Keywords: Electromagnetic Tissue Properties, Quantitative Susceptibility mapping, Echo Planar Imaging, Brain Masking, Artefacts, Electromagnetic Tissue Properties

Motivation: Artefacts in QSM reconstructions can persist even after optimal noise-based masking methods are used, especially in EPI-QSM, due to challenging regions caused by large field gradients rather than noise.

Goal(s): To reduce brain QSM artefacts using a novel, automated brain mask thresholding method.

Approach: Regions with high magnitude of the field gradients (MFG) were removed from the brain mask used for background field removal and/or susceptibility calculation.

Results: Thresholding the mask based on the MFG of the local field map was superior to thresholding the total field map. MFG-based thresholding reduced artifacts in 2D-EPI and 3D-GRE QSM compared to noise-based thresholding.

Impact: Thresholding the brain mask based on the magnitude of local field gradients improves brain QSM quality by reducing streaking artefacts compared to state-of-the-art noise-based thresholding. This automated MFG-based masking method particularly improves rapid 2D-EPI-QSM as well as conventional 3D-GRE QSM.

Keywords: Electromagnetic Tissue Properties, Image Reconstruction, Electrical Properties Tomography, EPT, Conductivity mapping

Motivation: To date, phase-based electrical properties tomography (EPT) has been performed using time-consuming (~5-minute) gradient-echo sequences. 

Goal(s): To calculate EPT conductivity maps from a rapid multi-echo 2D-EPI acquisition (TR~4s), overcoming slice-to-slice phase inconsistencies. 

Approach: We investigated the effect of four different methods to remove slice-to-slice inconsistencies from the phase offset (φ₀) of the multi-echo 2D-EPI data on conductivity maps in a numerical phantom and in vivo.

Results: Removing the median φ₀ in each slice in the brain provided high quality conductivity maps with clear delineation between grey matter, white matter and CSF. Tissue conductivity values showed good inter- and intra-subject repeatability

Impact: We developed a rapid, repeatable method for phase-based EPT from multi-echo 2D-EPI, overcoming slice-to-slice phase inconsistencies. This will facilitate clinical applications of EPT, particularly in studies already using multi-echo 2D-EPI for fMRI, and paves the way towards functional EPT.

Keywords: Electromagnetic Tissue Properties, Electromagnetic Tissue Properties, Metric

Motivation: To devise a metric to assess the extent of noise amplification in MR-EPT reconstruction algorithms.

Goal(s): We explore the correlation between the proposed metric and noise amplification for phase-based conductivity reconstructions.

Approach: This study conducted experiments using a concentric cylindrical simulated phantom with uniform electrical properties. Additionally, the proposed metric was applied to in-vivo data.

Results: This metric serves as an indicator of the reliability of the reconstructed conductivity maps. the size of the Laplacian kernel and the chosen weighting method significantly impact the metric.

Impact: This work reveals that the power of a designed MR-EPT reconstruction kernel acts as a noise error propagation factor in the MR-EPT conductivity reconstruction. Consequently, this map can offer insights into the reliability of reconstructed conductivity map.

Keywords: Electromagnetic Tissue Properties, Electromagnetic Tissue Properties, Electrical Conductivity, EPT, Conductivity Tensor Imaging, Metabolite Imaging

Motivation: Extracellular fluid within the brain is complex electrolyte comprising various ions. Disturbances in the balance of electrolytes have been associated with various pathological disorders.

Goal(s): The study aimed to provide quantitative visualization of mobility-weighted effective extracellular ion concentration at every pixel.

Approach: Three phantom experiments were conducted using CTI, precisely controlling ion concentrations, mobilities and cell-alike phantom validation.

Results: The measured values of ion concentrations in three phantoms were comparable to the intended values, demonstrating accuracy. The conductivity tensor imaging method could extract the contribution of ionic concentration and mobility in measured conductivity information.  

Impact: We proposed a method to derive the mobility-weighted effective extracellular ion concentration using conductivity tensor imaging (CTI). Ionizing radiation significantly alters the concentration of ions in tissues. Understanding cellular-level ion concentrations is critical for optimizing the effectiveness of radiation therapy.

Keywords: Electromagnetic Tissue Properties, Brain

Motivation: Ethanol is a sedative which reduces brain metabolism and activity. We hypothesise that brain tissue conductivity is related to brain activity and tested to see if ethanol ingestion reduces tissue conductivity.  

Goal(s): To demonstrate a relationship between baseline brain activity and tissue conductivity.

Approach: Using MREPT, data were acquired from 41 participants before and after ingestion of vodka and the resultant electrical conductivity maps were compared.

Results: Spatially similar decreased electrical conductivity was seen in most participants. The spectrum of response was unrelated to the amount of alcohol consumed or to breath alcohol reading. No increased conductivity was seen.  

Impact: The results support the hypothesis that tissue conductivity is related to brain activity. This suggests that changes in electrical conductivity may be used as a surrogate for baseline brain activity which could be a useful biomarker of injury or degeneration.

Keywords: Electromagnetic Tissue Properties, Electromagnetic Tissue Properties, Conductivity, EPT

Motivation: Electrical properties (EPs) are reconstructed from complex B₁⁺ maps. In-vivo reconstructed EPs values presented in literature show large variations, reducing the confidence in the quality/accuracy of the reconstruction methods.

Goal(s): To develop a method to compute complex B₁⁺ fields from EPs maps, which can the verify accuracy of EPs reconstructions.

Approach: Complex B₁⁺ maps are predicted using a finite difference-based approach. The difference between the predicted and measured fields is used as surrogate error of the estimated input EPs.

Results: The method shows accurate complex B₁⁺ field reconstructions in 2 minutes and the ability to localize errors in the input EPs maps.

Impact: Complex B₁⁺ fields are predicted using tissue electrical properties maps as input. This method provides a way to assess the quality/accuracy of in-vivo electrical properties reconstructions, providing a means to gain confidence in the reconstructed electrical properties values.

Keywords: Electromagnetic Tissue Properties, Electromagnetic Tissue Properties

Motivation: The research is motivated by the limitations of current EPT, either over-sensitive or over-simplified algorithm, prompting the development of a more accurate WF-EPT method. 

Goal(s): The study aims to enhance EPT by proposing WF-EPT with Dixon technique, seeking to improve accuracy in EPs mapping for clinical applications.

Approach: We fit measured EPs data to generate the WF-EPs model, and validate our approach via human imaging.

Results: Experiments showed relative errors of conductivity and relative permittivity of human liver were within 10.89% and 2.55% compared to literature values.

Impact: WF-EPT offers new insights for clinical EPT, potentially enhance applications in disease diagnosis and SAR estimation.

Keywords: Electromagnetic Tissue Properties, Multimodal, MR-EPT, functional electrical properties tomography, functional conductivity mapping

Motivation: Electrical properties tomography (EPT) can reveal brain tissue conductivity changes during functional activation. Previous attempts have used sequences with low resolution and limited coverage, and required separate acquisitions to generate functional MRI (fMRI) and EPT (fEPT).

Goal(s): To investigate the feasibility of high-resolution whole-brain fEPT and simultaneous fMRI using gradient-echo EPI (GRE-EPI). 

Approach: Two healthy volunteers were scanned using GRE-EPI during a visual stimulation paradigm. Conductivity maps calculated using phase-based EPT were analysed for functional activation.

Results: We observed small regions of positive and negative fEPT visual activation, co-localised with fMRI activations. These findings are reproducible across subjects and coil configurations.

Impact: Calculating whole-brain functional Electrical Properties Tomography (fEPT) and BOLD fMRI simultaneously using a high-resolution multi-echo GRE-EPI sequence will allow fMRI studies to reveal functional conductivity changes, opening up a new dimension with potential for new clinical and research applications. 

Keywords: Electromagnetic Tissue Properties, Electromagnetic Tissue Properties, MR-EPT, electrical conductivity mapping, electrical properties tomography, noise reduction

Motivation: Phase-based electrical properties tomography calculates conductivities by fitting the transceive phase weighted by a Gaussian function of the magnitude image with width δ. Currently, δ is selected empirically and its impact on conductivities is unknown.

Goal(s): To investigate the effect of δ on conductivity maps and develop a method to automatically select δ.

Approach: After evaluating relationships between δ and healthy brain conductivities at 3T, we calculated conductivities using a voxel-wise δ based on inverse phase noise and compared the results.

Results: Increasing δ decreased contrast and noise in conductivity maps. Our new method to calculate a varying δ automatically optimised conductivity maps.

Impact: We have developed a method to calculate a varying Gaussian weighting width  for EPT. This will enable automatic optimisation of conductivity maps rather than time-consuming empirical choice of δ, facilitating the use of phase-based EPT and broadening its applicability.

Keywords: Electromagnetic Tissue Properties, Tumor, Ionizing radiation

Motivation: Ionizing radiation produces ions inside the human body that can kill cancerous tissues by damaging DNA directly or creating charged particles that damage DNA.

Goal(s): Contrast mechanism of electrical conductivity is determined by the concentration and mobility of ions that make up tissues.

Approach: We applied MR-based conductivity imaging to mouse brain tumors to evaluate the responses in irradiated and non-irradiated tissues during the peri-irradiation period.

Results: MR-based conductivity images effectively showed acute response after irradiation in glial tumors.

Impact: High-frequency conductivity images can differentiate each brain tissues including viable tumors, tumor necrosis, and normal brain. It shows potential as a tool to differentiate the therapeutic effect of radiation by measuring absolute values and calculating percentage changes.

Keywords: Electromagnetic Tissue Properties, Electromagnetic Tissue Properties, Conductivity, EPT

Motivation: In Electrical Properties Tomography, often 2D reconstructions ignoring derivatives in the slice direction (often z) are performed instead of 3D reconstructions, without proper compensation.

Goal(s): In this work, we investigate the quantitative influence on the reconstructed conductivity.

Approach: This is done by experiments in a cylindrical phantom with homogeneous electrical properties in simulation and measurement. Furthermore, using simulations an indication is given of the importance of 3D reconstruction in several anatomical areas.

Results: The contribution of the third dimension on the reconstructed conductivity is shown to be highly dependent on sample geometry. Therefore, disregarding this can only be done in specific cases.

Impact: This work shows that the assumption of a negligible third dimension contribution as done in 2D EPT reconstruction is only accurate in specific cases. For most applications 3D reconstructions or proper compensation is needed.

Keywords: Electromagnetic Tissue Properties, Electromagnetic Tissue Properties

Motivation: The electrical conductivity (σ) has been proven as helpful for glioma and breast cancer characterization. Our prior study also suggests its potential utility in distinguishing benign and malignant thoracic lesions.

Goal(s): This study aimed to evaluate the added value of σ to thoracic lesion diagnosis by apparent diffusion coefficient (ADC).

Approach: In this prospective study, we used radiomics analysis to evaluate the prediction value of ADC and σ. The diagnostic performance of selected ADC, σ, and composite indices were compared.

Results: 7 independent features were identified in 21 lesions. Half of the top 4 features were from σ. All indices achieved excellent performance.

Impact: The results of this preliminary study highlight the potential usefulness of noninvasive electrical conductivity (σ) measurement in the characterization of thoracic lesions.

Keywords: Electromagnetic Tissue Properties, Electromagnetic Tissue Properties, MR EPT, conductivity, liver, brain, age-related changes

Motivation: Age-related study of electrical conductivity may be useful for personalized SAR modelling.

Goal(s): To evaluate in-vivo conductivity changes in a preliminary study in adults (brain and liver), using MR Electrical Properties Tomography, at 3T.

Approach: We used a UTE sequence to obtain RF-weighted images suitable for conductivity reconstruction. Data from 10 subjects were analyzed: in the brain, white matter and grey matter values were quantified; in the liver, fat volume fraction was also obtained for comparison.

Results: In-vivo conductivities were close to literature values, except for white matter which was higher in vivo. A significant dependency with age was found in the liver.

Impact: Knowledge of electrical properties of tissues is required for SAR modelling. Currently, values from ex-vivo/animal studies are used. In-vivo measurements by MR-EPT could bring valuable insights, in particular age-related changes, if existing, could be considered for MRI safety studies.

Keywords: Electromagnetic Tissue Properties, Electromagnetic Tissue Properties

Motivation: Plant-based models such as potatoes are employed for research on irreversible electroporation (IRE). Volumetric assessment of electroporation-mediated conductivity changes is desirable. MR-based electric properties tomography (EPT) provides volumetric conductivity assessment at the Larmor frequency. 

Goal(s): This study aimed to assess IRE-mediated conductivity changes in potato tissue by EPT at 64 MHz and by electrochemical impedance spectroscopy (EIS) between 1kHz and 1 MHz. 

Approach: Potato samples were electroporated with different pulse amplitudes and analyzed by EPT based on 3D FLAIR measurements and EIS. 

Results: EIS detected a clear conductivity rise in the low frequency range, while EPT did not detect significant conductivity changes.

Impact: MR-based electric properties tomography offers volumetric conductivity measurement method at the Larmor frequency, but was not found capable of detecting significant conductivity changes in potato tissue at 64 MHz. This has implications for treatment response assessment in basic electroporation research. 

Keywords: Electromagnetic Tissue Properties, Electromagnetic Tissue Properties

Motivation: Conductivity reconstructions based on polynomial fitting methods are mostly 2D leading to inaccurate reconstructions as information arising from the through-plane dimension is missing.

Goal(s): To include conductivity contributions from three-dimensions for deep-learning patch-based polynomial fitting reconstructions. 

Approach: A DL-informed polynomial fitting reconstruction method including $$$B_{1}^{+}$$$ magnitude information is presented. This method leverages neural networks to jointly predict optimal fitting coefficients enabling joint 2D-polynomial-fitting in three-orthogonal-planes, hence we call it 2.5D. 

Results: The proposed method demonstrates superior-performance compared to fitting-based 2D/3D fitting approaches and is computationally efficient for 3D-reconstructions.

Impact: A 2.5-dimensional neural network informed fitting approach is used for MR-based conductivity reconstructions. Conductivity reconstruction accuracy as well as structural information are improved compared to physics-based and deep learning-based fitting methods.

Keywords: Electromagnetic Tissue Properties, Brain

Motivation:  Psychosis has been studied from different perspectives, including genetic factors and dopamine dysfunction. However, those perspectives have been studied independently.

Goal(s): To investigate the relationship between genetic factors (i.e., Polygenic Risk Score, PSR) and magnetic tissue properties associated with dopamine (QSM, R2*) in a cohort of individuals with psychotic experiences.

Approach:  Analyze the potential correlations among QSM, R2* and PRS scores using linear mixed models in a cohort of patients and controls obtained from the UK Biobank.

Results: We identified significant predictors for QSM and R2* values with PRS, revealing differences in specific brain regions associated with dopamine pathways.

Impact: The changes found in the brain regions associated with dopamine pathways provide further evidence to support that psychosis may be related to a dopamine dysfunction, and those changes may also be related to genetic factors.