Abstract
Eddy currents and susceptibility differences are the most important
sources that interfere with the quality of MR images in the presence of an
artificial macroscopic object in the volume to be imaged. In this study,
both of these factors have been examined.
The findings show that the RF field is the most important cause of
induced eddy currents when gradients with relatively slow slew rates are
used. The induced eddy currents amplify or dampen the RF field with the
result that the flip angle changes. At the proximal end in the vicinity of
the hip prosthesis surface, there have been areas where the flip angle is
nearly threefold compared to the reference flip angle. Areas with
decreased flip angles have also been found near the surface of the
prosthesis top. The incompleteness of the image due to eddy currents
manifests as signal loss areas.
Two different methods based on MRI were developed to estimate the
susceptibility of a cylindrical object. One of them is based on
geometrical distortions in SE magnitude images, while the other takes
advantage of phase differences in GRE phase images. The estimate value of
the Profile™ test hip prosthesis is χ = (170 ± 13)
10-6.
A remapping method was selected to correct susceptibility image
distortions. Correction was accomplished with pixel shifts in the
frequency domain. The magnetic field distortions were measured using GRE
phase images. The method was tested by simulations and by imaging a hip
prosthesis in a water tank and in a human pelvis. The main limitations of
the method described here are the loss of a single-valued correction map
with higher susceptibility differences and the problems with phase
unwrapping in phase images. Modulation transfer functions (MTF) were
exploited to assess the effect of correction procedure. The corrected
image of a prosthesis in a human hip after total hip arthroplasty appears
to be equally sharp or slightly sharper than the corresponding original
images.
The computer programs written for this study are presented in an
appendix.
| Identifer | oai:union.ndltd.org:oulo.fi/oai:oulu.fi:isbn951-42-6827-X |
| Date | 27 November 2002 |
| Creators | Koivula, A. (Antero) |
| Publisher | University of Oulu |
| Source Sets | University of Oulu |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/doctoralThesis, info:eu-repo/semantics/publishedVersion |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess, © University of Oulu, 2002 |
| Relation | info:eu-repo/semantics/altIdentifier/pissn/0355-3221, info:eu-repo/semantics/altIdentifier/eissn/1796-2234 |
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