System and method for field map estimation
A system and method for determining a magnetic field map when using a magnetic resonance imaging (MRI) system to acquire images from a region of interest (ROI) of a subject. The method includes selecting a pulse sequence to elicit a plurality of echoes from the subject as medical imaging data from the subject. The method also includes optimizing an echo time for a dynamic range of interest during the pulse sequence (SB max ), a minimum signal-to-noise ratio (SNR 0 ) in the medical imaging data, and minimum T2* value in the ROI. The method further includes generating a magnetic field map estimation using the optimized echo times.
1. A magnetic resonance imaging (MRI) system comprising:
a magnet system configured to generate a polarizing magnetic field about at least a region of interest (ROI) in a subject arranged in the MRI system;
a plurality of gradient coils configured to apply a gradient field 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 a ROI;
a computer system programmed to:
select a pulse sequence to be performed by the plurality of gradient coils and RF system to elicit at least three echoes from the subject as medical imaging data;
compute angle locations for different pairs of the at least three echoes and magnetic field map estimates;
replicate the magnetic field map estimates over a dynamic range of interest associated with the pulse sequence;
identify magnetic field map estimates that provide a desired solution to a phase ambiguity function; and
using the magnetic field map estimates that provide the desired solution in order to indicate a magnetic field map solution based on one of the pair of echoes selected from the at least three echoes; and store the indicated magnetic field map solution in a memory for later use.
2. The MRI system of claim 1 wherein the computer system is further programmed to create a weighted combination of the magnetic field map estimates that provide the desired solution in order to generate the magnetic field map solution.
3. The MRI system of claim 1 wherein the computer system is configured to compute the angle locations for each of the different pairs of the at least three echoes (0, 1) as an angle location (x, y) that yields magnetic field map estimates, Δ{circumflex over (B)} 0,1 (x, y) in the following:
ɛ
0
,
1
=
Δ
B
⋒
0
,
1
-
Δ
B
=
n
0
,
1
+
ΔΩ
0
,
1
2
π
Δ
TE
0
,
1
where ε 0,1 is an estimate error, n 0,1 denotes a phase wrapping integer which is non-zero whenever 2πΔBΔTE 0,1 +ΔΩ 0,1 falls outside [−π,π) ΔΩ 0,1 =Ω 1 −Ω 0 is a difference between phase noise random variables in ψ k =2πΔAB(TE 0 +kΔTE)+Ω k +2πr k , where ΔTE is an echo spacing between each of the different pairs of the at least three echoes, k is an echo index, k=0, . . . , K−1, TE 0 is an echo time at k=0, and Ω k is a radian phase contribution of an additive noise term at time index k.
4. The MRI system of claim 1 wherein the at least three echoes is three echoes.
5. A magnetic resonance imaging (MRI) system comprising:
a magnet system configured to generate a polarizing magnetic field about at least a region of interest (ROI) in a subject arranged in the MRI system;
a plurality of gradient coils configured to apply a gradient field 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 a ROI;
a computer system programmed to:
select a pulse sequence to be performed by the plurality of gradient coils and RF system in order to elicit at least three echoes from the subject as medical imaging data;
select echoes from the at least three echoes to derive phase ambiguity functions that control an ambiguity of magnetic field values in the ROI;
using the selected echoes and the derived phase ambiguity functions in order to select phase-wrap free values associated with the magnetic field created by the magnet system, the plurality of gradient coils, and RF system;
using the phase-wrap free values in order to derive a magnetic field map; and
storing the derived magnetic field map for later use.
6. The MRI system of claim 5 wherein the computer system is further programmed to derive phase ambiguity functions that are designed to be maximally distant with respect to a defined metric, D, in order to maximize a distinctiveness of x 1 and x 2 , where:
x
1
=
α
1
f
Ω
1
(
2
πΔ
TE
0
,
1
ɛ
)
*
f
Ω
0
(
2
πΔ
TE
0
,
1
ɛ
)
*
∑
P
δ
(
ɛ
-
p
Δ
TE
0
,
1
)
and
x
2
=
α
2
f
Ω
2
(
2
πΔ
TE
0
,
2
ɛ
)
*
f
Ω
0
(
2
πΔ
TE
0
,
2
ɛ
)
*
∑
q
δ
(
ɛ
-
q
Δ
TE
0
,
2
)
where TE 0 and TE 1 represent at least two of the selected echoes, such that ΔTE 0,k =TE k −TE 0 is an echo spacing, s is an observed error in Hz, f Ω k ( ) is a probability density function of a radian phase contribution of noise, Ω k , * denotes a convolution operator, and α k is a probability density function scaling parameter over a domain of x k .
7. A method for determining a magnetic field map when using a magnetic resonance imaging (MRI) system to acquire images from a region of interest (ROI) of a subject, the steps of the method comprising:
a) selecting with a computer system, an MRI pulse sequence in order to elicit at least three echoes from the subject as medical imaging data that is obtained from the subject;
b) optimizing with the computer system, an echo time for a dynamic range of interest during the MRI pulse sequence (δB max ), a minimum signal-to-noise ratio (SNR 0 ) in the medical imaging data, and a minimum T2* value in the ROI;
c) generating with the computer system, a magnetic field map estimation for the selected MRI pulse sequence using the optimized echo times, and
d) storing the generated magnetic field map estimation in a memory of the computer system for later use.