An asymmetrical whole-body birdcage RF coil without RF shield for hyperpolarized Xe lung MR imaging at 1.5 T.

Purpose: This study describes the development and testing of an asymmetrical xenon-129 ( Xe) birdcage radiofrequency (RF) coil for Xe lung ventilation imaging at 1.5 Tesla, which allows proton ( H) system body coil transmit-receive functionality.

Methods: The Xe RF coil is a whole-body asymmetrical elliptical birdcage constructed without an outer RF shield to enable H imaging.

Impact of a parallel magnetic field on radiation dose beneath thin copper and aluminum foils.

Purpose: The RF coils for magnetic resonance image guided radiotherapy (MRIgRT) may be constructed using thin and/or low-density conductors, along with thinner enclosure materials. This work measures the surface dose increases for lightweight conductors and enclosure materials in a magnetic field parallel to a 6 MV photon beam.

Methods: Aluminum and copper foils (9-127 μm thick), as well as samples of polyimide (17 μm) and polyester (127 μm) films are positioned atop a polystyrene phantom.

How thin can you go? Performance of thin copper and aluminum RF coil conductors.

Purpose: To evaluate the impact of emerging conductor technology on RF coils. Performance and resulting image quality of thin or alternate conductors (eg, aluminum instead of copper) and thicknesses (9-600 μm) are compared in terms of SNR.

Methods: Eight prototype RF coils (15 cm × 15 cm square loops) were constructed and bench-tested to measure quality factor.

Optimal-permittivity Dielectric Liners for a 4.7T Transceiver Array.

Placing dielectric pads adjacent to the imaging region is an effective method to increase the signal locally and also increase the radio frequency magnetic field homogeneity in magnetic resonance imaging. The use of local high permittivity pads is becoming more common, and this work focuses on the effect of larger dielectric pads on the transmit/receive performance of an array (e.g.

The noise factor of receiver coil matching networks in MRI.

In typical MRI applications the dominant noise sources in the received signal are the sample, the coil loop and the preamplifier. We hypothesize that in some cases (e.g.

Neurochemical profile of the human cervical spinal cord determined by MRS.

MRS enables insight into the chemical composition of central nervous system tissue. However, technical challenges degrade the data quality when applied to the human spinal cord. Therefore, to date detection of only the most prominent metabolite resonances has been reported in the healthy human spinal cord.

CNR considerations for rapid real-time MRI tumor tracking in radiotherapy hybrid devices: Effects of B0 field strength.

Purpose: This work examines the subject of contrast-to-noise ratio (CNR), specifically between tumor and tissue background, and its dependence on the MRI field strength, B0. This examination is motivated by the recent interest and developments in MRI/radiotherapy hybrids where real-time imaging can be used to guide treatment beams. The ability to distinguish a tumor from background tissue is of primary importance in this field, and this work seeks to elucidate the complex relationship between the CNR and B0 that is too often assumed to be purely linear.

SNR efficiency of combined bipolar gradient echoes: Comparison of three-dimensional FLASH, MPRAGE, and multiparameter mapping with VFA-FLASH and MP2RAGE.

Purpose: High-bandwidth bipolar multiecho gradient echo sequences are increasingly popular in structural brain imaging because of reduced water-fat shifts, lower susceptibility effects, and improved signal-to-noise ratio (SNR) efficiency. In this study, we investigated the performance of three three-dimensional multiecho sequences (MPRAGE, MP2RAGE, and FLASH) with scan times < 9 min and 1-mm isotropic resolution against their single-echo, low-bandwidth counterparts at 3T. We also compared the performance of multiparameter mapping (PD, T , and T2*) with bipolar multiecho MP2RAGE versus the variable flip angle technique with multiecho FLASH (VFA-FLASH).

Analytical corrections of banding artifacts in driven equilibrium single pulse observation of T2 (DESPOT2).

Purpose: DESPOT2 is a single-component T mapping technique based on bSSFP imaging. It has seen limited application because of banding artifacts and magnetization transfer (MT) effects. In this work, acquisitions are optimized to minimize MT effects, while exact and approximate analytical equations enable automatic correction of banding artifacts within the T maps in mere seconds.

Algebraic method to synthesize specified modal currents in ladder resonators: Application to noncircular birdcage coils.

Purpose: Detectors such as birdcage coils often consist of networks of coupled resonant circuits that must produce specified magnetic field distributions. In many cases, such as quadrature asymmetric insert body coils, calculating the capacitance values required to achieve specified currents and frequencies simultaneously is a challenging task that previously had only approximate or computationally inefficient solutions.

Theory And Methods: A general algebraic method was developed that is applicable to linear networks having planar representations such as birdcage coils, transverse electromagnetic (TEM) coils, and numerous variants of ladder networks.

Experimental verification of SNR and parallel imaging improvements using composite arrays.

Composite MRI arrays consist of triplets where two orthogonal upright loops are placed over the same imaging area as a standard surface coil. The optimal height of the upright coils is approximately half the width for the 7 cm coils used in this work. Resistive and magnetic coupling is shown to be negligible within each coil triplet.

A system for automated noise parameter measurements on MR preamplifiers and application to high B(0) fields.

A noise figure and noise parameter measurement system was developed that consists of a combination spectrum and network analyzer, preamplifier, programmable power supply, noise source, tuning board, and desktop computer. The system uses the Y-factor method for noise figure calculation and allows calibrations to correct for a decrease in excess noise ratio between the noise source and device under test, second stage (system) noise, ambient temperature variations, and available gain of the device under test. Noise parameters are extracted by performing noise figure measurements at several source impedance values obtained by adjusting an electronically controlled tuner.

A dual-tuned transceive resonator for (13) C{(1) H} MRS: two open coils in one.

Proton-decoupled, (13) C nuclear MRS experiments require a RF coil that operates at the Larmor frequencies of both (13) C and (1) H. In this work, we designed, built and tested a single-unit, dual-tuned coil based on a half-birdcage open coil design. It was constructed as a low-pass network with a resonant trap in series with each leg.

Stretchable coil arrays: application to knee imaging under varying flexion angles.

To fit high-density receiver arrays for MRI closely around individual target anatomies, there is a need to provide a high degree of geometric adjustability with ease of handling and patient comfort. In this work, this is accomplished by the construction of a coil array that is stretchable such that it automatically conforms to a given anatomy's shape and size. Stretchability is implemented by creating the coil conductors from braided wire mounted on an elastic textile substrate.

Noise figure characterization of preamplifiers at NMR frequencies.

A method for characterizing the noise figure of preamplifiers at NMR frequencies is presented. The noise figure of preamplifiers as used for NMR and MRI detection varies with source impedance and with the operating frequency. Therefore, to characterize a preamplifier's noise behavior, it is necessary to perform noise measurements at the targeted frequency while varying the source impedance with high accuracy.

In-vivo assessment of tissue metabolite levels using 1H MRS and the Electric REference To access In vivo Concentrations (ERETIC) method.

Quantitative values of metabolite concentrations in (1)H magnetic resonance spectroscopy have been obtained using the Electric REference To access In vivo Concentrations (ERETIC) method, whereby a synthetic reference signal is injected during the acquisition of spectra. The method has been improved to enable quantification of metabolite concentrations in vivo. Optical signal transmission was used to eliminate random fluctuations in ERETIC signal coupling to the receiver coil due to changes in position of cables and highly dielectric human tissue.

Radio frequency shielding for a linac-MRI system.

The real-time operation of a linac-MRI system will require proper radio frequency (RF) shielding such that the MRI images can be acquired without extraneous RF noise from the linac. We report on the steps taken to successfully shield the linac from the MRI such that the two devices can operate independently of one another. RF power density levels are reported internally and externally to the RF cage which houses the linac and MRI.

A transmit/receive system for magnetic field monitoring of in vivo MRI.

Magnetic field monitoring with NMR probes has recently been introduced as a means of measuring the actual spatiotemporal magnetic field evolution during individual MR scans. Receive-only NMR probes as used thus far for this purpose impose significant practical limitations due to radiofrequency (RF) interference with the actual MR experiment. In this work these limitations are overcome with a transmit/receive (T/R) monitoring system based on RF-shielded NMR probes.

Travelling-wave nuclear magnetic resonance.

Nuclear magnetic resonance (NMR) is one of the most versatile experimental methods in chemistry, physics and biology, providing insight into the structure and dynamics of matter at the molecular scale. Its imaging variant-magnetic resonance imaging (MRI)-is widely used to examine the anatomy, physiology and metabolism of the human body. NMR signal detection is traditionally based on Faraday induction in one or multiple radio-frequency resonators that are brought into close proximity with the sample.

Mechanically adjustable coil array for wrist MRI.

In this work, the concept of mechanically adjustable MR receiver coil arrays is proposed and implemented for the specific case of human wrist imaging. An eight-channel wrist array for proton MRI at 3 Tesla was constructed and evaluated. The array adjusts to the individual anatomy by a mechanism that fits a configuration of flexible coil elements closely around the wrist.

Asymmetric quadrature split birdcage coil for hyperpolarized 3He lung MRI at 1.5T.

An asymmetric quadrature birdcage body coil for hyperpolarized (HP) (3)He lung imaging at 1.5T is presented. The coil is designed to rest on top of the patient support and be used as a temporary insert in a clinical system.

NMR probes for measuring magnetic fields and field dynamics in MR systems.

High-resolution magnetic field probes based on pulsed liquid-state NMR are presented. Static field measurements with an error of 10 nanotesla or less at 3 tesla are readily obtained in 100 ms. The further ability to measure dynamic magnetic fields results from using small ( approximately 1 microL) droplets of MR-active liquid surrounded by susceptibility-matched materials.

Spatiotemporal magnetic field monitoring for MR.

MR experiments frequently rely on signal encoding by the application of magnetic fields that vary in both space and time. The accurate interpretation of the resulting signals often requires knowledge of the exact spatiotemporal field evolution during the experiment. To better fulfill this need, a new approach is presented that enables measuring the field evolution concurrently with any MR sequence.

Modular design of receiver coil arrays.

We describe a modular and hence flexible system for connecting MR surface coils to create a receiver array. Up to 16 individual coils of different size and shape depending on the application are plugged into a connector box that houses the control electronics. Preamplification, matching and detuning circuitry are housed on a circuit board directly attached to each coil loop.

Preliminary experience with visualization of intracortical fibers by focused high-resolution diffusion tensor imaging.

Background And Purpose: The inherent low anisotropy of gray matter and the lack of adequate imaging sensitivity and resolution has, so far, impeded depiction of axonal fibers to their intracortical origin or termination. We tested the hypothesis that an experimental approach with high-resolution diffusion tensor imaging (DTI) provides anisotropic data for fiber tractography with sufficient sensitivity to visualize in vivo the fine distribution of white matter bundles at the intracortical level.

Materials And Methods: We conducted phantom measurements of signal-to-noise ratio (SNR) and obtained diffusion tensor maps of the occipital lobe in 6 healthy volunteers using a dedicated miniature phased array detector at 3T.