Method for magnetic resonance elastography using transient waveforms
A dynamic magnetic resonance elastography (“MRE”) method for quantifying liver stiffness using intrinsic transient waveforms imparted on the liver by the beating heart is provided. The method includes synchronizing motion-encoding gradients in an MRE pulse sequence to the subject's cardiac cycle.
1. A method for estimating mechanical properties of a tissue in a subject using a magnetic resonance imaging (MRI) system, the steps of the method comprising:
a) acquiring positive phase image data with the MRI system by performing a pulse sequence that employs a motion-encoding gradient substantially in-phase with transient wave motion propagating through the tissue, wherein the transient wave motion is generated by a source intrinsic to the subject;
b) acquiring negative phase image data with the MRI system by performing a pulse sequence that employs a motion-encoding gradient substantially out-of-phase with transient wave motion propagating through the tissue, wherein the transient wave motion is generated by a source intrinsic to the subject;
c) reconstructing positive and negative wave images from the acquired positive phase image data and the negative phase image data, respectively; and
d) calculating mechanical properties of the tissue using the reconstructed positive and negative wave images.
2. The method as recited in claim 1 in which the positive and negative phase image data are acquired using a spin-echo echo-planar imaging pulse sequence.
3. The method as recited in claim 1 in which the transient wave motion includes transient mechanical waves generated by beating of the subject's heart.
4. The method as recited in claim 3 in which steps a) and b) include selecting a frequency of the motion-encoding gradient based on spectral characteristics of the beating of the subject's heart.
5. The method as recited in claim 1 in which step a) includes starting the motion-encoding gradient such that the motion-encoding gradient is temporally in-phase with the subject's cardiac cycle.
6. The method as recited in claim 1 in which step b) includes starting the motion-encoding gradient such that the motion-encoding gradient is temporally out-of-phase with the subject's cardiac cycle.
7. The method as recited in claim 1 further comprising:
e) acquiring an electrocardiograph (ECG) signal from the subject; and
f) selecting a temporal offset with respect to the ECG signal.
8. The method as recited in claim 7 in which at least one of steps a) and b) includes performing a pulse sequence in which the motion-encoding gradient occurs at a time relative to the temporal offset selected in step f).
9. The method as recited in claim 8 in which the time relative to the temporal offset is such that the motion-encoding gradient is temporally centered about the temporal offset selected in step f).
10. A method for estimating mechanical properties of a tissue using a magnetic resonance imaging (MRI) system, the steps of the method comprising:
a) acquiring an electrocardiograph (ECG) signal;
b) selecting a temporal offset with respect to the ECG signal;
c) acquiring positive phase image data with the MRI system by performing a pulse sequence that employs a motion-encoding gradient substantially in-phase with transient wave motion propagating through the tissue, the motion-encoding gradient being timed to occur at the temporal offset, wherein the transient wave motion is generated by a source intrinsic to the subject;
d) acquiring negative phase image data with the MRI system by performing a pulse sequence that employs a motion-encoding gradient substantially out-of-phase with transient wave motion propagating through the tissue, the motion-encoding gradient being timed to occur at the temporal offset, wherein the transient wave motion is generated by a source intrinsic to the subject;
e) repeating steps c) and d) while selecting a different temporal offset for each repetition in order to acquire a positive phase image data set that includes positive phase image data corresponding to each temporal offset and a negative phase image data set that includes negative phase image data corresponding to each temporal offset;
f) reconstructing positive wave images and negative wave images from the acquired positive phase image data set and the negative phase image data set, respectively; and
g) calculating mechanical properties of the tissue using the reconstructed positive and negative wave images.
11. The method as recited in claim 10 in which step g) includes producing wave images using the positive waves images and the negative wave images, the wave images depicting a wavefront of the transient wave motion at each temporal offset.
12. The method as recited in claim 11 in which step g) includes calculating a wave speed from the wave images.
13. The method as recited in claim 10 in which the mechanical properties of the tissue include a stiffness of the tissue.
14. The method as recited in claim 10 in which the tissue is liver tissue.
15. The method as recited in claim 10 in which at least one of steps c) and d) includes performing a pulse sequence in which the motion-encoding gradient is timed to occur at the temporal offset selected in step b) such that the motion-encoding gradient is temporally centered about the temporal offset.