METHOD AND MAGNETIC RESONANCE APPARATUS FOR ACQUIRING MR DATA FROM MULTIPLE SLICES SIMULTANEOUSLY

Abstract

In a method and apparatus for acquiring magnetic resonance (MR) data from a predetermined volume within an examination object, a control protocol for a gradient echo sequence is selected that specifies that gradient moments produced in said gradient echo sequence be balanced along all three spatial directions. In this gradient echo sequence a slice selection gradient is activated in a slice selection direction that produces a balanced gradient moment, with simultaneous radiation of an RF pulse that simultaneously excites nuclear spins in multiple slices of the examination object, with said excitation being repeated according to a repetition time. A phase of MR signals to be acquired from a same one of said multiple layers is varied from repetition time-to-repetition time. An additional gradient is activated in the slice selection gradient that produces an additional gradient moment that is constant over consecutive repetition times and thus overrides the condition of the gradient moments of the gradient echo sequence being balanced along said slice selection direction. The MR signals are acquired during activation of a readout gradient.

Claims

1. A method for acquiring magnetic resonance (MR) data from a predetermined volume within an examination object, comprising: from a control computer, operating an MR data acquisition scanner according to a control protocol for a gradient echo sequence that specifies that gradient moments produced in said gradient echo sequence be balanced along all three spatial directions; from said control computer, operating said MR scanner in said gradient echo sequence by activating a slice selection gradient in a slice selection direction, said slice selection gradient producing a balanced gradient moment; from said control computer, operating said MR scanner in said gradient echo sequence by, simultaneously with activating said slice selection gradient, radiating an RF pulse that simultaneously excites nuclear spins in a plurality of slices of the examination object, with said excitation being repeated according to a repetition time; from said control computer, operating said MR scanner in said gradient echo sequence by varying a phase of MR signals to be acquired from a same one of said plurality of slices from repetition time-to-repetition time; from said control computer, operating said MR scanner in said gradient echo sequence by activating an additional gradient in the slice selection gradient, in addition to said slice selection gradient, said additional gradient producing an additional gradient moment that is constant over consecutive repetition times and that overrides the condition of the gradient moments of the gradient echo sequence being balanced along said slice selection direction; and from said control computer, operating said MR scanner in said gradient echo sequence by activating a readout gradient during which said MR signals are acquired.

2. A method as claimed in claim 1 comprising activating said additional gradient so as to be constant during all repetition times.

3. A method as claimed in claim 2 comprising activating said additional gradient at at least one time selected from the group consisting of before each RF excitation pulse and after each RF excitation pulse.

4. A method as claimed in claim 3 comprising maintaining a profile of said additional gradient so as to be identical over consecutive repetition times.

5. A method as claimed in claim 1 comprising in said control computer, determining said additional gradient moment dependent on at least one of a distance between at least two of the plurality of slices, and a difference in phase increment of said RF excitation pulses of said slices.

6. A method as claimed in claim 5 comprising in said control computer, determining said additional gradient moment DG according to: $\begin{array}{cc}\mathrm{DG}=\frac{\mathrm{PD}}{\gamma .Math.d},& \left(A.Math..Math.1\right)\end{array}$ wherein PD is the difference in phase increment of the RF excitation pulses of the slices, d is the distance between the slices and y is the gyromagnetic ratio of said nuclear spins.

7. A method as claimed in claim 1 wherein said MR scanner generates a basic magnetic field during said gradient echo sequence, said basic magnetic field having an isocenter, and wherein a first of said slices has a distance d0 in a predetermined direction from said isocenter and a second of said slices has a distance d1 in said predetermined direction from said isocenter, and wherein a k.sup.th RF excitation pulse of said first slice has a phase Po(k), with
P.sub.0(k)=−k*90°−k*Φ.sub.G+Φ.sub.C0 and wherein k.sup.th RF excitation pulse of the second layer has a phase P.sub.i(k), with
P.sub.1(k)=+k*90°−k*Φ.sub.G+Φ.sub.C1 wherein Φ.sub.C0 is a constant phase of the first slice and Φ.sub.C1 is a constant phase of the second slice, wherein Φ.sub.G is a phase increment that satisfies the equation: $.Math.{\Phi }_G=90.Math.°.Math.\frac{\left(d.Math..Math.0+d.Math..Math.1\right)}{\left(d.Math..Math.0-d.Math..Math.1\right)}$ wherein k begins at zero and runs over all rows of the respective slice.

8. A method as claimed in claim 7 wherein a phase Φ.sub.E of a receiver of the magnetic resonance scanner obeys the following equation in order to acquire said MR signals:
Φ.sub.E=X(k)+k*Φ0−k*Φ.sub.G wherein X(k)=180° if k is an odd number and otherwise X(k)=0°.

9. A method as claimed in claim 1 comprising varying the phase of the MR signals to be acquired for each of the slices to be acquired simultaneously.

10. A method as claimed in claim 1 comprising: varying the phase of the MR signals to be acquired by establishing a further gradient before and after the excitation pulse in the slice selection direction; and activating said further gradient before a respective RF excitation pulse with a gradient moment that corresponds to a gradient moment produced by the further gradient moment activated after the respective RF excitation pulse, and with the gradient moment produced by the further gradient varying over consecutive repetition times.

11. A method as claimed in claim 10 wherein said gradient moment produced by the further gradient in a first repetition time corresponds to a negative further gradient moment produced by the further gradient in a second repetition time that directly follows said first repetition time.

12. A method as claimed in claim 1 comprising varying the phase of the MR signals to be acquired by varying the phase of the RF excitation pulses that excite a same one of said slices.

13. A method as claimed in claim 12 comprising varying the phase of the RF excitation pulses for each of the slices.

14. A magnetic resonance (MR) apparatus comprising: an MR data acquisition scanner; a control computer configured to operate said MR data acquisition scanner according to a control protocol for a gradient echo sequence that specifies that gradient moments produced in said gradient echo sequence be balanced along all three spatial directions; said control computer being configured to operate said MR scanner in said gradient echo sequence by activating a slice selection gradient in a slice selection direction, said slice selection gradient producing a balanced gradient moment; said control computer being configured to operate said MR scanner in said gradient echo sequence by, simultaneously with activating said slice selection gradient, radiating an RF pulse that simultaneously excites nuclear spins in a plurality of slices of the examination object, with said excitation being repeated according to a repetition time; said control computer being configured to operate said MR scanner in said gradient echo sequence by varying a phase of MR signals to be acquired from a same one of said plurality of slices from repetition time-to-repetition time; said control computer being configured to operate said MR scanner in said gradient echo sequence by activating an additional gradient in the slice selection gradient, in addition to said slice selection gradient, said additional gradient producing an additional gradient moment that is constant over consecutive repetition times and that overrides the condition of the gradient moments of the gradient echo sequence being balanced along said slice selection direction; and said control computer being configured to operate said MR scanner in said gradient echo sequence by activating a readout gradient during which said MR signals are acquired.

15. A non-transitory, computer-readable data storage medium encoded with programming instructions, said storage medium being loaded into a control computer of a magnetic resonance (MR) apparatus that comprises an MR data acquisition scanner, said programming instructions causing said control computer to: operating said MR scanner according to a control protocol for a gradient echo sequence that specifies that gradient moments produced in said gradient echo sequence be balanced along all three spatial directions; operate said MR scanner in said gradient echo sequence to activate a slice selection gradient in a slice selection direction, said slice selection gradient producing a balanced gradient moment; operate said MR scanner in said gradient echo sequence to radiate simultaneously with activating said slice selection gradient, an RF pulse that simultaneously excites nuclear spins in a plurality of slices of the examination object, with said excitation being repeated according to a repetition time; operate said MR scanner in said gradient echo sequence to vary a phase of MR signals to be acquired from a same one of said plurality of slices from repetition time-to-repetition time; operate said MR scanner in said gradient echo sequence to activate an additional gradient in the slice selection gradient, in addition to said slice selection gradient, said additional gradient producing an additional gradient moment that is constant over consecutive repetition times and that overrides the condition of the gradient moments of the gradient echo sequence being balanced along said slice selection direction; and operate said MR scanner in said gradient echo sequence to activate a readout gradient during which said MR signals are acquired.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] FIG. 1 illustrates the problem to be solved by the invention, in the form of frequency bands of two slices to be captured simultaneously, the frequency bands being shifted relative to each other.

[0064] FIG. 2 illustrates the basis of the invention.

[0065] FIG. 3 illustrates a first variant for producing the additional gradient moment according to the invention.

[0066] FIG. 4 illustrates a second variant for producing the additional gradient moment according to the invention.

[0067] FIG. 5 illustrates a third variant for producing the additional gradient moment according to the invention.

[0068] FIG. 6 illustrates a further problem to be solved by the invention.

[0069] FIG. 7 schematically illustrates a magnetic resonance installation according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0070] FIG. 7 is a schematic illustration of a magnetic resonance apparatus 5 according to the invention. A basic field magnet 1 of a scanner S produces a temporally constant, strong magnetic field for polarizing and/or aligning the nuclear spins in an examination region of an object O, e.g. part of a human body to be examined. The object O is examined while lying on a table 23 in the scanner S. The high degree of homogeneity of the basic magnetic field, which is required for the magnetic resonance acquisition, is defined in a typically spherical measuring volume M in which the volume section of the human body to be examined is situated. In order to satisfy the homogeneity requirements and in particular to eliminate temporally invariable influences, shim plates of ferromagnetic material are attached at suitable positions. Temporally variable influences are eliminated by shim coils 2.

[0071] A cylindrical gradient coil system 3 composed of three sub-windings is situated in the basic field magnet 1. Each sub-winding is supplied with current by an amplifier for the purpose of producing a linear (and temporally variable) gradient field in a respective direction of a Cartesian coordinate system. The first sub-winding of the gradient coil system 3 produces a gradient G.sub.x in the x-direction, the second sub-winding produces a gradient G.sub.y in the y-direction and the third sub-winding produces a gradient G.sub.z in the z-direction. Each amplifier has a digital-analog converter, which is activated by a sequence controller 18 for the purpose of producing gradient pulses at the correct times.

[0072] Situated within the gradient coil system 3 is a radio-frequency antenna 4 (or a number thereof), which converts the radio-frequency pulses provided by a radio-frequency power amplifier into an alternating field that excites certain nuclei so that the nuclear spin thereof in the object O to be examined or a region thereof are deflected out of alignment with the basic magnetic field. Each radio-frequency antenna 4 has one or more RF transmit coils and one or more RF receive coils in an annular arrangement of component coils, the arrangement being preferably linear or in the form of a matrix. The RF receive coils of the respective radio-frequency antenna 4 also convert the alternating field originating from the excited nuclear spins, into a voltage (measured signal), which is supplied via an amplifier 7 to a radio-frequency receive channel 8 of a radio-frequency system 22. In general the magnetic resonance signals are spin echo signals provoked by a pulse sequence composed of one or more radio-frequency pulses and one or more gradient pulses. The radio-frequency system 22, which is part of a control computer 10 of the magnetic resonance apparatus 5, further has a transmit channel 9 in which the radio-frequency pulses for the excitation of magnetic nuclear resonance are produced. The respective radio-frequency pulses in this case are represented digitally as a sequence of complex numbers in the sequence controller 18 on the basis of a pulse sequence specified by the system computer 20. This sequence of numbers is supplied as a real part and an imaginary part via respective inputs 12 to a digital-analog converter in the radio-frequency system 22, and from there to a transmit channel 9. In the transmit channel 9, the pulse sequences are modulated onto a radio-frequency carrier signal having a base frequency that corresponds to the resonance frequency of the nuclear spin in the measuring volume.

[0073] The changeover from transmit to receive mode is effected by a transmit/receive diplexer 6. The RF transmit coils of the radio-frequency antenna(s) 4 direct the radio-frequency pulses for exciting the nuclear spin into the measuring volume M, and resulting echo signals are sampled by the RF receive coil(s). The nuclear resonance signals thus obtained are demodulated in a phase-sensitive manner onto an intermediate frequency in the receive channel 8′ (first demodulator) of the radio-frequency system 22, digitized in the analog-digital converter (ADC), and output via the output 11. This signal is also demodulated onto the frequency 0. The demodulation onto the frequency 0 and the separation into real and imaginary parts takes place in a second demodulator 8 after digitization in the digital domain. The measured data thus obtained via an output 11 are used by an image processor 17 to reconstruct an MR image. The management of the measured data, the image data and the control programs is performed by the system computer 20. On the basis of control program specifications, the sequence controller 18 checks the production of the currently desired pulse sequences and the corresponding sampling of k-space. In this case, the sequence controller 18 controls the switching of the gradients at the correct time, the emission of the radio-frequency pulses with defined phase amplitudes, and the receipt of the nuclear resonance signals. The time base for the radio-frequency system 22 and the sequence controller 18 is provided by a synthesizer 19. The selection of corresponding control programs for producing an MR image, said programs being stored e.g. on a DVD 21, and the representation of the produced MR image are effected via a terminal 13 having a keyboard 15, a mouse 16 and a display screen 14.

[0074] According to the present invention, the sequence controller 18 is configured so as to also switch the additional gradient.

[0075] FIG. 2 illustrates the basis of the invention. In the situation illustrated in FIG. 2, the isocenter of the basic magnetic field lies at the midpoint between the two slices S0, S1, whose MR signals are to be captured simultaneously, and therefore the two slices S0, S1 have the same distance in numerical terms from the isocenter 32. For example, as a result of a modified CAIPIRINHA method, the frequency bands 32 of the two slices S0, S1 are shifted by 180° relative to each other. It is intended quasi to cancel out this shift by adapting the local Larmor frequency 34 (i.e. by setting the Larmor frequency of the slices S0, S1 correspondingly). This would result in the frequency bands 32 of the two slices S0, S1 lying quasi one above the other, whereby the effective readout bandwidth would have a maximum value.

[0076] For the purpose of adapting the local Larmor frequency 34, provision is inventively made for producing an additional gradient moment ΔM, by which the frequency band shift Δω) between the frequency bands 32 of the slices S0, S1 is cancelled out. In other words, the Larmor frequency is increased (decreased) by the additional gradient moment ΔM to an extent that is proportional to the distance from the isocenter 33 in a predefined direction (counter to the predefined direction). In this case, the predefined direction in FIG. 2 points upwardly (from slice S1 to slice S1).

[0077] FIG. 3 illustrates a first inventive embodiment variant for producing the additional gradient moment ΔM.

[0078] The gradient echo sequence illustrated in FIG. 3 for one of a number of slices to be captured simultaneously has an RF excitation pulse 41, which is applied at the same time as a slice selection gradient 42 is present. After the RF excitation pulse 41, a phase coding gradient 44 and a readout gradient 43 are activated. While the readout gradient 43 is present, the readout of the MR signals takes place during a determined time period 45. The specified number of degrees (0° or 180°) in the respective time period 45 indicates the corresponding phase position of the MR signals.

[0079] In the embodiment variant according to FIG. 3, the inventive additional gradient moment is produced by an additional gradient 50 which is constantly present over the time. Since the inventive gradient echo sequence illustrated is a balanced sequence, the gradient moment produced by the gradients 42, 43, 44 in all three spatial directions (i.e. slice selection direction LS, readout direction RO and phase coding direction PC) per repetition time TR is zero. This condition is contravened by the inventive additional gradient 50.

[0080] More specifically, the gradient moment produced by the gradient portions 42a and 42c of the slice selection gradient 42 corresponds in size to the gradient moment produced by the gradient portion 42b of the slice selection gradient. Similarly, the gradient moment produced by the gradient portions 43a and 43c of the readout gradient 43 corresponds in size to the gradient moment produced by the gradient portion 43b. The gradient moments produced by the gradient portions 44a, 44b of the phase selection gradient 44 are likewise identical in size.

[0081] FIG. 4 illustrates a preferred inventive embodiment variant for producing the additional gradient moment.

[0082] In contrast to FIG. 3, the additional gradient moment in the embodiment variant illustrated in FIG. 4 is produced by an additional gradient 50, which is only present in the time periods before and after the RF excitation pulse 41, and is therefore not present in the time period during which the RF excitation pulse 41 is applied. It is also the cast that the additional gradient 50 is not present in the time period 45 during which the MR signals are read out. This means that the additional gradient 50 is established only in the so-called prephaser phase and in the so-called rephaser phase of the slice selection gradient 42. In other words, the additional gradient is divided into two portions 50a, 50b in this embodiment variant, the one portion 50a being superimposed on the prephaser portion 42a of the slice selection gradient 42, and the other portion 50b being superimposed on the rephaser portion 42c of the slice selection gradient 42. This embodiment has the advantage that the slice selection by the RF excitation pulse 41 and the readout of the MR signals are not disrupted by the additional gradient 50.

[0083] It should be noted that the additional gradient moment produced by the additional gradient 50 is equal in size for each repetition time TR. It is not necessary for the (profile of the) additional gradient(s) to be constant or identical during each repetition time in order to produce the same additional gradient moment during each repetition time TR. It is entirely possible to satisfy the condition that the additional gradient moment must be identical during each repetition time TR by various additional gradients within the respective repetition time TR. In the case of the embodiment variant illustrated in FIG. 4, however, the (profile of the) additional gradient(s) is also identical for all repetition times TR.

[0084] FIG. 5 illustrates a further embodiment variant according to the invention.

[0085] In contrast with the embodiment illustrated in FIG. 4, the additional gradient 50 in the embodiment illustrated in FIG. 5 is only activated immediately before the RF excitation pulse 41. This means that in this embodiment variant the additional gradient 50 is activated neither during the RF excitation pulse 41 nor during the readout of the MR signals, nor directly after the RF excitation pulse 41. In other words, the additional gradient 50 is activated only in the so-called prephaser phase of the slice selection gradient 42, such that the additional gradient 50 is only superimposed on the prephaser portion 42a of the slice selection gradient 42.

[0086] In this embodiment as well, the additional gradient moment produced per repetition time TR is constant over all repetition times TR. Although this is not necessary (cf. explanation of the embodiment variant illustrated in FIG. 4), the (profile of the) additional gradient(s) 50 is therefore identical for all repetition times TR.

[0087] According to a further inventive embodiment (not shown), the additional gradient 50 can also be activated only immediately after the RF excitation pulse 41. This means that in this embodiment variant the additional gradient 50 is switched neither during the RF excitation pulse 41 nor during the readout of the MR signals, nor directly before the RF excitation pulse 41. In other words, the additional gradient 50 is only switched in the so-called rephaser phase of the slice selection gradient 42, such that the additional gradient 50 is only superimposed on the rephaser portion 42c of the slice selection gradient 42.

[0088] FIG. 6 illustrates a characteristic feature of the present invention, occurring when the isocenter 33 is not situated at the midpoint between the two slices to be captured simultaneously S0, S1, which is generally the case. In this case, the captured MR signals accumulate an unwanted phase Φ.sub.G per repetition time TR.

[0089] This unwanted phase accumulation Φ.sub.G per TR can be avoided, specifically by determining this unwanted phase accumulation and then allowing for it when determining both the phase of the excitation pulses and the phase of the receiver. As a result, the captured MR signals no longer exhibit the unwanted phase accumulation Φ.sub.G.

[0090] 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.