A geometrically decoupled, twisted solenoid single‐axis gradient coil set for TRASE

Abstract

A highly efficient twisted solenoid coil was proposed recently for TRASE imaging for transverse B 0 geometries. This novel coil can be rotated to generate a phase gradient in any transverse direction, therefore, combining two such coils would double k-space coverage for single-axis encoding, resulting in higher spatial resolution. However, the strong inductive coupling between a pair of coaxial twisted solenoids must be overcome. Methods: Here, we demonstrate that two concentric twisted solenoids, designed using previously described Biot-Savart calculations, can be geometrically decoupled by attaching to each a regular solenoid in series. The regular solenoid geometry resulting in minimization of mutual inductance was determined from simulations using the FastHenry2 tool. The effects on TRASE encoding performance due to the regular solenoids were assessed from simulations and experiments. Results: The maximum resulting B 1 magnitude and phase distortions were 3.7% and 4.6 • , while a good isolation S 12 = −17.5 dB between the coil pair was obtained. TRASE experiments confirmed the double k-space coverage, and achieved a rapid spin echo train with 128 k-space points collected within 80 ms, allowing short T 2 samples to be accurately imaged. Conclusions: This study demonstrates that a pair of twisted solenoid phase gradient RF coils can be geometrically decoupled. Advantages over active PIN diode decoupling include faster switching, lower hardware complexity, and scalability. K E Y W O R D S geometric decoupling, RF coil, TRASE MRI, twisted solenoid 1 | INTRODUCTION 1.1 | TRASE overview and encoding principle Transmit Array Spatial Encoding (TRASE) is an MRI encoding approach that enables k-space data to be acquired using phase gradients in the radio frequency (RF) transmit fields (B 1). 1 Unlike conventional MRI encoding methods which rely on the use of main field (B 0) gradients, the substitution of B 0 gradients by B 1 phase gradients using simple RF technologies allows compact MRI systems to be made, particularly suitable for low-field, low-cost scenarios. 2 Recent work suggest that clinical-level millimeter spatial resolution is practically achievable, under both in vivo 3 and phantom situations. 1,2,4-6 | 1485 SUN et al. The fundamental concept of TRASE is that the pulse sequence consists of an echo train of 180 • refocusing pulses that are transmitted in an alternating pattern by switching the phase gradient B 1 field for each RF pulse. For onedimensional TRASE encoding, two different B 1 fields A and B with different phase gradients k A and k B are needed. These two phase gradients are transmitted alternatively down the spin echo train to impart a progressively increasing spatial phase modulation, with each k-space point collected by averaging the center points of each acquired echo. For higher dimensional TRASE encoding, more such B 1 phase gradient fields are needed, so for instance, three B 1 fields are sufficient for 2D TRASE encoding. Other reported phase gradient coil designs have included Helmholtz-M...

Fast spin‐echo approach for accelerated B1 gradient–based MRI

“…Several different groups have proposed similar techniques exploiting unique coil geometries to create more favorable fields for encoding spatial information without B 0 gradients. 38,39 For example, TRASE uses specific coil modes to produce a linear phase gradient with a uniform B + 1 amplitude. With this configuration, TRASE plays out trains of block-refocusing pulses, alternating between the different coil modes to encode single points in k-space.…”

“…Several different groups have proposed similar techniques exploiting unique coil geometries to create more favorable fields for encoding spatial information without B 0 gradients 38,39 . For example, TRASE uses specific coil modes to produce a linear phase gradient with a uniform $$$ {\mathrm{B}}_1^{+} $$$ amplitude.…”

A Short and Light, Sparse Dipolar Halbach Magnet for MRI

“…Therefore, TRASE removes the need for the entire B0 gradient system and, additionally, relaxes the main magnet homogeneity requirement, potentially increasing the compact and portable nature of current low-cost LF MRI systems. Recent developments of TRASE hardware enabled sub-millimeter 1D resolution (0.33 mm/pixel) in ~ 8.5 minutes using a conventional 0.2 T bi-planar permanent magnet with a twisted-solenoid RF coil set [33,34], a custom-made multi-channel RF amplifier [35], improved image reconstruction techniques [36][37][38] and a custom MRI console [39]. As seen above, the development of MRI technology enables new magnet designs.…”

A PXIe Based Scalable MRI Spectrometer for Inhomogeneous Imaging

“…To avoid using a gradient power amplifier for the third dimension encoding, we intended to use the transmit RF (B + 1 ) phase encoding. The phase gradient is produced by the TRansmit Array Spatial Encoding coil array (TRASE) [37][38][39][40][41], which consists of two nested cylindrical coils: a birdcage coil and a Maxwell coil, as shown in Figure 2.9 (a). The equation in Figure 2.9 (b) illustrates that the coils can be designed to approximate the cosine and sine for a linear slope.…”