Magnetic resonance spectroscopy with phase rotation

Abstract

In a method and magnetic resonance (MR) apparatus for acquiring an MR signal from an examination subject according to a pulse sequence, a first radio-frequency pulse is applied with a first phase and a gradient field is simultaneously applied in a first direction. Second and third radio-frequency pulses, with second and third phases, respectively, are applied simultaneously with a gradient field in a second direction. A fourth and a fifth radio-frequency pulse, with a fourth and a fifth phase, respectively, are applied and simultaneously with a gradient field in a third direction. A signal with a receiver phase is acquired =. The pulse sequence is repeated a number of times under phase rotation, wherein the third and fourth radio-frequency pulses in each repetition have the same phase, and the signals acquired in the repetition are added.

Claims

1. A method for acquiring a magnetic resonance (MR) signal from an examination subject, comprising: operating an MR data acquisition unit, comprising a radio-frequency (RF) transmitter and a gradient coil system and an RF receiver, according to a pulse sequence, while an examination subject is situated in the MR data acquisition unit; in said pulse sequence, operating said RF transmitter to radiate a first RF pulse that acts on a voxel in the examination subject, having a first phase, while simultaneously operating said gradient coil system to activate a gradient magnetic field in a first direction that also acts on said voxel; in said pulse sequence, operating said RF transmitter to radiate a second RF pulse that also acts on said voxel, having a second phase and to radiate a third RF pulse, having a third phase, while simultaneously operating said gradient coil system to activate a gradient magnetic field in a second direction that also acts on said voxel; in said pulse sequence, operating said RF transmitter to radiate a fourth RF pulse that also acts on said voxel, having a fourth phase, and a fifth RF pulse that also acts on said voxel having a fifth phase, while simultaneously operating said gradient coil system to activate a gradient magnetic field in a third direction that also acts on said voxel; in said pulse sequence, operating said RF receiver to acquire an MR signal from voxel in said examination subject resulting from a state of excitation of nuclear spins in said voxel in the examination subject produced by a combination of said first, second, third, fourth and fifth RF pulses, and entering the acquired MR signal into an electronic memory; operating said MR data acquisition unit by repeating said pulse sequence a plurality of times with a phase rotation of said first, second, third, fourth and fifth phases in each repetition, with said third and fourth phases being the same in each repetition, and thereby compiling a set of MR signals in said electronic memory respectively resulting from each repetition; and making said set of MR signals in said electronic memory available from said memory in electronic form as a data file for further processing thereof.

2. A method as claimed in claim 1 comprising, in said repetitions, rotating said first, second, and fifth phases according to a press phase rotation scheme selected from the group consisting of a 4-step press phase rotation scheme, and 8-step press phase rotation scheme, and 16-step press phase rotation scheme.

3. A method as claimed in claim 1 comprising rotating said third and fourth phases through at least two phases in said repetitions wherein, for each rotated phase of said third and fourth phases, said first, second and fifth phases proceed through a phase rotation.

4. A method as claimed in claim 1 comprising rotating said third and fourth phases through at least two phases in said repetitions wherein, for each rotated phase of third and fourth phases, said first, second and fifth phases proceed through a phase rotation selected from the group consisting of an 8-step EXOR, a 16-step EXOR, an EXORCYCLE, and CYCLOPS.

5. A method as claimed in claim 1 comprising, in said pulse sequence, radiating said second, third, fourth and fifth RF pulses respectively as refocusing pulses.

6. A method as claimed in claim 5 comprising, in said pulse sequence, radiating said second, third, fourth and fifth RF pulses respectively as adiabatic refocusing pulses.

7. A method as claimed in claim 1 comprising radiating said first RF pulses as an excitation pulse.

8. A method as claimed in claim 7 comprising radiating said first RF pulse as a non-adiabatic excitation pulse.

9. A method as claimed in claim 1 comprising selecting said pulse sequence from the group consisting of a LASER pulse sequence and a semi-LASER pulse sequence.

10. A method as claimed in claim 1 comprising operating said MR data acquisition unit to implement the respective phase rotation of said first, second, and fifth phases at a time of acquisition of said MR signal in each repetition.

11. A method as claimed in claim 1 comprising providing said data file to a computer and, in said computer, adding the respective MR signals in said data file together to form a sum, and Fourier transforming said sum.

12. A method as claimed in claim 1 comprising providing said data file to a computer and, in said computer, Fourier transforming each MR signal in said data file to obtain respective Fourier-transformed signals, and adding said Fourier-transformed signals together.

13. A magnetic resonance apparatus comprising: an MR data acquisition unit comprising a radio-frequency (RF) transmitter and a gradient coil system and an RF receiver; a control computer configured to operate said MR data acquisition unit according to a pulse sequence, while an examination subject is situated in the MR data acquisition unit; an electronic memory; said control computer being configured to operate said RF transmitter in said pulse sequence to radiate a first RF pulse that acts on a voxel in the examination subject, having a first phase, while simultaneously operating said gradient coil system to activate a gradient magnetic field in a first direction that also acts on said voxel; said control computer being configured to operate said RF transmitter in said pulse sequence to radiate a second RF pulse that also acts on said voxel, having a second phase and to radiate a third RF pulse, having a third phase, while simultaneously operating said gradient coil system to activate a gradient magnetic field in a second direction that also acts on said voxel; said control computer being configured to operate said RF transmitter in said pulse sequence to radiate a fourth RF pulse that also acts on said voxel, having a fourth phase, and a fifth RF pulse that also acts on said voxel, having a fifth phase, while simultaneously operating said gradient coil system to activate a gradient magnetic field in a third direction that also acts on said voxel; said control computer being configured to operate said RF receiver in said pulse sequence to acquire an MR signal from said voxel in said examination subject resulting from a state of excitation of nuclear spins in said voxel in the examination subject produced by a combination of said first, second, third, fourth and fifth RF pulses, and to enter the acquired MR signal into said electronic memory; said control computer being configured to operate said MR data acquisition unit by repeating said pulse sequence a plurality of times with a phase rotation of said first, second, third, fourth and fifth phases in each repetition, with said third and fourth phases being the same in each repetition, and thereby compile a set of MR signals in said electronic memory respectively resulting from each repetition; and said control computer being configured to make said set of MR signals in said electronic memory available from said memory in electronic form as a data file for further processing thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a magnetic resonance system.

(2) FIG. 2 shows a sequence diagram of a PRESS sequence.

(3) FIG. 3 shows a sequence diagram of a semi-LASER sequence.

(4) FIG. 4 shows a spectrum, acquired by means of a semi-LASER sequence, of a voxel in the frontal lobe, without phase rotation.

(5) FIG. 5 shows a spectrum of the same voxel as in FIG. 4, acquired by a semi-LASER sequence using a 32-step phase rotation scheme according to the present invention.

(6) FIG. 6 shows a spectrum of a voxel in the vicinity of the cranial bone, acquired without phase rotation.

(7) FIG. 7 shows a spectrum of the same voxel as in FIG. 6, acquired by means of a semi-LASER sequence using a 32-step phase rotation scheme.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) FIG. 1 schematically depicts a magnetic resonance system 1 having three gradient coils 2, 3 and 4, two radio-frequency coils 5 and 6, and a control computer 7. For clarity, other components of the magnetic resonance system 1, such as the patient table, are not shown. In this arrangement the radio-frequency coil 5 is embodied as an excitation coil, and the radio-frequency coil 6 as a detection (reception) coil. In this case the radio-frequency coil 6 is usually tailored to fit specific sections of the examination subject, e.g. as a head coil, chest coil or knee coil. The excitation coil 5 is also known as a body coil and is less sensitive than the radio-frequency coil 6, but is homogeneous over a greater range. This partitioning of the radio-frequency coils 5, 6 is typical (but not mandatory for the invention) for magnetic resonance systems 1 in the medical field, though not in the case of devices having bores in the range from several to approx. 30 cm, where the same radio-frequency coil is often used for both excitation and detection. Whether the excitation coil is simultaneously the detection coil is immaterial for the purposes of the invention and more or less device-dependent.

(9) The gradient coils 2, 3 and 4 generate gradient fields in directions oriented orthogonally to one another. In order to generate a resulting gradient in a specified direction, in particular the slice direction in the case of the invention, for the first, the second and third, and the fourth and fifth RF pulses, the gradient fields of two gradient coils or of all three gradient coils 2, 3 and 4 can also be superimposed. A gradient field is therefore identical with the gradient field of a single gradient coil only in exceptional cases, while in most cases it is an overlay composed of a plurality of gradient fields.

(10) The described method is realized in the form of software in the control computer 7. The control computer 7 is preferably part of an overall computer system, or is a mobile computer or a workstation or console acting as a control device for the magnetic resonance system 1. In particular the control computer 7 can have a data memory and a processor, in particular a CPU. The described method can be stored in the form of software on a digital data medium, for example an optical data medium, a USB stick or a hard disk.

(11) The method according to the invention is preferably carried out automatically following positioning of the patient, after a user has selected an appropriate voxel within the body of the patient.

(12) FIG. 2 shows a sequence diagram intended to explain the known PRESS sequence. In this case, a 90 pulse with phase .sub.1 and a gradient field 20 are initially applied simultaneously in the x-direction. A slice through the examination region is excited as a result. The magnetization generated in this way is refocused by a slice-selective 180 pulse with phase .sub.2, a gradient 22 being applied simultaneously in the y-direction, in any case orthogonally to the first slice-selective gradient. The thus resulting echo is again refocused by means of a second 180 pulse with phase .sub.3, a gradient now being applied in the z-direction. Spoiler gradients S are applied before and after each 180 pulse in order to dephase the magnetization outside of the voxel of interest which is produced as a result of the overlapping of the three excited slices. A further echo 10 is then generated at time point TE. This wanted echo is acquired by switching on the receiver or the receiver ADC, once again with a specific phase.

(13) It can be shown that the phase of the wanted signal, in other words the echo from the three pulses, has the phase .sub.12.sub.2+2.sub.3.

(14) FIG. 3 shows a sequence diagram of a semi-LASER sequence 12, which is an extension of the PRESS sequence using adiabatic pulses. In this case a normal excitation pulse .sub.1 with phase .sub.1 is emitted at time point t.sub.1, while a gradient field 24 is simultaneously applied in the z-direction. Since the magnetization generated by means of the RF pulse starts to dephase in the x-y plane due to the z gradient, the z gradient is subsequently reversed in order to rephase the spins once more. At the same time a spoiler gradient S is applied in the x-direction. An adiabatic pulse, preferably an AFP pulse .sub.2 with phase .sub.2, is emitted at time point t.sub.2. A gradient field 20 is simultaneously applied in the x-direction. This is followed by a further AFP pulse .sub.3 with phase .sub.3, which is likewise emitted while a gradient 20 is applied in the x-direction. A slice selection is therefore performed in the x-direction by means of said two RF pulses .sub.2 and .sub.3. A spoiler gradient S is inserted between the two pulses. Two adiabatic refocusing pulses .sub.4 and .sub.5, with phases .sub.4 and .sub.5, are in turn emitted at time points t.sub.4 and t.sub.5, respectively, while a gradient field is simultaneously applied in the y-direction. Spoiler gradients are inserted in this case too between the two RF pulses .sub.4 and .sub.5, as well as after the last refocusing pulse .sub.5. This results at time point TE in a (wanted) echo 10, which is acquired by means of the receiver with a receiver phase .sub.Rec.

(15) Table 1 generically shows the phase rotation scheme according to the invention for the semi-Laser sequence shown in FIG. 3. In the table, .sub.Rec denotes the phase of the receiver or the analog-to-digital converter (ADC). According to Table 1, the phase angles .sub.1, .sub.2 and .sub.5 of the excitation pulse .sub.1 and of the first and fourth refocusing pulse pass through a PRESS phase rotation scheme, for example an 8-step EXOR scheme or an EXORCYCLE scheme. The phase rotation scheme can have an arbitrary number of steps, for example 4, 8, 16 or 32. In this case .sub.3 and .sub.4, i.e. the phases of the second and third refocusing pulse, are each identical to one another and also remain identical during the entire first pass through the PRESS phase rotation scheme, e.g. on the value .sub.1. This value can be e.g. 0, 90, 180, 270, but also an odd value, such as 22.5 or 45, for example.

(16) Following this, the selected Press phase rotation scheme is repeated once again for the excitation pulse .sub.1 and the first and fourth refocusing pulse .sub.2 and .sub.5, the second and third refocusing pulse having a different phase .sub.2 which is nonetheless again identical for each pulse and remains identical over the entire PRESS phase rotation scheme. If .sub.1=0, then .sub.2 can be for example 180, but also e.g. 22.5, 45 or 180. Arbitrary combinations of values for .sub.1 and .sub.2 are conceivable and practicable.

(17) TABLE-US-00002 TABLE 1 First example of a phase rotation scheme according to the invention .sub.1 .sub.2 .sub.3 .sub.4 .sub.5 .sub.Rec PRESS PRESS .sub.1 .sub.1 PRESS .sub.1 2.sub.2 + 2.sub.5 phase phase .sub.1 .sub.1 phase .sub.1 2.sub.2 + 2.sub.5 rotation rotation .sub.1 .sub.1 rotation .sub.1 2.sub.2 + 2.sub.5 scheme scheme .sub.1 .sub.1 scheme .sub.1 2.sub.2 + 2.sub.5 PRESS PRESS .sub.2 .sub.2 PRESS .sub.1 2.sub.2 + 2.sub.5 phase phase .sub.2 .sub.2 phase .sub.1 2.sub.2 + 2.sub.5 rotation rotation .sub.2 .sub.2 rotation .sub.1 2.sub.2 + 2.sub.5 scheme scheme .sub.2 .sub.2 scheme .sub.1 2.sub.2 + 2.sub.5 . . . . . . . . . . . . . . . . . .

(18) Accordingly, the acquisition can be terminated and the signals acquired in each case added together. On the other hand, a further pass through the Press phase rotation scheme can still be executed using a further value .sub.3 for the phases of the second and third refocusing pulse, or even a fourth pass using a further value .sub.4 for the phase of the second and third refocusing pulse. The respective receiver phase .sub.Rec must be set in accordance with the specified shape for each acquisition so that the respective wanted signals are added together.

(19) An actual example of a phase rotation according to the invention is shown in Table 2. Therein, the excitation pulse and the first and fourth refocusing pulses in each case pass through a 16-step EXOR scheme, while the second and third refocusing pulses are held on .sub.3=.sub.4=0. A second pass through the 16-step EXOR scheme is then executed using .sub.3=.sub.4=180.

(20) TABLE-US-00003 TABLE 2 Second example of a phase rotation scheme according to the invention Step No. .sub.1 .sub.2 .sub.3 .sub.4 .sub.5 .sub.Rec 1 90 90 0 0 0 270 2 90 180 0 0 180 90 3 90 270 0 0 0 270 4 90 0 0 0 180 90 5 90 90 0 0 0 270 6 90 180 0 0 180 90 7 90 270 0 0 0 270 8 90 0 0 0 180 90 9 270 90 0 0 90 270 10 270 180 0 0 270 90 11 270 270 0 0 90 270 12 270 0 0 0 270 90 13 270 90 0 0 90 270 14 270 180 0 0 270 90 15 270 270 0 0 90 270 16 270 0 0 0 270 90 17 90 90 180 180 0 270 18 90 90 180 180 180 90 19 90 180 180 180 0 270 20 90 270 180 180 180 90 21 90 0 180 180 0 270 22 90 90 180 180 180 90 23 90 180 180 180 0 270 24 90 270 180 180 180 90 25 270 90 180 180 90 270 26 270 180 180 180 270 90 27 270 270 180 180 90 270 28 270 0 180 180 270 90 29 270 90 180 180 90 270 30 270 180 180 180 270 90 31 270 270 180 180 90 270 32 270 0 180 180 270 90

(21) The 32-step phase rotation scheme shown in Table 2 was implemented and tested in vivo using the following parameters: Repetition time=2 sec., echo time (TE)=135 msec., 128 repetitions, voxel size=20 mm20 mm20 mm20 mm. The adiabatic refocusing pulses were AFP pulses using hyperbolic secant modulation. This resulted in an overall measurement time of about 4 min. The measurements were carried out on a 3T scanner (MAGNETOM Skyra, Siemens Healthcare, Erlangen, Germany). The same measurement was performed with and without phase rotation.

(22) FIGS. 4 and 5 show corresponding 1H spectra of a voxel in the frontal lobe, a region where unwanted echoes due to the high susceptibility jumps at the air-filled sinuses are a problem.

(23) FIG. 4 shows the spectrum without phase rotation. It can be seen here that massive unwanted echoes are present above approx. 4.2 ppm in the region 30. The signal strength exceeds even that of the highest metabolite peak of NAA (N-acetylaspartate).

(24) FIG. 5 shows the spectrum 34 of the same voxel, acquired by means of the same sequence, but using the phase rotation scheme according to the invention. It can be seen in this case that the unwanted echoes in the region 30 have been massively reduced. In contrast, the metabolites in the brain matter, in particular NAA, creatine (Cr) and choline (Cho), are now clearly visible, as also is the signal of inositol and the second peak of creatine (Cr2).

(25) FIG. 6 shows a spectrum of a voxel in the occiput in the vicinity of the cranial bone in order to observe contamination originating from the nearby fat tissue. FIG. 6 shows the spectrum without phase rotation, with a substantial contamination being visible in the region 32 between 0.5 and 1.5 ppm.

(26) FIG. 7, in contrast, shows the result of the measurement with phase rotation, the spectrum 34, where the contamination by lipids has been substantially reduced in the region 32.

(27) Accordingly it was possible to demonstrate that by means of the phase rotation according to the invention two problems in connection with semi-LASER SVS can be resolved: unwanted echoes and contamination from outside of the selected voxel.

(28) 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.