Method and apparatus for magnetic resonance imaging

Abstract

To enable improved adjustment of at least one shim channel for magnetic resonance imaging of an examination region of an examination object by operation of a magnetic resonance apparatus that has a shim arrangement with a first shim channel volume having at least one first shim channel and a second shim channel volume having at least one second shim channel, the examination region is divided into multiple of sections, multiple first shim parameter sets are determined for the at least one first shim channel, with one first shim parameter set among the multiple first shim parameter sets being ascertained for each of the multiple sections, a second shim parameter set is ascertained for the at least one second shim channel, taking into account the ascertained multiple first shim parameter sets, and magnetic resonance image data of the examination region are acquired, but before this acquisition, the at least one second shim channel is adjusted using the second shim parameter set and the at least one first shim channel is adjusted for acquiring the magnetic resonance image data from a specific section of the multiple sections using a first shim parameter set ascertained for that specific section.

Claims

1. A method for acquiring magnetic resonance (MR) data from an examination region of an examination object situated in an MR scanner that comprises a shim unit, said shim unit comprising a first shim channel volume having at least one first shim channel and a second shim channel volume having at least one second shim channel, said method comprising: in a computer, dividing said examination region into multiple sections; in said computer, determining a plurality of first shim parameter sets for said first shim channel, with one first shim channel parameter set among said plurality of first shim channel parameter sets being determined for each section among said multiple sections; in said computer, determining a second shim parameter set for said second shim channel dependent on said plurality of determined first shim parameter sets; before acquiring said magnetic resonance data from the examination region, adjusting said second shim channel using said second shim parameter set and adjusting said first shim channel for acquiring the magnetic resonance data from a specific section among said multiple sections using a first shim parameter set determined for that specific section; and operating said MR scanner to acquire MR data from said specific section using the adjusted first shim channel and the adjusted second shim channel.

2. A method as claimed in claim 1 wherein said first shim channel volume and said second shim channel volume are disjunct.

3. A method as claimed in claim 1 comprising, before determining said plurality of first shim parameter sets, operating said MR scanner to acquire a first B0 field map that represents a basic magnetic field in said MR scanner, and determining said plurality of first shim parameter sets using said first B0 field map.

4. A method as claimed in claim 3 comprising acquiring said first B0 field map using raw data comprising at least three echo signals that are respectively acquired following an excitation of nuclear spins in the examination region.

5. A method as claimed in claim 3 comprising, in said computer, calculating a second B0 field map using said first B0 field map and said plurality of first shim parameter sets, and determining said second shim parameter set using said plurality of first shim parameter sets and said second B0 field map.

6. A method as claimed in claim 5 comprising calculating said second B0 field map using said first B0 field map and said plurality of first shim parameter sets by offsetting B0 field contributions, resulting from said plurality of first shim parameter sets, against said first B0 field map.

7. A method as claimed in claim 6 comprising, in said computer, offsetting a spatial portion of said first B0 field map against said B0 field contributions against a determined first shim parameter set among said plurality of first shim parameter sets that has been specifically determined for said section among said multiple sections, that corresponds to said spatial portion.

8. A method as claimed in claim 5 comprising, in said computer, calculating a third B0 field map using said first B0 field map and said second shim parameter set, and determining a plurality of adjusted first shim parameter sets for said first shim channel using said third B0 field map, with said first shim channel being adjusted using said plurality of adjusted first shim parameter sets for acquiring said MR data.

9. A method as claimed in claim 1 comprising operating said MR scanner to acquire said MR data by sequential steps comprising: adjusting said second shim channel using said second shim parameter set; thereafter adjusting said first shim channel using a first shim parameter set determined for a first section among said multiple sections; thereafter acquiring MR data from said first section; thereafter adjusting said first shim channel using a first shim parameter set determined for a second section among said multiple sections; and thereafter acquiring MR data from said second section.

10. A method as claimed in claim 9 comprising operating said MR scanner to cause a second settling time to elapse between adjustment of said second shim channel and acquiring said MR data from said first section, and to cause a first settling time to elapse between adjusting said first shim channel using said first shim parameter set and acquiring the MR data from the first section, with said first settling time being shorter than said second settling time.

11. A method as claimed in claim 9 comprising, after adjusting said second shim channel, operating said MR scanner to acquire a fourth B0 field map, and determining a plurality of altered first shim parameter sets for said first shim channel using said fourth B0 field map and, during acquisition of said MR data, adjusting said first shim channel using said plurality of altered first shim parameter sets.

12. A magnetic resonance (MR) apparatus comprising: an MR scanner comprising a shim unit, said shim unit comprising a first shim channel volume having at least one first shim channel and a second shim channel volume having at least one second shim channel; a computer configured to divide an examination region of an examination subject into multiple sections; said computer being configured to determine a plurality of first shim parameter sets for said first shim channel, with one first shim channel parameter set among said plurality of first shim channel parameter sets being determined for each section among said multiple sections; said computer being configured to determine a second shim parameter set for said second shim channel dependent on said plurality of determined first shim parameter sets; said computer being configured to adjust, before acquiring said magnetic resonance data from the examination region, said second shim channel using said second shim parameter set and to adjust said first shim channel for acquiring the magnetic resonance data from a specific section among said multiple sections using a first shim parameter set determined for that specific section; and said computer being configured to operate said MR scanner to acquire MR data from said specific section using the adjusted first shim channel and the adjusted second shim channel.

13. A non-transitory, computer-readable data storage medium encoded with programming instructions for operating a magnetic resonance (MR) apparatus to acquire MR data from an examination region of an examination object situated in an MR scanner of the MR apparatus that comprises a shim unit, said shim unit comprising a first shim channel volume having at least one first shim channel and a second shim channel volume having at least one second shim channel, said MR apparatus comprising a control computer and said programming instructions causing said control computer to: divide said examination region into multiple sections; determine a plurality of first shim parameter sets for said first shim channel, with one first shim channel parameter set among said plurality of first shim channel parameter sets being determined for each section among said multiple sections; determine a second shim parameter set for said second shim channel dependent on said plurality of determined first shim parameter sets; before acquiring said magnetic resonance data from the examination region, adjust said second shim channel using said second shim parameter set and adjust said first shim channel for acquiring the magnetic resonance data from a specific section among said multiple sections using a first shim parameter set determined for that specific section; and operate said MR scanner to acquire MR data from said specific section using the adjusted first shim channel and the adjusted second shim channel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically illustrates an inventive magnetic resonance apparatus.

(2) FIG. 2 is a flowchart of a first embodiment of the inventive method.

(3) FIG. 3 is a flowchart of a second embodiment of the inventive method.

(4) FIG. 4 is a flowchart of a third embodiment of the inventive method.

(5) FIG. 5 is a flowchart of a fourth embodiment of the inventive method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) FIG. 1 schematically shows an inventive magnetic resonance apparatus 11. The magnetic resonance apparatus 11 has a data acquisition scanner that includes a magnet unit 13, having a basic field magnet 17 that generates a strong, constant basic magnetic field 18. The scanner 32 has a cylindrical patient-receiving region 14 for receiving an examination object 15, in the present case a patient, with the patient-receiving region 14 being cylindrically surrounded in a circumferential direction by the magnet unit 13. The patient 15 can be moved by a patient-positioning device 16 of the magnetic resonance apparatus 11 into the patient-receiving region 14. The patient-positioning device 16 has for this purpose an examination table arranged so as to move inside the scanner 32. The magnet unit 13 is shielded from the outside by a housing shell 31.

(7) The scanner 32 also has a gradient coil arrangement 19 that generates magnetic field gradients that are used for spatial encoding during imaging. The gradient coil arrangement 19 is controlled by a gradient control processor 28. The scanner 32 also has a radio-frequency antenna 20, which is designed in the illustrated case as a body coil permanently integrated in the scanner 32, and a radio-frequency antenna control processor 29 that operates the antenna 20 so as to excite nuclear spins in the object 15 so as to cause the excited nuclear spins to deviate from the polarization that is established in the basic magnetic field 18 generated by the basic field magnet 17. The radio-frequency antenna unit 20 is controlled by the radio-frequency antenna control processor 29 and to radiate radio-frequency magnetic resonance sequences into an examination space that is essentially formed by the patient-receiving region 14. The radio-frequency antenna unit 20 is also designed to receive magnetic resonance signals, in particular from the patient 15 that occur as the excited nuclear spins relax.

(8) For controlling the basic field magnet 17, the gradient control processor 28 and the radio-frequency antenna control processor 29, the magnetic resonance apparatus 11 has a control computer 24. The control computer 24 centrally controls the magnetic resonance apparatus 11, such as, to carry out a predetermined imaging gradient echo sequence. Control information, such as imaging parameters, and reconstructed magnetic resonance images, can be supplied for a user at an output interface device 25, in the present case a display monitor, of the magnetic resonance apparatus 11. Furthermore, the magnetic resonance apparatus 11 has an input interface device 26, via which a user can enter information and/or parameters during a scanning process. The control computer 24 can include the gradient control processor 28 and/or radio-frequency antenna control processor 29 and/or the output interface device 25 and/or the input interface device 26. In the illustrated case the control computer 24 has a division processor 38, a first ascertaining processor 33 and a second ascertaining processor 34. These processors can be separate from each other or combined.

(9) The scanner also has a shim device 35, having a first shim channel volume having at least one first shim channel 36, and a second shim channel volume having at least one second shim channel 37.

(10) The first shim channel volume includes, for example, three shim channels 36. In the case illustrated in FIG. 1, the three first shim channels 36 are formed by the three gradient coils of the gradient coil arrangement 19. These three first shim channels 36 generate shim fields in the x-direction, y-direction and in the z-direction. As an example, the x-direction proceeds along a horizontal body axis of the examination object 15 lying on his or her back, the y-direction proceeds along the vertical body axis of the examination object 15, and the z-direction proceeds along a sagittal axis of the examination object 15. For different reasons, the gradient coils and their gradient amplifiers are designed such that the gradient fields can be varied (switched) on a timescale of a few microseconds. In other words, this time is short compared to the time that elapses between acquisition of data from different sections and/or slices of an examination region. The three first shim channels 36 of the first order thus can be used for dynamic shimming. The first shim channel volume also has an RF center frequency. This RF center frequency reproduces the mean deviation of the resonance frequency within the section, to which the respective first shim parameter set is allocated, from the RF center frequency that was adjusted during acquisition of the first B0 field map. The RF center frequency can be formally treated like a shim channel, namely a shim channel of the 0.sup.th order.

(11) The second shim channel volume has, for example, five dedicated second shim channels 37. These five second shim channels 37 can generate shim fields of the second order. These shim fields are proportional to the current flowing through the second shim channels 37 and the spatial characteristic of the shim fields can be described, for example, in a good approximation by z.sup.2−(x.sup.2+y.sup.2)/2), xz, yz, (x.sup.2−y.sup.2)/2 and xy. The amplifiers associated with the second shim channels 37 are designed such that a settling time in the order of one second or longer elapses between the change in the current through the second shim channel 37 and the adjustment of the desired field. This settling time is longer than the typical time between acquisition of data from different sections and/or slices of an examination region. The five second shim channels 37 of the second order thus cannot typically be used for dynamic shimming. Of course the second shim channels 37 can also include shim channels of a higher order, which generate, for example, shim fields of a third or fourth order. The first shim channel volume and the second shim channel volume are disjunct in the illustrated case.

(12) The magnetic resonance apparatus 11 is designed, together with the image data acquisition scanner 32, the control computer 24 and shim arrangement 35 to implement the inventive method for magnetic resonance imaging.

(13) The illustrated magnetic resonance apparatus 11 can of course have further components that magnetic resonance apparatuses conventionally have. The general operation of a magnetic resonance apparatus is known to those skilled in the art, moreover, so a detailed description of the further components is not necessary herein.

(14) FIG. 2 is a flowchart of a first embodiment of an inventive method for magnetic resonance imaging in an examination region of an examination object 15 by operation of the magnetic resonance apparatus 11 in particular the scanner 32 and shim unit 35 thereof.

(15) In a first method step 39, the examination region is divided into a plurality of sections by the division processor 38. For example, each excitation slice of a slice stack forms a section of the plurality of sections.

(16) In a further method step 40, a number of first shim parameter sets is ascertained for the at least one first shim channel 36, wherein one first shim parameter set respectively of the multiple first shim parameter sets is ascertained respectively for the multiple sections by the first ascertaining processor 33.

(17) In a further method step 41, a number of second shim parameter sets is ascertained for the at least one second shim channel 37 by taking into account the multiple first ascertained shim parameter sets, by the second ascertaining processor 34.

(18) In a further method step 42, magnetic resonance image data of the examination region of the examination object 15 are acquired by operation of the image data acquisition scanner 32, wherein, before acquisition of the magnetic resonance image data, the at least one second shim channel 37 is adjusted using the second shim parameter set, and the at least one first shim channel 36 is adjusted for acquiring the magnetic resonance image data from a specific section of the multiple sections using a first shim parameter set ascertained for the specific section.

(19) The acquired magnetic resonance image data can be presented on the display monitor of the output interface device 25 and/or stored in a database.

(20) FIG. 3 is a flowchart of a second embodiment of the inventive method for magnetic resonance imaging in an examination region of an examination object 15 by operation of the magnetic resonance apparatus 11.

(21) The following description is essentially limited to the differences from the exemplary embodiment in FIG. 2, with reference being made with respect to unchanging method steps to the description of the exemplary embodiment in FIG. 2. Method steps that essentially stay the same are basically numbered with the same reference numerals.

(22) The embodiment of the inventive method shown in FIG. 3 essentially includes method steps 39, 40, 41, 42 of the first embodiment of the inventive method according to FIG. 2. The embodiment of the inventive method shown in FIG. 3 has additional method steps and substeps. An alternative method sequence to FIG. 3 is also conceivable that has just some of the additional method steps and/or substeps shown in FIG. 2. Of course an alternative method sequence to FIG. 3 can also have additional method steps and/or substeps.

(23) In the example of FIG. 3 it is assumed for a better overview that the examination region is divided into two sections in the first method step 39, i.e., for example, two individual slices. The concept may of course be arbitrarily expanded to a division into more sections.

(24) In a further method step 43 a first B0 field map is acquired before ascertaining the plurality of first shim parameter sets. The first B0 field map reproduces local deviations from an ideal constant basic magnetic field 18 within the multiple sections of the examination region. The B0 field map can be a frequency map that reproduces a local deviation of a resonance frequency from a system frequency. A frequency map Δf(x,y,z) and a B0 field map ΔB.sub.0(x,y,z) may typically be converted such that:
Δf(x,y,z)=(γ/(2π)×ΔB.sub.0(x,y,z)

(25) Here γ/(2π) is the gyromagnetic ratio, which for protons is 42.576 MHz/T.

(26) The plurality of first shim parameter sets is ascertained in a further method step 40 using the first B0 field map. In a first substep 40-1 of the further method step 40 a first section shim parameter set is ascertained using the first B0 field map and in a second substep 40-2 of the further method step 40 a second section shim parameter set is ascertained using the first B0 field map. The first section shim parameter set is, in particular, a first shim parameter set that has been ascertained for the first section. The second section shim parameter set is a first shim parameter set that has been ascertained for the second section. Of course further section shim data sets can also be ascertained. In particular, a section shim parameter set can be ascertained for each slice of the examination region.

(27) The first shim parameter set, which was ascertained for the first section, Δf.sub.0.sup.(1), ΔG.sub.x.sup.(1), ΔG.sub.y.sup.(1), ΔG.sub.z.sup.(1) has a first resonance frequency Δf.sub.0.sup.(1) for the frequency adjustment, a first x-gradient offset current ΔG.sub.x.sup.(1) for the gradient coil in the x-direction, a first-y-gradient offset current ΔG.sub.y.sup.(1) for the gradient coil in the y-direction and a first z-gradient offset current ΔG.sub.z.sup.(1) for the gradient coil in the z-direction. The first shim parameter set, which was ascertained for the first section, Δf.sub.0.sup.(1), ΔG.sub.x.sup.(1), ΔG.sub.y.sup.(1), ΔG.sub.z.sup.(1) is advantageously chosen such that local field deviations ΔB.sub.0(x,y,z) in the first section are optimally compensated.

(28) The first shim parameter set, which was ascertained for the second section, Δf.sub.0.sup.(2), ΔG.sub.x.sup.(2), ΔG.sub.y.sup.(2), ΔG.sub.z.sup.(2) comprises, in particular, a second resonance frequency Δf.sub.0.sup.(2) for the frequency adjustment, a second x-gradient offset current ΔG.sub.x.sup.(2) for the gradient coil in the x-direction, a second y-gradient offset current ΔG.sub.y.sup.(2) for the gradient coil in the y-direction and a second z-gradient offset current ΔG.sub.z.sup.(2) for the gradient coil in the z-direction. The first shim parameter set, which was ascertained for the second section, Δf.sub.0.sup.(2), ΔG.sub.x.sup.(2), ΔG.sub.y.sup.(2), ΔG.sub.z.sup.(2) is advantageously chosen such that local field deviations ΔB.sub.0(x,y,z) in the second section are optimally compensated.

(29) To ascertain the shim parameter sets for the first and/or second section an equation with the following form can be deployed for each pixel of the first B0 field map within the first section and/or the second section:

(30) $-\Delta B_o\left(x,y,z\right)=\frac{2\pi }{\gamma }\Delta f_0+\Delta G_x\left(x-x_0\right)+\Delta G_y\left(y-y_0\right)+\Delta G_z\left(z-z_0\right)$
wherein (x,y,z) is the coordinate of the respective pixel, (x0,y0,z0) the coordinate of an isocenter of the gradient coil arrangement 19 within the first B0 field map. The isocenter is the location at which the field contribution of the gradient coil arrangement 19 is zero. ΔG.sub.x indicates a change in the gradient field in the x-direction of the first B0 field map with respect to the adjustment during acquisition of the B0 field map. Accordingly, ΔG.sub.y and ΔG.sub.z indicate a change in the gradient field in the y and z-directions respectively of the B0 field map with respect to the corresponding adjustment during acquisition of the B0 field map. Δf.sub.0 indicates a change in an RF center frequency with respect to the adjustment during acquisition of the B0 field map.

(31) As a whole, these equations form an overdetermined linear equation system for each section of the examination region, and this can be solved using standard methods. The four unknowns Δf.sub.0.sup.(1), ΔG.sub.x.sup.(1), ΔG.sub.y.sup.(1), ΔG.sub.z.sup.(1) of the first shim parameter set for the first section or Δf.sub.0.sup.(2), ΔG.sub.x.sup.(2), ΔG.sub.y.sup.(2), ΔG.sub.z.sup.(2) of the first shim parameter set for the second section therefore can each be ascertained. If creation of the B0 field map is based on the scan of the phase difference between acquired MR images with a different echo time then no information about the local B0 field deviation can be obtained from pixels that essentially contain only noise. Pixels of this kind should therefore be excluded from the solution to the equation systems. This can occur by way of background segmenting and/or weighting of the individual equations proportionally to the pixel value of a magnitude image at the location (x,y,z). The magnitude image can likewise be extracted from the magnetic resonance scan data that is acquired in order to create the first B0 field map.

(32) In a further method step 44 a second B0 field map is calculated using the first B0 field map and using the plurality of first shim parameter sets. The second B0 field map is calculated using the first B0 field map and using the plurality of first shim parameter sets such that B0 field contributions, resulting from the plurality of first shim parameter sets, are offset against the first B0 field map. Here, in particular, a spatial section of the first B0 field map is offset against the B0 field contributions, resulting from an appropriate suitable first shim parameter set of the plurality of first shim parameter sets, with the appropriate first shim parameter set having been specifically ascertained for the section of the plurality of sections that corresponds to the spatial section.

(33) In the illustrated case a first spatial section of the second B0 field map is calculated in a first substep 44-1 of the further method step 44 using the first spatial section of the first B0 field map and the first shim parameter set ascertained for the first section. In a second substep 44-2 of the further method step 44 a second spatial section of the second B0 field map is calculated using the second spatial section of the first B0 field map and the first shim parameter set ascertained for the second section. The second B0 field map then comprises the two ascertained spatial sections.

(34) To calculate the second B0 field map the B0 field contributions of the first shim parameter set ascertained for the respective section for the at least one first shim channel 36 are added to the first B0 field map pixel-by-pixel, for example for each section. This means that the second B0 field map represents a new virtual B0 field map ΔB.sub.0′(x,y,z), to which the following applies:

(35) $\Delta B_0^{\prime }\left(x,y,z\right)=\Delta B_o\left(x,y,z\right)+\{\begin{array}{c}\frac{2\pi }{\gamma }\Delta f_0^{\left(1\right)}+{\mathrm{\Delta G}}_x^{\left(1\right)}\left(x-x_0\right)+{\mathrm{\Delta G}}_y^{\left(1\right)}\left(y-y_0\right)+{\mathrm{\Delta G}}_z^{\left(1\right)}\left(z-z_0\right),\mathrm{if}\left(x,y,z\right)\in V1\\ \frac{2\pi }{\gamma }\Delta f_0^{\left(2\right)}+{\mathrm{\Delta G}}_x^{\left(2\right)}\left(x-x_0\right)+{\mathrm{\Delta G}}_y^{\left(2\right)}\left(y-y_0\right)+{\mathrm{\Delta G}}_z^{\left(2\right)}\left(z-z_0\right),\mathrm{if}\left(x,y,z\right)\in V2\end{array}$

(36) Therein V1 denotes the first section, V2 the second section and the element sign from the set theory here represents “the pixel considered having coordinates (x,y,z) belongs to the first section V1 or to the second section V2”. The second (virtual) B0 field map ΔB.sub.0′(x,y,z) will, generally, have lower B0 field deviations than the original scanned first B0 field map ΔB.sub.0(x,y,z). Locally within a section the second B0 field map can approximately reproduce the B0 field characteristic following adjustment of the at least one first shim channel 37 using the respective first shim parameter sets.

(37) In further method step 41, ascertaining the second shim parameter set, the plurality of first shim parameter sets is taken into account when ascertaining the second shim parameter such that the second shim parameter set is ascertained using the second B0 field map. The second shim parameter set is advantageously ascertained such that the B0 field reproduced by the second B0 field map ΔB.sub.0′(x,y,z) is optimally compensated.

(38) The field contribution ΔB.sub.j(x,y,z) of the j.sup.th shim channel of the at least one second shim channel 37 at location (x,y,z) is proportional to the shim current I.sub.j in the j.sup.th shim channel:
ΔB.sub.j(x,y,z)=C.sub.j(x,y,z).Math.P.sub.j.Math.I.sub.j

(39) C.sub.j(x,y,z) is the known standardized field distribution of the second shim channel j and P.sub.j the likewise known location-independent sensitivity of the second shim channel j. The index j assumes values of 1, . . . , N2, where N2 is the number of second shim channels 37. To ascertain the unknown shim currents I.sub.1, . . . , I.sub.N2 an equation with the following form can be deployed for each pixel in the examination region:

(40) $-\Delta B_o^{\prime }\left(x,y,z\right)=\frac{2\pi }{\gamma }\Delta f_0+\underset{j=1}{\overset{N2}{.Math.}}\Delta B_j\left(x,y,z\right)=\frac{2\pi }{\gamma }\Delta f_0+\underset{j=1}{\overset{N2}{.Math.}}{C}_j\left(x,y,z\right).Math.P_j.Math.\Delta I_j$

(41) The examination region is here intended to mean the combined volume of the two sections. As a whole said equations therefore again form an overdetermined linear equation system: overdetermined because the number of pixels in the examination region is generally very much greater than the number of unknowns. The sought unknowns in this equation system are the N2 values ΔI.sub.j which indicate the change in the current in the j.sup.th shim channel compared to the adjustment during acquisition of the first B0 field map, and again a term Δf.sub.0 without location dependency. The overdetermined equation system can be solved, for example, within the sense of the smallest quadratic deviation using standard methods. The result of step 41 is therefore a second shim parameter set Δf.sub.0, I.sub.1, . . . , I.sub.N2, for adjustment of the at least one second shim channel 37.

(42) The magnetic resonance image data are typically acquired in further method step 42 in a nested 2D multi-slice scan. The number of slices can typically be between 5 and 50. Each of these slices can have a thickness of typically 2 mm to 10 mm. The field of view of a slice is typically between 200×200 mm.sup.2 (for example in the case of an axial head examination) and 400×400 mm.sup.2 (for example in the case of an abdominal examination). As a rule, the slices are oriented parallel to each other with a gap of 0 to 50 percent of the slice thickness between adjacent slices. Each of these slices and/or a section of each slice can represent a section of the examination region. In the illustrated case there are only two sections of the examination region for the sake of clarity, however. The time between acquisition of data of different slices is TR/N, where TR is the repetition time and N the number of slices from which data is acquired in a repetition interval. The repetition time TR indicates the time between successive excitations of a specific slice. Depending on the underlying sequence technique, the TR time lies between a few ms and a few seconds. The time between acquisition of data from different slices is accordingly reduced by a factor N. The parameters described here are only examples. The magnetic resonance image data can also be acquired in further method step 42 using different scanning parameters to those described.

(43) Acquiring the magnetic resonance image data in further method step 42 has a first substep 42-1 in which firstly the at least one second shim channel 37 is adjusted using the second shim parameter set. For example, the five second shim channels 37 are adjusted using the shim currents I.sub.1, . . . , I.sub.5.

(44) In a second substep 42-2 of the further method step 42 a second settling time elapses between adjustment of the at least one second shim channel 37 and acquisition of the magnetic resonance image data from the first section in a fourth substep 42-4. This second settling time is longer than a first settling time which elapses between adjustment of the at least one first shim channel 36 using the first shim parameter set, which was ascertained for the first section, and acquisition of the magnetic resonance image data from the first section. The settling time is waited for until the actual field contribution of the at least one second shim channel 37 matches the calculated field contribution.

(45) In a third substep 42-3 of further method step 42 the at least one first shim channel 36 is then adjusted using the first shim parameter set which was ascertained for the first section. In the example the gradient offset currents ΔG.sub.x.sup.(1), ΔG.sub.y.sup.(1), ΔG.sub.z.sup.(1) are therefore added to the gradient offset currents which were adjusted during acquisition of the first B0 field map. Furthermore, the RF enter frequency can be changed by Δf.sub.0.sup.(1)+Δf.sub.0 with respect to the value during acquisition of the first B0 field map. Here Δf.sub.0.sup.(1) is the change in center frequency ascertained in step M3 for the first section and Δf.sub.0 is the constant term without location dependency from the second shim parameter set.

(46) In a fourth substep 42-4 of the further method step 42 magnetic resonance image data is then acquired from the first section. The at least one first shim channel 36 remains adjusted for acquisition of the magnetic resonance image data from the first section using the first shim parameter set which was ascertained for the first section.

(47) In a fifth substep 42-5 of the further method step 42 the at least one first shim channel 36 is then adjusted using the first shim parameter set which was ascertained for the second section. For this purpose the gradient offset currents ΔG.sub.x.sup.(2), ΔG.sub.y.sup.(2), ΔG.sub.z.sup.(2) are added to the gradient offset currents which were adjusted during acquisition of the first B0 field map. Adjustment of the RF center frequency is changed by Δf.sub.0.sup.(2)+Δf.sub.0 with respect to the adjustment during acquisition of the first B0 field map.

(48) It should be noted in this connection that when adjusting the at least one first shim channel 36 for a specific section only the adjustments between transmitting and receiving paths of the magnetic resonance sequence can be active. Whether, for example, the gradient offset currents are switched on before a first radio-frequency excitation pulse within a section or, for example, only with a slice selection gradient of the first radio-frequency excitation pulse, is irrelevant if physical effects as a consequence of the altered gradient current are not taken into account. Due to unavoidable eddy currents as a consequence of the altered gradient current, which decay exponentially with a typical time constant, it can be expedient to adjust the at least one first shim channel 36 using the shim parameter sets as early as possible without extending the total scanning time.

(49) In a sixth substep 42-6 of further method step 42 magnetic resonance image data are then acquired from the second section. The at least one first shim channel 36 remains adjusted for acquisition of the magnetic resonance image data from the second section using the first shim parameter set which was ascertained for the second section.

(50) The method steps 42-3 to 42-6 can be repeated until all magnetic resonance data is acquired from the two sections which is necessary for reconstruction of the images.

(51) In a seventh substep 42-7 of the further method step 42 the magnetic resonance image data, which map the first section, and the magnetic resonance image data, which map the second section, are combined to form an overall magnetic resonance image data set. Magnetic resonance images from the two sections can then be reconstructed from this overall magnetic resonance image data set. The images can then be output, for example, on the display unit 25 and/or stored in a database.

(52) FIG. 4 is a flowchart of a third embodiment of the inventive method for magnetic resonance imaging in an examination region of an examination object 15 by means of a magnetic resonance apparatus 11.

(53) The following description is essentially limited to the differences from the exemplary embodiment in FIG. 3, with reference being made in respect of unchanging method steps to the description of the exemplary embodiment in FIG. 3. Method steps that essentially stay the same are basically numbered with the same reference numerals.

(54) In contrast to the approach shown in FIG. 3, the at least one first shim channel 36 is not adjusted during acquisition of the magnetic resonance image data in further method step 42 using the number of first shim parameter sets ascertained in further method step 40.

(55) Instead, according to FIG. 4, a third B0 field map is calculated in a further method step 45 using the first B0 field map and the second shim parameter set. This can take account of the fact that the modified shim currents in the at least one second shim channel 37 influence the B0 field in the sections of the examination region. When calculating the first shim parameter sets of the at least one first shim channel 36 in further method step 40 this contribution is not initially taken into account. For this reason the known field contributions of the at least one second shim channel 37 are added to the original first B0 field map ΔB.sub.0(x,y,z), with a third B0 field map ΔB.sub.0″(x,y,z) being generated:

(56) $\Delta B_0^{″}\left(x,y,z\right)=\Delta B_o\left(x,y,z\right)+\frac{2\pi }{\gamma }\Delta f_0+\underset{j=1}{\overset{N2}{.Math.}}\Delta C_j\left(x,y,z\right).Math.P_j.Math.\Delta I_j$

(57) The new third B0 field map ΔB.sub.0″(x,y,z) at least approximately reproduces (apart from the location-independent term (2π/γ)Δf.sub.0) the field characteristic following adjustment of the at least one second shim channel 37.

(58) In a further method step 46 a number of adjusted first shim parameter sets is then ascertained for the at least one first shim channel 36 using the third B0 field map. In a first substep 46-1 of the further method step 46 an adjusted first shim parameter set is therefore ascertained for the first section, and in a second substep 46-2 of the further method step 46 an adjusted second shim parameter set is ascertained for the second section. The result is a new set of shim adjustments Δf.sub.0′.sup.(1), ΔG.sub.x′.sup.(1), ΔG.sub.y′.sup.(1), ΔG.sub.z′.sup.(1) for the first section and Δf.sub.0′.sup.(2), ΔG.sub.x′.sup.(2), ΔG.sub.y′.sup.(2), ΔG.sub.z′.sup.(2) for the second section.

(59) During acquisition of the magnetic resonance image data in further method step 42 the at least one first shim channel 36 is adjusted using the number of adjusted first shim parameter sets. In a third substep 42-3 of the further method step 42 the at least one first shim channel 36 is therefore adjusted for the first section using the adjusted first shim parameter set. In a fifth substep 42-5 of the further method step 42 the at least one first shim channel 37 is then adjusted for the second section using the adjusted second shim parameter set.

(60) It should be noted that the adjusted first shim parameter sets and adjusted second shim parameter sets can also be ascertained iteratively. For this purpose, method steps 45, 46, 44, 41 are each performed in the given sequence in an i.sup.th iteration step. In method step 45 a B0″(i) field map is calculated using the first B0 field map and the second shim parameter set of the preceding iteration step. In method step 46 adjusted first shim parameter sets are calculated for the individual sections using the B0 field map B0″(i) calculated in respective method step 45. In method step 44 a further B0 field map B0′(i) is calculated using the B0″(i) field map (step 45) calculated in the respective iteration step and the adjusted first shim parameter sets for the individual sections ascertained respectively in method step 46. In method step 41 an adjusted second shim parameter set is in each case calculated on the basis of the B0 field map B0′(i) calculated in method step 41 of the i.sup.th iteration step. An abort criterion of the iterative method is if, compared to the adjusted first shim parameter set of the (i−1).sup.th iteration step, the change in the adjusted first shim parameter sets of the i.sup.th step of each section is smaller for the corresponding section than a first threshold set predefined for the first shim channels and, compared to the second shim parameter set calculated in the (i−1).sup.th step, the change in the second shim parameter set calculated in the i.sup.th iteration step is smaller than a second threshold set predefined for the second shim channels, or if a predefined maximum number of iteration steps is exceeded.

(61) FIG. 5 is a flowchart of a fourth embodiment of an inventive method for magnetic resonance imaging in an examination region of an examination object 15 by means of a magnetic resonance apparatus 11.

(62) The following description is essentially limited to the differences from the exemplary embodiment in FIG. 3, with reference being made in respect of unchanging method steps to the description of the exemplary embodiment in FIG. 3. Method steps that essentially stay the same are basically numbered with the same reference numerals.

(63) In contrast to the approach shown in FIG. 3, the at least one first shim channel 36 is not adjusted for the section before acquisition of the magnetic resonance image data from a section in a further method step 42 using the first shim parameter set ascertained in further method step 40. This embodiment also takes into account the influence of the at least one second shim channel 37 in the calculation of the first shim parameter sets for the at least one first shim channel 36. In contrast to the third embodiment according to FIG. 4, the field characteristic following adjustment of the at least one second shim channel 37 is not calculated, however, but actually scanned.

(64) Instead, according to FIG. 5, a fourth B0 field map is acquired in a further substep 42-X of the further method step 42 following adjustment of the at least one second shim channel 37 in a first substep 42-1 of the further method step 42. The fourth B0 field map is acquired, in particular, at the earliest when the second settling time in a further substep 42-2 of further method step 42 has elapsed.

(65) In a further substep 42-Y of further method step 42 a plurality of altered first shim parameter sets is then ascertained for the at least one first shim channel 36 using the fourth B0 field map. In a first sub-substep 42Y-1 of substep 42-Y an adjusted first shim parameter set is therefore ascertained for the first section, and in a second sub-substep 42Y-2 of substep 42-Y an adjusted second shim parameter set is ascertained for the second section. The result is again a center frequency Δf.sub.0″.sup.(1), and a set of first shim parameter sets ΔG.sub.x″.sup.(1), ΔG.sub.y″.sup.(1), ΔG.sub.z″.sup.(1) for the at least one first shim channel 36 for each section.

(66) The at least one first shim channel 36 is adjusted using the plurality of adjusted first shim parameter sets during acquisition of the magnetic resonance image data in further method step 42. In a third substep 42-3 of further method step 42 the at least one first shim channel 36 is therefore adjusted for the first section using the adjusted first shim parameter set. In a fifth substep 42-5 of further method step 42 the at least one first shim channel 37 is then adjusted for the second section using the adjusted second shim parameter set.

(67) The first B0 field map and/or fourth B0 field map can be acquired using raw data which comprises at least three echo signals which are each acquired following an excitation of the examination region.

(68) The method steps of the inventive method shown in FIG. 2-5 are performed by the computer 24. For this purpose, the computer 24 has the requisite software and/or computer programs that are stored in a memory of the computer 24. The software and/or computer programs comprise program means which are configured to carry out the inventive method when the computer program and/or the software are run in the computer 24 by operation of a processor of the computer 24.

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