MR imaging with suppresion of flow artifacts

Abstract

At least two gradient echo signals are generated at two different echo times by subjecting a portion of a body (10) in an MR examination region (1) to an imaging sequence of RF pulses and switched magnetic field gradients. The 0th moment of the readout magnetic field gradient essentially vanishes at the time of a first gradient echo while the 1st moment of the readout gradient is non-zero. Both the 0th and 1st moments of the readout magnetic field gradient essentially vanish at a time of a second gradient echo. Gradient echo signals are acquired. Acquiring the gradient echo signals is repeated for a plurality of phase encoding steps. A first MR image is reconstructed from the gradient echo signals of the first gradient echo and a second MR image is reconstructed from the gradient echo signals of the second gradient echo. Ghosting artefacts in the first and/or second MR image are identified by comparing the first and second MR images.

Claims

1. A method of MR imaging of a body placed in the examination volume of a MR device, the method comprising the steps of: a) generating at least two gradient echo signals at two different echo times by subjecting a portion of the body to an imaging sequence of RF pulses and switched magnetic field gradients, wherein the 0.sup.th moment of the readout magnetic field gradient essentially vanishes at the time of the first gradient echo, the 1.sup.st moment of the readout gradient being non-zero at the time of the first gradient echo, while both the 0.sup.th and 1.sup.st moments of the readout magnetic field gradient essentially vanish at the time of the second gradient echo; b) acquiring the gradient echo signals; c) repeating steps a) and b) for a plurality of phase encoding steps; d) reconstructing a first MR image from the gradient echo signals of the first gradient echo and a second MR image from the gradient echo signals of the second gradient echo; and e) identifying ghosting artefacts in the first and/or second MR image by comparing the first and second MR images.

2. The method of claim 1, wherein the ghosting artefacts are identified on the basis of local intensity losses and/or local intensity gains in the first MR image compared with the second MR image.

3. The method of claim 1, the ghosting artefacts are eliminated in the first and/or second MR image.

4. The method of claim 1, wherein at least one final MR image is computed by combining the first and second MR images.

5. The method of claim 1, wherein the gradient echo signals are part of a Dixon imaging method and comprise contributions from at least two chemical species having different MR spectra, wherein the echo times are selected such that the signal contributions from the at least two chemical species are more in phase at the time of the second gradient echo than at the time of the first gradient echo.

6. The method of claim 5, wherein the signal contributions of the at least two chemical species are separated such that the at least one final MR image comprises contributions from only one of the chemical species.

7. The method of claim 1, wherein generating the at least two gradient echo signals includes: following one of the RF pulses, applying switched magnetic field gradients including a first readout gradient pulse followed immediately by a second readout gradient pulse of opposite polarity to the first readout gradient pulse; wherein the time of the first echo being during the first readout gradient pulse when both the 0.sup.th moment of the switched magnetic field gradient essentially vanishes and the 1.sup.st moment of the switched magnetic field gradient is non-zero; and wherein the time of the second echo is during the second readout gradient pulse when both the 0.sup.th and 1.sup.st moments of the switched magnetic field gradients essentially vanish.

8. A magnetic resonance (MR) device comprising: at least one main magnet coil configured to generate a uniform, steady magnetic field B.sub.0 within an examination volume, a number of gradient coils configured to generate switched magnetic field gradients in different spatial directions within the examination volume, at least one RF coil configured to generate RF pulses within the examination volume and/or for receiving MR signals from a body of a patient positioned in the examination volume, a control computer configured to control the at least one RF coil and the gradient coils to implement an imaging sequence including: a) subjecting a portion of the body disposed in the examination region to an RF pulse followed by switched magnetic field readout gradients including readout gradient pulses of alternating polarity such that a first gradient echo is formed during a first of the readout gradient pulses when a 0.sup.th moment of the switched magnetic field readout gradients essentially vanishes and a 1.sup.st moment of the switched magnetic field readout gradients is non-zero and a second gradient echo is formed during a second of the readout gradient pulses of opposite polarity to the first readout gradient pulse when both the 0.sup.th and 1.sup.st moments of the switched magnetic field readout gradients essentially vanish; b) acquiring the first and second gradient echo signals from the first and second gradient echoes; c) repeating steps a) and b) for a plurality of phase encoding steps to generate a plurality of first and second gradient echoes with different phase encodings; and a reconstruction processor configured to: reconstruct a first MR image from the first gradient echo signals from the first gradient echo and a second MR image from the second gradient echo signals from the second gradient echo; and identify ghosting artefacts in the first and/or second MR image by comparing the first and second MR images.

9. The MR device of claim 8, wherein the first and second echoes include contributions from first and second chemical species having different MR spectra, and wherein the RF pulses and the switched magnetic field gradients are timed such that signal contributions from the first and second chemical species are more in phase in one of the first and second gradient echoes than in the other of the first and second gradient echoes, and further including: at least one of additively combining the first and second images to generate a first species image and subtractively combining the first and second images to generate a second species image.

10. A non-transitory computer-readable medium carrying software code for controlling one or more computer processors of a magnetic resonance imaging device to generate an imaging sequence including: a) subjecting a portion of a subject disposed in an examination region to an RF pulse followed by switched magnetic field readout gradients to induce a first gradient echo formed when a 0.sup.th moment of the switched magnetic field readout gradients essentially vanishes and a 1.sup.st moment of the switched magnetic field readout gradients is non-zero and a second gradient echo is formed when both the 0.sup.th and 1.sup.st moments of the switched magnetic field readout gradients essentially vanish; b) acquiring first and second gradient echo signals from the first and second gradient echoes; c) repeating steps a) and b) for a plurality of phase encoding steps to generate a plurality of first and second gradient echoes with different phase encodings; d) reconstructing a first magnetic resonance image from the first gradient echo signals and a second magnetic resonance image from the second gradient echo signals; and e) comparing the first and second magnetic resonance images to identify ghosting artifacts.

11. The non-transitory computer-readable medium of claim 10, wherein the first and second echoes include contributions from first and second chemical species having different MR spectra, and wherein the RF pulses and the switched magnetic field gradients are timed such that signal contributions from the first and second chemical species are more in phase in one of the first and second gradient echoes than in the other of the first and second gradient echoes, and the computer software is further configured to control the one or more processors of the magnetic resonance imaging device to: at least one of additively combining the first and second images to generate a first species image and subtractively combining the first and second images to generate a second species image.

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 shows a MR device for carrying out the method of the invention;

(3) FIG. 2 illustrates the switching of the readout gradient for generation of gradient echo signals according to the invention;

(4) FIG. 3 shows first and second MR images reconstructed according to the invention, together with a difference image of the first and second MR images;

(5) FIG. 4 shows water and fat images derived in a conventional manner from the first and second MR images shown in FIG. 3;

(6) FIG. 5 shows water and fat images derived from the first and second MR images shown in FIG. 3 with eliminated ghosting artefacts according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(7) With reference to FIG. 1, a MR device 1 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, and—where applicable—3.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.

(8) 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.

(9) 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, together with 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.

(10) 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.

(11) 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 preamplifier (not shown). The receiver 14 is connected to the RF coils 9, 11, 12 and 13 via send-/receive switch 8.

(12) A host computer 15 controls the shimming coils 2′ as well as the gradient pulse amplifier 3 and the transmitter 7 to generate any of a plurality of MR imaging sequences, such as echo planar imaging (EPI), echo volume imaging, gradient and spin echo imaging, fast spin echo imaging, and the like. For the selected sequence, the receiver 14 receives a single or 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.

(13) Ultimately, the digital raw image data are reconstructed into an image representation by a reconstruction processor 17 which applies a Fourier transform or other appropriate reconstruction algorithms, such as SENSE or SMASH. The MR image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, or the like. The image is then stored in an image memory where it may be accessed for converting slices, projections, or other portions of the image representation into appropriate format for visualization, for example via a video monitor 18 which provides a man-readable display of the resultant MR image.

(14) In accordance with the invention, at least two gradient echo signals are generated at two different echo times, wherein the 0.sup.th moment of the readout magnetic field gradient essentially vanishes at the time of the first gradient echo, the first moment of the readout gradient being non-zero at the time of the first gradient echo, while both the 0.sup.th and 1.sup.st moments of the readout magnetic field gradient essentially vanish at the time of the second gradient echo. This is schematically illustrated in FIG. 2. Shown are the readout magnetic field gradient G.sub.readout of the dual-echo acquisition scheme of the invention, and the 0.sup.th moment M.sub.0 and the 1.sup.st moment M.sub.1 of the readout magnetic field gradient. The 0.sup.th moment M.sub.0 vanishes at both echo times (indicated by dashed lines in FIG. 2), but the 1.sup.st moment M.sub.1 vanishes only at the second echo time. The same holds for all odd and even echoes in a multi-echo acquisition according to the invention. An inherent flow compensation of the second gradient echo is achieved due to the gradient moment rephasing effect. The 0.sup.th moment M.sub.0 of the readout magnetic field gradient is defined as the integral of the readout magnetic field gradient over time. M.sub.0 equals zero at the times of the first and second gradient echoes. The 1.sup.st moment M.sub.1 of the readout magnetic field gradient is defined as the integral of the product of the readout magnetic field gradient and time over time. M.sub.1 equals zero only at the time of the second gradient echo. This renders the gradient echo signals of the first gradient echo far more prone to flow artefacts than the gradient echo signals of the second gradient echo.

(15) FIG. 3 shows first (left) and second (middle) MR images reconstructed from the gradient echo signals of the first gradient echo and the gradient echo signals of the second gradient echo respectively. The depicted first and second MR images are selected slices of a three-dimensional contrast-enhanced MR angiography acquisition. Substantial ghosting (indicated by white arrows) is present in the first MR image (left) as expected from the above discussion of the method of the invention. No ghosting artefacts can be seen in the second MR image (middle). The right image in FIG. 3 is the difference of the first (left) and second (middle) MR images. The difference image can be used for identifying the ghosting artefacts in the first MR image as the difference image shows local intensity losses as well as local intensity gains in the first MR image due to ghosting. The ghosting artefacts can then be eliminated in the first MR image by minimizing the identified intensity losses and gains.

(16) The depicted MR images of the abdomen of a patient were acquired with a three-dimensional spoiled dual-gradient-echo sequence at a main magnetic field strength of 1.5 T, wherein the echo times of the first and second gradient echoes were 1.8 ms and 3.0 ms respectively. MR signals of water and fat are slightly more out of phase at the first echo time than at the second echo time.

(17) FIG. 4 shows a water MR image (left) and a fat MR image (right) derived from the first and second MR images shown in FIG. 2 using a conventional two-point Dixon separation method, i.e. without elimination of ghosting artefacts. As can be seen in FIG. 4, ghosting artefacts due to flow propagate into both images (white arrows).

(18) FIG. 5 shows a water MR image (left) and a fat MR image (right) derived again from the first and second MR images shown in FIG. 2, wherein use is made of the method of the invention for eliminating the ghosting artefacts. By detecting a decreased local image intensity in the second MR image compared to the first MR image and by eliminating the local image intensity losses in the first MR image, the ghosting artefacts are mostly suppressed. Since the echo times are selected such that the signal contributions from water and fat are more in phase at the time of the second gradient echo than at the time of the first gradient echo, water and fat rephase and thus cannot account for signal losses towards the second echo time. Thus, intensity losses can be attributed to ghosting artefacts that are present in the first MR image, but suppressed in the second MR image due to the gradient moment rephasing effect. By eliminating the identified ghosting artefacts, a significantly improved image quality is achieved as compared with the images shown in FIG. 4 without a prolongation of echo and repetition times.