Helmholtz-type Coil Research Articles

Comparison of different wireless coils for 1.5 T bilateral breast MRI

In this work, we compare three types of transceive wireless coils: Helmholtz-type coil, metamaterial-inspired coil and their combination. Each transceive coil is electromagnetically coupled to a body coil “birdcage” type of a 1.5 T MR scanner and improves bilateral breast imaging performance. While Helmholtz-type coil and metamaterial-inspired coil based on coupled split loop resonators are linearly polarized, their combination allows to couple with both linear components of the radiofrequency magnetic field, providing a more significant effect of a local boosting of the body coil’s transmit efficiency and radiofrequency safety in comparison with birdcage coil only.

Evaluation of superconducting pickup coils with high Q for 700 MHz NMR

In this study, we propose a helmholtz-type coil using a high-temperature superconductor (HTS) for 700 MHz NMR pickup coils with a high quality factor (Q). We analyzed the coil microwave resonant properties of the helmholtz-type one turn coil made using an HTS (DyBa2Cu3O7−δ: DyBCO coil) and the same type of coil made from normal metal (Cu coil) through electromagnetic simulations. The Q of the DyBCO coil at 77 K and the Cu coil at 77 K were 3295 and 482 in the simulations. The one turn coils were fabricated using 200-μm-thick DyBCO thin film deposited on an r-sapphire substrate and 3^m-thick Cu thin film on an r-sapphire substrate. The Q of the DyBCO coil was 2920 at 77 K, and that of the Cu coil was 423 at 77 K. These experimental results indicate that the DyBCO coils are more advantageous than the Cu coils for high-sensitivity NMR measurements.

B1 transmit phase gradient coil for single-axis TRASE RF encoding

PurposeTRASE (Transmit Array Spatial Encoding) MRI uses RF transmit phase gradients instead of B0 field gradients for k-space traversal and high-resolution MR image formation. Transmit coil performance is a key determinant of TRASE image quality. The purpose of this work is to design an optimized RF transmit phase gradient array for spatial encoding in a transverse direction (x- or y- axis) for a 0.2T vertical B0 field MRI system, using a single transmitter channel. This requires the generation of two transmit B1 RF fields with uniform amplitude and positive and negative linear phase gradients respectively over the imaging volume. Materials and MethodsA two-element array consisting of a double Maxwell-type coil and a Helmholtz-type coil was designed using 3D field simulations. The phase gradient polarity is set by the relative phase of the RF signals driving the simultaneously energized elements. ResultsField mapping and 1D TRASE imaging experiments confirmed that the constructed coil produced the fields and operated as designed. A substantially larger imaging volume relative to that obtainable from a non-optimized Maxwell-Helmholtz design was achieved. ConclusionThe Maxwell (sine)–Helmholtz (cosine) approach has proven successful for a horizontal phase gradient coil. A similar approach may be useful for other phase-gradient coil designs.

SQUID NDE for in situ inspection of copper heat exchanger tubes

An eddy-current-based SQUID NDE system was constructed for in situ inspectionof copper heat exchanger tubes using an HTS SQUID gradiometer and aHelmholtz-type-coil inducer. Thin copper tubes of 6.35 mm in diameter and0.8 mm in thickness were selected from products as specimens. Artificial flaws of100 µm in width, 15–25 mmin length and 10–50 µm in depth were made on the surfaces of the tubes. The tubes were moved at a velocity of1.6–32 m min−1 by a motor through the Helmholtz-type coil, which generated an excitation field of10 µT at 3 kHz. The SQUID NDE system could detect an anomalous magnetic response due to a flaw of10 µm in depth on thetube moving at 32 m min−1.

A new instrumental set-up for polarized neutron scattering experiments

Neutron scattering with polarization analysis is a powerful tool to determine magnetic structures and excitations. A common setup is to mount the sample at the center of a Helmholtz-type coil which can provide a magnetic field of any direction at the sample position and also a guide field along the neutron flight paths around the sample. Recent experiments showed quite a high demand for measurements at low momentum transfers. For the corresponding low scattering angles air scattering gives rise to a very large background. For this reason we have extended the standard setup to a combination of a large vacuum tank surrounded by electrical coils. The vacuum tank eliminates the air scattering and we can use the polarization analysis down to the lowest accessible momentum transfers. The coils themselves also show some new features: In contrary to the classic (symmetric) coil distribution we use an asymmetric setup which gives the advantage of a larger scattering window. Due to a more sophisticated current distribution this modified coil arrangement needs not to be rotated for different scattering conditions. The whole set-up will soon be available at IN12, a cold neutrons three-axis spectrometer operated by FZ Jülich in collaboration with CEA Grenoble as a CRG-B instrument at the Institut Laue Langevin in Grenoble.

A microchannel plate system for accelerator mass spectrometry measurements

The position-sensitive focal plane detector system for the trace element accelerator mass spectrometer at the US Naval Research Laboratory consists of 12 microchannel plate detector modules. Each module provides a 10×2 cm 2 active area and together they cover 80% of the split-pole spectrograph focal plane. The MCP detects electrons produced when an MeV ion beam passes through a thin carbon foil placed at 45° with respect to the beam. The electric charge is amplified and deposited on a double-delay line anode, with differential timing at the two ends providing horizontal position. Vertical position is evaluated from top-to-bottom charge partitioning. The collected charge is converted to timing signals through specially designed readout electronics based on the MQT300A LeCroy integrated circuit. A 400-μm horizontal resolution was obtained from ion beam generated secondary electrons accelerated to 2.5 keV. A Helmholtz-type coil pair was built for further confining the secondary electrons with a magnetic field.

Instrumental setup for polarized neutron scattering experiments

Neutron scattering with polarization analysis is a powerful tool to determine magnetic structures and excitations. A common setup is to mount the sample at the center of a Helmholtz-type coil which can provide a magnetic field of any direction at the sample position and also a guide field along the neutron flight paths around the sample. We have extended the standard setup to a combination of a large vacuum tank surrounded by the electrical coils. The vacuum tank eliminates the air scattering, and we can use the polarization analysis down to the lowest accessible momentum transfers. The coils themselves also show some appealing features: In contrast to the classic (symmetric) coil distribution, we use an asymmetric setup which gives the advantage of a larger scattering window. Due to a more sophisticated current distribution, this modified Helmholtz coil does not need to be rotated for different scattering conditions. The whole setup is currently being constructed at the IN12 instrument, a three-axis spectrometer for cold neutrons, operated by FZ Jülich in collaboration with CEA Grenoble as a CRG-B instrument at the Institut Laue Langevin in Grenoble.