MR imaging using apt contrast enhancement and sampling at multiple echo times

Patent metadata

Field	Value
Publication number	US-9766313-B2
Application number	US-201314387421-A
Country	US
Kind code	B2
Filing date	Mar 21, 2013
Priority date	Apr 3, 2012
Publication date	Sep 19, 2017
Grant date	Sep 19, 2017

Abstract

The invention relates to a method of CEST or APT MR imaging of at least a portion of a body ( 10 ) placed in a main magnetic field B 0 within the examination volume of a MR device. The method of the invention comprises the following steps: •a) subjecting the portion of the body ( 10 ) to a saturation RF pulse at a saturation frequency offset; •b) subjecting the portion of the body ( 10 ) to an imaging sequence comprising at least one excitation/refocusing RF pulse and switched magnetic field gradients, whereby MR signals are acquired from the portion of the body ( 10 ) as spin echo signals; •c) repeating steps a) and b) two or more times, wherein the saturation frequency offset and/or a echo time shift in the imaging sequence are varied, such that a different combination of saturation frequency offset and echo time shift is applied in two or more of the repetitions; •d) reconstructing a MR image and/or B 0 field homogeneity corrected APT/CEST images from the acquired MR signals. Moreover, the invention relates to a MR device ( 1 ) for carrying out the method of the invention and to a computer program to be run on a MR device.

First claim

The invention claimed is:

1. A method of MR imaging of at least a portion of a body placed in a main magnetic field B 0 within the examination volume of a MR device, the method comprising the following steps:
a) subjecting the portion of the body to a saturation RF pulse at a saturation frequency offset respective to the resonance frequency of water protons;
b) subjecting the portion of the body to an imaging sequence comprising excitation and refocusing RF pulses and switched magnetic field gradients, whereby MR signals are acquired from the portion of the body as spin echo signals;
c) repeating steps a) and b) two or more times, wherein at least one of the saturation frequency offset and an echo time shift in the imaging sequence are varied, such that a different combination of saturation frequency offset and echo time shift is applied in two or more of the repetitions;
d) reconstructing a MR image as B 0 field homogeneity corrected amide proton transfer (APT)/Chemical Exchange Saturation Transfer (CEST) images from the acquired MR signals;
wherein the reconstructing includes determining a spatial variation of B 0 within the portion of the body from the acquired MR signals using a multi-point Dixon technique based on MR signal acquisitions with different saturation frequency offsets and different echo time shifts.

2. The method of claim 1 wherein
the repeating of steps a) and b) includes repeating with a number of offset-values for the saturation frequency offset and a number of shift-values for the echo-time shift are selected and
for each of the respective different selected offset-values a different shift value for the echo time shift is applied in the imaging sequence.

3. The method of claim 2 , wherein the applied offset-values and the applied shift values effect a sparse sampling of a plane spanned by offset values and shift values.

4. The method of claim 1 , wherein the spatial variation of B 0 within the portion of the body is determined from the acquired MR signals using the multi-point Dixon technique based on the MR signal acquisitions with the different saturation frequency offsets that are positive with respect to the resonance frequency of water protons.

5. The method of claim 1 , wherein the reconstruction of the MR image includes deriving a spatial distribution of amide protons within the portion of the body from an asymmetry analysis of the amplitude of the acquired MR signals as a function of the saturation frequency offset respective to the resonance frequency of water protons, which asymmetry analysis involves a saturation frequency offset correction based on the determined spatial variation of B 0 .

6. The method of claim 5 , wherein the reconstruction of the MR image includes deriving a spatial pH distribution within the portion of the body from an asymmetry analysis of the amplitude of the acquired MR signals as a function of the saturation frequency offset respective to the resonance frequency of water protons, which asymmetry analysis involves a saturation frequency offset correction based on the determined spatial variation of B 0 .

7. The method of claim 1 , wherein saturation RF pulses are applied in different repetitions of steps a) and b) at positive and negative saturation frequency offsets around the resonance frequency of water protons.

8. The method of claim 1 , wherein steps a) and b) are repeated two or more times with the same saturation frequency offset and with a different echo time shift in two or more of the repetitions.

9. The method of claim 1 , wherein steps a) and b) are repeated two or more times with a different saturation frequency offset and with a different echo time shift in two or more of the repetitions.

10. The method of claim 1 , wherein the repeating of steps a) and b) generates exactly one combination of saturation frequency offset and echo time shift for each saturation frequency offset.

11. The method of claim 1 , wherein the determining of the spatial variation of B 0 within the portion of the body produces a single B 0 map which is used in the B 0 field homogeneity correction of all of the B 0 field homogeneity corrected APT/CEST images.

12. 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 a body of a patient positioned in the examination volume; and
a computer programmed to control the temporal succession of RF pulses generated by the at least one RF coil and switched magnetic field gradients generated by the gradient coils and to reconstruct an MR image from the received MR signals by performing the following steps:
a) subjecting the portion of the body to a saturation RF pulse generated by the at least one RF coil at a saturation frequency offset with respect to the resonance frequency of water protons;
b) subjecting the portion of the body to an imaging sequence comprising excitation and refocusing RF pulses generated by the at least one RF coil and switched magnetic field gradients generated by the gradient coils, whereby MR signals are acquired from the portion of the body as spin echo signals;
c) repeating steps a) and b) two or more times, wherein at least one of the saturation frequency offset and an echo time shift in the imaging sequence are varied, such that a different combination of saturation frequency offset and echo time shift is applied in two or more of the repetitions;
d) reconstructing an MR image as B 0 field homogeneity corrected amide proton transfer (APT)/Chemical Exchange Saturation Transfer (CEST) images from the acquired MR signals;
wherein the reconstructing includes determining a spatial variation of B 0 within the portion of the body from the acquired MR signals using a Dixon technique based on MR signal acquisitions with different saturation frequency offsets and different echo time shifts to produce a single B 0 map which is used in the B 0 field homogeneity correction of all of the B 0 field homogeneity corrected APT/CEST images.

13. A non-transitory data carrier storing a computer program to be run on a magnetic resonance (MR) device, which computer program comprises instructions for causing the MR device to perform a method including:
a) generating a saturation RF pulse at a saturation frequency offset with respect to the resonance frequency of water protons;
b) generating an imaging sequence comprising excitation and refocusing RF pulses and switched magnetic field gradients, whereby MR signals are acquired from the portion of the body as spin echo signals;
c) repeating steps a) and b) two or more times, wherein at least one of the saturation frequency offset and an echo time shift in the imaging sequence are varied, such that a different combination of saturation frequency offset and echo time shift is applied in each of the two or more of the repetitions and the repetitions generate exactly one combination of saturation frequency offset and echo time shift for each saturation frequency offset;
d) reconstructing an MR image as B 0 field homogeneity corrected amide proton transfer (APT)/Chemical Exchange Saturation Transfer (CEST) images from the acquired MR signals;
wherein the reconstructing includes determining a spatial variation of B 0 within the portion of the body from the acquired MR signals using a Dixon technique based on MR signal acquisitions with different saturation fre

Assignees

• Koninklijke Philips Nv

Inventors

• Eggers Holger
• Keupp Jochen

Classifications

• A61B5/055

  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging · CPC title

• G01R33/5617

  using RF refocusing, e.g. RARE · CPC title

• G01R33/5605 Primary

  by transferring coherence or polarization from a spin species to another, e.g. creating magnetization transfer contrast [MTC], polarization transfer using nuclear Overhauser enhancement [NOE] · CPC title

• G01R33/243

  Spatial mapping of the polarizing magnetic field · CPC title

• G01R33/4828 Primary

  Resolving the MR signals of different chemical species, e.g. water-fat imaging · CPC title