Configuration-based electrical properties tomography

Purpose: To introduce phase-based conductivity mapping from a configuration space analysis.Methods: The frequency response function of balanced SSFP (bSSFP) is used to perform a configuration space analysis. It is shown that the transceive phase for conductivity mapping can be directly obtained by a simple fast Fourier transform of a series of phase-cycled bSSFP scans. For validation, transceive phase and off-resonance mapping with fast Fourier transform is compared with phase estimation using a recently proposed method, termed PLANET. Experiments were performed in phantoms and for in vivo brain imaging at 3 T using a quadrature head coil.Results: For fast Fourier transform, aliasing can lead to systematic phase errors. This bias, however, decreases rapidly with increasing sampling points. Interestingly, Monte Carlo simulations revealed a lower uncertainty for the transceive phase and the off-resonance using fast Fourier transform as compared with PLANET. Both methods, however, essentially retrieve the same phase information from a set of phase-cycled bSSFP scans. As a result, configuration-based conductivity mapping was successfully performed using eight phase-cycled bSSFP scans in the phantoms and for brain tissues. Overall, the retrieved values were in good agreement with ex-pectations. Conductivity estimation and mapping of the field inhomogeneities can therefore be performed in conjunction with the estimation of other quantitative pa-rameters, such as relaxation, using configuration theory.Conclusions: Phase-based conductivity mapping can be estimated directly from a simple Fourier analysis, such as in conjunction with relaxometry, using a series of phase-cycled bSSFP scans

Phantom measurement data for 'Configuration-based electrical properties tomography', Iyyakkunnel et al. (2021)

This dataset contains the phantom bSSFP measurement data used in the published article Iyyakkunnel et al., 'Configuration-based electrical properties tomography', Magn Reson Med. 2021;85:1855–1864 (doi: 10.1002/mrm.28542). The acquisitions were made with a 3 T MRI system (Magnetom Prisma; Siemens Healthcare, Erlangen, Germany) using a dual-tuned 1H/23Na quadrature head coil for transmission and reception (Rapid Biomedical, Rimpar, Germany). The data includes the magnitude and phase measurements for eight phase-cycled scans (in dicom (.dcm) format). The RF phase increment for the phase cycled scans corresponds to 0°, 45°, 90°, 135°, 180°, 225°, 270° and 315°. For further measurement details, please refer to the mentioned original article

Methodological advances in electrical properties tomography

Electrical properties tomography (EPT) aims to non-invasively measure the electrical properties (EPs) of biological tissues using conventional magnetic resonance imaging (MRI). EPT has gained increased interest over the last decade thanks to its potential in various applications, ranging from the optimization of radio frequency (RF) irradiation for increased safety, to the diagnosis and staging of pathologies, such as cancer, as demonstrated in several studies. In MRI, the electrical properties of the tissue, in addition to the coil and body geometries, affect the RF magnetic field used for spin excitation. EPT reconstructs the EPs from local changes in the spatial distribution of this field, commonly known as B1+. Typically, a measurement of B1+ is used to correct for effects resulting from inhomogeneous excitation. Measurements used for this purpose are often performed with fast sequences and thus, at low resolution. EPT, however, needs high-resolution measurements to be sensitive to the local variations, often translating into long acquisition times when relying on existing B1+ mapping methods. In addition, both the amplitude and phase of B1+ are required for accurate EP estimation. These components are generally measured separately, adding to the overall needed scan time. Such long scan times are a large factor in hindering the translation of EPT into the clinics. This thesis investigated strategies to reduce the scan time for B1+ mapping methods that are suitable for EPT. In particular, a high signal-to-noise ratio and high precision requirements must be met. For this reason, three methods are proposed to improve the time efficiency of the B1+ acquisitions. Two of the implemented methods reduce the scan time by combining B1+ amplitude and phase measurement, virtually halving the acquisition time. The third method increases efficiency by measuring one B1+ component in conjunction with other relevant quantitative parameters. The proposed methods showed promising results in healthy volunteers indicating a potential translation into clinics. In conclusion, this thesis successfully addressed the challenge of reducing the measurement time for EPT by proposing three methods with increased time efficiency. Combined with more accurate reconstruction algorithms, these methods could serve as a promising starting point for integrating EPT into clinical practice

Effect of 3 T magnetic field on RF plasma sputtering in an ITER-relevant first mirror unit

The first mirror (FM) cleaning operations in ITER are expected to be executed in the presence of a $\sim 3$ T magnetic field. In the RF plasma cleaning configuration, this would have a significant influence on the plasma properties, ion energy, angle of incidence as well as flux spatial distribution. To this end, RF discharges were excited in an ITER-sized mock-up of a first mirror unit (FMU) consisting of a powered first mirror M1 and a grounded second mirror M2 placed in a homogeneous 3 T magnetic field. The plasma discharge was confined in a beam extending in the direction of the magnetic field, consequently wetting a limited portion of the FMU walls. In the DC-decoupled scheme (without λ/4 filter), this considerably influenced the self-bias voltage VDC that develops on M1. Changing the angle α between M1 normal and magnetic field, modified the plasma wetted wall area Ag and the resulting VDC varied by over two orders of magnitude. Plasma exposure experiments were also done in the DC-coupled scheme (with λ/4 filter), wherein the angle and wetted surface determined the area of wall sputtered. Increasing α led to an increase in the sputtered wall area Ag, and consequently the wall deposition on grounded M2. However, in all the cases M1 was entirely clean with the exception of edge deposits in some. In contrast, both M1 and M2 are coated with wall deposits in the absence of a magnetic field and a similar plasma exposure. The results show that plasma cleaning with λ/4 filter in a 3 T magnetic field at ITER could potentially prevent the parasitic wall deposition on FMs. The results also highlight the importance of FM orientation in the magnetic field and the wetted area in the plasma cleaning in both the DC-coupled and decoupled schemes within the ITER diagnostic systems

Morphological changes of tungsten surfaces by low-flux helium plasma treatment and helium incorporation via magnetron sputtering.

The effect of helium on the tungsten microstructure was investigated first by exposure to a radio frequency driven helium plasma with fluxes of the order of 1 × 10(19) m(-2) s(-1) and second by helium incorporation via magnetron sputtering. Roughening of the surface and the creation of pinholes were observed when exposing poly- and nanocrystalline tungsten samples to low-flux plasma. A coating process using an excess of helium besides argon in the process gas mixture leads to a porous thin film and a granular surface structure whereas gas mixture ratios of up to 50% He/Ar (in terms of their partial pressures) lead to a dense structure. The presence of helium in the deposited film was confirmed with glow-discharge optical emission spectroscopy and thermal desorption measurements. Latter revealed that the highest fraction of the embedded helium atoms desorb at approximately 1500 K. Identical plasma treatments at various temperatures showed strongest modifications of the surface at 1500 K, which is attributed to the massive activation of helium singly bond to a single vacancy inside the film. Thus, an efficient way of preparing nanostructured tungsten surfaces and porous tungsten films at low fluxes was found

Morphological changes of tungsten surfaces by low-flux helium plasma treatment and helium incorporation via magnetron sputtering

The effect of helium on the tungsten microstructure was investigated first by exposure to a radio frequency driven helium plasma with fluxes of the order of 1 × 10(19) m(-2) s(-1) and second by helium incorporation via magnetron sputtering. Roughening of the surface and the creation of pinholes were observed when exposing poly- and nanocrystalline tungsten samples to low-flux plasma. A coating process using an excess of helium besides argon in the process gas mixture leads to a porous thin film and a granular surface structure whereas gas mixture ratios of up to 50% He/Ar (in terms of their partial pressures) lead to a dense structure. The presence of helium in the deposited film was confirmed with glow-discharge optical emission spectroscopy and thermal desorption measurements. Latter revealed that the highest fraction of the embedded helium atoms desorb at approximately 1500 K. Identical plasma treatments at various temperatures showed strongest modifications of the surface at 1500 K, which is attributed to the massive activation of helium singly bond to a single vacancy inside the film. Thus, an efficient way of preparing nanostructured tungsten surfaces and porous tungsten films at low fluxes was found