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Proceedings Paper

Use of (N-1)-D expansions for N-D phase unwrapping in MRI
Author(s): Philip J. Bones; Laura J. King; Rick P. Millane
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Paper Abstract

In MRI the presence of metal implants causes severe artifacts in images and interferes with the usual techniques used to separate fat signals from other tissues. In the Dixon method, three images are acquired at different echo times to enable the variation in the magnetic field to be estimated. However, the estimate is represented as the phase of a complex quantity and therefore suffers from wrapping. High field gradients near the metal mean that the phase estimate is undersampled and therefore challenging to unwrap.

We have developed POP, phase estimation by onion peeling, an algorithm which unwraps the phase along 1-D paths for a 2-D image obtained with the Dixon method. The unwrapping is initially performed along a closed path enclosing the implant and well separated from it. The recovered phase is expanded using a smooth periodic basis along the path. Then, path-by-path, the estimate is applied to the next path and then the expansion coefficients are estimated to best fit the wrapped measurements. We have successfully tested POP on MRI images of specially constructed phantoms and on a group of patients with hip implants.

In principle, POP can be extended to 3-D imaging. In that case, POP would entail representing phase with a suitably smooth basis over a series of surfaces enclosing the implant (the "onion skins"), again beginning the phase estimation well away from the implant. An approach for this is proposed.

Results are presented for fat and water separation for 2-D images of phantoms and actual patients. The practicality of the method and its employment in clinical MRI are discussed.

Paper Details

Date Published: 6 September 2017
PDF: 11 pages
Proc. SPIE 10410, Unconventional and Indirect Imaging, Image Reconstruction, and Wavefront Sensing 2017, 104100G (6 September 2017); doi: 10.1117/12.2275878
Show Author Affiliations
Philip J. Bones, Univ. of Canterbury (New Zealand)
Laura J. King, Univ. of Canterbury (New Zealand)
Rick P. Millane, Univ. of Canterbury (New Zealand)

Published in SPIE Proceedings Vol. 10410:
Unconventional and Indirect Imaging, Image Reconstruction, and Wavefront Sensing 2017
Jean J. Dolne; Rick P. Millane, Editor(s)

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