Patent ID: 6392412
Filing Date: 2002-05-21
Classification: G01R

Abstract:
A method of determining and correcting higher order magnetic field inhomogeneities of a nearly homogeneous magnetic field B0 in an investigation volume of a magnetic resonance apparatus, wherein magnetic resonance is excited in a sample located in the investigation volume through a radio frequency pulse, at least one additional, linear magnetic gradient field is applied and a magnetic resonance signal is measured, the method comprising the steps of:a) irradiating a first excitation radio frequency pulse onto the sample; b) applying a first phase gradient Gix1 in a predetermined direction x; c) measuring, at a fixed time tdx1 after said first excitation radio frequency pulse, a value Six1 of a magnetic resonance signal from the sample; d) digitizing and storing said value Six1 measured in step c); e) irradiating a second excitation radio frequency pulse onto the sample; f) applying a second phase gradient Gix2 in said predetermined direction x; g) measuring, at a fixed time tdx2 after said second radio frequency excitation pulse, a value Six2 of a magnetic resonance signal from the sample, wherein Gix2 and tdx2 are selected such that the integrals &Integral;0tÎ´x&it;â€ƒ&it;1&it;â€ƒ&it;Gx&it;â€ƒ&it;1&it;&dd;t=&Integral;0tÎ´x&it;â€ƒ&it;2&it;Gx&it;â€ƒ&it;2&it;â€ƒ&it;&dd;tâ€ƒare equal; h) digitizing and storing said value Six2; i) repeating steps a) to h) a plurality of times with systematically altered strength of said phase gradient Gix; j) combining, in dependence on an associated gradient strength Gix, values of said measured resonance signals Six1 , into a quasi-spin echo data set Sx1; k) combining, in dependence on said associated gradient strength Gix, said values of said measured resonance signals Six2, into a quasi-spin echo data set Sx2; l) Fourier transforming said data set Sx1; m) Fourier transforming said data set Sx2, wherein a difference &phgr;ix1-&phgr;ix2 between phases of the signals Six1 and Six2 represent a measure of a profile of magnetic field inhomogeneities along said direction x; and n) applying, in subsequent measurements of magnetic resonance in the apparatus, a correction magnetic field B(x) in the investigation volume for homogenizing the magnetic field B0 to compensate for said magnetic field inhomogeneities determined in step m).