Electrical impedance information provides crucial information for characterizing biological tissues, and also important information aiding electrophysiological source imaging. Novel means are being investigated to image electrical impedance distribution of the biological tissues with applications to cancer detection and functional imaging.
We have proposed and developed a novel approach — magnetoacoustic tomography with magnetic induction (MAT-MI) — by integrating magnetism and ultrasound. MAT-MI generates magnetic stimulation and then measures ultrasound response for bioimpedance imaging. In MAT-MI, the object is placed in a static magnetic field and a pulsed magnetic field. The pulsed magnetic field induces eddy current in the object. Consequently, the object will emit ultrasonic waves through the Lorenz force produced by the combination of the eddy current and the static magnetic field. The acoustic waves are then collected by the detectors located around the object for image reconstruction. Theoretical (Xu & He, Physics Med. & Biol., 2005, Abstract; Xia et al., IEEE-TMI, 2009, pdf) and experimental (Li et al., J Applied Physics, 2006, pdf; Xia et al., Applied Physics Letters, 2007, Abstract) studies indicate that MAT-MI promises to provide high spatial resolution in imaging electrical impedance distribution. Our simulation results further demonstrate the feasibility to reconstruct conductivity distribution with great contrast without the sacrifice of spatial resolution (Li et al., IEEE-TBME, 2007, pdf). We are further improving the imaging performance and exploring its applications in cancer detection.
Fig. 1 Illustration of the concept of the MAT-MI. (from Xia et al., IEEE-TMI, 2009)
Magnetic Resonance Electrical Impedance Tomography
Magnetic Resonance Electrical Impedance Tomography (MREIT) is a recently developed imaging modality that reconstructs the electrical impedance distribution from the magnetic flux density generated by the injected current into a volume conductor. In MREIT, a low frequency current is used through a pair of electrodes, and its induced magnetic flux density can be measured by a MRI scanner distributed over the entire conductive media. Such magnetic flux distribution depends on the electrical impedance distribution of the media and, thus, the electrical impedance tomography within the entire media can be reconstructed noninvasively. We have been developing novel MREIT reconstruction algorithms (Gao et al., Physics Med. & Biol., 2005, Abstract, 2006, Abstract), which use only one directional measurement of the magnetic flux density for head impedance imaging. Electrical impedance distribution of biological tissues can provide useful information about their physiological and pathological status. The future development of MREIT is to improve its sensitivity to small electrical impedance changes and its robustness to measurement noise.
Fig. 1 An example of simulated magnetic flux density (Bz) in a realistic geometry human head model. Note that our algorithm uses only one magnetic flux density measurement (Bz) to reconstruct electrical impedance image. (from Gao et al., Physics in Med. & Biol., 2006)