Water fat separated magnetic resonance imaging method and system using steady-state free-precession
Water-fat separated magnetic resonance (MR) images with balanced steady-state free precession (SSFP) are produced. The acquired SSFP signals are isolated into multiple echo components in which the phase arrangements between the water and fat signals are controlled by appropriately selecting the TR and TE values of the SSFP imaging sequence. From the isolated echo components, the effects of the field inhomogeneities are corrected and water and fat images are separated.
1. A method of separating and identifying water and fat magnetic resonance (MR) images of a body with balanced steady state free-precession (SSFP), said method comprising:
using an MRI apparatus to:
(a) select a time of repetition (TR) and a time to echo period (TE) for a phase-cycled SSFP sequence in accordance with a desired water-fat phase arrangement for echo components after isolation, said echo components respectively corresponding to different echo-formation pathways and containing signals emanating from both fat and water nuclei;
(b) apply the SSFP sequence with the selected TR and TE at said selected values to obtain MR SSFP data from the body being imaged;
(c) isolate the acquired SSFP data into multiple of said echo components in which water and fat signals have the desired phase arrangements;
(d) generate a field correction map from the isolated echo components data or using a field map already available from the MRI apparatus;
(e) apply the field correction map to correct the isolated echo components data wherein water and fat signals in the data are orthogonal or substantially orthogonal; and
(f) generate water and fat images from the phase-corrected isolated echo components data.
2. A method as in claim 1 wherein the TR and the TE are selected such that water and fat signals are orthogonal in S 0 and anti-parallel in S −1 after echo isolation.
3. A method as in claim 2 further comprising:
using said MRI apparatus to:
(g) apply the SSFP sequence with the selected TR and TE to obtain MR SSFP data from the body being imaged;
(h) isolate the SSFP data to have S 0 and S −1 , wherein water and fat signals are orthogonal in S 0 and anti-parallel in S −1 ;
(i) generate a field correction map from S −1 according to:
−2φ=unwrap {arg( S −1 2 )};
(j) apply the field map to correct S 0 to have:
S
0
corr
=
S
0
ⅇ
-
ⅈϕ
TE
TR
-
TE
; and
(k) generate water and fat images from S 0 corr according to:
I W =|real { S 0 corr }|
I F =|imag { S 0 corr }|.
4. A method as in claim 1 wherein the TR and TE are selected with TR=2TE and such that water and fat signals are orthogonal in all echo components after isolation.
5. A method as in claim 4 further comprising:
using said MRI apparatus to:
(g) apply the SSFP sequence with the selected TR and TE to obtain MR SSFP data from the body being imaged;
(h) isolate the SSFP data to have S −1 , S 0 and S 1 , wherein water and fat signals are orthogonal;
(i) construct a combinatory data of S 0 ·S 1 ;
(j) generate a field correction map from the combinatory data according to:
4φ=unwrap {arg( S 0 ·S 1 )}
(k) apply the field correction map to correct S −1 and S 0 to have:
S 0 corr =S 0 e −iφ
S −1 corr =S −1 e iφ
(l) generating water and fat images from S 0 corr and/or S −1 corr according to at least one of:
I W =|real( S 0 corr )|
I F =|imag ( S 0 corr )|
and
I W =|real( S −1 corr )|
I F =|imag ( S −1 corr )|
and
I
W
=
1
2
real
(
S
0
corr
)
+
1
2
real
(
S
-
1
corr
)
I
F
=
1
2
imag
(
S
0
corr
)
+
1
2
imag
(
S
-
1
corr
)
.
6. A method as in claim 1 wherein a master phase-cycled SSFP sequence is accompanied with a low spatial resolution assist SSFP imaging sequence.
7. A method as in claim 6 wherein TR and TE of the master SSFP sequence, designated herein as TR m and TE m respectively, are selected with TR m equal to 2TE m and such that water and fat signals are substantially orthogonal in all echo components, and
TR and TE of the assist SSFP imaging sequence, herein designated as TR a and TE a respectively, are selected such that water and fat signals are anti-parallel in S −1 after echo isolation.
8. A method as in claim 7 further comprising:
using said MRI apparatus to:
(g) apply both the master SSFP sequence and the assist SSFP imaging sequence with the selected TR and TE combinations to obtain MR data from the body being imaged;
(h) isolate the SSFP data to have S 0 m and S −1 m from the master scan data, and S −1 a from the assist scan data;
(i) generate a field correction map from S −1 a according to:
−2φ=unwrap {arg( S −1 a ·S −1 a )};
(j) apply the field correction map to correct S 0 m and S −1 m of the master scan data to have:
S
0
m
,
corr
=
S
0
ⅇ
-
ⅈϕ
TE
m
TR
a
-
TE
a
S
-
1
m
,
corr
=
S
-
1
ⅇ
ⅈϕ
TR
m
-
TE
m
TR
a
-
TE
a
;
and
(k) generate water and fat images from at least one of S 0 m,corr and S −1 m,corr according to at least one of:
I W =|Real( S 0 m,corr )−Imag( S 0 m,corr )·cot(ψ 0 m )|
I
F
=
Imag
(
S
0
m
,
corr
)
sin
(
ψ
0
m
)
and
I W =|Real( S −1 m,corr )−Imag( S −1 m,corr )·cot(ψ −1 m )|
I
F
=
Imag
(
S
-
1
m
,
corr
)
sin
(
ψ
-
1
m
)
and
I
W
=
1
2
Real
(
S
0
m
,
corr
)
-
Imag
(
S
0
m
,
corr
)
·
cot
(
ψ
0
m
)
+
1
2
Real
(
S
-
1
m
,
corr
)
-
Imag
(
S
-
1
m
,
corr
)
·
cot
(
ψ
-
1
m
)
I
F
=
1
2
Imag
(
S
0
m
,
corr
)
sin
(
ψ
0
m
)
+
1
2
Imag
(
S
-
1
m
,
corr
)
sin
(
ψ
-
1
m
)
with
ψ 0 m =2πΔfTE m
ψ −1 m =2 πΔf ( TR m −TE m ).
9. A method as in claim 1 wherein TR and TE are selected with TR equal to 2 times TE, and such that water and fat signals are significantly orthogonal in all echo components after echo isolation.
10. A method as in claim 9 further comprising:
using said MRI apparatus to:
(g) applying the SSFP sequence with the selected TR and TE to obtain MR data from the body being imaged;
(h) isolating the SSFP data to have S 0 and S −1 echo components;
(i) obtaining a B 0 field map, ΔB 0 , that is available from the scanner;
(j) applying the B 0 field map to S 0 and S −1 to have:
S 0 corr =S 0 e −iγΔB 0 TE
S −1 corr =S −1 e iγΔB 0 (TR−TE) ; and
(k) generating water and fat images from at least one of S 0 corr and S −1 corr according to at least one of:
I W =|Real( S 0 corr )−Imag( S 0 corr )·cot(2π ΔfTE )|
I
F
=
Imag
(
S
0
corr
)
sin
(
2
π
Δ
fTE
)
and
I W =|Real( S −1 corr )−Imag( S −1 corr )·cot(2 πΔf ( TR−TE ))|
I
F
=
Imag
(
S
-
1
corr
)
sin
(
2
π
Δ
f
(
TR
-
TE
)
)
and
I
W
=
1
2
Real
(
S
0
corr
)
-
Imag
(
S
0
corr
)
·
cot
(
2
π
Δ
fTE
)
+
1
2
Real
(
S
-
1
corr
)
-
Imag
(
S
-
1
corr
)
·
cot
(
2
π
Δ
f
(
TR
-
TE
)
)
I
F
=
1
2
Imag
(
S
0
corr
)
sin
(
2
π
Δ
fTE
)
+
1
2
Imag
(
S
-
1
corr
)
sin
(
2
π
Δ
f
(
TR
-
TE
)
)
.