B Phase mapping at 7 T and its application for in vivo electrical conductivity mapping

Abstract

In this study, a new approach to measure local electrical conductivity in tissue is presented, which is based on the propagating B In recent years, the used frequency of radio frequency (RF) pulses in MRI has increased considerably as a consequence of the increase in magnetic field strengths. As an increased frequency leads to a shorter wavelength, the electromagnetic fields (EM-fields) induced by the RF pulse should more and more be regarded as waves (1,2) impinging onto a very dielectrically heterogeneous object: the human body. As a wave propagates in the human body, it is being affected by the local dielectric properties which are tissue dependent. The interaction between the applied EM-fields and the body leads to a variation of the EM-fields in amplitude (3,4) as well as in phase (5); this is the origin of B 1 field inhomogeneities. The variations in the B + 1 field can be separated in two categories: body-scale effects and local effects. A recent simulation study has shown that the B + 1 field variations depend predominantly on the global dielectric properties and the contour of the body (6). Small deviations were found; these were attributed to local variations in dielectric properties. Furthermore, it was found that these local B + 1 variations can amount up to 10-20% in magnitude of the average B + 1 field at 3 T (7). These findings indicate that the local B + 1 field variations contain information about the local dielectric environment; in this study we aim to exploit this information. As the B

CSI-EPT: A novel contrast source approach to MRI based electric properties tomography and patient-specific SAR

“…Furthermore, exact knowledge of B + 1 phase is assumed. In practice, however, measurements of the B + 1 phase are based on assumptions regarding the object and coil geometry [3,4].…”

“…Van Lier et al [4] demonstrated the feasibility of EPT using phase only information and Sodickson et al [5] introduced the so-called Local Maxwell Tomography (LMT) technique, which is free of assumptions on the B + 1 phase. What these currently used methods have in common is that local field equations (Maxwell's equations or Helmholtz's equation) are used as a basis for tissue parameter retrieval.…”

Non-Invasive in vivo Loss Tangent Imaging: Thermal Sensitivity Estimation at the Larmor Frequency

“…Recently, a novel non-invasive technique called MREPT (Magnetic resonance electric properties tomography) was introduced for the measurement of electrical properties (12)(13)(14). MREPT yields quantitative values for conductivity and permittivity, determined at the MR Larmor frequency.…”

“…The multi-echo readout scheme removed the effect of B 0 inhomogeneity, allowing the B 1 + phase (TE = 0 phase) to be retrieved through linear least squares fitting of the phase values. The approximation that the B 1 + phase is half the value of the transceive phase value was used herein (13,19,20).…”

Quantitative Conductivity Estimation Error due to Statistical Noise in Complex B1+Map

“…In addition, conductivity reconstruction using only the B 1 + phase has been introduced (13) and widely used (14,15), however it can provide accurate conductivity values under the assumption that the spatial variation of B 1 + magnitude is negligible. For all these applications and measurements, the conductivity value is needed to be estimated under an acceptable error level.…”