System and method for quantitative magnetization transfer imaging based on spin-lock

Patent metadata

Field	Value
Publication number	US-11280867-B2
Application number	US-202015929627-A
Country	US
Kind code	B2
Filing date	May 13, 2020
Priority date	Nov 8, 2019
Publication date	Mar 22, 2022
Grant date	Mar 22, 2022

Abstract

Systems and methods for fast and robust quantification of magnetization transfer (MT) using off-resonance spin-lock MRI. The techniques can be insensitive to variations of the inherent relaxation rates R1 (1/T1) and R2 (1/T2) of the free-water pool and to variations of the chemical exchange pool. The techniques can also be robust in the presence of inhomogeneity in the B1 RF and/or B0 magnetic fields.

First claim

What is claimed is:

1. A method for quantifying magnetization transfer using a magnetic resonance imaging (MRI) apparatus, the method comprising:
performing a plurality of image acquisition processes to produce a plurality of MRI images, wherein each image acquisition process (i) in the plurality of image acquisition process includes applying an off-resonance spin-lock pulse having an RF amplitude (ω 1 (i) ) and a frequency offset (Δω (i) ); and
computing, based on the plurality of MRI images one or more parameters of magnetization transfer, wherein the one or more parameters of magnetization transfer include a parameter R mpfsl defined as R mpfsl =R 1ρ (2) −R 1ρ (1) , wherein:
R 1ρ (1) is a first relaxation rate in a rotating frame responsive to a spin-lock pulse having a first RF amplitude ω 1 (1) and a first frequency offset Δω (1) ;
R 1ρ (2) is a second relaxation rate in the rotating frame responsive to a spin-lock pulse having a second RF amplitude ω 1 (2) and a second frequency offset Δω (2) ;
the first RF amplitude ω 1 (1) is different from the second RF amplitude ω 1 (2) ; and
the first RF amplitude ω 1 (1) , the first frequency offset Δω (1) , the second RF amplitude ω 1 (2) , and the second frequency offset Δω (2) are chosen such that Δω (1) /ω 1 (1) =Δω (2) /ω 1 (2) .

2. The method of claim 1 further comprising:
computing one or more additional parameters of magnetization transfer based on the parameter R mpfsl .

3. The method of claim 2 wherein the one or more additional parameters of magnetization transfer includes a pool size ratio (f b ) of a bound pool of protons bound to semi-solid macromolecules that is computed from the parameter R mpfsl according to the equation

R

m
⁢
pfsl

=

k

b
⁢
a

2

⁢

f
b

⁡

(

1
+

f
b

)

⁢

(

1

(

1
+

f
b

)

⁢

k

b
⁢
a

+

R
rfc

(
1
)

-

1

(

1
+

f
b

)

⁢

k

b
⁢
a

+

R
rfc

(
2
)

)

,

wherein k ba is a magnetization exchange rate between a free-water pool and the bound pool, R rfc (1) is a parameter representing a saturation rate of the bound pool at the first off-resonance frequency Δω (1) and the first RF amplitude ω 1 (1) , R rfc (2) is a parameter representing a saturation rate of the bound pool at the second off-resonance frequency Δω (2) and the second RF amplitude ω 1 (2) .

4. The method of claim 3 wherein the saturation rate parameter R rfc is a function of RF amplitude (ω 1 ), off-resonance frequency (Δω), and a transverse relaxation time of the bound pool (T 2b ).

5. The method of claim 4 wherein the magnetization exchange rate k ba and the transverse relaxation time of the bound pool T 2b are treated as constants that do not depend on the off-resonance frequency Δω or the RF amplitude ω 1 .

6. The method of claim 4 wherein the magnetization exchange rate k ba and the transverse relaxation time of the bound pool T 2b are treated as variables that are determined by fitting to the plurality of MRI images.

7. The method of claim 2 wherein the one or more additional parameters of magnetization transfer include a macromolecular proton fraction (MPF) indicating a fraction of protons that are bound to semi-solid macromolecules.

8. The method of claim 1 wherein performing the plurality of image acquisition processes includes:
performing a first image acquisition process using a first spin-lock pulse at the first RF amplitude ω 1 (1) and the first frequency offset Δω (1) without a toggling RF pulse, wherein the first image acquisition process produces a first MRI image (M noTog (1) );
performing a second image acquisition process using a second spin-lock pulse at the first RF amplitude ω 1 (1) and the first frequency offset Δω (1) with a toggling RF pulse, wherein the second image acquisition process produces a second MRI image (M Tog (1) );
performing a third image acquisition process using a third spin-lock pulse at the second RF amplitude ω 1 (2) and the second frequency offset Δω (2) without a toggling RF pulse, wherein the third image acquisition process produces a third MRI image (M noTog (2) ); and
performing a fourth image acquisition process using a fourth spin-lock pulse at the second RF amplitude ω 1 (2) and the second frequency offset Δω (2) with a toggling RF pulse, wherein the

Assignees

• Univ Hong Kong Chinese

Inventors

• Chen Weitian
• Hou Jian
• Jiang Baiyan

Classifications

• A61B5/055

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

• G01R33/4828

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

• G01R33/48

  NMR imaging systems · 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