Abstracts (Continue in Part XXXIII)
Proceedings of the International Society for Magnetic Resonance in Medicine
Ghostine In the phase map correction method, ghosting is removed by thresholding the image. However, if a ghost cannot be removed, and is close to the object, or overlaps the object, then it interferes with the correction. across the two images. If the ghost is differently positioned in the two images, it will contribute differently to the integrals, causing artifacts, as described below. This effect occurs even for high SNR images. Eddy Currents Eddy currents can cause phase evolution that is
... ot linear in time and this produces inaccuracy in the inhomogeneous field map calculated by the phase map correction method. This typically results in a poor correction of the gross shape of the object. As the gradient reversal method does not depend on phase, it is not affected by eddy currents. Sensitivity to Imaging Sequence The phase map method requires measurement of a small phase change due to magnetic field inhomogeneities for every pixel in the object and so a full Fourier spin echo experiment is required. This has the disadvantage of a greater minimum echo time than for a half Fourier or gradient echo EPI experiment of the same resolution, often leading to excessive T2 contrast. Using a lower resolution phase map obtained from the central portion of partial Fourier data, results in a poor correction. The gradient reversal method is less sensitive to the experiment type and works well with images obtained from half Fourier spin echo EPI experiments. However, it has been observed that the correction of gradient echo EP images is generally not as good as in spin echo images. This results from intra-voxel dephasing in the gradient echo experiments. Artifacts in the Corrected Image Phase unwrapping is a crucial step in the phase map method and any errors at this stage can cause discontinuities and shearing in the corrected image. This problem can occur in images that contain voids or unconnected regions. One solution is to use a three-point phase measurement to unwrap each pixel independently, but this increases the number of images required. Phase spikes due to high intensity noise pixels can also cause artifacts but may be removed by Fourier filtering the phase map or using a polynomial approximation ', The gradient reversal method can produce artifacts due to noise and ghosting. This manifests itself as reduced resolution and contrast in the corrected image, and the appearance of signal in the centre of low intensity areas within the object. Both these artifacts are due to a mismatch between the intensity line integrals of the distorted images. References [l] Fraass,B.A., et al. Int .