T2-weighted MR imaging with elimination of non-T2-weighted signal contributions

Abstract

An object positioned in an examination volume of a magnetic resonance (MR) device (1) is T2-weighted MR imaged such that the MR image is essentially free from interfering contributions from MR signals without T2 weighting. The object (10) is subject to a first T2 preparation sequence (T2PREP1) including an excitation RF pulse (21), one or more refocusing RF pulses (22), and a tip-up RF pulse (23). The object (10) is subject to a first readout sequence (RO1) including at least one excitation RF pulse and switched magnetic field gradients for acquiring a first set of MR signals. The object (10) is subject to a second T2 preparation sequence (T2PREP2) including an excitation RF pulse (21), one or more refocusing RF pulses (22), and a tip-up RF pulse (23). At least one of the RF pulses (21, 22, 23) of the second T2 preparation sequence (T2PREP2) has a different phase than the corresponding RF pulse (21, 22, 23) of the first T2 preparation sequence (T2PREP1). The object (10) is subject to a second readout sequence (RO2) including at least one excitation RF pulse and switched magnetic field gradients for acquiring a second set of MR signals. The MR image is reconstructed from the first and second sets of MR signals.

Claims

1. A method of magnetic resonance (MR) imaging of an object positioned in an examination volume of a MR device, the method comprising the steps of: a) subjecting the object to a first T.sub.2 preparation sequence (T2PREP1) comprising an excitation RF pulse, one or more refocusing RF pulses, and a tip-up RF pulse; b) subjecting the object to a first readout sequence (RO1) comprising switched magnetic field gradients for acquiring a first set of MR signals including T.sub.2-weighted contributions and interference components; c) subjecting the object to a second T.sub.2 preparation sequence (T2PREP2) comprising an excitation RF pulse, one or more refocusing RF pulses, and a tip-up RF pulse, wherein the excitation RF pulse of the second T.sub.2 preparation sequence (T2PREP2) has a different phase than the excitation RF pulse of the first T.sub.2 preparation sequence (T2PREP1); d) subjecting the object to a second readout sequence (RO2) comprising switched magnetic field gradients for acquiring a second set of MR signals including T.sub.2-weighted contributions and interference components; e) reconstructing a MR image from the first and second sets of MR signals using subtractive combination to eliminate the interference components from the final MR image.

2. The method of claim 1, wherein the excitation RF pulses of the first and second T.sub.2 preparation sequences (T2PREP1, T2PREP2) have opposite phases.

3. The method of claim 1, wherein steps a) through d) are repeated a number of times for sampling a given k-space region before reconstructing the MR image in step e).

4. The method of claim 1, wherein the first and second T.sub.2 preparation sequences (T2PREP1, T2PREP2) are spatially non-selective.

5. The method of claim 2, further including eliminating the interference component by subtracting the first and second sets of MR signals to form a set of difference MR signals and wherein the MR image is reconstructed from the set of difference MR signals.

6. The method of claim 1, wherein in first MR image is reconstructed from the first set of MR signals and a second MR image is reconstructed from the second set of MR signals and the first second MR images are subtracted to form the MR image.

7. The method of claim 1, wherein the first and second readout sequences are gradient echo sequences.

8. The method of claim 1, wherein the first and second readout sequences are zero echo time sequences, each comprising: i) setting a readout magnetic field gradient having a readout direction and a readout strength; ii) acquiring the MR signals of the first and second sets of signals in the presence of the readout magnetic field gradient; iii) rotating the readout magnetic field; and iv) sampling a spherical volume in k-space by repeating steps i) through iii) a plurality of times.

9. A magnetic resonance (MR) device comprising at least one main magnet coil for generating a uniform, steady magnetic field within an examination volume, a number of gradient coils for generating switched magnetic field gradients in different spatial directions within the examination volume, at least one RF coil for generating RF pulses within the examination volume and/or for receiving MR signals from an object positioned in the examination volume, a control unit for controlling the temporal succession of RF pulses and switched magnetic field gradients, and a reconstruction unit, wherein the MR device is configured to perform the following steps: a) subjecting the object to a first T.sub.2 preparation sequence (T2PREP1) comprising an excitation RF pulse, one or more refocusing RF pulses, and a tip-up RF pulse; b) subjecting the object to a first readout sequence (RO1) comprising switched magnetic field gradients for acquiring a first set of MR signals; c) subjecting the object to a second T.sub.2 preparation sequence (T2PREP2) comprising an excitation RF pulse, one or more refocusing RF pulses, and a tip-up RF pulse, wherein at least one of the excitation RF pulse, one of the revocusing RF pulses, and the tip-up RF pulse of the second T.sub.2 preparation sequence (T2PREP2) has a different phase than the corresponding RF pulse of the first T.sub.2 preparation sequence (T2PREP1); d) subjecting the object to a second readout sequence (RO2) switched magnetic field gradients for acquiring a second set of MR signals; e) wherein the MR signals of the first and second sets of MR signals include T.sub.2-weighted components of the MR signals of the first and second sets of data having different phases; f) eliminating the interferences from the T.sub.2-weighted components from the first and second sets of MR signals and reconstructing a MR image from the first and second sets of MR signals with said interferences eliminated.

10. A non-transitory computer-readable medium storing a computer program which when run on a control processor of a magnetic resonance (MR) device, controls the MR device to: a) generating a first T.sub.2 preparation sequence (T2PREP1) comprising an excitation RF pulse, one or more refocusing RF pulses, and a tip-up RF pulse; b) generating a first readout sequence (RO1) including switched magnetic field gradients for acquiring a first set of MR signals including T.sub.2-weighted contributions of a first RF phase and interference; c) generating a second T.sub.2 preparation sequence (T2PREP2) comprising an excitation RF pulse, one or more refocusing RF pulses, and a tip-up RF pulse, wherein at least one of the RF pulses of the second T.sub.2 preparation sequence (T2PREP2) has a different RF phase than the corresponding RF pulse of the first T.sub.2 preparation sequence (T2PREP1); d) generating a second readout sequence (RO2) comprising switched magnetic field gradients for acquiring a second set of MR signals including T.sub.2-weighted contributions of a second RF phase and interference, the first and second RF phases being different; e) distinguishing the T.sub.2-weighted contributions from the interference in the first and second sets of MR signals on the basis of the different first and second RF phases of the acquired first and second sets of RF signals; f) eliminating the interferences from the first and second sets of MR signals; and g) reconstructing a MR image from the first and second sets of MR signals from which said interference have been eliminated.

11. The MR device of claim 9, wherein at least one of the excitation RF pulses, the refocusing RF pulses, and the tip-up RF pulses of the first and second T.sub.2 preparation sequences have opposite phases, wherein the MR signals of the first and second sets of MR signals have the opposite phase, and wherein eliminating the interferences in step f) includes subtracting the MR signals of the first and second sets of MR signals to generate a difference set of MR signals, and wherein reconstructing the MR image includes reconstructing the difference set of MR signals.

12. The non-transitory computer-readable medium of claim 10, wherein the MR device is further controlled to: generate the first and second T.sub.2 preparation sets of MR signals such that the excitation RF pulses, the refocusing RF pulses, and the tip-up RF pulses of the first and second T.sub.2 preparation sequences have opposite phases, wherein the MR signals of the first and second sets of MR signals have the opposite phase, and wherein eliminating the interferences in step f) includes subtracting the MR signals of the first and second sets of MR signals to generate a difference set of MR signals, and wherein reconstructing the MR image includes reconstructing the difference set of MR signals.

13. A magnetic resonance (MR) device comprising at least one main magnet coil for generating a steady magnetic field within an examination volume, gradient coils for generating switched magnetic field gradients in different spatial directions within the examination volume, at least one RF coil for generating RF pulses within the examination volume and/or for receiving MR signals from an object positioned in the examination volume, a control processor configured to control a temporal succession of RF pulses and switched magnetic field gradients and reconstruct an MR image, wherein the MR device is configured to perform the following steps: a) subjecting the object to a first T.sub.2 preparation sequence (T2PREP1) including an excitation RF pulse, one or more refocusing RF pulses, and a tip-up RF pulse with a first phase; b) subjecting the object to a first readout sequence (RO1) including switched magnetic field gradients for acquiring a first set of MR signals including T.sub.2-weighted contributions of the first phase and interference; c) subjecting the object to a second T.sub.2 preparation sequence (T2PREP2) including an excitation RF pulse, one or more refocusing RF pulses, and a tip-up RF pulse, at least one of the excitation RF pulse, the one or more refocusing RF pulses and the tip-up pulse have a second phase, wherein the second phase is opposite to the first phase; d) subjecting the object to a second readout sequence (RO2) including switched magnetic field gradients for acquiring a second set of MR signals including T.sub.2-weighted contributions and interference, the T.sub.2-weighted contributions of the first and second sets of MR signals being of opposite phase; e) subtracting the first and second sets of MR signals to form a set of difference MR signals including T.sub.2-weighted contributions with the interferences eliminated; and, f) reconstructing a MR image from the difference sets of MR signals with the interferences eliminated.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The enclosed drawings disclose preferred embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention. In the drawings:

(2) FIG. 1 schematically shows a MR device for carrying out the method of the invention;

(3) FIG. 2 shows a diagram illustrating the T.sub.2-weighted MR imaging procedure of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) With reference to FIG. 1, a MR device 1 which can be used for carrying out the method of the invention is shown. The device comprises superconducting or resistive main magnet coils 2 such that a substantially uniform, temporally constant main magnetic field B.sub.0 is created along a z-axis through an examination volume. The device further comprises a set of (1.sup.st, 2.sup.nd, andwhere applicable3.sup.rd order) shimming coils 2, wherein the current flow through the individual shimming coils of the set 2 is controllable for the purpose of minimizing B.sub.0 deviations within the examination volume.

(5) A magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially and otherwise encode the magnetic resonance, saturate spins, and the like to perform MR imaging.

(6) More specifically, a gradient pulse amplifier 3 applies current pulses to selected ones of whole-body gradient coils 4, 5 and 6 along x, y and z-axes of the examination volume. A digital RF frequency transmitter 7 transmits RF pulses or pulse packets, via a send-/receive switch 8, to a -body RF coil 9 to transmit RF pulses into the examination volume. A typical MR imaging sequence is composed of a packet of RF pulse segments of short duration which taken together with each other and any applied magnetic field gradients achieve a selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance and select a portion of a body 10 positioned in the examination volume. The MR signals are also picked up by the body RF coil 9.

(7) For generation of MR images of limited regions of the body 10 by means of parallel imaging, a set of local array RF coils 11, 12, 13 are placed contiguous to the region selected for imaging. The array coils 11, 12, 13 can be used to receive MR signals induced by body-coil RF transmissions.

(8) The resultant MR signals are picked up by the body RF coil 9 and/or by the array RF coils 11, 12, 13 and demodulated by a receiver 14 preferably including a pre-amplifier (not shown). The receiver 14 is connected to the RF coils 9, 11, 12 and 13 via send-/receive switch 8.

(9) A host computer 15 controls the current flow through the shimming coils 2 as well as the gradient pulse amplifier 3 and the transmitter 7 to generate imaging sequences according to the invention. The receiver 14 receives a plurality of MR data lines in rapid succession following each RF excitation pulse. A data acquisition system 16 performs analog-to-digital conversion of the received signals and converts each MR data line to a digital format suitable for further processing. In modern MR devices the data acquisition system 16 is a separate computer which is specialized in acquisition of raw image data.

(10) Ultimately, the digital raw image data is reconstructed into an image representation by a reconstruction processor 17 which applies an appropriate reconstruction algorithm. The image is then stored in an image memory where it may be accessed for converting projections or other portions of the image representation into appropriate format for visualization, for example via a video monitor 18 which provides a human-readable display of the resultant MR image.

(11) FIG. 2 shows a diagram illustrating the imaging procedure of the invention. The method starts with a first T.sub.2 preparation sequence T2PREP1 comprising an excitation RF pulse 21, two refocusing RF pulses 22, and a tip-up RF pulse 23. Thereafter, a first readout sequence RO1 is applied, which is a ZTE sequence. A readout gradient (not depicted) is set before radiation of a short, hard, small flip-angle excitation RF pulse. The acquisition of a free induction decay (FID) signal starts immediately after radiation of this excitation RF pulse. After the FID readout, the next readout gradient is set before the next hard excitation RF pulse is applied and so forth. The readout direction is incrementally varied from repetition to repetition until a spherical volume in k-space is sampled to the required extent. The FID signals acquired during the first readout sequence RO1 form a first set of MR signals. This first set of MR signal includes a T.sub.2-weighted signal contribution 24 and an interfering signal contribution 25 resulting from increasing longitudinal magnetization during MR signal acquisition. As a next step, a second T.sub.2 preparation sequence T2PREP2 is applied which comprises an excitation RF pulse 21. The excitation RF pulses 21 and 21 have opposite phases (i.e. a phase difference of 180). The second T.sub.2 preparation sequence T2PREP2 uses refocusing RF pulses 22 and a tip-up RF pulse 23 having the same phases like the corresponding RF pulses of the first T.sub.2 preparation sequence T2PREP1. In a second readout sequence RO2, a second set of MR signals is acquired comprising a T.sub.2-weighted component 24 and an interfering component 25 resulting from increasing longitudinal magnetization as well. The first and second sets of MR signals are acquired with identical readout directions. The T.sub.2-weighted MR signal components 24 and 24 have opposite signs, while the sign of the interfering MR signal contributions 25, 25 is the same in both acquisitions RO1, RO2. The curves 24, 24, 25, 25 schematically illustrate the amplitude of the respective MR signal contributions as a function of time t during the first and second readout sequences RO1, RO2. The interfering MR signal contributions 25, 25 are eliminated by subtracting 30 the first and second sets of MR signals to form a set of difference MR signals, from which a MR image is finally reconstructed by reconstruction processor 17. The final MR image is thus entirely T.sub.2-weighted without any contribution from non-T.sub.2-weighted MR signal components.

(12) The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.