6 research outputs found
SNR-Enhanced, Rapid Electrical Conductivity Mapping Using Echo-Shifted MRI
Magnetic resonance electrical impedance tomography (MREIT) permits high-spatial resolution electrical conductivity mapping of biological tissues, and its quantification accuracy hinges on the signal-to-noise ratio (SNR) of the current-induced magnetic flux density (Bz). The purpose of this work was to achieve Bz SNR-enhanced rapid conductivity imaging by developing an echo-shifted steady-state incoherent imaging-based MREIT technique. In the proposed pulse sequence, the free-induction-decay signal is shifted in time over multiple imaging slices, and as a result is exposed to a plurality of injecting current pulses before forming an echo. Thus, the proposed multi-slice echo-shifting strategy allows a high SNR for Bz for a given number of current injections. However, with increasing the time of echo formation, the Bz SNR will also be compromised by T2*-related signal loss. Hence, numerical simulations were performed to evaluate the relationship between the echo-shifting and the Bz SNR, and subsequently to determine the optimal imaging parameters. Experimental studies were conducted to evaluate the effectiveness of the proposed method over conventional spin-echo-based MREIT. Compared with the reference spin-echo MREIT, the proposed echo-shifting-based method improves the efficiency in both data acquisition and current injection while retaining the accuracy of conductivity quantification. The results suggest the feasibility of the proposed MREIT method as a practical means for conductivity mapping
SNR-Enhanced, Rapid Electrical Conductivity Mapping Using Echo-Shifted MRI
Magnetic resonance electrical impedance tomography (MREIT) permits high-spatial resolution electrical conductivity mapping of biological tissues, and its quantification accuracy hinges on the signal-to-noise ratio (SNR) of the current-induced magnetic flux density (Bz). The purpose of this work was to achieve Bz SNR-enhanced rapid conductivity imaging by developing an echo-shifted steady-state incoherent imaging-based MREIT technique. In the proposed pulse sequence, the free-induction-decay signal is shifted in time over multiple imaging slices, and as a result is exposed to a plurality of injecting current pulses before forming an echo. Thus, the proposed multi-slice echo-shifting strategy allows a high SNR for Bz for a given number of current injections. However, with increasing the time of echo formation, the Bz SNR will also be compromised by T2*-related signal loss. Hence, numerical simulations were performed to evaluate the relationship between the echo-shifting and the Bz SNR, and subsequently to determine the optimal imaging parameters. Experimental studies were conducted to evaluate the effectiveness of the proposed method over conventional spin-echo-based MREIT. Compared with the reference spin-echo MREIT, the proposed echo-shifting-based method improves the efficiency in both data acquisition and current injection while retaining the accuracy of conductivity quantification. The results suggest the feasibility of the proposed MREIT method as a practical means for conductivity mapping
Cerebral oxygen metabolism from MRI susceptibility
This article provides an overview of MRI methods exploiting magnetic susceptibility properties of blood to assess cerebral oxygen metabolism, including the tissue oxygen extraction fraction (OEF) and the cerebral metabolic rate of oxygen (CMRO2). The first section is devoted to describing blood magnetic susceptibility and its effect on the MRI signal. Blood circulating in the vasculature can have diamagnetic (oxyhemoglobin) or paramagnetic properties (deoxyhemoglobin). The overall balance between oxygenated and deoxygenated hemoglobin determines the induced magnetic field which, in turn, modulates the transverse relaxation decay of the MRI signal via additional phase accumulation. The following sections of this review then illustrate the principles underpinning susceptibility-based techniques for quantifying OEF and CMRO2. Here, it is detailed whether these techniques provide global (OxFlow) or local (Quantitative Susceptibility Mapping - QSM, calibrated BOLD - cBOLD, quantitative BOLD - qBOLD, QSM+qBOLD) measurements of OEF or CMRO2, and what signal components (magnitude or phase) and tissue pools they consider (intravascular or extravascular). Validations studies and potential limitations of each method are also described. The latter include (but are not limited to) challenges in the experimental setup, the accuracy of signal modeling, and assumptions on the measured signal. The last section outlines the clinical uses of these techniques in healthy aging and neurodegenerative diseases and contextualizes these reports relative to results from gold-standard PET
Alternating steady state free precession for estimation of current-induced magnetic flux density: A feasibility study
Purpose: To develop a novel, current-controlled alternating
steady-state free precession (SSFP)-based conductivity imaging
method and corresponding MR signal models to estimate
current-induced magnetic flux density (Bz) and conductivity
distribution.
Methods: In the proposed method, an SSFP pulse sequence,
which is in sync with alternating current pulses, produces dual
oscillating steady states while yielding nonlinear relation
between signal phase and Bz. A ratiometric signal model
between the states was analytically derived using the Bloch
equation, wherein Bz was estimated by solving a nonlinear
inverse problem for conductivity estimation. A theoretical analysis
on the signal-to-noise ratio of Bz was given. Numerical
and experimental studies were performed using SSFP-FID and
SSFP-ECHO with current pulses positioned either before or
after signal encoding to investigate the feasibility of the proposed
method in conductivity estimation.
Results: Given all SSFP variants herein, SSFP-FID with alternating
current pulses applied before signal encoding exhibits
the highest Bz signal-to-noise ratio and conductivity contrast.
Additionally, compared with conventional conductivity imaging,
the proposed method benefits from rapid SSFP acquisition
without apparent loss of conductivity contrast.
Conclusion: We successfully demonstrated the feasibility of
the proposed method in estimating current-induced Bz and
conductivity distribution. It can be a promising, rapid imaging
strategy for quantitative conductivity imaging. Magn Reson
Med 75(5) : 2009-2019. (C) 2015 Wiley Periodicals, Inc.1331sciescopu