SIMULTANEOUS MULTI-SLICE MULTI-ECHO TURBO SPIN ECHO (TSE) IMAGING

Abstract

In a method and apparatus for acquiring magnetic resonance (MR) raw data, an MR data acquisition scanner is operated to execute a turbo spin echo (TSE) or a turbo gradient spin echo (TGSE) sequence wherein nuclear spins are excited in multiple slices of the examination object simultaneously by radiating at least one radio-frequency (RF) pulse from an RF radiator of the MR data acquisition scanner, thereby causing the excited nuclear spins in said multiple slices to produce an echo train. A multi-band refocusing pulse is radiated that refocuses nuclear spins in at least one of said multiple slices that follows a first of the multiple slices, and readout gradients are activated to acquire MR signals, with respectively different contrasts, at respectively different readout times of the echo train. The read out MR signals are entered into an electronic memory organized as k-space.

Claims

1. A method for acquiring magnetic resonance (MR) raw data, comprising: operating an MR data acquisition scanner to execute an accelerated spin echo sequence while an examination object is situated in the MR data acquisition scanner; in said accelerated spin echo sequence, exciting nuclear spins in multiple slices of the examination object simultaneously by radiating at least one radio-frequency (RF) pulse from an RF radiator of said MR data acquisition scanner, said multiple slices comprising at least one first slice and at least one additional slice and thereby causing the excited nuclear spins in said multiple slices to produce an echo train; in said accelerated spin echo sequence, activating a multi-band refocusing pulse that refocuses the nuclear spins in all excited slices, and activating read out gradients to acquire MR signals, with respectively different contrasts, at respectively different readout times of said echo train; providing the read out MR signals to a computer and, from said computer, entering the readout MR signals as k-space data into an electronic memory organized as k-space; and from said computer, making the k-space data available from said electronic memory in electronic form, as a data file.

2. A method as claimed in claim 1 comprising providing said data file to an image reconstruction computer and, in said image reconstruction computer, separating the k-space data respectively for said multiple slices by executing a partial parallel acquisition separation algorithm and reconstructing respective individual images of said slices in said computer, and making the individual reconstructed images of the respective slices available as respective data files from said image reconstruction computer.

3. A method as claimed in claim 2 comprising executing a slice GRAPPA (Generalized Autocalibration Partially Parallel Acquisitions) separation algorithm as said partial parallel acquisition algorithm.

4. A method as claimed in claim 1 comprising exciting said nuclear spins in said multiple slices by initially exciting nuclear spins at least in said first slice or slices with either a single band RF pulse or multi-band RF pulse respectively and thereafter refocusing said nuclear spins in said first slice or slices for a selected number of repetitions with a single band refocusing pulse or a multi-band refocusing pulse respectively, and thereafter exciting nuclear spins in said at least one additional slice or slices with a single band RF pulse or a multi-band RF pulse respectively, and thereafter refocusing the nuclear spins in said first slice or slices and said at least one additional slice or slices for a selected number of repetitions by radiating a multi-band refocusing pulse that targets said first slice or slices and said at least one additional slice or slices and the possibility of acquiring different contrasts between the refocused signals from said first slice or slices and the refocused signals from said at least one additional slice or slices.

5. A method as claimed in claim 4 comprising acquiring a navigator echo from said first slice or slices after refocusing said first slice or slices.

6. A method as claimed in claim 4 comprising acquiring a navigator echo from said second slice or slices after refocusing said second slice or slices.

7. A method as claimed in claim 1 comprising operating said MR data acquisition scanner to execute said accelerated spin echo sequence as a sequence selected from the group consisting of a turbo spin echo (TSE) sequence and a turbo gradient spin echo (TGSE) sequence.

8. A magnetic resonance (MR) apparatus comprising: an MR data acquisition scanner comprising a radio-frequency (RF) radiator and a gradient coil arrangement; a control computer configured to operate said MR data acquisition scanner to execute an accelerated spin echo sequence while an examination object is situated in the MR data acquisition scanner; said control computer being configured to operate said MR data acquisition scanner in said accelerated spin echo sequence to excite nuclear spins in multiple slices of the examination object simultaneously by radiating at least one radio-frequency (RF) pulse from said RF radiator of said MR data acquisition scanner, said multiple slices comprising a first slice or slices and at least one additional slice or slices and thereby causing the excited nuclear spins in said multiple slices to produce an echo train; said control computer being configured to operate said MR data acquisition scanner in said accelerated spin echo sequence by operating said RF radiator to activate a multi-band refocusing pulse that refocuses the nuclear spins in said slice or slices and at least one additional slice or slices, and to operate said gradient coil arrangement to activate readout gradients to acquire MR signals, with respectively different contrasts, at respectively different readout times of said echo train; an electronic memory organized as k-space; said control computer being configured to the read out MR signals as k-space data into said electronic memory; and said computer being configured to make the k-space data available from said electronic memory in electronic form, as a data file.

9. An apparatus as claimed in claim 8 comprising an image4 reconstruction computer provided with said data file, said image reconstruction computer being configured to separate the k-space data respectively for said multiple slices by executing a partial parallel acquisition separation algorithm and to reconstruct respective individual images of said slices, and to make the individual reconstructed images of the respective slices available as respective data files from said image reconstruction computer.

10. An apparatus as claimed in claim 9 wherein said image reconstruction computer is configured to execute a slice GRAPPA (Generalized Autocalibration Partially Parallel Acquisitions) separation algorithm as said partial parallel acquisition algorithm.

11. An apparatus as claimed in claim 9 wherein said control computer is configured to operate said MR data acquisition scanner to excite said multiple slices by initially exciting nuclear spins at least in said first slice or slices with a single band RF pulse or a multi-band RF pulse respectively and thereafter refocusing said nuclear spins in said first slice or slices for a selected number of repetitions with a single band refocusing pulse or a multi-band refocusing pulse respectively, and thereafter exciting nuclear spins in said at least one additional slice or slices with a single band RF pulse or a multi-band RF pulse respectively, and thereafter refocusing the nuclear spins in said first slice or slices and said at least one additional slice or slices for a selected number of repetitions by radiating a multi-band refocusing pulse that targets said first slice or slices and said at least one additional slice or slices.

12. An apparatus as claimed in claim 11 wherein said control computer is configured to operate said MR data acquisition scanner to acquire a navigator echo from said first slice or slices after refocusing said first slice or slices.

13. An apparatus as claimed in claim 12 wherein said control computer is configured to operate said MR data acquisition scanner to acquire a navigator echo from said second slice or slices after refocusing said second slice or slices.

14. An apparatus as claimed in claim 8 wherein said control computer is configured to operate said MR data acquisition scanner to execute said accelerated spin echo sequence as a sequence selected from the group consisting of a turbo spin echo (TSE) sequence and a turbo gradient spin echo (TGSE) sequence.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1, as noted above, illustrates a conventional turbo spin echo (TSE) sequence for operating an MR scanner.

[0026] FIGS. 2A and 2B, as noted above, respectively illustrate different modulation transfer functions (MTF) and point spread functions (PSF) that result from different reordering of the k-space lines, used with the conventional TSE sequence shown in FIG. 1.

[0027] FIG. 3, as noted above, schematically illustrates a conventional simultaneous multi-slice (SMS) data acquisition and separation of the acquired data in order to reconstruct individual images of the multiple slices.

[0028] FIG. 4 is a block diagram of the basic components of a magnetic resonance apparatus constructed and operating in accordance with the present invention.

[0029] FIG. 5 schematically illustrates an embodiment of an SMS acquisition sequence in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] FIG. 4 schematically illustrates a magnetic resonance apparatus 5 (a magnetic resonance imaging or tomography device). A basic field magnet 1 generates, a temporally constant strong magnetic field for the polarization or alignment of the nuclear spin in a region of an examination subject O, such as a portion of a human body that is to be examined, lying on a table 23 in order to be moved into the magnetic resonance apparatus 5. The high degree of homogeneity in the basic magnetic field necessary for the magnetic resonance measurement (data acquisition) is defined in a typically sphere-shaped measurement volume M, in which the portion of the human body that is to be examined is placed. In order to support the homogeneity requirements temporally constant effects are eliminated by shim-plates made of ferromagnetic materials are placed at appropriate positions. Temporally variable effects are eliminated by shim-coils 2 and an appropriate control unit 27 for the shim-coils 2.

[0031] A cylindrically shaped gradient coil system 3 is incorporated in the basic field magnet 1, composed of three windings. Each winding is supplied by a corresponding amplifier 24-26 with power for generating a linear gradient field in a respective axis of a Cartesian coordinate system. The first partial winding of the gradient field system 3 generates a gradient Gx in the x-axis, the second partial winding generates a gradient Gy in the y-axis, and the third partial winding generates a gradient Gz in the z-axis. Each amplifier 24-26 has a digital-analog converter (DAC), controlled by a sequencer 18 for the accurately-times generation of gradient pulses.

[0032] A radio-frequency antenna 4 is located within the gradient field system 3, which converts the radio-frequency pulses provided by a radio-frequency power amplifier into a magnetic alternating field for the excitation of the nuclei by tipping (“flipping”) the spins in the subject or the region thereof to be examined, from the alignment produced by the basic magnetic field. The radio-frequency antenna 4 is composed of one or more RF transmitting coils and one or more RF receiving coils in the form of an annular, linear or matrix type configuration of coils. The alternating field based on the precessing nuclear spin, i.e. the nuclear spin echo signal normally produced from a pulse sequence composed of one or more radio-frequency pulses and one or more gradient pulses, is also converted by the RF receiving coils of the radio-frequency antenna 4 into a voltage (measurement signal), which is transmitted to a radio-frequency system 22 via an amplifier 7 of a radio-frequency receiver channel 8, 8′. The radio-frequency system 22 furthermore has a transmitting channel 9, in which the radio-frequency pulses for the excitation of the magnetic nuclear resonance are generated. For this purpose, the respective radio-frequency pulses are digitally depicted in the sequencer 18 as a series of complex numbers, based on a given pulse sequence provided by the system computer 20. This number series is sent via an input 12, in each case, as real and imaginary number components to a digital-analog converter (DAC) in the radio-frequency system 22 and from there to the transmitting channel 9. The pulse sequences are modulated in the transmitting channel 9 to a radio-frequency carrier signal, the base frequency of which corresponds to the resonance frequency of the nuclear spin in the measurement volume. The modulated pulse sequences of the RF transmitter coil are transmitted to the radio-frequency antenna 4 via an amplifier 28.

[0033] Switching from transmitting to receiving operation occurs via a transmission-receiving switch 6. The RF transmitting coil of the radio-frequency antenna 4 radiates the radio-frequency pulse for the excitation of the nuclear spin in the measurement volume M and scans the resulting echo signals via the RF receiving coils. The corresponding magnetic resonance signals obtained thereby are demodulated to an intermediate frequency in a phase sensitive manner in a first demodulator 8′ of the receiving channel of the radio-frequency system 22, and digitalized in an analog-digital converter (ADC). This signal is then demodulated to the base frequency. The demodulation to the base frequency and the separation into real and imaginary parts occurs after digitization in the spatial domain in a second demodulator 8, which emits the demodulated data via outputs 11 to an image processor 17. In an image processor 17, an MR image is reconstructed from the measurement data obtained in this manner through the use of the method according to the invention, which includes computation of at least one disturbance matrix and the inversion thereof, in the image processor 17. The management of the measurement data, the image data, and the control program occurs via the system computer 20. The sequencer 18 controls the generation of the desired pulse sequences and the corresponding scanning of k-space with control programs, in particular, in accordance with the method according to the invention. The sequencer 18 controls accurately-timed switching (activation) of the gradients, the transmission of the radio-frequency pulse with a defined phase amplitude, and the reception of the magnetic resonance signals. The time base for the radio-frequency system 22 and the sequencer 18 is provided by a synthesizer 19. The selection of appropriate control programs for the generation of an MR image, which are stored, for example, on a DVD 21, as well as other user inputs such as a desired number n of adjacent clusters, which are to collectively cover the desired k-space, and the display of the generated MR images, occurs via a terminal 13, which includes units for enabling input entries, such as, e.g. a keyboard 15, and/or a mouse 16, and a unit for enabling a display, such as, e.g. a display screen.

[0034] The components within the dot-dash outline S are commonly called a magnetic resonance scanner.

[0035] FIG. 5 is a sequence diagram of the SMS data acquisition procedure in accordance with the invention, with the radiated RF pulses, the detected ADC signals and the readout gradient being schematically illustrated. FIG. 5 shows the example of a two-fold slice-accelerated acquisition (Slice 1 and Slice 2 are the slices to be acquired simultaneously). The first excitation pulse targets only Slice 1. Conditions known as CPMG (Carr, Purcell, Melboom, Gill) conditions are well known in the field of magnetic resonance imaging. When these conditions are satisfied in the context of a TSE sequence, the primary and stimulated echoes occur only at the midpoint between two consecutive refocusing pulses, and have the same phase. In order to fulfill the CPMG conditions in the sequence shown in FIG. 5, the spins for Slice 1 are refocused at every echo spacing ES period. After the desired shift time between the two TEs to be acquired has passed, Slice 2 is excited between two refocusing pulses. The first two refocusing pulses refocus Slice 1 only while the third refocusing pulse is a multi-band (MB) pulse that refocuses Slices 1 and 2 simultaneously. After that point in time, a standard TSE echo train follows, with all refocusing pulses being MB pulses, and thus all echoes being acquired from Slices 1 and 2 simultaneously.

[0036] Depending on the desired difference among TEs, navigator echoes can be acquired for Slice 1 in the period before the excitation of Slice 2. These may be non-phase encoded echoes (such as for phase correction between different echo train segments), or phase-encoded echoes with arbitrary phrase encoding steps (such as for a GRAPPA calibration scan or a motion-correction scan).

[0037] A readout dephaser, which targets the spins of Slice 1, is activated before the excitation pulse for Slice 2, so as to avoid the readout prephaser, after the excitation of Slice 2, dephasing the spins of Slice 1, and instead prephases the spins in both Slice 1 and Slice 2.

[0038] At a later point in time, the echo train is repeated with Slices 1 and 2 being swapped (exchanged).

[0039] If a slice acceleration factor greater than 2 is used, at least one type of the SB pulses can be radiated as MB pulses. For example, in addition to Slice 1, a third slice (Slice 3) is also excited simultaneously, leading to TE.sub.1 for Slices 1 and 3, and TE.sub.2. In a second repetition, Slice 2 is excited first and Slices 1 and 3 are subsequently excited. In a different scenario different echo times can be achieved for each slice.

[0040] Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.