934 | © 2018 International Society for Magnetic Resonance in Medicine wileyonlinelibrary.com/journal/mrm Magn Reson Med. 2019;81:934–946.
Received: 16 March 2018
|
Revised: 6 June 2018
|
Accepted: 12 June 2018
DOI: 10.1002/mrm.27446
FULL PAPER
bSSFP phase correction and its use in magnetic resonance
electrical properties tomography
Safa Ozdemir
|
Yusuf Ziya Ider
Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey
Correspondence
Yusuf Ziya Ider, Department of Electrical
and Electronics Engineering, Bilkent
University, Ankara, Turkey.
Email: ider@ee.bilkent.edu.tr
Funding information
This study was supported by TUBITAK
114E522 research grant. Experimental
data were acquired using the facilities of
UMRAM, Bilkent University, Ankara.
Purpose: Balanced steady‐state free precession (bSSFP) sequence is widely used
because of its high SNR and high speed. However, bSSFP images suffer from “band-
ing artifact” caused by B
0
inhomogeneity. In this article, we propose a method to
remove this artifact in bSSFP phase images and investigate the usage of the corrected
phase images in phase‐based magnetic resonance electrical properties tomography
(MREPT).
Theory and Methods: Two bSSFP phase images, obtained with different excitation
frequencies, are collaged to get rid of the regions containing banding artifacts. Phase
of the collaged bSSFP image is the sum of the transceive phase of the RF system and
an error term that depends on B
0
and T
2
. By using B
0
and T
2
maps, this error is elimi-
nated from bSSFP phase images by using pixel‐wise corrections. Conductivity maps
are obtained from the uncorrected and the corrected phase images using the phase‐
based cr‐MREPT method.
Results: Phantom and human experiment results of the proposed method are illus-
trated for both phase images and conductivity maps. It is shown that uncorrected
phase images yield unacceptable conductivity images. When only B
0
information is
used for phase correction conductivity, reconstructions are substantially improved,
and yet T
2
information is still needed to fully recover accurate and undistorted con-
ductivity images.
Conclusions: With the proposed technique, B
0
sensitivity of the bSSFP phase im-
ages can be removed by using B
0
and T
2
maps. It is also shown that corrected bSSFP
phase images are of sufficient quality to be used in conductivity imaging.
KEYWORDS
banding artifact, bSSFP, conductivity, MREPT, phase‐based
1
|
INTRODUCTION
Impedance imaging aims at reconstructing conductivity, σ,
and permittivity, ϵ, of the tissues. Earlier methods of imped-
ance imaging are electrical impedance tomography (EIT)
1,2
and magnetic induction tomography (MIT)
3
that are used to
induce currents in the object by using either surface electrodes
(EIT) or external coils (MIT). However, these methods yield
images that have low spatial resolution in interior regions be-
cause the measurements (i.e., surface potentials) are not very
sensitive to electrical property perturbations relatively far
from the surface. To overcome this weakness, magnetic res-
onance electrical impedance tomography (MREIT) has been
introduced.
4-9
In MREIT, current is induced by using surface
electrodes in the frequency range of 10 Hz–10 kHz and the
resulting magnetic field is measured by MRI to reconstruct