System and method for determining patient parameters using radio frequency phase increments in magnetic resonance imaging
A system and method are provided for controlling a magnetic resonance imaging system to perform a gradient echo pulse sequence that includes varying a phase of an RF pulse of the gradient echo pulse sequence between repetitions and acquire complex MR data. The method includes processing the complex MR data to determine signal contributions from transverse relaxation (T2) in the subject, generating a quantitative T2 map of the subject using the signal contributions from T2 in the subject, and displaying the quantitative T2 map.
1. A magnetic resonance imaging (MRI) system comprising:
a magnet system configured to generate a polarizing magnetic field about at least a portion of a subject arranged in the MRI system;
a plurality of gradient coils configured to apply magnetic gradients to the polarizing magnetic field;
a radio frequency (RF) system configured to apply an excitation field to the subject and acquire MR image data from the subject;
a computer system programmed to:
control the plurality of gradient coils and the RF system to perform a gradient echo pulse sequence that includes varying a phase of an RF pulse of the gradient echo pulse sequence between repetitions and acquire complex MR data;
process the complex MR data to determine signal contributions from transverse relaxation (T2) in the subject;
generate a quantitative T2 map of the subject using the signal contributions from T2 in the subject;
a display configured to receive the quantitative T2 map from the computer and display the quantitative T2 map.
2. The MRI system of claim 1 , wherein the computer system is configured to vary the phase using at least two phase increments that are equal in magnitude and opposite in sign and wherein the computer system is configured to separate signal contributions from T2 from background phase effects by subtracting MR data acquired with the phase increments that are equal in magnitude and opposite in sign.
3. The MRI system of claim 1 , wherein the computer system is configured to determine signal contributions of T2 and T1 using at least one of a model and lookup table.
4. The MRI system of claim 1 , wherein the computer system is configured to determine signal contributions from T2 using:
θ
(
Δ
Φ
,
α
,
TR
|
T
1
,
T
2
)
=
(
∠S
(
Δ
Φ
,
α
,
TR
|
T
1
,
T
2
)
-
∠S
(
-
Δ
Φ
,
α
,
TR
|
T
1
,
T
2
)
)
2
;
where θ(ΔΦ, α,TR| T 1 , T 2 ) is signal phase of the complex MR data as a function of the phase increment (ΔΦ), flip angle (α) repetition time of the pulse sequence (TR), latitudinal relaxation time (T1), and longitudinal relaxation time (T2), and S(T1,T2;ΔΦ, α,TR) is a gradient signal acquired using the gradient echo pulse sequence.
5. The MRI system of claim 1 , wherein the computer system is configured to determine signal contributions of T2 and T1 using:
S
1
S
2
=
η
(
Δ
Φ
1
,
α
1
,
TR
|
T
1
,
T
2
)
η
(
Δ
Φ
2
,
α
2
,
TR
|
T
1
,
T
2
)
e
i
(
θ
(
ΔΦ
1
,
α
1
,
TR
|
T
1
,
T
2
)
-
(
θ
(
ΔΦ
2
,
α
2
,
TR
|
T
1
,
T
2
)
)
where θ(ΔΦ, α,TR| T 1 ,T 2 ) is signal phase of the complex MR data as a function of the phase increment ( 66 Φ), flip angle (α) repetition time of the pulse sequence (TR), latitudinal relaxation time (T1), and longitudinal relaxation time (T2); where η(ΔΦ,α,TR|T 1 , T 2 ) is the signal magnitude relative to magnetization at thermal equilibrium (M 0 ); and S(T1,T2;ΔΦ, α,TR) is a gradient signal acquired using the gradient echo pulse sequence.
6. The MRI system of claim 1 , wherein the computer system is configured to include a gradient in at least some repetitions of the gradient echo pulse sequence to encode diffusion weighting in the complex MR data.
7. The MRI system of claim 6 , wherein the computer system is configured to generate at least one of a diffusion-weighted image or an apparent diffusion coefficient (ADC) map of the subject from the complex MR data.
8. The MRI system of claim 1 , wherein the computer system is configured to control the plurality of gradient coils and the RF system to perform the gradient echo pulse sequence to acquire the complex MR data, wherein a flip angle and the phase of the RF pulse is varied between repetitions and wherein computer system is further configured to determine at least one of the flip angle, or B1 inhomogeneity from the data.
9. The MRI system of claim 1 , wherein the phase using a phase increment that is less than 12 degrees.
10. The MRI system of claim 1 , wherein the computer system is configured to generate the quantitative T2 map using a lookup table to derive estimates of T1 and T2.
11. The MRI system of claim 1 , wherein the display is configured to display an image of the subject.
12. The MRI system of claim 10 , wherein the image of the subject includes at least one of a T1-weighted image, a T2-weighted image, a B 1 map, or a diffusion-weighted image displayed by the display.
13. A method for producing at least one of an image or a quantitative map of a subject comprising:
controlling a magnetic resonance imaging system to perform a gradient echo pulse sequence that includes varying a phase of an RF pulse of the gradient echo pulse sequence between repetitions and acquire complex MR data;
processing the complex MR data to determine signal contributions from transverse relaxation (T2) in the subject;
generating a quantitative T2 map of the subject using the signal contributions from T2 in the subject;
displaying the quantitative T2 map.
14. The method of claim 13 , wherein varying the phase includes using at least two phase increments that are equal in magnitude and opposite in sign and further comprising separating signal contributions from T2 from background phase effects by subtracting MR data acquired with the phase increments that are equal in magnitude and opposite in sign.
15. The method of claim 13 , further comprising determining signal contributions of T2 using:
θ
(
Δ
Φ
,
α
,
TR
|
T
1
,
T
2
)
=
(
∠S
(
Δ
Φ
,
α
,
TR
|
T
1
,
T
2
)
-
∠S
(
-
Δ
Φ
,
α
,
TR
|
T
1
,
T
2
)
)
2
;
where θ(ΔΦ, α,TR T 1 , T 2 ) is signal phase of the complex MR data as a function of the phase increment (ΔΦ), flip angle (α) repetition time of the pulse sequence (TR), latitudinal relaxation time (T1), and longitudinal relaxation time (T2); and S(T1,T2;ΔΦ, α,TR) is a gradient signal acquired using the gradient echo pulse sequence.
16. The method of claim 13 , further comprising determining signal contributions of T2 using:
S
1
S
2
=
η
(
Δ
Φ
1
,
α
1
,
TR
|
T
1
,
T
2
)
η
(
Δ
Φ
2
,
α
2
,
TR
|
T
1
,
T
2
)
e
i
(
θ
(
ΔΦ
1
,
α
1
,
TR
|
T
1
,
T
2
)
-
(
θ
(
ΔΦ
2
,
α
2
,
TR
|
T
1
,
T
2
)
)
where θ(ΔΦ, α,TR T 1 , T 2 ) is signal phase of the complex MR data as a function of the phase increment (ΔΦ), flip angle (α) repetition time of the pulse sequence (TR), latitudinal relaxation time (T1), and longitudinal relaxation time (T2); where η(ΔΦ,α,TR|T 1 , T 2 ) is the signal magnitude relative to magnetization at thermal equilibrium (M 0 ); and S(T1,T2;ΔΦ, α,TR) is a gradient signal acquired using the gradient echo pulse sequence.
17. The method of claim 13 , wherein each repetition of the gradient echo pulse sequence includes varying RF phase and flip angle of the RF pulse and further comprises determining signal contributions of T2 and T1 using at least one of a model and lookup table.
18. The method of claim 13 , further comprising including a gradient in at least some repetitions of the gradient echo pulse sequence to encode diffusion weighting in the complex MR data.
19. The method of claim 18 , further comprising generating at least one of a diffusion-weighted image or an apparent diffusion coefficient (ADC) map of the subject from the complex MR data.
20. The method of claim 13 , wherein the computer system is configured to control the plurality of gradient coils and the RF system to perform at least two repetitions of the gradient echo pulse sequence to acquire the MRI data, wherein a flip angle and the phase of the RF pulse is varied between repetitions and wherein computer system is further configured to determine the flip angle from the data.
21. The method of claim 13 , wherein the phase is varied using an increment that is less than 20 degrees.
22. The method of claim 13 , further comprising generating the quantitative T2 map using a lookup table to derive estimates of T1 and T2.
23. The method of claim 13 , further comprising reconstructing an image of the subject from the complex MR data and displaying the image.
24. The method of claim 23 , wherein the image of the subject includes at least one of a T1-weighted image, a T2-weighted image, B 1 inhomogeneity map, flip angle map, and a diffusion-weighted image.