MR imaging with B1 mapping

Patent metadata

Field	Value
Publication number	US-9977106-B2
Application number	US-201314371887-A
Country	US
Kind code	B2
Filing date	Jan 4, 2013
Priority date	Jan 12, 2012
Publication date	May 22, 2018
Grant date	May 22, 2018

Abstract

In an MR imaging method and apparatus, a portion of a body placed in an examination volume of an 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) two preparation RF pulses (α) radiated toward the portion of the body during a preparation period ( 21 ), and ii) reading RF pulses (β) radiated toward the portion of the body during an acquisition period ( 22 ) temporally subsequent to the preparation period ( 21 ). FID signals (I 1 ) and stimulated echo signals (I 2 ) are acquired during the acquisition period ( 22 ) with equal T 2 *-weighting. A B 1 map indicating a spatial distribution of the RF field of the preparation RF pulses within the portion of the body is derived from the acquired FID (I 1 ) and stimulated echo (I 2 ) signals.

First claim

The invention claimed is:

1. A method of MR imaging of at least a portion of a body placed in an examination volume of an MR device, the method comprising:
subjecting the portion of the body to a stimulated echo imaging sequence of RF pulses and switched magnetic field gradients, which stimulated echo sequence includes:
i) radiating at least two preparation RF pulses (α) toward the portion of the body via at least one RF coil during a preparation period,
ii) radiating one or more reading RF pulses (β) toward the portion of the body via the at least one RF coil during an acquisition period temporally subsequent to the preparation period,
iii) with at least one gradient magnetic field coil, subjecting the portion of the body to the switched magnetic field gradient during the preparation and acquisition periods, and
iv) acquiring one or more FID signals (I 1 ) and one or more simulated echo signals (I 2 ) via the at least one RF coil during the acquisition period;

wherein a timing among the preparation and reading RF pulses and the switched magnetic field gradients is selected such that the one or more FID signals (I 1 ) and one or more stimulated echo signals (I 2 ) acquired during the acquisition periods have an equal T* weighting;
deriving a B 1 map indicating a spatial distribution of an RF field of the RF pulses within the portion of the body from the acquired FID (I 1 ) and stimulated echo (I 2 ) signals;
reconstructing separate water images and fat images from signal contributions from water spins and signal contributions from fat spins superimposed in the FID signals (I 1 ) and the stimulated echo signals (I 2 );
controlling a display device to display the water and fat images.

2. The method of claim 1 , wherein the signal contributions from water spins and signal contributions from fat spins are essentially in phase in the FID signals (I 1 ) and in the stimulated echo signals (I 2 ).

3. A method of MR imaging of at least a portion of a body placed in an examination volume of an MR device, the method comprising:
subjecting the portion of the body to a stimulated echo imaging sequence of RF pulses and switched magnetic field gradients, which stimulated echo sequence includes:
i) radiating at least two preparation RF pulses (α) towards the portion of the body via two or more RF coils, during a preparation period, and
ii) one or more reading RF pulses (β) radiated toward the portion of the body during an acquisition period temporally subsequent to the preparation period;

acquiring one or more FID signals (I 1 ) and one or more stimulated echo signals (I 2 ) during the acquisition period;
wherein a timing among the preparation and reading RF pulses is selected such that one or more FID signals (I 1 ) and one or more stimulated echo signals (I 2 ) acquired during the acquisition period have an equal T 2 * weighting;
deriving a B 1 map from the acquired FID (I 1 ) and stimulated echo (I 2 ) signals for each RF coil or set of RF coils, each B 1 map indicating a spatial distribution of the RF field of the RF pulses irradiated via the respective RF coil or set of RF coils.

4. The method of claim 3 , wherein the FID (I 1 ) and stimulated echo (I 2 ) signals are acquired as gradient-recalled echo signals.

5. The method of claim 3 , wherein the B 1 map is derived from the voxel-wise intensity ratio of the FID (I 1 ) and stimulated echo (I 2 ) signals.

6. The method of claim 3 , further including:
deriving RF shim settings from the B 1 maps.

7. A method of magnetic resonance (MR) imaging comprising:
subjecting a portion of a body placed in an examination volume of an MR device to a stimulated echo imaging sequence, the stimulated echo sequence including:
radiating at least two preparation RF pulses (α) toward the portion of the body via at least one RF coil during a preparation period,
radiating one or more reading RF pulses (β) toward the portion of the body with the at least one RF coil during an acquisition period temporally subsequent to the preparation period,
applying magnetic field gradient pulses during the preparation and acquisition periods, and
acquiring one or more FID signals (I 1 ) and one or more stimulated echo signals (I 2 ) acquired during the acquisition period;

wherein a timing among the preparation and reading RF pulses is selected such that the one or more FID signals (I 1 ) and one or more stimulated echo signals (I 2 ) acquired during the acquisition period have an equal T* weighting;
subjecting the portion of the body to a navigator sequence comprising one or more navigator RF pulses with the one or more RF coils and switched magnetic field gradients and acquiring navigator signals with the one or more RF coils from the body portion, which navigator sequence is applied at least once before, during or after the stimulated echo imaging sequence;
deriving motion data from the acquired navigator signals, which motion data reflect motion of the portion of the body;
deriving at least one B 1 map of at least one motion state of the portion of the body indicating a spatial distribution of an RF field of the RF pulses within the portion of the body from the acquired FID (I 1 ) and stimulated echo (I 2 ) signals and the motion data.

8. The method of claim 7 , wherein a plurality of B 1 maps are derived from the FID (I 1 ) and stimulated echo (I 2 ) signals acquired during repetitions of the stimulated echo sequence and the navigator sequence, each B 1 map corresponding to a different one of a plurality of motional states.

9. The method of claim 7 , further including:
determining local specific RF absorption rate (SAR) within the portion of the body from the at least one B 1 map.

10. The method of claim 9 , further including:
controlling an amplitude and phase of RF pulses radiated toward the portion of the body in the preparation and acquisition periods to maintain the local specific RF absorption rate below preselected limits.

11. The method of claim 7 , further including:
reconstructing the acquired HD and stimulated echo signals into an image; and
controlling a display to display the reconstructed image.

12. The method of claim 11 , wherein a direct stimulated echo signal (ISTE) and a conjugate stimulated echo signal (ISTE*) are acquired during the acquisition period.

13. The method of claim 12 , wherein the direct stimulated echo signal (ISTE) and the conjugate stimulated echo signal (ISTE*) are acquired as gradient-recalled echo signals.

14. A method of magnetic resonance (MR) imaging comprising:
repeatedly subjecting a portion of a body placed in an examination volume of an MR device to a stimulated echo imaging sequence several times with a repetition time shorter than a longitudinal nuclear relaxation time, the stimulated echo sequence including:
i) applying, with at least one RF coil, at least two preparation RF pulses (α) to the portion of the body during a preparation period, wherein the at least two preparation RF pulses (α) have a phase difference which is varied with each repetition of the stimulated echo sequence,
ii) applying, with the at least one RF coil, one or more reading RF pulses (β) to the portion of the body during an acquisition period temporally subsequent to the preparation period,
iii) switching a gradient magnetic field coil to apply gradient magnetic field pulses to the portion of the body at least during the acquisition period, and
iv) acquiring, with the at least one RF oil, FID signals (I 1 ) and stimulated echo signals (I 2 ) during the acquisition period;

wherein the preparation and reading RF pulses are timed such that FID signals (I 1 ) a

Assignees

• Koninklijke Philips Nv

Inventors

• Nehrke Kay
• Boernert Peter
• Graesslin Ingmar
• Katscher Ulrich
• Leussler Christoph
• Eggers Holger

Classifications

• G01R33/243 Primary

  Spatial mapping of the polarizing magnetic field · CPC title

• G01R33/50 Primary

  based on the determination of relaxation times {, e.g. T1 measurement by IR sequences; T2 measurement by multiple-echo sequences} · CPC title

• G01R33/54

  Signal processing systems, e.g. using pulse sequences {; Generation or control of pulse sequences; Operator console} · CPC title

• G01R33/246

  Spatial mapping of the RF magnetic field B1 · CPC title

• G01R33/24 Primary

  for measuring direction or magnitude of magnetic fields or magnetic flux · CPC title