Method for R
A method for measuring transverse relaxation rate, R 2 *, corrected for the presence of macroscopic magnetic field inhomogeneities with a magnetic resonance imaging (MRI) system is provided. The method accounts for additional signal decay that occurs as a result of macroscopic variations in the main magnetic field, B 0 , of the MRI system, and also mitigates susceptibility-based errors and introduction of increased noise in the R 2 * measurements. Image data are acquired by sampling multiple different echo signals occurring at respectively different echo times. A B 0 field inhomogeneity map is estimated by fitting the acquired image data to an initial signal model. Using the estimated field map, a revised signal model that accounts for signal from multiple different chemical species and for signal decay resulting from macroscopic variations in the B 0 field is formed. Corrected R 2 * values for the different chemical species are then estimated by fitting the acquired image data to the revised signal model.
1. A method for measuring a transverse magnetization relaxation rate that is corrected for macroscopic magnetic field inhomogeneities in a subject with a magnetic resonance imaging (MRI) system, the steps of the method comprising:
a) acquiring image data with the MRI system by sampling a plurality of different echo signals occurring at a respective plurality of different echo times;
b) forming a plurality of initial signal models by forming for each of the plurality of different echo signals, an initial signal model that accounts for relative signal components for each of a plurality of different chemical species;
c) estimating magnetic field inhomogeneity values associated with the MRI system by fitting the image data acquired in step a) to the initial signal models formed in step b);
d) forming a plurality of revised signal models by forming for each of the plurality of different echo signals using the magnetic field inhomogeneity values estimated in step c), a revised signal model that accounts for relative signal components for each of a plurality of different chemical species and for signal decay resulting from macroscopic variations in a main magnetic field of the MRI system, the macroscopic variations in the main magnetic field of the MRI system being different from the magnetic field inhomogeneity values estimated in step c); and
e) estimating a transverse relaxation rate for at least one of the plurality of different chemical species by fitting the image data acquired in step a) to the revised signal models formed in step d).
2. The method as recited in claim 1 in which forming the revised signal models in step d) includes calculating a spatial response function using the magnetic field inhomogeneity values estimated in step c).
3. The method as recited in claim 2 in which the spatial response function models the signal decay resulting from macroscopic variations in the main magnetic field of the MRI system as a sinc modulation.
4. The method as recited in claim 2 in which calculating the spatial response function in step c) includes calculating a variation in the magnetic field inhomogeneity values estimated in step c).
5. The method as recited in claim 4 in which the variation in the magnetic field inhomogeneity values is calculated in step c) by calculating a gradient of the magnetic field inhomogeneity values estimated in step c).
6. The method as recited in claim 1 in which the plurality of different chemical species includes at least two of water, fat, silicone, hyperpolarized carbon-13, metabolites containing hyperpolarized carbon-13, hyperpolarized xenon-129, hyperpolarized helium-3, acetone, choline, and lactate.
7. The method as recited in claim 1 in which the plurality of different chemical species comprises water and fat, and in which the initial signal models formed in step b) account for a fat spectrum having multiple spectral peaks.
8. The method as recited in claim 7 in which the transverse magnetization relaxation rate estimated in step e) is a transverse relaxation rate of water.
9. The method as recited in claim 7 in which step d) includes simultaneously estimating a transverse magnetization relaxation rate for water and a transverse magnetization relaxation rate for fat.
10. The method as recited in claim 1 further comprising:
f) producing an R 2 * map that is corrected for susceptibility effects using the transverse magnetization relaxation rate estimated in step e).
11. The method as recited in claim 1 in which the magnetic field inhomogeneity values estimated in step c) are indicative of magnetic field inhomogeneities along less than three spatial directions.
12. The method as recited in claim 11 in which the magnetic field inhomogeneity values estimated in step c) are indicative of a magnetic field inhomogeneities along only a direction transverse to an image slice.
13. The method as recited in claim 1 in which the revised signal modles formed in step d) account for both signal decay resulting from macroscopic variations in a main magnetic field of the MRI system and the magnetic field inhomogeneity values estimated in step c) using the initial signal models that were formed in step b).
14. The method as recited in claim 1 in which the revised signal models formed in step d) model the signal decay resulting from macroscopic variations in a main magnetic field of the MRI system using a non-exponential decay term.