Gap cycling for SWIFT.

PURPOSE: SWIFT (SWeep Imaging with Fourier Transformation) is a non-Cartesian MRI method with unique features and capabilities. In SWIFT, radiofrequency (RF) excitation and reception are performed nearly simultaneously, by rapidly switching between transmit and receive during a frequency-swept RF pulse. Because both the transmitted pulse and data acquisition are simultaneously amplitude-modulated in SWIFT (in contrast to continuous RF excitation and uninterrupted data acquisition in more familiar MRI sequences), crosstalk between different frequency bands occurs in the data. This crosstalk leads to a "bulls-eye" artifact in SWIFT images. We present a method to cancel this interband crosstalk by cycling the pulse and receive gap positions relative to the un-gapped pulse shape. We call this strategy "gap cycling." THEORY AND METHODS: We carry out theoretical analysis, simulation and experiments to characterize the signal chain, resulting artifacts, and their elimination for SWIFT. RESULTS: Theoretical analysis reveals the mechanism for gap-cycling's effectiveness in canceling interband crosstalk in the received data. We show phantom and in vivo results demonstrating bulls-eye artifact free images. CONCLUSION: Gap cycling is an effective method to remove bulls-eye artifact resulting from interband crosstalk in SWIFT data.

Automated tuning of an eight-channel cardiac transceive array at 7 tesla using piezoelectric actuators.

PURPOSE: Ultra-high field (UHF) MR scanning in the body requires novel coil designs due to B1 field inhomogeneities. In the transverse electromagnetic field (TEM) design, maximum B1 transmit power can only be achieved if each individual transmit element is tuned and matched for different coil loads, which requires a considerable amount of valuable scanner time. METHODS: An integrated system for autotuning a multichannel parallel transmit (pTx) cardiac TEM array was devised, using piezoelectric actuators, power monitoring equipment and control software. The reproducibility and performance of the system were tested and the power responses of the coil elements were profiled. An automated optimization method was devised and evaluated. RESULTS: The time required to tune an eight-element pTx cardiac RF array was reduced from a mean of 30 min to less than 10 min with the use of this system. CONCLUSION: Piezoelectric actuators are an attractive means of tuning RF coil arrays to yield more efficient B1 transmission into the subject. An automated mechanism for tuning these elements provides a practical solution for cardiac imaging at UHF, bringing this technology closer to clinical use.

High-spatial- and high-temporal-resolution dynamic contrast-enhanced MR breast imaging with sweep imaging with Fourier transformation: a pilot study.

PURPOSE: To report the results of sweep imaging with Fourier transformation (SWIFT) magnetic resonance (MR) imaging for diagnostic breast imaging. MATERIALS AND METHODS: Informed consent was obtained from all participants under one of two institutional review board-approved, HIPAA-compliant protocols. Twelve female patients (age range, 19-54 years; mean age, 41.2 years) and eight normal control subjects (age range, 22-56 years; mean age, 43.2 years) enrolled and completed the study from January 28, 2011, to March 5, 2013. Patients had previous lesions that were Breast Imaging Reporting and Data System 4 and 5 based on mammography and/or ultrasonographic imaging. Contrast-enhanced SWIFT imaging was completed by using a 4-T research MR imaging system. Noncontrast studies were completed in the normal control subjects. One of two sized single-breast SWIFT-compatible transceiver coils was used for nine patients and five controls. Three patients and five control subjects used a SWIFT-compatible dual breast coil. Temporal resolution was 5.9-7.5 seconds. Spatial resolution was 1.00 mm isotropic, with later examinations at 0.67 mm isotropic, and dual breast at 1.00 mm or 0.75 mm isotropic resolution. RESULTS: Two nonblinded breast radiologists reported SWIFT image findings of normal breast tissue, benign fibroadenomas (six of six lesions), and malignant lesions (10 of 12 lesions) concordant with other imaging modalities and pathologic reports. Two lesions in two patients were not visualized because of coil field of view. The images yielded by SWIFT showed the presence and extent of known breast lesions. CONCLUSION: The SWIFT technique could become an important addition to breast imaging modalities because it provides high spatial resolution at all points during the dynamic contrast-enhanced examination.

Stepped impedance resonators for high-field magnetic resonance imaging.

Multi-element volume radio-frequency (RF) coils are an integral aspect of the growing field of high-field magnetic resonance imaging. In these systems, a popular volume coil of choice has become the transverse electromagnetic (TEM) transceiver coil consisting of microstrip resonators. In this paper, to further advance this design approach, a new microstrip resonator strategy in which the transmission line is segmented into alternating impedance sections, referred to as stepped impedance resonators (SIRs), is investigated. Single-element simulation results in free space and in a phantom at 7 T (298 MHz) demonstrate the rationale and feasibility of the SIR design strategy. Simulation and image results at 7 T in a phantom and human head illustrate the improvements in a transmit magnetic field, as well as RF efficiency (transmit magnetic field versus specific absorption rate) when two different SIR designs are incorporated in 8-element volume coil configurations and compared to a volume coil consisting of microstrip elements.

Inversion recovery at 7 T in the human myocardium: measurement of T(1), inversion efficiency and B(1) (+).

At clinical MRI field strengths (1.5 and 3 T), quantitative maps of the longitudinal relaxation time T1 of the myocardium reveal diseased tissue without requiring contrast agents. Cardiac T1 maps can be measured by Look-Locker inversion recovery sequences such as ShMOLLI at 1.5 and 3 T. Cardiovascular MRI at a field strength of 7 T has recently become feasible, but doubts have remained as to whether magnetization inversion is possible in the heart due to subject heating and technical limitations. This work extends the repertoire of 7 T cardiovascular MRI by implementing an adiabatic inversion pulse optimized for use in the heart at 7 T. A "ShMOLLI+IE" adaptation of the ShMOLLI pulse sequence has been introduced together with new postprocessing that accounts for the possibility of incomplete magnetization inversion. These methods were validated in phantoms and then used in a study of six healthy volunteers to determine the degree of magnetization inversion and the T1 of normal myocardium at 7 T within a 22-heartbeat breathhold. Using a scanner with 16 × 1 kW radiofrequency outputs, inversion efficiencies ranging from -0.79 to -0.83 (intrasegment means; perfect 180° would give -1) were attainable across the myocardium. The myocardial T1 was 1925 ± 48 ms (mean ± standard deviation).

Performance of external and internal coil configurations for prostate investigations at 7 T.

Three different coil configurations were evaluated through simulation and experimentally to determine safe operating limits and evaluate subject size-dependent performance for prostate imaging at 7 T. The coils included a transceiver endorectal coil (trERC), a 16-channel transceiver external surface array (trESA) and a trESA combined with a receive-only ERC (trESA+roERC). Although the transmit B(1) (B 1+) homogeneity was far superior for the trESA, the maximum achievable B 1+ is subject size dependent and limited by transmit chain losses and amplifier performance. For the trERC, limitations in transmit homogeneity greatly compromised image quality and limited coverage of the prostate. Despite these challenges, the high peak B 1+ close to the trERC and subject size-independent performance provides potential advantages especially for spectroscopic localization where high-bandwidth radiofrequency pulses are required. On the receive side, the combined trESA+roERC provided the highest signal-to-noise ratio and improved homogeneity over the trERC resulting in better visualization of the prostate and surrounding anatomy. In addition, the parallel imaging performance of the trESA+roERC holds strong promise for diffusion-weighted imaging and dynamic contrast-enhanced MRI.

A 32-channel lattice transmission line array for parallel transmit and receive MRI at 7 tesla.

Transmit and receive RF coil arrays have proven to be particularly beneficial for ultra-high-field MR. Transmit coil arrays enable such techniques as B(1) (+) shimming to substantially improve transmit B(1) homogeneity compared to conventional volume coil designs, and receive coil arrays offer enhanced parallel imaging performance and SNR. Concentric coil arrangements hold promise for developing transceiver arrays incorporating large numbers of coil elements. At magnetic field strengths of 7 tesla and higher where the Larmor frequencies of interest can exceed 300 MHz, the coil array design must also overcome the problem of the coil conductor length approaching the RF wavelength. In this study, a novel concentric arrangement of resonance elements built from capacitively-shortened half-wavelength transmission lines is presented. This approach was utilized to construct an array with whole-brain coverage using 16 transceiver elements and 16 receive-only elements, resulting in a coil with a total of 16 transmit and 32 receive channels.

Whole-body imaging at 7T: preliminary results.

The objective of this study was to investigate the feasibility of whole-body imaging at 7T. To achieve this objective, new technology and methods were developed. Radio frequency (RF) field distribution and specific absorption rate (SAR) were first explored through numerical modeling. A body coil was then designed and built. Multichannel transmit and receive coils were also developed and implemented. With this new technology in hand, an imaging survey of the "landscape" of the human body at 7T was conducted. Cardiac imaging at 7T appeared to be possible. The potential for breast imaging and spectroscopy was demonstrated. Preliminary results of the first human body imaging at 7T suggest both promise and directions for further development.

Multinuclear NMR investigation of probe construction materials at 9.4T.

This work investigates probe construction materials for their signal contribution to ultrashort echo time spectroscopy and imaging. (1)H, (13)C, and (31)P spectra were obtained at a field strength of 9.4 T for 16 materials considered for use in probe and holder design and construction. Four of the materials were found to be suited for the construction of NMR probes, housing of RF coils, and holders for in vivo experiments.

A geometrically adjustable 16-channel transmit/receive transmission line array for improved RF efficiency and parallel imaging performance at 7 Tesla.

A novel geometrically adjustable transceiver array system is presented. A key feature of the geometrically adjustable array was the introduction of decoupling capacitors that allow for automatic change in capacitance dependent on neighboring resonant element distance. The 16-element head array version of such an adjustable coil based on transmission line technology was compared to fixed geometry transmission line arrays (TLAs) of various sizes at 7T. The focus of this comparison was on parallel imaging performance, RF transmit efficiency, and signal-to-noise ratio (SNR). Significant gains in parallel imaging performance and SNR were observed for the new coil and attributed to its adjustability and to the design of the individual elements with a three-sided ground plane.

Effects of urinary macromolecules on hydroxyapatite crystal formation.

Particle size analysis was combined with titration data obtained in constant-composition, hydroxyapatite (HA)-seeded, crystal growth assays. With addition of large amounts of HA (250 microg), titration rates were linear, new crystal formation was minimal, and aggregation effects could be detected. With addition of small amounts of HA (62.5 microg), nucleation of new HA was observed. The effects of urinary macromolecules, i.e., osteopontin (OPN), recombinant glutathione-S-transferase-OPN (G-OPN), Tamm-Horsfall protein, chondroitin sulfate, human serum albumin, mixed urinary macromolecules from a stone-former (SFU1), mixed urinary macromolecules from a normal individual (NU1), and polyaspartic acid (PA), were examined in this system. Crystal growth inhibition, as measured by the slope of linear titration curves in this system, was observed with PA, G-OPN, OPN, SFU1, and NU1. All of the macromolecules tested inhibited aggregation, including Tamm-Horsfall protein, which did not inhibit growth. As reflected by the ratio of the final number of particles to the initial number in the 62.5-microg seed addition, the macromolecules that were most effective in inhibiting growth, i.e., OPN, G-OPN, PA, SFU1, and NU1, actually increased secondary nucleation. Recombinant G-OPN demonstrated less inhibitory activity than did OPN isolated from cell culture. Chondroitin sulfate and human serum albumin exhibited no significant effects on the various components of HA crystallization under these conditions. SFU1 and NU1 slowed growth and increased secondary nucleation to similar degrees, and neither exhibited any measurable effect on aggregation. Therefore, crystal surface sites that participate in nucleation, growth, and aggregation processes are affected independently by macromolecules, presumably because of differences in their structural features. These results illustrate the utility of combining these techniques to provide a much greater understanding of crystallization behavior than that possible with either analysis alone.