Efficient off-resonance correction method and system for spiral imaging with improved accuracy
Automatic off-resonance correction for spiral MR imaging is performed in two stages. In the first stage, we acquire a low resolution map using two single shot spirals instead of estimating it using an automatic method. In the second stage, we developed a modified version of automatic off-resonance correction with a frequency constraint from the first stage map.
1. A method for imaging an object, the method comprising:
(a) providing a scanner for scanning the object using spiral scanning;
(b) using the scanner to collect a plurality of spiral scans of the object;
(c) acquiring a low-resolution field map from the plurality of spiral scans;
(d) acquiring image data from the object by scanning the object with the scanner;
(e) performing automatic off-resonance correction on the image data in accordance with the low-resolution field map; and
(f) imaging the object in accordance with the automatic off-resonance correction.
2. The method of claim 1 , wherein the plurality of spiral scans comprise two single shot spirals.
3. The method of claim 2 , wherein the two single shot spirals have different echo times.
4. The method of claim 3 , wherein the different echo times occur during an approach to steady state.
5. The method of claim 1 , wherein the automatic off-resonance correction is performed using searching frequencies constrained by the low-resolution field map.
6. The method of claim 5 , wherein the automatic off-resonance correction comprises searching only for nonlinear components of off-resonance frequencies.
7. The method of claim 6 , wherein the automatic off-resonance correction further comprises forming a polynomial approximation to the low-resolution field map and performing the automatic off-resonance correction in accordance with the polynomial approximation.
8. A system for imaging an object, the system comprising:
a scanner for scanning the object using spiral scanning;
a processor, in communication with the scanner, for using the scanner to collect a plurality of spiral scans of the object, acquiring a low-resolution field map from the plurality of spiral scans, acquiring image data from the object by scanning the object with the scanner, performing automatic off-resonance correction on the image data in accordance with the low-resolution field map, and imaging the object in accordance with the automatic off-resonance correction; and
an output, in communication with the processor, for outputting an image of the object formed by the processor.
9. The system of claim 8 , wherein the plurality of spiral scans comprise two single shot spirals.
10. The system of claim 9 , wherein the two single shot spirals have different echo times.
11. The system of claim 10 , wherein the different echo times occur during an approach to steady state.
12. The system of claim 8 , wherein the automatic off-resonance correction is performed using searching frequencies constrained by the low-resolution field map.
13. The system of claim 12 , wherein the automatic off-resonance correction comprises searching only for nonlinear components of off-resonance frequencies.
14. The system of claim 13 , wherein the automatic off-resonance correction further comprises forming a polynomial approximation to the low-resolution field map and performing the automatic off-resonance correction in accordance with the polynomial approximation.