions in order to determine said B.sub.0 value, or determine said B.sub.0 value and at least one other parameter value; and simulate said signal evolutions in said computer only in a reduced B.sub.0 value range, wherein said B.sub.0 value range is limited to values of (1/T.sub.r)/2 to +(1/T.sub.r)/2, wherein T.sub.r is a repetition time for acquiring said image data sets.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically illustrates a magnetic resonance apparatus.

(2) FIG. 2 shows a FLASH measurement sequence.

(3) FIG. 3 shows a FISP measurement sequence.

(4) FIG. 4 shows a TrueFISP measurement sequence.

(5) FIG. 5 shows a recording method having multiple measurement sequences.

(6) FIGS. 6-8 respectively show various magnetic resonance data sets.

(7) FIG. 9 shows a B.sub.0 map and a susceptibility map.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) FIG. 1 shows a magnetic resonance apparatus 1 having a transmit coil arrangement 2. The transmit coil arrangement 2 can be designed as a body coil, but can also be a transmit coil array. The transmit coil arrangement 2 is shown by dashed lines.

(9) For data acquisition, the magnetic resonance apparatus 1 has a reception coil arrangement 3. The reception coil arrangement 3 is preferably a coil array having coils 4, 5, 6 and 7. The coils 4, 5, 6 and 7 therefore detect MR signals simultaneously and therefore in parallel.

(10) To control the data acquisitions (scans), the magnetic resonance apparatus 1 has a control computer 8.

(11) As part of the control computer 8 or independently thereof, the magnetic resonance apparatus 1 also has a non-transitory data storage medium 9 on which computer programs 10 for carrying out magnetic resonance measurements are stored.

(12) For clarity, other components of the magnetic resonance apparatus 1 such as e.g. gradient coils or patient table are not shown.

(13) FIG. 2 shows a FLASH measurement sequence diagram 11. The gradient axes are as usual labeled G.sub.R for the read direction, G.sub.P for the phase encoding direction and G.sub.S for the slice selection direction. ACQ denotes the axis for the radio-frequency pulses and the acquisition windows.

(14) A FLASH is a gradient-echo-based sequence using a radio-frequency pulse 12 whose flip angle is less than 90. A T.sub.2* contrast can be set via the echo time T.sub.E and a T.sub.1 contrast via the repetition time T.sub.R. The radio-frequency pulse typically has a flip angle of between 4 and 30 for weighted measurements.

(15) In order to excite a single slice using the radio-frequency pulse 12, a slice selection gradient 13 is applied simultaneously with the radio-frequency pulse 12 in the slice selection direction G.sub.S. In order to compensate its dephasing effect on the magnetization in the transverse plane, the slice selection gradient 13 is directly followed by a slice rephasing gradient 14.

(16) In the phase encoding direction G.sub.P a phase encoding gradient 15 is used. Like the read gradient 16 in the read direction G.sub.R, this is applied in an oscillating manner. This is preferably performed in order to sample the k-space spirally. As already described above, Cartesian or radial sampling can also be performed instead.

(17) MR signals 17 can be acquired accordingly.

(18) It is important to note in this context that an entire image data set is recorded in the repetition time T.sub.R. The FLASH measurement sequence 11like the other measurement sequences discussedis therefore a kind of single-shot sequence, as a single radio-frequency pulse 12 suffices to obtain a complete image data set.

(19) The raw data set thus acquired can be converted into an image data set by a non-uniform Fourier transform. It may be prone to artifacts, but this is sufficient for matching.

(20) The second radio-frequency pulse 12 on the right side of the drawing shows that, after recording of the first image data set, the second image data set is commenced without a pause. As will be described in more detail below, the second radio-frequency pulse 12 can have a flip angle different from that of the first radio-frequency pulse 12. In addition, the phase for implementing a phase cycle can change. SNR problems can be reduced by parallel imaging, as less k-space data has to be recorded, thereby enabling the repetition time T.sub.R to be reduced.

(21) FIG. 3 shows a FISP measurement sequence 18. For this, only one radio-frequency pulse 19 is likewise used to record a complete image data set.

(22) As specified for the FLASH measurement sequence 11, a slice selection gradient 13, a slice rephasing gradient 14, a phase encoding gradient 15 and a read gradient 16 are present.

(23) In addition, a phase rewind gradient 21 is present. This ensures that the sum of the gradient moments in the phase direction equals zero over a repetition time T.sub.R.

(24) In the slice direction G.sub.S, the sum of the gradient moments is non-zero over a repetition time T.sub.R.

(25) In the read direction G.sub.R, the gradients are balanced, but this is not obligatory. The sum of the gradients in the read direction G.sub.R can therefore also be non-zero over a repetition time. As spiral trajectories are recorded, the resulting total moment is always the same, as the individual gradient moments always have the same evolution over a repetition time.

(26) The second image data set is begun with the radio-frequency pulse 20. This preferably has the same phase as the preceding radio-frequency pulse 19, but has a different flip angle.

(27) FIG. 4 shows a TrueFISP measurement sequence 22. Here reference can basically be made to the remarks concerning the FLASH measurement sequence 11 and also the FISP measurement sequence 18.

(28) In addition to the components already mentioned, the TrueFISP measurement sequence 22 involves a read-rewind gradient 23 and a slice dephasing gradient 24. As a result, the TrueFISP measurement sequence 22 is fully balanced over a repetition time T.sub.R, i.e. the sums of the gradient moments are equal to zero over a repetition time T.sub.R. Also in the case of the TrueFISP measurement sequence 22, the absolute values of the flip angles of the radio-frequency pulses 19 and 20 vary.

(29) As already described, the TrueFISP measurement sequence 22 can have phase cycles. As already described, a 90 phase cycle can be used. The first radio-frequency pulse 19 then has a phase , the second radio-frequency pulse 20 a phase (+180), the third radio-frequency pulse 25 a phase (+90), the fourth radio-frequency pulse a phase (+270), the fifth radio-frequency pulse a phase (+180, the sixth radio-frequency pulse a phase (+360), etc.

(30) A 180 phase cycle then jumps with 180 increments instead of with 90 increments and a 270 phase cycle with 270 increments.

(31) For all the measurement sequences 11, 18 and 22, the scheme for recording an image data set and the radio-frequency pulse including gradients in the slice selection direction G.sub.S of the next image data set is shown in order to clarify the procedure.

(32) FIG. 5 schematically illustrates a recording method for recording a magnetic resonance data set. Here the number of the recorded image data set is plotted on the axis 26 and different variables are plotted on the axis 27. Plotted as the first variable is the flip angle in from 0 at the origin to 90 at the axis point 28. The axis 26 runs from the image data set 1 to the image data set 3100.

(33) The 3000 image data sets are distributed over eleven sections 29, 30, 31, 32, 33, 34, 35, 36, 37, 38 and 39.

(34) In the first section 29, the FISP sequence flip angle that was used for the recording is plotted via the curve 40 for two hundred image data sets. As described in connection with FIG. 3, a complete image data set is recorded after the application of a radio-frequency pulse having a particular flip angle and then the next radio-frequency pulse having the next flip angle is applied and another image data set is acquired. FIG. 5 accordingly shows in section 29 a flip angle distribution which corresponds to a sine curve. The maximum flip angle is 24 and constant phases are used.

(35) For the hundredth image data set, a line 41 has been inserted purely by way of example. The corresponding flip angle is the maximum flip angle of the curve 40.

(36) In the second section 30, four hundred image data sets are acquired using the TrueFISP sequence 22 according to FIG. 4. Flip angles according to the curves 42 and 43 are used here.

(37) For the curve 42 these extend to 45 and for the curve 43 to 72.

(38) Also for section 30, at the flip angle for the four hundredth image data set a line 44 has been inserted solely as an example. Here the flip angle is 1.

(39) A particular feature of section 30 is the use of two different phase cycles. For running through the flip angles of the curve 42, a 00 phase cycle i.e. no phase cycle is used and for running through the curve 43, a 180 phase cycle. A 00 phase cycle denotes a fixed phase.

(40) In the following section 31, in the curve 45 the flip angles for recording four hundred and fifty image data sets using a FLASH sequence 11 are indicated. These are smaller than in the FISP or TrueFISP sequence and run up to 6. Their distribution is also a sin.sup.2 distribution.

(41) In addition to the variation of the flip angles, during repeated running of the FLASH sequence, a phase cycle for implementing RF spoiling is applied. As described, the phase is here increased by multiples of 117.

(42) The succession of the measurement sequences 11, 18 and 22 together constitute a block 45. This is used a total of three times in FIG. 5. Here the focus is solely on the type of sequence, but not on the number of image data sets or the flip angle curves (profiles).

(43) In section 32, 200 image data sets are again recorded using a FISP sequence 18. As in section 29, the phase is constant, but the maximum flip angle is 45. These lie on the curve 46.

(44) In section 33, 200 image data sets follow which are to be acquired using a TrueFISP sequence 22. Here a 90 phase cycle is used, the maximum flip angle is 50. The flip angles are plotted on the curve 47.

(45) The next approximately 450 image data sets in section 34 are to be recorded using a FLASH sequence, as in section 31. The curve 48 shows a sin.sup.2 distribution with a maximum value of 14.

(46) Curve 49 in section 35 runs to 72 and shows the flip angles of the radio-frequency pulse 19 when using a FISP sequence 18 for the third time. The phase is also constant in this run.

(47) For acquiring another two hundred image data sets using a TrueFISP sequence 22 according to FIG. 4, a 270 phase cycle is used. The flip angles that are plotted in the curve 50 run to 65.

(48) The next approx. 450 image data sets in section 37 are recorded using the FLASH sequence 11 according to FIG. 2. The curve 51 represents a flip angle evolution of up to 20, again sin.sup.2-distributed.

(49) In the last section 38, there are two curves 52 and 53 for recording image data sets using a FISP sequence. These again represent flip angle evolutions. As already in the preceding sections, a constant phase is used for the FISP measurement sequence 18.

(50) To summarize, regardless of the specific number of images and the respective maximum flip angles, a sin.sup.2-distributed flip angle evolution is preferably used in all the sections. As described above, considerably fewer image data sets can also be acquired in a section, but preferably at least 10.

(51) FIGS. 6 to 8 show a magnetic resonance data set 54 comprising image data sets 55, 56 and 57. These are illustrative of the 3000 image data sets obtained using the method according to FIG. 5. The necessary post-processing is generally known.

(52) The image data sets 55, 56 and 57 each depict a region of interest 58. The image data set 55 has been recorded using the FISP measurement sequence 18, image data set 56 using the TrueFISP measurement sequence 22 and image data set 57 using the FLASH sequence 11. The flip angle is in each case one of the possible flip angles from the curves 40 to 53. However, the signal also depends on the past history.

(53) The evaluation proceeds pixel by pixel. The pixel 59 is charted purely by way of example. In all the image data sets 55 to 57 the pixel at the same location, namely the pixel 59, is used to obtain a signal evolution. For the other pixels, a signal evolution is determined and evaluated in each case. The regions 60 in which only noise signal is present can be detected and omitted in order to minimize the evaluation time e.g. on the basis of a threshold value.

(54) FIG. 9 shows a B.sub.0 map 61 which can be determined from the magnetic resonance data set 54. As described above, the pixel-by-pixel evaluation enables a B.sub.0 value, a B.sub.1 value, a T.sub.1 value and a T.sub.2 value to be determined for each pixel 59. Other parameters are also basically possible. The B.sub.0 map 61 can be composed from all the B.sub.0 values determined, as is normal for creating parameter maps.

(55) As described, the B.sub.0 map 61 can be calculated via a reference B.sub.0 map and an extension B.sub.0 map.

(56) A susceptibility map 62 can be calculated from the B.sub.0 map 61 by the use of a low-pass filter. This can avoid the use of phase maps.

(57) 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.