Multi-compartment separation in magnetic resonance using transient steady-state free precession imaging
View Patent ↗Disclosed is a method of quantitatively separating tissue signals based on relaxation time differences. The method uses the transient signal decay in steady-state free precession (balanced SSFP) imaging to provide an alternative to standard CPMG methods of T 2 -relaxometry. The balanced SSFP technique allows 3-4 times the temporal resolution of CPMG, and also slows the short T 2 decay so that it can be more accurately measured.
1. A method of separating species signals in a composite magnetic resonance imaging signal comprising the steps of:
a) applying a series of steady-state free precession (balanced SSFP) pulse sequences, wherein evolution of the steady-state in each balanced SSFP pulse sequence follows a smooth exponential path,
b) measuring magnetic resonance signals during transient periods for the balanced SSFP sequences as steady-state signals evolve, and
c) fitting the transient response of the measured signals to a model to identify the smallest number of discrete exponential terms which provide a satisfactory representation of the measured data, which utilizes a curve-fitting algorithm.
2. The method as defined by claim 1 wherein the curve-fitting algorithm utilizes non-negative least-squares.
3. The method as defined by claim 2 wherein the model in step c) is defined by:
M
(
t
)
=
∑
i
=
1
N
C
i
ⅇ
-
t
τ
i
+
M
ss
where M(t) is the signal intensity as a function of time,
N is the number τ points used in the fit,
C i is relative amount of material with an exponential term constant τ i ,
M s s is steady-state signal in a voxel, and
τ is the exponential term in the fitting model.
4. The method as defined by claim 2 wherein a single data frame is acquired repeatedly over decay of the magnetic resonance signals.
5. The method as defined by claim 4 wherein before step a) a plurality of preparation pulses are applied, wherein an inversion pulse is applied with the preparation pulses and magnetization starts at a negative value.
6. The method as defined by claim 4 wherein before step a) a plurality of preparation pulses are applied and magnetization is saturated thereby.
7. The method as defined by claim 4 wherein before step a) a plurality of preparation pulses are applied and magnetization starts in a steady state and is inverted in the steady state.
8. The method as defined by claim 2 wherein multiple data frames are acquired repeatedly over decay of the magnetic resonance signals.
9. The method as defined by claim 8 wherein before step a) a plurality of preparation pulses are applied, wherein an inversion pulse is applied with the preparation pulses and magnetization starts at a negative value.
10. The method as defined by claim 8 wherein before step a) a plurality of preparation pulses are applied and magnetization is saturated thereby.
11. The method as defined by claim 8 wherein before step a) a plurality of preparation pulses are applied and magnetization starts in a steady state and is inverted in the steady state.
12. The method as defined by claim 2 wherein before step a) a plurality of preparation pulses are applied, wherein an inversion pulse is applied with the preparation pulses and magnetization starts at a negative value.
13. The method as defined by claim 2 wherein before step a) a plurality of preparation pulses are applied and magnetization is saturated thereby.
14. The method as defined by claim 2 wherein before step a) a plurality of preparation pulses are applied and magnetization starts in a steady state and is inverted in the steady state.
15. The method as defined by claim 1 wherein the model in step c) is defined by:
M
(
t
)
=
∑
i
=
1
M
C
i
ⅇ
t
τ
i
+
M
ss
where M(t) is the signal intensity as a function of time,
N is the number τ points used in the fit,
C i is relative amount of material with an exponential term constant τ i ,
M ss is steady-state signal in a voxel, and
τ is the exponential term in the fitting model.
16. The method as defined by claim 1 wherein before step a) a plurality of preparation pulses are applied, wherein an inversion pulse is applied with the preparation pulses and magnetization staffs at a negative value.
17. The method as defined by claim 1 wherein before step a) a plurality of preparation pulses are applied and magnetization is saturated thereby.
18. The method as defined by claim 1 wherein before step a) a plurality of preparation pulses are applied and magnetization staffs in a steady state and is inverted in the steady state.