Parallel MR imaging with RF coil sensitivity mapping

Abstract

The invention relates to a method of MR imaging of an object (10). The problem of the invention is to provide an improved MR imaging technique that enables fast and robust determination of spatial sensitivity profiles of RF receiving antennas (11, 12, 13) used in parallel imaging as well as B1 and/or B0 mapping. The method of the invention comprises subjecting the object (10) to a stimulated echo sequence. Two or more stimulated echo signals (STE, STE*) are acquired, namely a direct stimulated echo signal (STE) and a conjugated stimulated echo signal (STE*), wherein at least one of the stimulated echo signals (STE, STE*) is received in parallel via an array of two or more RF receiving antennas (11, 12, 13) having different spatial sensitivity profiles, and wherein at least another one of the stimulated echo signals (STE, STE*) is received via a body RF coil (9) having an essentially homogeneous spatial sensitivity profile. Sensitivity maps indicating the spatial sensitivity profiles of the individual RF receiving antennas (11, 12, 13) of the array are derived by comparing the stimulated echo signals (STE, STE*) received via the array of RF receiving antennas (11, 12, 13) with the stimulated echo signals (STE, STE*) received via the body RF coil (9). Moreover, the invention relates to a MR device (1) and to a computer program for a MR device (1).

Claims

1. A method of magnetic resonance (MR) imaging of an object placed in the examination volume of a MR device, the method comprising: subjecting the object to an imaging sequence of RF pulses and switched magnetic field gradients, which imaging sequence is a stimulated echo sequence including: i) at least two preparation RF pulses () radiated toward the object during a magnetization preparation period, and ii) one or more reading RF pulses () radiated toward the object during an acquisition period temporally subsequent to the magnetization preparation period; acquiring two or more stimulated echo signals (STE, STE*) after each reading RF pulse () during the acquisition period, namely a direct stimulated echo signal (STE) and a conjugated stimulated echo signal (STE*), wherein at least one of the stimulated echo signals (STE, STE*) is received in parallel via an array of two or more RF receiving antennas having different spatial sensitivity profiles, and wherein at least another one of the stimulated echo signals (STE, STE*) is received via a body RF coil having an essentially homogeneous spatial sensitivity profile; and deriving sensitivity maps indicating the spatial sensitivity profiles of the individual RF receiving antennas of the array by comparing the stimulated echo signals (STE, STE*) received via the array of RF receiving antennas with the stimulated echo signals (STE, STE*) received via the body RF coil.

2. The method of claim 1, wherein one or more FID signals are acquired during the acquisition period.

3. The method of claim 2, wherein a B1 map is derived from the voxel-wise intensity ratios of the FID and stimulated echo signals.

4. The method of claim 1, wherein a plurality of FID and stimulated echo (STE, STE*) MR signals are generated by means of a plurality of consecutive reading RF pulses ().

5. The method of claim 1, wherein the FID signals and/or the two or more stimulated echo signals (STE, STE*) are acquired as gradient-recalled echo signals.

6. The method of claim 1, wherein a B.sub.0 map indicating a spatial distribution of the main magnetic field within the examination volume is derived from the acquired FID and stimulated echo signals (STE, STE*).

7. The method of claim 1, wherein the parameters of the imaging sequence are selected such that the contributions from water spins and from fat spins to the stimulated echo signals (STE, STE*), from which the spatial sensitivity profiles are derived, are essentially identical.

8. 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 for reconstructing MR images from the received MR signals, wherein the MR device is configured to perform the following steps: subjecting the object to an imaging sequence of RF pulses and switched magnetic field gradients, which imaging sequence is a stimulated echo sequence including: i) at least two preparation RF pulses () radiated toward the object during a magnetization preparation period, and ii) one or more reading RF pulses () radiated toward the object during an acquisition period temporally subsequent to the magnetization preparation period; acquiring two or more stimulated echo signals (STE, STE*) after each reading RF pulse () during the acquisition period, namely a direct stimulated echo signal (STE) and a conjugated stimulated echo signal (STE*), wherein at least one of the stimulated echo signals (STE, STE*) is received in parallel via an array of two or more RF receiving antennas having different spatial sensitivity profiles, and wherein at least another one of the stimulated echo signals (STE, STE*) is received via a body RF coil having an essentially homogeneous spatial sensitivity profile; and deriving sensitivity maps indicating the spatial sensitivity profiles of the individual RF receiving antennas of the array by comparing the stimulated echo signals (STE, STE*) received via the array of RF receiving antennas with the stimulated echo signals (STE, STE*) received via the body RF coil.

9. A non-transitory computer readable storage medium configured to store a computer program to be run on a magnetic resonance (MR) device, which computer program comprises instructions for: generating an imaging sequence of RF pulses and switched magnetic field gradients, which imaging sequence is a stimulated echo sequence including: i) at least two preparation RF pulses () radiated during a magnetization preparation period, and ii) one or more reading RF pulses () radiated during an acquisition period temporally subsequent to the magnetization preparation period; acquiring two or more stimulated echo signals (STE, STE*) after each reading RF pulse () during the acquisition period, namely a direct stimulated echo signal (STE) and a conjugated stimulated echo signal (STE*), wherein at least one of the stimulated echo signals (STE, STE*) is received in parallel via an array of two or more RF receiving antennas having different spatial sensitivity profiles, and wherein at least another one of the stimulated echo signals (STE, STE*) is received via a body RF coil having an essentially homogeneous spatial sensitivity profile; and deriving sensitivity maps indicating the spatial sensitivity profiles of the individual RF receiving antennas of the array by comparing the stimulated echo signals (STE, STE*) received via the array of RF receiving antennas with the stimulated echo signals (STE, STE*) received via the body RF coil.

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 methods of the invention;

(3) FIG. 2 shows a schematic diagram illustrating an imaging sequence according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) 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, 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 which has an essentially homogeneous spatial sensitivity.

(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 having different spatial sensitivity profiles 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. In parallel transmit applications, the array RF coils 11, 12, 13 may also be used for RF transmission, for example for the purpose of RF shimming.

(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 preamplifier (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 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.

(10) Ultimately, the digital raw image data is reconstructed into an image representation by a reconstruction processor 17 which applies a Fourier transform or other appropriate reconstruction algorithms, such like 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.

(11) FIG. 2 shows a diagram illustrating an imaging sequence according to the invention.

(12) The depicted imaging sequence is a stimulated echo sequence which is subdivided into a magnetization preparation period 21 and an acquisition period 22. Two preparation RF pulses having a flip angle of a are applied during the magnetization preparation period 21. The two preparation RF pulses are separated by a time interval T.sub.S. A de-phaser magnetic field gradient G.sub.m2 is applied between the two preparation RF pulses. A sequence of reading RF pulses having flip-angle are generated during the acquisition period 22, which is temporally subsequent to the preparation period 21. Each reading RF pulse creates an FID signal, a stimulated echo STE and a conjugated stimulated echo STE* that are acquired as separate gradient-recalled echoes quasi-simultaneously under a single readout gradient lobe. The acquisition order is determined by the polarity of the de-phaser gradient G.sub.m2. The timing of the three gradient echoes is controlled by the switched gradients G.sub.m1, G.sub.m (illustrated by the lower phase graph .sub.Gr).

(13) Directly after the stimulated echo preparation period 21, the longitudinal nuclear magnetization is given by:
M.sub.z,FID=cos.sup.2().Math.M.sub.0
M.sub.z,STE=sin.sup.2().Math.M.sub.0
M.sub.z,STE*=sin.sup.2().Math.M.sub.0

(14) Where M.sub.z,FID denotes the unprepared (i.e. in-phase) longitudinal magnetization, and M.sub.z,STE and M.sub.z,STE* denote the two mirrored stimulated echo-prepared (i.e. de-phased) longitudinal magnetization contributions. The transverse magnetization components (i.e. the FID from the first RF pulse , the FID and the spin echo from second RF pulse ) are spoiled by a strong crusher gradient and will not be further considered.

(15) According to the invention, the stimulated echo STE is received in parallel via the array of array RF coils 11, 12, 13 having different spatial sensitivity profiles. The conjugated stimulated echo signal STE* is received via the body RF coil 9 which has an essentially homogeneous spatial sensitivity. The two stimulated echoes STE, STE* are separated by a time interval of approximately 1-2 ms, which is sufficiently long to switch signal reception between the array of RF coils 11, 12, 13 and the body RF coil 9 by means of send/receive switch 8. The FID signal can be received, for example, also in parallel via the array RF coils 11, 12, 13.

(16) The reading RF pulse of the imaging sequence thus generates three transverse signal contributions:
I.sub.FID=S.sub.A.Math.C(t)sin()M.sub.z,FID
I.sub.STE=S.sub.A.Math.C(tT.sub.S)sin()M.sub.z,STE
I.sub.STE*=S.sub.BC.Math.C(t+T.sub.S)sin()M.sub.z,STE*

(17) Therein S.sub.A and S.sub.BC each represent a complex system constant which comprises the receive coil sensitivity for the respective array RF coil (S.sub.A) and for the body RF coil (S.sub.BC) for a given voxel, and is the nominal flip angle of the reading RF pulse. Furthermore, as mentioned above, T.sub.S is the time interval separating the two RF pulses in the preparation phase and C describes the static signal de-phasing for a given voxel due to susceptibility and chemical shift effects:

(18) $C\left(t\right)={}_V^{}\left(r\right)e^{-i\left(r\right).Math.t}\mathrm{dr}$

(19) Wherein (r) and (r) denote proton density and off-resonance frequency offset, and the integral describes the summation over the given voxel. While the STE signal I.sub.STE refocuses as a stimulated echo, the STE* signal I.sub.STE* further de-phases, and, hence, is therefore typically discarded in a conventional stimulated echo experiment. However, the imaging sequence shown in FIG. 2 employs tailored switched magnetic field gradients to acquire all three signal contributions as separate re-called gradient echoes at deliberately chosen echo times. The relations for the gradient areas A of the measurement gradient G.sub.m, the re-phaser gradient G.sub.m1 and the stimulated echo de-phaser gradient G.sub.m2 are:
A(G.sub.m1)=1.5 A(G.sub.m)
A(G.sub.m2)=A(G.sub.m)

(20) The first equation ensures that the gradient echo of the FID is refocused at the centre of the second readout gradient G.sub.m. The second equation ensures that the direct stimulated echo STE and the conjugated stimulated echo STE* are refocused at the centre of the first and third readout gradient G.sub.m, respectively. The acquisition order (STE-FID-STE* or STE*-FID-STE) is determined by the polarity of the stimulated echo de-phaser gradient G.sub.m2 relative to the readout gradient G.sub.m.

(21) Within the general sequence timing constraints resulting from e.g. acquisition bandwidth or RF and gradient power limitations, the gradient echoes times (i.e. time of the gradient echo top) may be independently selected to obtain a desired spectral encoding for the different echoes. For example, an equidistant timing scheme for the three gradient echoes could be applied by concatenating the three readout gradients G.sub.m to a single, constant gradient lobe. If, additionally, the time interval between the STE/STE* and the FID signals, T, is chosen equal to T.sub.S, the STE and STE* signals have the same spectral encoding time, namely TE.sub.FID and differ only in the T.sub.2 evolution time, which is TE.sub.FID+2T.sub.S for the stimulated echo STE and TE.sub.FID for the conjugated echo STE*. Hence, MR images reconstructed from the STE and STE* signals are identical and differ only by T.sub.2 relaxation and their respective system constants S.sub.A and S.sub.BC. As mentioned above, the timing can be selected such that the two stimulated echoes STE, STE* are separated by a time interval of only 1-2 ms such that T.sub.2 relaxation can be neglected. The spatial sensitivity maps of the array RF coils 11, 12, 13 can thus be derived directly from the voxel-wise ratio of the stimulated echoes STE and STE*(I.sub.STE/I.sub.STE*, see above) which can be calculated for each individual RF receiving coil 11, 12, 13 of the array.

(22) In addition, as in the known DREAM approach (see above), the flip angle of the stimulated echo preparation RF pulses (and thus the B.sub.1 map) can be derived from the ratio of the acquired stimulated echo and FID signals according to:
=arctan{square root over (2|I.sub.STE/I.sub.FID|)}

(23) If T.sub.S is set to T.sub.S=T+TE.sub.FID, a B.sub.0 phase map can be derived from the phase of the two signals:
.sub.B.sub.0=arg(I.sub.STE.Math.I*.sub.FID)

(24) This applies under the provision that the FID is acquired at a fat/water in-phase echo time (e.g. 2.3 ms at 3 Tesla). In this case, the FID and the stimulated echoes are acquired at different water/fat in-phase spectral encoding times (e.g. 4.6 ms for STE*, 2.3 ms for FID and 0 ms for STE at 3 Tesla). Thus the STE and STE* signals would also differ by T.sub.2* relaxation, but T.sub.2* and T.sub.2 effects at least partly cancel out, since the STE stimulated echo has a stronger T.sub.2- and a weaker T.sub.2*-weighting than the STE* stimulated echo.