According to the MR method in general, the body of a patient or in general an object has to be arranged in a strong, uniform magnetic field whose direction at the same time defines an axis (normally the z-axis) of the coordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins independent of the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so called Larmor frequency, or MR frequency). From a microscopic point of view, the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicular to the z-axis, also referred to as longitudinal axis, so that the magnetization performs a precessional motion about the z-axis. The precessional motion describes the surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse. In the case of the so called 90 degree pulse, the spins are deflected from the z-axis to the transverse plane (flip angle 90 degrees).
After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z-direction is built up again with a first time constant T1 (spin-lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z-direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of receiving RF coils which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied, after application of, for example a 90 degree pulse, by a transition of the nuclear spins induced by local magnetic field inhomogeneities from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (de-phasing). The de-phasing can be compensated by means of a refocusing pulse, for example 180 degree pulse. This produces an echo signal (spin echo) in the receiving coils.
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superimposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving coils corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to an MR image by means of Fourier transformation.
Electric properties tomography (EPT) is a recently developed approach to determine electric conductivity σ, permittivity ε and local SAR (specific absorption rate) in vivo using standard MR systems. For EPT, the spatial components of the magnetic field of the transmission/reception RF coil involved are measured and post-processed. One substantial advantage of EPT compared to the well known electric impedance tomography (EIT) or MR-EIT is that EPT does not apply any external currents to the patients or objects to be examined. Optimally, all three spatial components of the RF coil's magnetic field are measured and post-processed. Typically, the spatial amplitude distribution of one of these three components can be measured exactly, namely the positive circularly polarized component H+. However, determination of the two other components, namely the negative circularly polarized magnetic field component H− and Hz are rather difficult to determine. Furthermore, MR imaging always yields a mixture of the spatial phase distributions τ and ρ, corresponding to the transmit sensitivity H+ and the receive sensitivity H−, respectively.
An electric impedance imaging system to explore the electrical conductivity and permittivity distribution of an object is known for example from WO 2007/017779.