Steady-state Pulse Sequence Research Articles

Signal Processing Steady-State Surface NMR Data

Surface nuclear magnetic resonance (NMR) is a unique technique to study groundwater as it provides direct insights into water content and pore-scale properties. However, surface NMR suffers from an extremely weak signal that limits mapping speeds and can prohibit measurements in high ambient noise environments. It has recently been demonstrated that steady-state sequences can deliver orders of magnitude signal-to-noise ratio enhancement in comparison to standard methods. Most notably, the steady-state approach collapses the surface NMR signal an extremely narrow frequency band, which leads to vastly improved noise rejection. In this paper, we develop the signal processing chain for steady-state surface NMR data. Each step in the signal processing chain, transient culling, pulse windowing, de-spiking, powerline harmonics subtraction and spectral analysis are explained in detail. We demonstrate how error estimation can be efficiently performed using the power spectral density in the vicinity of the transmitting frequency. We also demonstrate how off-resonance excitation can be determined by locating the peak spectrum of surface NMR signal with an array of phase-corrected discrete Fourier transforms. The presented signal processing methods can be easily implemented and applied to extract signal features for quality control and inversion procedures. We give a field example where data collected with steady-state pulse sequences are processed and inverted.

Explicit phenomenological solutions for magnetization exposed to an arbitrary NMR diffusion steady state pulse sequence.

Explicit phenomenological solutions to recurrence relations for the bulk transverse and longitudinal magnetization found using the Torrey-Bloch equations with relaxation effects are used to investigate nuclear magnetic resonance (NMR) diffusion measurements. Of particular interest are steady state NMR (self-)diffusion measurements that reduce experimental time that can extend the techniques to quickly reacting systems. The solutions for bulk transverse and longitudinal magnetization presented here are used to investigate the average behavior of the transverse and longitudinal magnetization in forming a steady state and are used to derive new expressions for the steady state longitudinal magnetization. These solutions can be applied to a noninteracting spin 1/2 ensemble undergoing free diffusion exposed to an arbitrary NMR pulse sequence containing arbitrary magnetic field gradient waveforms. The closed algebraic form method presented here has an advantage over iterative procedures for calculating transverse and longitudinal magnetization for the analysis and development of steady state pulse sequences. Previous theoretical results for steady state diffusion measurements are also reproduced. The Mathematica code for these solutions is provided in the supplementary material.

Steady‐state imaging with inhomogeneous magnetization transfer contrast using multiband radiofrequency pulses

PurposeInhomogeneous magnetization transfer (ihMT) is an emerging form of MRI contrast that may offer high specificity for myelinated tissue. Existing ihMT and pulsed MT sequences often use separate radiofrequency pulses for saturation and signal excitation. This study investigates the use of nonselective multiband radiofrequency pulses for simultaneous off‐resonance saturation and on‐resonance excitation specifically for generation of ihMT contrast within rapid steady‐state pulse sequences.Theory and MethodsA matrix‐based signal modeling approach was developed and applied for both balanced steady state free precession and spoiled gradient echo sequences, accounting specifically for multiband pulses. Phantom experiments were performed using a combination of balanced steady state free precession and spoiled gradient echo sequences, and compared with model fits. A human brain imaging exam was performed using balanced steady state free precession sequences to demonstrate the achieved contrast.ResultsA simple signal model derived assuming instantaneous radiofrequency pulses was shown to agree well with full integration of the governing equations and provided fits to phantom data for materials with strong ihMT contrast (PL161 root mean square error = 0.9%, and hair conditioner root mean square error = 2.4%). In vivo ihMT ratio images showed the expected white matter contrast that has been seen by other ihMT investigations, and the observed ihMT ratios corresponded well with predictions.ConclusionsihMT contrast can be generated by integrating multiband radiofrequency pulses directly into both spoiled gradient echo and balanced steady state free precession sequences, and the presented signal modeling approach can be used to understand the acquired signals.

1H time domain NMR real time monitoring of polyacrylamide hydrogels synthesis

Synthesis reactions of polyacrylamide based hydrogels were automatically monitored in situ by time-domain nuclear magnetic resonance, through a steady-state pulse sequence. Four hydrogel formulations with different amounts of monomer and crosslinking agent were tested and the proposed method demonstrated sensitivity to each formulation without the need for a priori calibration of the spectrometer, even when the hydrogels exhibited substantially different structural characteristics. The results obtained by the proposed method were compared for validation with those generated by traditional reaction monitoring methods, such as UV–Vis spectroscopy, and exhibited similar results, suggesting that time-domain nuclear magnetic constitutes an interesting alternative for the monitoring of solution crosslinking reactions.

Steerable Catheter Microcoils for Interventional MRI: Reducing Resistive Heating

The aims of this study were to assess resistive heating of microwires used for remote catheter steering in interventional magnetic resonance imaging and to investigate the use of alumina to facilitate heat transfer to saline flowing in the catheter lumen. A microcoil was fabricated using a laser lathe onto polyimide-tipped or alumina-tipped endovascular catheters. In vitro testing was performed on a 1.5-T magnetic resonance system using a vessel phantom, body radiofrequency coil, and steady-state pulse sequence. Resistive heating was measured with water flowing over a polyimide-tip catheter or saline flowing through the lumen of an alumina-tip catheter. Preliminary in vivo testing in porcine common carotid arteries was conducted with normal blood flow or after arterial ligation when current was applied to an alumina-tip catheter for up to 5 minutes. After application of up to 1 W of direct current power, clinically significant temperature increases were noted with the polyimide-tip catheter: 23°C/W at zero flow, 13°C/W at 0.28 cm(3)/s, and 7.9°C/W at 1 cm(3)/s. Using the alumina-tip catheter, the effluent temperature rise using the lowest flow rate (0.12 cm(3)/s) was 2.3°C/W. In vivo testing demonstrated no thermal injury to vessel walls at normal and zero arterial flow. Resistive heating in current carrying wire pairs can be dissipated by saline coolant flowing within the lumen of a catheter tip composed of material that facilitates heat transfer.

A Dual Echo Steady State Pulse Sequence in Evaluation of Brain Cerebrospinal Fluid Flow

This paper summarizes our experience of exploiting a Dual Echo Steady State (DESS) pulse sequence for the fast qualitative assessment of cerebrospinal fluid (CSF) flow dynamics at the aqueduct of Sylvius and Monro's foramina levels of the brain liquor pathways using a low-field magnetic resonance imager. Normal and pathologic CSF flow patterns are discussed. The capabilities of the described method as an effective extra tool for deriving the additional diagnostic information via visualization of flow voids in the brain CSF pathways are shown.

MRT-Sicherheitsaspekte und -Artefakte von Vorhofseptum-Okkludersystemen bei 1,5 Tesla: eine Phantomstudie

To investigate the magnetic resonance safety and magnetic resonance artifact intensities of patent foramen ovale and atrial septal occluders in a phantom study. Seven different commercial occluder models (Amplatzer ASD, Angel Wing ASD, Helex Septal Occluder, Cardia PFO-Star Generation I & III, Sideris Buttoned Device, Starflex ASD) were tested in a 1.5 Tesla whole-body MR scanner. Following a deflection test, the non-magnetic devices were imaged using different MRI pulse sequences including conventional spin echo, spoiled gradient echo as well as fast refocused steady-state pulse sequences (trueFISP) commonly used for cardiac MRI. Additionally, device temperatures were measured in the MR scanner during two imaging protocols (trueFISP, HASTE) with an MR-compatible fiber-optical thermometer. One ferromagnetic device (Sideris Buttoned Device) did not pass the deflection test and was excluded from further experiments. For typical cardiac imaging protocols, the remaining non-ferromagnetic devices showed only minor image artifacts with artifact sizes not exceeding 2 mm next to a strut. Heating was not observed in the devices. Non-ferromagnetic occluder models used in this study can safely be imaged with typical cardiac MRI protocols at 1.5 Tesla. The device artifact sizes are small enough to allow anatomical and functional MRI even in the immediate vicinity of the occluders.

The Frequency Offset Effects of NQR of Spin I = 1 for Remote Detection

The frequency offset effects of the signal interference during steady-state pulse sequences for remote nuclear quadrupole resonance (NQR) of 14N nuclei were investigated for the purposes of optimization in remote NQR. The experimental investigations with the separation of interfering NQR signals in sequences for observing signals {α0° |β90° −τ−α180° |β90° −τ} and {α0° |β90° −τ−α0° |β270° −τ} were carried out. The applications of the spherical tensor method for pure NQR of spin I = 1 for the investigation of these sequences and one pulse sequence, two pulse sequences are presented, too.

MR-based three-dimensional presentation of cartilage thickness in the femoral head.

The purpose of our study was to visualize the hyaline cartilage of the femoral head and to evaluate the distribution of the thickness by three-dimensional reconstruction of MRI data. The MRI was performed in 10 normal volunteers, 1 patient with osteonecrosis and 4 with advanced osteoarthritis. A fast 3D spoiled gradient-recalled acquisition in the steady state pulse sequence (TR 22 ms/TE 5.6 ms/no. of excitations 2) with fat suppression was used for data collection. Coronal and sagittal images were obtained with 3-mm effective slice thickness, 16-cm field of view (FOV) and 256x192 matrix. The MR images were reconstructed in three dimensions for evaluating the distribution of the cartilage thickness. In all normal volunteers, 1 patient with osteonecrosis and three advanced osteoarthritis, 3D reconstruction was successful, but in 1 case of osteoarthritis, 3D reconstruction failed because of the narrow joint space. In normal volunteers, the cartilage thickness is thickest in the central portion around the ligamentum teres (mean 2.8 mm). The medial portion and the lateral portion are almost of the same thickness (medial 1.3 mm, lateral 1.1 mm). In 3 cases of osteoarthritis, the cartilage became thinner in the lateral portions (<0.6 mm), but was unchanged in the central and medial portions. Three-dimensional reconstruction of MRI data is useful for evaluating the distribution of the cartilage thickness of the femoral head objectively.

Assessing tumor angiogenesis using macromolecular MR imaging contrast media.

MRI enhanced with a macromolecular contrast medium (MMCM) has previously been shown to estimate tumor microvascular characteristics that correlate closely with histologic microvascular density, an established surrogate of tumor angiogenesis. A similar MMCM-enhanced MRI technique has now been used to investigate the acute tumor microvascular effects of antibody-mediated inhibition of vascular endothelial growth factor (VEGF), a well-studied and potent angiogenesis stimulator. Athymic rats xenografted with a human breast carcinoma (MDA-MB-435) were imaged after administration of albumin-gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA30) using a heavily T1-weighted three dimensional-spoiled gradient-refocused acquisition in a steady-state pulse sequence before and 24 hours after treatment with anti-VEGF antibody (single dose of 1 mg). Changes in longitudinal relaxivity (delta R1) were analyzed using a bidirectional two-compartment kinetic model to estimate tumor fractional blood volume (fBV) and permeability surface area product (PS). Data showed a significant decrease (P < 0.05) of tumor PS with respect to macromolecular contrast medium at 24 hours after treatment with anti-VEGF antibody. No significant change was observed in fBV. Suppression of tumor microvascular permeability induced by anti-VEGF antibody can be detected and quantified by MMCM-enhanced MRI. MRI grading of tumor angiogenesis and monitoring of anti-angiogenesis interventions could find wide clinical application.

Enhancement of MR angiography with iron oxide: preliminary studies in whole-blood phantom and in animals.

We hypothesized that the previously observed T1 effect of a small monocrystalline iron oxide preparation can be exploited to decrease T1 relaxation time of blood. Such a decrease, particularly if present for a long time, could be used to improve the quality of MR angiograms. To test the hypothesis, we performed phantom studies and in vivo animal experiments. The effect of the monocrystalline iron oxide preparation on the MR signal intensity (spoiled gradient-recalled acquisition in the steady state pulse sequences, various timing parameters) of human whole blood was first tested in a phantom (dose range of monocrystalline iron oxide preparation, 0-3 mumol Fe/ml). Subsequent experiments were performed in rats (n = 7) and in rabbits (n = 6) to determine whether predicted changes in signal intensity could be observed in vivo. Dose optimization studies in rats indicate that injected doses of 15-50 mumol Fe/kg (0.8-2.8 mg Fe/kg) of monocrystalline iron oxide preparation resulted in threefold to fourfold increases of aortic signal-to-noise ratio. Because of its long plasma half-life (180 min in rats), the iron oxide preparation markedly improved the quality of images of the vasculature of the lungs, abdomen, and extremities. Our experimental results suggest that this and possibly other iron oxide preparations are alternatives to compounds containing macromolecular gadolinium and could be useful for clinical MR angiography.

Optimization of submillimeter-resolution MR imaging methods for the inner ear.

Submillimeter-resolution magnetic resonance (MR) imaging of the inner ear is valuable for diagnosis and treatment planning. Its main advantage for investigations of underlying disease is that it can directly depict the fluid spaces of the membranous labyrinth rather than define only the bony canal, as does computed tomography. A systematic evaluation of factors influencing high-resolution three-dimensional (3D) gradient-echo imaging of the inner ear with a standard clinical MR system is presented. This includes the evaluation of various radio-frequency coils, the design of steady-state pulse sequences, and the optimization of acquisition parameters. A quantitative analysis was facilitated by computer simulations and image processing. The highest signal-to-noise ratio for the membranous labyrinth was obtained with a single 3-inch (7.6-cm) receiver coil and a 3D GRASS (gradient-recalled acquisition in the steady state) sequence with the minimal achievable TR msec/TE msec of 25/7 and a 40 degrees--60 degrees flip angle, which yielded acceptable images with minimal voxel volumes of 0.1 mm3 in 14 minutes.

5034692 Magnetic resonance imaging method for acquiring flux-compensated, T2-weighted images

A method for obtaining an image in a magnetic resonance imaging apparatus uses a steady state pulse sequence which is based on a combination of a FISP pulse sequence and a PSIF pulse sequence. To signals are obtained by executing a sequence of steps once with alternating phase relation of the RF pulses, which excite the nuclear spins, and once with non-altering phase relation. By subtracting these signals, a pure PSIF signal is obtained, and the steady state is not disturbed by the moving spins. The flux phase is also compensated by the additional introduction of gradient motion refocusing (GMR) pulses, so that images of pulsating examination subjects, such as cerebral spin fluid, can be obtained free of flux artifacts.

Patellofemoral joint: evaluation during active flexion with ultrafast spoiled GRASS MR imaging.

An ultrafast spoiled gradient-recalled acquisition in the steady state pulse sequence was developed that permits multiple images to be obtained at a temporal resolution suitable for examining the patellofemoral joint during active flexion. This pulse sequence was used to perform kinematic magnetic resonance imaging of patellar alignment and tracking in five healthy subjects and seven patients with a provisional clinical diagnosis of abnormal patellofemoral joints.

Application of a steady-state pulse sequence for solvent elimination in 17O NMR

Application of a steady-state pulse sequence for solvent elimination in 17O NMR

Theory of T1 relaxation measurements in pure nuclear quadrupole resonance for spins / = 1

A quantitative analysis of the effect of spin lattice relaxation rates on pure quadrupole resonance pulse experiments for spins with I = 1 is given. A model is presented in which the three dimensional relaxation problem is reduced to a two dimensional one. In this model any possible population distribution of the energy levels corresponds to a point in a plane, the ``ν plane.'' The response of the system to spin-lattice relaxation effects and to rf pulses are described as translations of points in this plane. The experimental signal intensities are directly related to the coordinates of those points. The following cases are treated quantitatively: (i) CW saturation, (ii) CW saturation followed by a 90° pulse, (iii) two-pulse sequences, and (iv) continuous steady state pulse sequences. For each case explicit equations for the signal intensity as function of the experimental parameters are given. The analysis is made for both single crystals and powders.