dc.contributor.author |
Mohsin, Syed Ali |
|
dc.date.accessioned |
2017-11-27T10:07:56Z |
|
dc.date.accessioned |
2020-04-09T16:31:19Z |
|
dc.date.available |
2020-04-09T16:31:19Z |
|
dc.date.issued |
2008 |
|
dc.identifier.uri |
http://142.54.178.187:9060/xmlui/handle/123456789/2456 |
|
dc.description.abstract |
The radiofrequency field used in Magnetic Resonance Imaging is scattered by
implanted medical devices. The scattered field is concentrated in the tissue surrounding
the implant and conduction currents will flow in the tissue resulting in potentially
hazardous heating. Patients with medical implants can undergo diagnostic or
interventional MRI procedures and thus the scattering of the MRI RF field by medical
implants merits a detailed investigation. In this thesis, scattering by various types of
implants has been investigated.
The scattered field of a deep brain stimulation lead can be very intense near the
electrodes stimulating the brain. The lead is just like an antenna excited by an incident
electromagnetic field in a dissipative medium. The greatest concern regarding MRI
induced heating is when the lead length approaches the resonant length. The factors that
determine the resonant length of a lead are examined. The finite element method is used
to find the near field for the lead immersed in nonhomogeneous tissue and connected to
an implantable pulse generator as well as for varying distances of the connecting portion
of the lead from the air-tissue interface. Electric field, SAR, dissipated powers and
induced temperature rise distributions have been obtained in the brain tissue surrounding
the electrodes. It is shown that the presence of the IPG can significantly change the
induced temperature rise and that the near proximity of the air-tissue interface results in a
reduction in the induced temperature rise. The computed values are in good agreement
with in-vitro measurements made in the laboratory. Similar analyses and computations
have been carried out for an implanted vagus nerve stimulation lead device. Current
distributions in the twin-strand lead have been computed. SAR and temperature rise
distributions have been obtained around the twin electrodes which are placed on the left
vagus nerve.
A model implant embedded in nonhomogeneous tissue has been investigated. The nature
of the embedding tissue is varied and the current distribution in the implant, the scattered
field, and the temperature rise distributions in the tissue surrounding the electrodes has
been computed. It has been found that the induced temperature rise is significantly lower
for tissue with a lower conductivity and permittivity such as fat than for tissue with a
higher conductivity and permittivity such as muscle. The interaction of the MRI RF field
with orthopedic implants is investigated. As specific case studies, the scattered fields due
to a bone support frame implant and a hip joint implant are computed. It is found that the
greatest MRI- induced heating occurs at the tips of long metal parts where the length and
thickness of a metal part and its tips determine the amount of induced heating. For the
bone support frame, the induced surface current density distributions on the steel pins and
the spatial electric field distributions in the surrounding tissue have been obtained. For
the hip joint, the maximum temperature rise is at the elliptical tip of the long cylindrical
limb of the joint where it joins the femur. The spatial electric field and temperature rise
distributions around intravascular stents of various lengths have also been obtained. The
maximum temperature rise occurs in the tissue surrounding the tips of a stent. The
induced heating effect increases with increasing length. |
en_US |
dc.description.sponsorship |
Higher Education Commission, Pakistan |
en_US |
dc.language.iso |
en |
en_US |
dc.publisher |
University of Engineering and Technology Lahore Pakistan |
en_US |
dc.subject |
Applied Sciences |
en_US |
dc.title |
SCATTERING OF THE MAGNETIC RESONANCE IMAGING RADIOFREQUENCY FIELD BY IMPLANTED MEDICAL DEVICES |
en_US |
dc.type |
Thesis |
en_US |