Electric properties tomography: Biochemical, physical and technical background, evaluation and clinical applications

Purpose In this work we demonstrate how sequence parameter settings influence the accuracy and precision in T 1 , T 2 , and off‐resonance maps obtained with the PLANET method for a single‐component signal model. In addition, the performance of the method for the particular case of a two‐component relaxation model for white matter tissue was assessed. Methods Numerical simulations were performed to investigate the influence of sequence parameter settings on the accuracy and precision in the estimated parameters for a single‐component model, as well as for a two‐component white matter model. Phantom and in vivo experiments were performed for validation. In addition, the effects of Gibbs ringing were investigated. Results By making a proper choice for sequence parameter settings, accurate and precise parameter estimation can be achieved for a single‐component signal model over a wide range of relaxation times at realistic SNR levels. Due to the presence of a second myelin‐related signal component in white matter, an underestimation of approximately 30% in T 1 and T 2 was observed, predicted by simulations and confirmed by measurements. Gibbs ringing artifacts correction improved the precision and accuracy of the parameter estimates. Conclusion For a single‐component signal model there is a broad “sweet spot” of sequence parameter combinations for which a high accuracy and precision in the parameter estimates is achieved over a wide range of relaxation times. For a multicomponent signal model, the single‐component PLANET reconstruction results in systematic errors in the parameter estimates as expected.

Section: Discussionmentioning

confidence: 99%

On the accuracy and precision of PLANET for multiparametric MRI using phase‐cycled bSSFP imaging

Shcherbakova

Berg

Moonen

et al. 2018

“…It has been argued that bSSFP is the most favorable phase‐measuring method for MREPT because of its speed, motion insensitivity, automatic eddy current compensation, and high SNR . However, bSSFP phase is susceptible to off‐resonance and T 2 effects, and it is vulnerable especially in certain regions where banding artifact occurs.…”

Section: Discussionmentioning

confidence: 99%

bSSFP phase correction and its use in magnetic resonance electrical properties tomography

Ozdemir

Ider

2018

Purpose Balanced steady‐state free precession (bSSFP) sequence is widely used because of its high SNR and high speed. However, bSSFP images suffer from “banding artifact” caused by B0 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 B0 and T2. By using B0 and T2 maps, this error is eliminated 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 illustrated for both phase images and conductivity maps. It is shown that uncorrected phase images yield unacceptable conductivity images. When only B0 information is used for phase correction conductivity, reconstructions are substantially improved, and yet T2 information is still needed to fully recover accurate and undistorted conductivity images. Conclusions With the proposed technique, B0 sensitivity of the bSSFP phase images can be removed by using B0 and T2 maps. It is also shown that corrected bSSFP phase images are of sufficient quality to be used in conductivity imaging.

“…In particular, in our study, for sodium MR imaging, a twisted projection read out was implemented, which allows for much shorter TE (0.4 ms) compared to the gradient echo sequence in the reference study (1.36 ms), and additional sodium coil inhomogeneity correction was performed to deliver high‐quality quantitative sodium images. Finally, MREPT‐based conductivity mapping seems more suitable for low field strength where the assumption that the transmit and receive phases are the same is more valid …”

Section: Discussionmentioning

confidence: 99%

Correlation of quantitative conductivity mapping and total tissue sodium concentration at 3T/4T

Liao

Lechea

Magill

et al. 2019

Purpose To investigate the correlation between electrical conductivity and sodium concentration, both measured in vivo, in the human brain. Methods Conductivity measurements were performed on samples with different sodium (Na+) and agarose concentrations using a dielectric probe, and the correlation between conductivity and Na+ content was evaluated. Subsequently, brain conductivity and total Na+ content maps were measured in 8 healthy subjects using phase‐based MREPT and sodium MRI, respectively. After co‐registration and spatial normalization to the 1 mm 152 MNI brain atlas, the relationship between conductivity and tissue sodium concentration (TSC) was examined within different brain regions. Results The conductivities of agarose gels increased linearly with NaCl concentration, while remaining almost independent of agarose content. When measured in healthy subjects, conductivities showed positive correlation with total tissue sodium concentration (R = 0.39, P < 0.005). The same trend was found in gray matter (R = 0.36, P < 0.005) and in white matter (R = 0.28, P < 0.05). Conclusion Tissue conductivity shows a positive correlation with total sodium concentration. Conductivity might serve as a novel technique to visualize the total tissue electrolyte concentration, although refinements in the consideration of e.g., tissue water content, would be necessary to improve the quantitative value.