Transmit Array Spatial Encoding (TRASE) is an MRI technique that utilizes multiple radiofrequency (RF) phase-gradient fields (B1) for k-space encoding. Here, we demonstrate that the recently developed Radial TRASE variant mitigates the primary limitation of earlier versions, which required very high isolation between RF coil elements. Whereas earlier 2D Cartesian TRASE used three or four RF coils, 2D Radial TRASE requires only two coils, together with mechanical rotation. The reduced coil count and 1D nature of Radial TRASE encoding contribute to a simpler implementation, easier reconstruction, and reduced artifacts. Motivated by encouraging early experimental results, this study systematically evaluated Radial TRASE under imperfect B1 field conditions to establish practical design criteria for the RF coils and their isolation. Bloch-equation-based simulations were performed using ideal twisted solenoid (TS) phase-gradient fields. The TS geometry was varied to quantify the trade-off between increasing the phase-gradient strength (and thus resolution) and decreasing B1 uniformity. Coil coupling was modelled by varying the current amplitude within the idle coil relative to the active coil. Reconstructed images were compared to a reference using the structural similarity index (SSIM). Simulations indicate a modulation-to-radius ratio (A/a) of approximately 0.7 is optimal for TS coil design with a refocusing flip-angle error within ±15°. Under these conditions, isolation levels between -20 and -16.5 dB are shown to produce minimal Radial TRASE distortions, whereas prior studies indicate -30 dB is required for three-coil Cartesian TRASE. This represents a far more achievable isolation in practice, reinforcing the practicality of Radial TRASE imaging.

PURPOSE:Two-dimensional (2D) transmit array spatial encoding (TRASE) previously required four radiofrequency fields; however, interactions between transmit (Tx) array elements caused significant challenges for 2D imaging. Here, we present a low-cost, 2D radial encoding scheme (Radial TRASE) using a simplified two-coil array. THEORY AND METHODS:The system consists of two B1 phase gradient coils capable of encoding any one transverse direction. By incremental mechanical rotation over a 90° range, the encoding axis can be changed, allowing a complete radial k-space acquisition. As a first demonstration, a wrist-sized coil pair was experimentally verified on a 2-MHz Halbach magnet, incorporating a static B0 slice-selection gradient. Although a high level of isolation is achievable geometrically, for a more robust implementation, we demonstrate the capability of active digital decoupling in eliminating residual coupling through a parallel-transmit system. RESULTS:Radial TRASE-encoded images of water phantoms were acquired, achieving a resolution better than 1.67 mm. Rotation of the Tx array was performed during the recovery period, which caused no imaging delays. All acquired images show minimal distortions, indicating the advantage of the simplified Tx array. The active digital decoupling technique is demonstrated to eliminate residual coupled currents, effectively increasing the isolation of the two-coil array to -50 dB. Sequential axial slice images were demonstrated using a uniform B0 coil to shift the slice position. CONCLUSION:Two-coil Radial TRASE can encode a 2D slice without rapidly switched B0 gradients. Compared with previous three-coil or four-coil Cartesian TRASE, the design and isolation of the Tx array are significantly simplified.

Astronauts experience detrimental loss to bone and muscle during long-term space flight. MRI is desired to monitor bone and muscle loss, but current portable systems remain unsuitable for the International Space Station (ISS). We built a 25 kg dipolar Halbach magnet dedicated to the transmit array spatial encoding (TRASE) method over a cylindrical region of interest (ROI), which satisfies the requirements for MRI on the ISS. Still, the natural axial gradient is not well suited to slice selection because it has a quadratic-type profile producing unwanted aliasing. We propose a technique to implement a linear axial B 0 gradient for encoding.

TRASE is an MR imaging sequence that achieves k-space encoding through the use of phase gradients in the RF transmit field. The twisted solenoid is the most efficient RF transmit coil used for TRASE encoding; however, the geometry results in a long coil with a relatively short imaging volume. We introduce a new truncated design for the twisted solenoid to increase the usable imaging volume relative to the coil’s size. Simulation studies conducted indicate the truncated design can create similar imaging volumes to the untruncated version whilst significantly reducing the coils length by as much as a half.

Recently designed dipolar Halbach magnets used in portable MRI systems are much lighter and more compact than standard permanent or superconductive magnets. However, improved designs and manufacturing techniques aiming at lower weight and smaller external size are an area of continual interest especially for application to space flight. Most Halbach magnet design techniques aim to optimize homogeneity suitable for MRI over a diameter-spherical volume (DSV) that requires the aspect ratio (length/inner diameter) to be larger than 1.5:1. Furthermore, current magnet construction techniques often use low-coercivity magnetic pieces and imperfect formers that produce a mismatch in the intended designs. As a result, Halbach magnets require complex shimming methods to improve the magnetic field homogeneity, causing further size and weight increase. Here, we propose to reduce the weight and the aspect ratio of the Halbach magnet by optimizing homogeneity over a cylindrical region of interest (ROI) rather than a DSV, applying a genetic algorithm, high-coercivity ferromagnets (N40UH) and a robust construction technique. The assembled 67 mT magnet, with aspect ratio ~ 1:1, produces almost identical homogeneity (11152 ppm) as simulations (11451 ppm) within a 12.7 cm diameter, 1 cm long cylinder ROI. The magnet structure was 3D printed ring-by-ring and assembled coaxially. The magnet can be disassembled for transportation.

Transmit Array Spatial Encoding (TRASE) is a novel MRI technique that encodes spatial information by introducing phase gradients in the transmit RF (B1) magnetic field. Since TRASE relies on the use of multiple RF fields (B1 fields with different phase gradients) for k-space traversal, a TRASE pulse sequence requires RF pulses that are produced by switching between the transmit coils (B1 fields). However, interactions among the transmit RF coils can cause un-driven coils to produce unwanted B1 fields that impair the spatial encoding. Therefore, TRASE is sensitive to B1 field perturbations arising from inductive coupling among the RF transmit coils and any B1 field isolation (coil decoupling) technique requires an understanding of the effects of the B1 field interactions. The purpose of this study was to investigate the effects of B1 field coupling using Bloch equation based simulations and to determine the acceptable level of B1 field interactions for 2D TRASE imaging. The simulations show that 2D TRASE MRI (using a 3-coil setup) displays ideal performance for pairwise coupling constant lower than k = 0.01 while having acceptable performance up to k = 0.1. This translates into S12 measurements of range ~(- 50 dB to -30 dB) required for successful 2D TRASE MRI in this study. This result is of crucial importance for designers of practical TRASE transmit array systems.

A radiofrequency (RF) power amplifier is an essential component of any magnetic resonance imaging (MRI) system. Unfortunately, no commercial amplifier exists to fulfill the needs of the transmit array spatial encoding (TRASE) MRI technique, requiring high duty cycle, high RF output power and independently controlled multi-channel capability. Thus, an RF amplifier for TRASE MRI is needed. A dual-channel RF power amplifier dedicated for TRASE at 0.22 T (9.27 MHz) was designed and constructed using commercially available components. The amplifier was tested on the bench and used a 0.22 T MRI system with a twisted solenoid and saddle RF coil combination capable of a single-axis TRASE. The amplifier is capable of sequential, dual-channel operation up to 50% duty cycle, 1 kW peak output and highly stable 100 μs RF pulse trains. High spatial resolution one-dimensional TRASE was obtained with the power amplifier to demonstrate its capability. The constructed amplifier is the first prototype that meets the requirements of TRASE rectifying limitations of duty cycle and timing presented by commercial RF amplifiers. The amplifier makes possible future high resolution in vivo TRASE MRI.

PurposeTRASE uses phase gradients in the RF transmit field to encode MRI data. A highly efficient twisted solenoid coil was proposed recently for TRASE imaging for transverse 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.MethodsHere, 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.ResultsThe maximum resulting magnitude and phase distortions were 3.7% and , while a good isolation 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 samples to be accurately imaged.ConclusionsThis 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.

Transmit Array Spatial Encoding (TRASE) is an MRI technique that uses radio-frequency (RF) magnetic field (B1) phase gradients for spatial encoding. A TRASE pulse sequence consists of a long echo train in which each echo samples a different k-space point. Due to the need for accurate refocusing, TRASE imaging performance depends on |B1| homogeneity. Although the CPMG echo train is often relied on to provide immunity against B1 flip angle errors, this does not apply to TRASE echo trains. Due to the spatially dependent B1 phases involved in TRASE imaging, the CPMG condition, where all spins flip about the y-axis in the rotating frame, can only be achieved at one single location within the sample. Moreover, CPMG only preserves one component of the transverse magnetization, the y-component, whereas TRASE requires both components to be retained. Here we investigate the performance of a set of variants of a 1-dimensional (1D) TRASE sequence under conditions of |B1| errors. We varied the B1 transmit pulse RF waveform phases in an effort to optimize the TRASE imaging point spread function (PSF). The performance of 256 sequence variants, including those previously reported in the literature was studied. Both Bloch equation simulations and experimental confirmations were completed. Off-resonance (B0 inhomogeneity) effects were not considered so that the effects of B1 inhomogeneity alone could be understood. Results show that, using optimum transmit pulse phases, high quality image encoding is achievable over ∼90% of the Nyquist field-of-view (FOV) for a practically realizable variation in B1 amplitude (Δ|B1|⩽±11%). This improves significantly upon the performance of a previously-reported sequence which generated ∼75% usable FOV within the Nyquist FOV.

The nonlocality of entangled quantum mechanical systems is incompatible with the standard interpretation of special relativity as a single 4D Minkowskian metric spacetime. The difficulty is that the definition of a spacetime interval between any pair of events precludes any form of nonlocal interaction, even the relatively benign non-signaling correlations. By an application of the relativity principle, and the use of the space ←→ time symmetry of the Lorentz boost I propose here a reinterpretation of special relativistic spacetime. This new ontology consists of a set of coexisting 3+1D spaces (‘framespaces’), each containing unique content in the form of a complex density. These spaces are related by the Lorentz boost, and coupled pairwise in a manner dictated by the Lorentz transformation. The inter-space coupling acting on the spacetime content gives rise to a nonlocal wave phenomenon, which is identified as quantum wave mechanics. The interspace coupling strength is then inversely proportional to Planck’s constant. The coexistence of multiple spaces is interpreted as momentum superposition, implying that momentum is the fundamental physical basis of quantum superposition. This new spacetime interpretation of quantum mechanics has many consequences, including explanations of quantum non-locality, the spacetime role of Planck’s constant, quantum measurement as a symmetry-breaking process and the redundancy of description of gauge theory.

Some MRI applications require the generation of a series high power RF pulses in which the spatial transmit B1 field pattern over the sample is modified between one pulse and the next. This requirement may be realized by a RF transmit array with the capability to enable and disable individual elements to switch between field patterns with switching times of the order 10 mu s. Our application is for a TRASE ("Transmit Array Spatial Encoding") array for which short high power pulses are necessary to achieve high resolution (mm-level) spatial encoding. We present designs for coil array, coil switching circuits, and a high power PIN diode driver together capable of robust and rapid switching of short (similar to 120 mu s) high power pulses for a 24 cm TRASE phase gradient coil suitable for imaging extremities at 8 MHz. We describe in detail the selection of suitable coil components and switch circuit designs to satisfy a specific requirement for maximum B1 field strength, and provide all circuit designs.

TRASE is an MRI k-space encoding method that uses radio-frequency (RF or B1) transmit phase gradient fields to achieve millimeter-level spatial resolution. Image quality is critically dependent upon the efficient generation of B1 fields with uniform magnitude and strong phase gradients. We present the design of a new family of phase gradient transmit coil based upon a solenoid twisted about a transverse axis. This design has many attractive geometric, electrical and magnetic characteristics, including the capability to spatially encode in the direction of the main static B0 field without obstructing access to the bore. Analytical, numerical simulation and experimental results are presented, including demonstration of 1-dimensional TRASE encoding without the use of PIN diode switches. Twisted solenoid coils significantly expand the capabilities of TRASE MRI.

Since the Lorentz boost is symmetric under exchange of x and ct, special relativistic phenomena will also manifest this symmetry. Firstly, simultaneity becomes paired with 'colocality' ('at the same place'), and the 'Relativity of Colocality' becomes the dual to the well-known 'Relativity of Simultaneity'. Further, Lorentz time contraction arises from reversal of the observation conditions pertaining to time dilation, expressible figuratively as 'Moving clocks run slow, but moving time runs fast'. Symmetry also dictates that the most fundamental observational modes are: (1) the simultaneous observation of length, a process involving both the relativity of simultaneity and length contraction; and (2) the colocal measurement of duration, involving both the relativity of colocality and time contraction. Only the first of these modes is well known. The adoption of this symmetrical lexicon provides a necessary logical basis for interpretational studies of observation and measurement in special relativity.

ABSTRACTBreast cancer is the most frequently diagnosed cancer in women. High field studies have shown the diagnostic value of breast MRI, but the examination costs greatly exceed those of competing conventional mammography. Low field MRI offers typical MRI contrast at substantially lower cost, but has suffered from lower spatial resolution. Specificity of breast MRI can potentially be increased by acquiring MR imaging with higher spatial or temporal resolution, but the signal‐to‐noise ratio (SNR) achievable in a given imaging time becomes limiting. SNR for the particular pulse sequence and magnet field strength is strongly influenced by the characteristics of the radio‐frequency coil. An optimal breast coil should yield excellent SNR but also generate a homogeneous B1 field, while allowing imaging of the both breasts simultaneously and maintaining patient comfort. RF receiver coil design is a key determinant of image quality, thus to address this we have designed and constructed a low field breast imaging coil. The coil was tested with a 4‐post 0.2T MRI providing high quality breast images. Designed and constructed saddle rf coil allows to obtain good quality image of the breast using low 0.2 T MRI system within 2 minutes. The coil provides patient comfort as breast compression is not required and minimizes artefacts caused by respiration or motion. A high contrast, low‐cost and pain‐free breast examination using optimized low field MRI system has the potential to serve a large patient population for whom current technologies have deficiencies. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 46B: 3–7, 2016

PurposeThe numerical feasibility of reconstructing MRI signals generated by RF coils that produce B1 fields with a non-linearly varying spatial phase is explored.TheoryA global linear spatial phase variation of B1 is difficult to produce from current confined to RF coils. Here we use regularized least squares inversion, in place of the usual Fourier transform, to reconstruct signals generated in B1 fields with non-linear phase variation.MethodsRF encoded signals were simulated for three RF coil configurations: ideal linear, parallel conductors and, circular coil pairs. The simulated signals were reconstructed by Fourier transform and by regularized least squares.ResultsThe Fourier reconstruction of simulated RF encoded signals from the parallel conductor coil set showed minor distortions over the reconstruction of signals from the ideal linear coil set but the Fourier reconstruction of signals from the circular coil set produced severe geometric distortion. Least squares inversion in all cases produced reconstruction errors comparable to the Fourier reconstruction of the simulated signal from the ideal linear coil set.ConclusionMRI signals encoded in B1 fields with non-linearly varying spatial phase may be accurately reconstructed using regularized least squares thus pointing the way to the use of simple RF coil designs for RF encoded MRI.

Cylindrical quadrature radio frequency (RF) coils are widely used in magnetic resonance imaging and spectroscopy due to their high sensitivity and field uniformity. However, the field geometry is unsuitable for use in low-field open magnetic resonance imaging (MRI) systems with vertical B-0 field configurations. Therefore, a new design is proposed. A quadrature RF coil that combines Alderman-Grant and Helmholtz designs was constructed to produce two independent modes, both orthogonal to the main magnetic field. The coil provides good RF homogeneity over a 20 x 15 x 15 cm volume and operates as both a transmit and receive coil. The application of the coil for 0.2 Tesla permanent magnet with a vertical B-0 field is shown. The proposed coil may be applied to MR imaging of larger objects at low vertical magnetic fields.

BACKGROUND:Glioma is the most common and most difficult to treat brain cancer. Despite many efforts treatment, efficacy remains low. As neurosurgical removal is the standard procedure for glioma, a method, allowing for both early detection and exact determination of the location, size and extent of the tumor, could improve a patient's positive response to therapy. NEW METHOD:We propose application of susceptibility weighted molecular magnetic resonance imaging using, targeted contrast agents, based on superparamagnetic iron oxide nanoparticles, for imaging of the, glioma rim, namely brain-tumor interface. Iron oxide attached to the targeted cells increases, susceptibility differences at the boundary between tumor and normal tissue, providing the opportunity, to utilize susceptibility weighted imaging for improved tumor delineation. We investigated potential, enhancement of the tumor-brain contrast, including tumor core and rim when using susceptibility, weighted MRI for molecular imaging of glioma. RESULTS:There were significant differences in contrast-to-noise ratio before, 12 and 120min after contrast, agent injection between standard gradient echo pulse sequence and susceptibility weighted molecular, magnetic resonance imaging for the core-brain, tumor rim-core and tumor rim-brain areas. COMPARISON WITH EXISTING METHODS:Currently, the most common MRI contrast agent used for glioma diagnosis is a non-specific, gadolinium-based agent providing T1-weighted enhancement. Susceptibility-weighted magnetic, resonance imaging is much less efficient when no targeted superparamagnetic contrast agents are, used. CONCLUSION:The improved determination of glioma extent provided by SWI offers an important new tool for, diagnosis and surgical planning.

Purpose: TRASE (Transmit Array Spatial Encoding) MRI uses RF transmit phase gradients instead of B-0 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 0.2 T vertical B-0 field MRI system, using a single transmitter channel. This requires the generation of two transmit B-1 RF fields with uniform amplitude and positive and negative linear phase gradients respectively over the imaging volume.Materials and Methods: A 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.Results: Field 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.Conclusion: The 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. (c) 2013 Elsevier Inc. All rights reserved.