System and method for estimating phase measurements in magnetic resonance imaging
Described here are systems and methods for estimating phase measurements obtained using a magnetic resonance imaging (MRI) system such that phase ambiguities in the measurements are significantly mitigated. Echo time spacings are determined by optimizing phase ambiguity functions associated with the echo time spacings. Data is then acquired using a multi-echo pulse sequence that utilizes the determined echo spacings. Phase measurements are then estimated and images are reconstructed using a reconstruction technique that disambiguates the phase ambiguities in the phase measurements.
1. A method for estimating a phase of a magnetic resonance signal using a magnetic resonance imaging (MRI) system, the steps of the method comprising:
a) determining with a computer system at least two echo time spacings that define a duration of time between pairs of echo times, the at least two echo time spacings being determined by optimizing phase ambiguity functions that are associated with the at least two echo time spacings;
b) acquiring data by directing a radio frequency (RF) system and a gradient system of the MRI system to perform a pulse sequence that includes forming echoes at a plurality of echo times that are spaced apart in time using the determined at least two echo time spacings; and
c) estimating with a data processing server of the MRI system a phase value at each voxel location in an image matrix using the acquired data and the determined at least two echo time spacings.
2. The method as recited in claim 1 in which the data acquired in step b) is acquired from a subject in which a fluid is flowing, and in which the method further comprises determining with the data processing server a flow value associated with the fluid at voxel locations in the image matrix using the phase values estimated in step c).
3. The method as recited in claim 2 in which the fluid is at least one of blood, cerebrospinal fluid, and air.
4. The method as recited in claim 1 in which the data acquired in step b) is acquired from a subject, and in which the method further comprises computing with the data processing server at least one of a physiological parameter of the subject and a measure of a physiological function of the subject using the phase values estimated in step c).
5. The method as recited in claim 4 in which the measure of the physiological function is a measure associated with neuronal activity.
6. The method as recited in claim 1 in which step a) includes optimizing the phase ambiguity functions with the computer system by maximizing a distinctiveness of each phase ambiguity function with the computer system.
7. The method as recited in claim 6 in which the distinctiveness of each phase ambiguity function is maximized by maximizing distances between the phase ambiguity functions with respect to a defined distance metric.
8. The method as recited in claim 7 in which the defined distance metric includes an L 2 distance.
9. The method as recited in claim 1 in which step a) includes determining with the computer system the at least two echo spacings using a numerical routine that takes into account at least one of a minimum expected signal-to-noise ratio (SNR), a minimum expected transverse relaxation time, a minimum attainable echo time spacing for a given pulse sequence, and a maximum expected dynamic range in a phase slope.
10. A method for resolving phase ambiguities in magnetic resonance signals acquired using a magnetic resonance imaging (MRI) system and a multichannel receiver, the steps of the method comprising:
a) determining with a computer system at least two echo time spacings that define a duration of time between pairs of echo times, the at least two echo time spacings being determined by optimizing phase ambiguity functions of receiver elements in the multichannel receiver that are associated with the at least two echo time spacings;
b) acquiring data on each of a plurality of channels of the multichannel receiver by directing a radio frequency (RF) system and a gradient system of the MRI system to perform a pulse sequence that includes forming echoes at a plurality of echo times that are spaced apart in time using the determined at least two echo time spacings;
c) combining with a data processing server of the MRI system the data associated with each of the plurality of channels of the multichannel receiver; and
d) reconstructing with the data processing server an image from the combined data.
11. The method as recited in claim 10 in which step a) includes optimizing the phase ambiguity functions with the computer system by maximizing a distinctiveness of each phase ambiguity function with the computer system.
12. The method as recited in claim 11 in which the distinctiveness of each phase ambiguity function is maximized by maximizing distances between the phase ambiguity functions with respect to a defined distance metric.
13. The method as recited in claim 12 in which the defined distance metric includes an L 2 distance.
14. The method as recited in claim 10 in which step a) includes determining with the data processing server the at least two echo spacings using a numerical routine that takes into account at least one of a minimum expected signal-to-noise ratio (SNR), a minimum expected transverse relaxation time, a minimum attainable echo time spacing for a given pulse sequence, and a maximum expected dynamic range in a phase slope.