MR imaging with B1 mapping

Abstract

A method of MR imaging, wherein a portion of a body placed in the examination volume of a MR device is subjected to an imaging sequence of RF pulses and switched magnetic field gradients. The imaging sequence is a stimulated echo sequence including i) at least two preparation RF pulses () radiated toward the portion of the body during a preparation period, and ii) one or more reading RF pulses () radiated toward the portion of the body during an acquisition period temporally subsequent to the preparation period. One or more FID signals and one or more stimulated echo signals are acquired during the acquisition period. A B1 map indicating the spatial distribution of the RF field of the RF pulses within the portion of the body is derived from the acquired FID and stimulated echo signals.

Claims

1. Method of MR imaging of at least a portion of a body placed in the examination volume of a MR device, the method comprising the steps of: subjecting the portion of the body to a suppression sequence of at least one RF pulse for suppression of MR signals emanating from blood; subjecting the portion of the body 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 portion of the body during a preparation period, and ii) one or more reading RF pulses radiated toward the portion of the body during an acquisition period temporally subsequent to the preparation period, acquiring one or more FID signals and one or more stimulated echo signals during the acquisition period; and deriving at least one B.sub.1 map indicating the spatial distribution of the RF field of the RF pulses within the portion of the body from the acquired FID and stimulated echo signals.

2. Method of claim 1, wherein the B.sub.1 map is derived from the voxel-wise intensity ratio of the FID and stimulated echo signals.

3. Method of claim 1, wherein the RF pulses are radiated toward the portion of the body via two or more RF coils, wherein the B.sub.1 map indicates the spatial distribution of the RF field of the RF pulses radiated via the two or more RF coils.

4. Method of claim 3, wherein RF shim settings are derived from the B.sub.1 map, wherein the amplitudes and phases of the RF pulses radiated toward the portion of the body via the two or more RF coils are controlled according to the RF shim settings.

5. Method of claim 4, wherein a threshold-based masking is applied to the B.sub.1 map prior to deriving the RF shim settings.

6. Method of claim 1, wherein a plurality of FID and stimulated echo MR signals are generated by means of a plurality of consecutive reading RF pulses.

7. Method of claim 1, wherein the suppression sequence and/or the imaging sequence are ECG-gated.

8. Method of claim 1, wherein the suppression sequence comprises at least one black-blood preparation pre-pulse, wherein the imaging sequence is applied after a time delay after the black-blood saturation pre-pulse.

9. 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 a body of a patient 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 arranged to perform the following steps: subjecting the portion of the body to a suppression sequence of at least one RF pulse for suppression of MR signals emanating from blood; subjecting the portion of the body 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 portion of the body during a preparation period, and ii) one or more reading RF pulses radiated toward the portion of the body during an acquisition period temporally subsequent to the preparation period, acquiring one or more FID signals and one or more stimulated echo signals during the acquisition period; and deriving at least one B.sub.1 map indicating the spatial distribution of the RF field of the RF pulses within the portion of the body from the acquired FID and stimulated echo signals.

10. Computer program to be run on a MR device, which computer program comprises instructions for: generating a suppression sequence of at least one RF pulse for suppression of MR signals emanating from blood; 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 preparation period, and ii) one or more reading RF pulses radiated during an acquisition period temporally subsequent to the preparation period, acquiring one or more FID signals and one or more stimulated echo signals during the acquisition period; and deriving at least one B.sub.1 map indicating the spatial distribution of the RF field of the RF pulses from the acquired FID and stimulated echo signals.

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;

(4) FIG. 3 shows a timing diagram illustrating the combination of the imaging sequence with a black-blood preparation pre-pulse applied according to the invention within the diastolic phase of a single heartbeat.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(5) 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.

(6) 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.

(7) Most 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.

(8) 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. 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. 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. The depicted imaging sequence is a stimulated echo sequence which is subdivided into a preparation period 21 and an acquisition period 22. Two preparation RF pulses having a flip angle of are applied during the preparation period 21. The two preparation RF pulses are separated by a time interval T.sub.E. A de-phaser magnetic field gradient G.sub.mc2 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. An FID signal I.sub.1 and a stimulated echo signal I.sub.2 are acquired after each reading pulse as gradient-recalled echoes.

(12) Directly after the preparation sequence 21, the longitudinal magnetization is given by:

(13) $M_{z1}={\cos }^2\left(\right).Math.M_0$$M_{z2}=\frac{1}{2}{\sin }^2\left(\right).Math.M_0,$
wherein M.sub.z1 and M.sub.z2 denote the un-prepared (i.e. in-phase) and the stimulated echo-prepared (i.e. de-phased) longitudinal magnetization, respectively. In accordance with the invention, both the FID signal I.sub.1 generated from M.sub.z1 and the stimulated echo signal I.sub.2 generated from M.sub.z2 are acquired at different points in time T.sub.E1 and T.sub.E1+T, respectively. The delay T between the two echoes I.sub.1, I.sub.2 is determined by the relation:
T=a.sub.mc2/G.sub.m,
wherein A.sub.mc2 denotes the gradient-time area of the de-phaser gradient G.sub.mc2 and G.sub.m denotes the strength of the readout magnetic field gradient. Neglecting T.sub.1- and T.sub.2-effects, the two acquired echo signals I.sub.1 and I.sub.2 are given by:
I.sub.1=S.Math.C(T.sub.E1)sin()M.sub.z1
I.sub.2=S.Math.C(T.sub.E1+TT.sub.g)sin()M.sub.s2,
wherein S represents a complex system constant, which is equal for both echo signals I.sub.1 and I.sub.2 and which is determined e.g. by transmit and receive coil sensitivities for a given voxel. is the nominal flip angle of the reading RF pulses. C describes the static signal de-phasing for a given voxel due to susceptibility and chemical shift effects:

(14) $C\left(t\right)={}_V\left(r\right)e^{-i\left(r\right).Math.t}\mathrm{dr},$
wherein and denote the proton density and the off-resonance frequency offset, respectively. The integral describes the summation over the given voxel. By applying the timing scheme
T.sub.E=2T.sub.E1+T
the measured echo signals I.sub.1 and I.sub.2 are given by:
I.sub.1=S.Math.C(T.sub.E1)sin()M.sub.z1
I.sub.2=S.Math.C(T.sub.E1+TT.sub.g)sin()M.sub.s2,
Thus, the de-phasing term C is identical for both echo signals, apart from the mirrored phase. For example by selecting T.sub.E1=2.3 ms at a main magnetic field strength of 3 Tesla, signal contributions from water spins and signal contributions from fat spins are essentially in phase for both echoes I.sub.1, I.sub.2. Combining the above equations yields:
|I.sub.2/I.sub.1|=tan.sup.2()/2
Thus, the unknown flip angle of the stimulated echo preparation RF pulses can be derived from the ratio of the acquired echo signals according to:
=arctan{square root over (2|I.sub.2/I.sub.1|)}
It has to be noted that for B.sub.1 mapping also the alternative timing scheme T.sub.E=T can be employed, which results in identical de-phasing terms, i.e. without mirrored phase. However, this variant results in a longer T and, hence, a longer overall repetition time of the sequence.

(15) The imaging sequence shown in FIG. 2 is actually a basic version of the known DREAM B.sub.1 mapping sequence (Magnetic Resonance in Medicine, 68, 1517-1526, 2012).

(16) FIG. 3 shows a schematic diagram illustrating the application of the DREAM imaging sequence IMG as illustrated in FIG. 2 in combination with a suppression sequence, which is a black-blood preparation pulse BB in the depicted embodiment, the black-blood preparation pulse BB being radiated prior to the imaging sequence in order to suppress contributions from blood spins to the FID and stimulated echo signals I.sub.1, I.sub.2. The time interval T.sub.i between the black blood sequence BB and the imaging sequence is adjusted to approximately null the blood signal contribution to the stimulated echo and FID signals I.sub.1, I.sub.2 underlying the B.sub.1 maps. Other magnetization preparation pulses may be added to further improve the B.sub.1 mapping quality.

(17) The generation of the black-blood preparation pulse BB (and consequently also the generation of the imaging sequence IMG) is ECG-triggered. The time delay T.sub.D is selected to acquire the stimulated echo and FID signals I.sub.1, I.sub.2 in the diastolic phase of the heartbeat.

(18) The RF pulses of the imaging sequence IMG are radiated toward the body 10 in parallel via the RF coils 11, 12, 13, wherein the obtained B.sub.1 maps indicate the spatial distribution of the RF field of the RF pulses radiated via the respective RF coils 11, 12, 13. The B.sub.1 maps are employed for RF shimming RF shim settings are derived from the B.sub.1 maps, wherein the amplitudes and phases of the RF pulses are controlled according to the RF shim settings for each RF coil 11, 12, 13 in order to achieve an optimum homogeneous distribution of the B.sub.1 field within the imaged region. In order to avoid flow-artifacts, the obtained B.sub.1 maps used for computing the RF shim settings are masked automatically using a simple threshold applied to the MR images reconstructed from the FID and stimulated echo signals I.sub.1, I.sub.2 respectively.

(19) The approach of the invention allows a two-dimensional B.sub.1 map to be acquired in a single heart beat, which makes it possible to integrate the sequence into the clinical workflow of a parallel transmit MR imaging system, as described above. B.sub.1 maps may be acquired for several slices, orientations, and/or transmit channels in clinically acceptable breath hold durations. The proposed magnetization preparation-based blood suppression scheme facilitates masking of the blood pool signal, thus improving the accuracy of the B.sub.1 maps and, hence, the quality of RF shimming, considering only reliable B.sub.1 estimates.

(20) More advanced black-blood preparation pulses suitable for multi-slice excitation could be employed in a straight forward manner, further increasing the flexibility of the approach.