Consistent non-cartesian off-axis MRI quality: calibrating and removing multiple sources of demodulation phase errors.
The consistency of off-axis MRI with non-Cartesian sequences across a large number of scanners is highly variable. Improper timing alignment of the gradient fields, data acquisition system, and real-time frequency demodulation reference signal, which are necessary for off-axis imaging, is an important source of this variability. In addition, eddy currents and anisotropic gradient delays cause deviations in k-space trajectories that in turn make the demodulation reference signals inaccurate. A method is presented to quickly measure the timing error in the frequency demodulation reference signal and separate it from anisotropic gradient delays. k-Space deviations, as measured with a previous gradient calibration technique, are shown to be a second source of demodulation phase errors that degrade image quality. Using the timing delay and k-space deviations, a retrospective phase correction is applied to each k-space sample before the data are regridded during reconstruction. The timing delays of four MR scanners were measured to be 4.2-7.5 micros below the manufacturer's suggested delay. Significant degradation in 3D radial (3D projection reconstruction (PR)) knee and breast images are retrospectively corrected while a partial prospective correction is applied for spiral imaging. The method allows for more consistent performance of non-Cartesian sequences across multiple scanners without operator intervention.
Jung
Youngkyoo
Y
Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, USA. yjung2@wisc.edu
Jashnani
Yogesh
Y
Kijowski
Richard
R
Block
Walter F
WF
D000465:Algorithms; D016880:Anisotropy; D016477:Artifacts; D002138:Calibration; D006801:Humans; D021621:Imaging, Three-Dimensional; D007717:Knee / Q000033:anatomy & histology; D008279:Magnetic Resonance Imaging / Q000379:methods* / Q000592:standards*; D019047:Phantoms, Imaging; D011786:Quality Control; D012815:Signal Processing, Computer-Assisted / Q000295:instrumentation; D013997:Time Factors