B0 Gradient Research Articles

An Improved Postprocessing Method to Mitigate the Macroscopic Cross-Slice B0 Field Effect on R2* Measurements in the Mouse Brain at 7T.

The MR transverse relaxation rate, R2*, has been widely used to detect iron and myelin content in tissue. However, it is also sensitive to macroscopic B0 inhomogeneities. One approach to correct for the B0 effect is to fit gradient-echo signals with the three-parameter model, a sinc function-weighted monoexponential decay. However, such three-parameter models are subject to increased noise sensitivity. To address this issue, this study presents a two-stage fitting procedure based on the three-parameter model to mitigate the B0 effect and reduce the noise sensitivity of R2* measurement in the mouse brain at 7T. MRI scans were performed on eight healthy mice. The gradient-echo signals were fitted with the two-stage fitting procedure to generate R2corr_t*. The signals were also fitted with the monoexponential and three-parameter models to generate R2nocorr* and R2corr*, respectively. Regions of interest (ROIs), including the corpus callosum, internal capsule, somatosensory cortex, caudo-putamen, thalamus, and lateral ventricle, were selected to evaluate the within-ROI mean and standard deviation (SD) of the R2* measurements. The results showed that the Akaike information criterion of the monoexponential model was significantly reduced by using the three-parameter model in the selected ROIs (p = 0.0039-0.0078). However, the within-ROI SD of R2corr* using the three-parameter model was significantly higher than that of the R2nocorr* in the internal capsule, caudo-putamen, and thalamus regions (p = 0.0039), a consequence partially due to the increased noise sensitivity of the three-parameter model. With the two-stage fitting procedure, the within-ROI SD of R2corr* was significantly reduced by 7.7-30.2% in all ROIs, except for the somatosensory cortex region with a fast in-plane variation of the B0 gradient field (p = 0.0039-0.0078). These results support the utilization of the two-stage fitting procedure to mitigate the B0 effect and reduce noise sensitivity for R2* measurement in the mouse brain.

Advancements in MR hardware systems and magnetic field control: B0 shimming, RF coils, and gradient techniques for enhancing magnetic resonance imaging and spectroscopy.

High magnetic field homogeneity is critical for magnetic resonance imaging (MRI), functional MRI, and magnetic resonance spectroscopy (MRS) applications. B0 inhomogeneity during MR scans is a long-standing problem resulting from magnet imperfections and site conditions, with the main issue being the inhomogeneity across the human body caused by differences in magnetic susceptibilities between tissues, resulting in signal loss, image distortion, and poor spectral resolution. Through a combination of passive and active shim techniques, as well as technological advances employing multi-coil techniques, optimal coil design, motion tracking, and real-time modifications, improved field homogeneity and image quality have been achieved in MRI/MRS. The integration of RF and shim coils brings a high shim efficiency due to the proximity of participants. This technique will potentially be applied to high-density RF coils with a high-density shim array for improved B0 homogeneity. Simultaneous shimming and image encoding can be achieved using multi-coil array, which also enables the development of novel encoding methods using advanced magnetic field control. Field monitoring enables the capture and real-time compensation for dynamic field perturbance beyond the static background inhomogeneity. These advancements have the potential to better use the scanner performance to enhance diagnostic capabilities and broaden applications of MRI/MRS in a variety of clinical and research settings. The purpose of this paper is to provide an overview of the latest advances in B0 magnetic field shimming and magnetic field control techniques as well as MR hardware, and to emphasize their significance and potential impact on improving the data quality of MRI/MRS.

Individually tailored spatial-spectral pulsed CEST MRI for ratiometric mapping of myocardial energetic species at 3T.

CEST MRI has been used to probe changes in cardiac metabolism via assessment of CEST contrast from Cr. However, B1 variation across the myocardium leads to spatially variable Cr CEST contrast in healthy myocardium. We developed a spatial-spectral (SPSP) saturation pulsed CEST protocol to compensate for B1 variation. Flip angle maps were used to individually tailor SPSP pulses comprised of a train of one-dimensional spatially selective subpulses selective along the principal B1 gradient dimension. Complete Z-spectra in the hearts of (n = 10) healthy individuals were acquired using conventional Gaussian saturation and SPSP schemes and supported by phantom studies. In simulations, the use of SPSP pulses reduced the average SD of the effective saturation B1 values within the myocardium (n = 10) from 0.12 ± 0.02 μT to 0.05 ± 0.01 μT (p < 0.01) and reduced the average SD of Cr CEST contrast in vivo from 10.0 ± 4.3% to 6.1 ± 3.5% (p < 0.05). Results from the hearts of human subjects showed a significant reduction of CEST contrast distribution at 2 ppm, as well as amplitude, when using SPSP saturation. Corresponding phantom experiments revealed PCr-specific contrast generation at body temperature when SPSP saturation was used but combined PCr and Cr contrast generation when Gaussian saturation was used. The use of SPSP saturation pulsed CEST resulted in PCr-specific contrast generation and enabled ratiometric mapping of PCr to total Cr CEST contrast in the human heart at 3T.

Fast spin-echo approach for accelerated B1 gradient-based MRI.

To expand on the previously developed -encoding technique, frequency-modulated Rabi-encoded echoes (FREE), to perform accelerated image acquisition by collecting multiple lines of k-space in an echo train. FREE uses adiabatic full-passage pulses and a spatially varying RF field to encode unique spatial information without the use of traditional B0 gradients. The original implementation relied on acquiring single lines of k-space, leading to long acquisitions. In this work, an acceleration scheme is presented in which multiple echoes are acquired in a single shot, analogous to conventional fast spin-echo sequences. Theoretical analysis and computer simulations investigated the feasibility of this approach and presented a framework to analyze important imaging parameters of FREE-based sequences. Experimentally, the multi-echo approach was compared with conventional phase-encoded images of the human visual cortex using a simple surface transceiver coil. Finally, different contrasts demonstrated the clinical versatility of the new accelerated sequence. Images were acquired with an acceleration factor of 3.9, compared with the previous implementation of FREE, without exceeding specific absorption rate limits. Different contrasts can easily be acquired without major modifications, including inversion recovery-type images. FREE initially illustrated the feasibility of performing slice-selective 2D imaging of the human brain without the need for a B0 gradient along the y-direction. The multi-echo version maintains the advantages that encoding provides but represents an important step toward improving the clinical feasibility of such sequences. Additional acceleration and more advanced reconstruction techniques could further improve the clinical viability of FREE-based techniques.

Design of a three-loop asymmetric coil producing a homogeneous radiofrequency B1 field gradient along the axis of a vertical sample tube

A coil system generating a vertical radio-frequency (rf) field gradient (B1 gradient) has been built for surrounding, in a horizontal magnet, a vertical sample (object) of axial symmetry. The system comprises three coaxial loops with an overall shape either spherical or ellipsoidal. The geometry has been theoretically and experimentally devised for producing a very uniform gradient (cancellation of B1 derivatives from second order up to sixth order) in the central region where a vertical receiver/transmitter coil is installed. The latter is of the saddle-shaped type and is geometrically and electrically decoupled from the gradient coil system. This receiver/transmitter coil not only ensures an optimal signal reception but, in addition, is able to deliver perfectly homogeneous rf hard pulses which are mandatory in most NMR experiments. In its present design, the system delivers a uniform gradient in a limited region but could be extended at will. Its main advantages over static field gradients (B0 gradients) appear clearly in the case of very short transverse relaxation times. This property has been emphasized in the case of experiments leading to the measurement of diffusion coefficients. Also, this system would be suitable for chemical shift imaging (CSI) experiments as confirmed by a preliminary test experiment.

A truncated twisted solenoid RF phase gradient transmit coil for TRASE MRI

Transmit array spatial encoding (TRASE) is an MR imaging technique that achieves k-space encoding through the use of phase gradients in the RF transmit field. Without requiring B0 gradient fields, TRASE MRI can be performed using significantly cheaper bi-planar permanent magnets or Halbach arrays. For TRASE encoding with these magnets, the twisted solenoid has been demonstrated as the most efficient RF transmit coil; however, this specific geometry results in a long coil with a relatively short imaging volume. We introduce a new truncated design to increase the usable imaging volume relative to the coil length. Based on simulations of optimal parameters, a 200 mm long, 100 mm inner diameter coil pair was constructed with an imaging volume 100 mm in length and 80 mm in diameter. The coil pair was tested using an un-shimmed 2.84 MHz Halbach array. Results indicate the truncated design can create a similar imaging volume and quality to the untruncated version whilst significantly reducing the length of the coil by as much as a half.

B1 -gradient-based MRI using frequency-modulated Rabi-encoded echoes.

Reduce expense and increase accessibility of MRI by eliminating pulsed field (B0 ) gradient hardware. A radiofrequency imaging method is described that enables spatial encoding without B0 gradients. This method, herein referred to as frequency-modulated Rabi-encoded echoes (FREE), utilizes adiabatic full passage pulses and a gradient in the RF field (B1 ) to produce spatially dependent phase modulation, equivalent to conventional phase encoding. In this work, Cartesian phase encoding was accomplished using FREE in a multi-shot double spin-echo sequence. Theoretical analysis and computer simulations investigated the influence of resonance offset and B1 -gradient steepness and magnitude on reconstruction quality, which limit other radiofrequency imaging methodologies. Experimentally, FREE was compared to conventional phase-encoded MRI on human visual cortex using a simple surface transceiver coil. Image distortions occurred in FREE when using nonlinear B1 fields where the phase dependence becomes nonlinear, but with minimal change in signal intensity. Resonance offset effects were minimal for Larmor frequencies within the adiabatic full-passage pulse bandwidth. For the first time, FREE enabled slice-selective 2D imaging of the human brain without a B0 gradient in the y-direction. FREE achieved high resolution in regions where the B1 gradient was steepest, whereas images were distorted in regions where nonlinearity in the B1 gradient was significant. Given that FREE experiences no significant signal loss due to B1 nonlinearities and resonance offset, image distortions shown in this work might be corrected in the future based on B1 and B0 maps.

Effects of B0 eddy currents on imaging isocenter shifts in 0.35‐T MRI‐guided radiotherapy (MR‐IGRT) system

PurposeThe purpose of this study was to measure gantry angle‐related eddy currents in a 0.35‐T MRI‐Linac and determine if B0 (zeroth order) eddy currents are the primary cause of gantry angle‐dependent imaging isocenter shifts vs other potential causes like B0 inhomogeneities and gradient (first order) eddy currents. For conventional Cartesian acquisitions, B0 eddy currents can cause imaging isocenter shifts along both phase encode and readout directions. Gradient eddy currents can cause spatial distortion along both the phase encode and readout directions. Center frequency offsets can cause imaging isocenter shifts along the readout direction that vary with readout gradient polarity.MethodsMRI‐related eddy currents and imaging isocenter shifts were measured on a 0.35‐T MRI‐Linac at gantry angles from 0° to 330° in increments of 30°. All measurements were made after gradient shimming and center frequency tuning at each planned gantry angle. Eddy current and field homogeneity measurements were conducted using a 24‐cm diameter spherical phantom. Gradient and B0 eddy currents were calculated from the free induction decays (FIDs) resulting from selective excitation of slices located ±5 cm from isocenter. B0 eddy currents were also calculated from FIDs acquired with nonselective excitation and compared with B0 eddy current values derived using selective excitation. B0 inhomogeneities and center frequency offsets were measured by acquiring FIDs with nonselective excitation. Imaging isocenter shifts were measured using a 33x33x10.5 cm3 uniformity linearity (grid) phantom and a 3D true fast imaging with steady‐state precession (TrueFISP) sequence used in MRI‐guided radiation therapy. Eddy currents were compared to vendor specifications and correlated with the imaging isocenter shifts. Measurements were conducted before and after the MRI‐Linac’s waveguide was replaced with an updated design to reduce eddy currents.ResultsB0 eddy currents were highly correlated (r = 0.986, P &lt;&lt; 0.001) for measurements made with vs without selective excitation. Transverse (X and Y) axis B0 eddy currents before and after the waveguide upgrade were out of specification (specification: ≤0.1 μT m/mT for delays &lt; 10 ms) for most of the measured gantry angles. Gradient eddy currents before and after the upgrade were within specifications for the measured gantry angles (≤0.1% for delays &lt; 10 ms). B0 eddy currents and imaging isocenter shifts were highly correlated (r = 0.965, P &lt;&lt; 0.001). After the Linac waveguide upgrade, root mean square (RMS) peak B0 and gradient eddy currents dropped 45% and 11%, respectively, for delays &lt;10 ms, while imaging isocenter shifts dropped 53%. Isocenter shifts were observed in both phase encode and readout directions. Center frequency offsets were &lt;26 Hz while B0 inhomogeneities were &lt;33 Hz full width at half maximum (FWHM).ConclusionsImaging isocenter shifts measured in a 0.35‐T MRI‐Linac were highly correlated with B0 eddy currents. The eddy currents and imaging isocenter shifts decreased after the MRI‐Linac’s waveguide was replaced.

Quantitative Evaluations of Geometrical Distortion Corrections in Cortical Surface-Based Analysis of High-Resolution Functional MRI Data at 7T.

BackgroundAlthough 7T functional MRI (fMRI) provides better signal‐to‐noise ratio and higher spatial resolution than 3T fMRI, geometric distortions become more challenging because fMRI is more susceptible to distortions than structural MRI. Accurate alignment of 7T fMRI to structural MRI data is critical for precise cortical surface‐based analysis.PurposeTo quantify the effectiveness of distortion corrections of 7T fMRI data.Study TypeProspective.SubjectsFifteen healthy individuals aged 19–26 years (mean: 21.9 years).Field Strength/SequenceMultiband gradient‐echo echo‐planar imaging sequence at 7T; 3D T1/T2‐weighted sequences (magnetization prepared rapid acquisition with gradient echo [MPRAGE] and sampling perfection with application optimized contrast using different flip angle evolution [SPACE]) at 3T.AssessmentfMRI data at 7T were registered to cortical surfaces reconstructed from 3T structural data acquired in the same subjects. Distortions induced by B0 inhomogeneity and gradient nonlinearity (B0 and gradient distortions) were evaluated as cortical fallout (misregistration of noncortical areas) and displacement (misregistration along gray matter).Statistical TestsRepeated measures analyses of variance with post‐hoc t‐tests with Bonferroni correction.ResultsThe accuracy of fully corrected fMRI images based on the intensity distribution was 89.2%. Without any corrections, 9.7% of vertices in the whole surfaces were fallout and the average displacement was 0.96 mm for the rest of the vertices. B0 and gradient distortion corrections significantly reduced the fallout (to 2.1% and 8.7%) and displacement (to 0.29 mm and 0.86 mm). These corrections were effective even around regions with moderate distortions (the somatosensory and visual cortices for B0 distortion, and the anterior frontal, inferior temporal, and posterior occipital cortices for gradient distortion).Data ConclusionB0 distortion correction is crucial for surface‐based analysis of fine‐resolution fMRI at 7T. Gradient distortion correction should be considered when regions of interest include regions distant from the isocenter of scanners.Evidence Level1Technical Efficacy Stage1

A comparative study of gradient nonlinearity correction strategies for processing diffusion data obtained with ultra-strong gradient MRI scanners.

PurposeThe analysis of diffusion data obtained under large gradient nonlinearities necessitates corrections during data reconstruction and analysis. While two such preprocessing pipelines have been proposed, no comparative studies assessing their performance exist. Furthermore, both pipelines neglect the impact of subject motion during acquisition, which, in the presence of gradient nonlinearities, induces spatio‐temporal B‐matrix variations. Here, spatio‐temporal B‐matrix tracking (STB) is proposed and its performance compared to established pipelines.MethodsDiffusion tensor MRI (DT‐MRI) was performed using a 300 mT/m gradient system. Data were acquired with volunteers positioned in regions with pronounced gradient nonlinearities, and used to compare the performance of six different processing pipelines, including STB.ResultsUp to 30% errors were observed in DT‐MRI parameter estimates when neglecting gradient nonlinearities. Moreover, the order in which B0 inhomogeneity, eddy current and gradient nonlinearity corrections were performed was found to impact the consistency of parameter estimates significantly. Although, no pipeline emerged as a clear winner, the STB approach seemed to yield the most consistent parameter estimates under large gradient nonlinearities.ConclusionsUnder large gradient nonlinearities, the choice of preprocessing pipeline significantly impacts the estimated diffusion parameters. Motion‐induced spatio‐temporal B‐matrix variations can lead to systematic bias in the parameter estimates, that can be ameliorated using the proposed STB framework.

Imaging of Stroke in Rodents Using a Clinical Scanner and Inductively Coupled Specially Designed Receiver Coils.

Imaging of small laboratory animals in clinical MRI scanners is feasible but challenging. Compared with dedicated preclinical systems, clinical scanners have relatively low B0 field (1.5-3.0 T) and gradient strength (40-60 mT/m). This work explored the use of wireless inductively coupled coils (ICCs) combined with appropriate pulse sequence parameters to overcome these two drawbacks, with a special emphasis on the optimization of the coil passive detuning circuit for this application. A Bengal rose photothrombotic stroke model was used to induce cortical infarction in rats and mice. Animals were imaged in a 3T scanner using T2 and T1-weighted sequences. In all animals, the ICCs allowed acquisition of high-quality images of the infarcted brain at acute and chronic stages. Images obtained with the ICCs showed a substantial increase in SNR compared to clinical coils (by factors of 6 in the rat brain and 16-17 in the mouse brain), and the absence of wires made the animal preparation workflow straightforward.

Feasibility of head-tilted brain scan to reduce susceptibility-induced signal loss in the prefrontal cortex in gradient echo-based imaging

Susceptibility-induced static field (B0) inhomogeneity near the nasal cavity degrades high-field MRI image quality. Many studies have addressed this problem by hardware- or sequence-based methods to improve local B0 shimming or minimize the impact of inhomogeneity. Here, we investigate the feasibility of the head-tilted brain scan as an easily accessible way to reduce B0 inhomogeneity and associated gradient echo signal loss in the prefrontal cortex (PFC). We exploit the fact that the region of intense local B0 gradient can be steered away from the PFC by head reorientation with respect to the main magnetic field. We found that the required chin-up head tilting by a substantial angle (> 30°) can be readily achieved for a group of healthy subjects when their back was raised by about 10 cm. Eleven subjects were scanned at 3T, using a standard 20 channel head-neck coil, for whole-head B0 mapping and gradient-echo EPI-based functional MRI (fMRI) performing a reward-punishment task in normal and tilted head orientations. Additionally, multi-echo gradient echo and resting-state fMRI scans were performed on six subjects in both orientations. Head-tilted sessions, which lasted for at least 20 min, were well-tolerated by all subjects and demonstrated a marked reduction of localized signal loss in the gradient echo-based images and EPI images in the PFC compared to normal orientation scans. Imaging in tilted orientation reduced the group-averaged B0 standard deviation and peak B0 gradient in the orbital gyrus beyond what was possible with simulated 3rd order shimming. The behavioral performance in the head-tilted fMRI scans indicated that the subjects were able to perform a cognitive task with little difficulty, and the tilted fMRI scans successfully produced a robust whole-brain functional activation map consistent with the literature. Our study proposes that the back-raised, head-tilted imaging can benefit the shimming of the prefrontal brain regions while being compatible with moderate-length neuroimaging scans on healthy, cooperating subjects.

Experimental setup for bulk susceptibility effect-minimized, multi-orientation MRI of ex vivo tissue samples.

Many conventional ex vivo magnetic resonance imaging (MRI) setups utilize cylindrical or other nonspherical tissue containers which can cause static field (B0 ) inhomogeneity affecting the accuracy of the measurements in an orientation-dependent manner. In this work we demonstrate an experimental method to obtain MRI of ex vivo tissue samples held in a spherical container in order to minimize bulk susceptibility-induced B0 inhomogeneity in arbitrary orientations. B0 inhomogeneity caused by tissue-air susceptibility mismatch can be theoretically eliminated if the surface of susceptibility discontinuity is spherical. This situation can be approximated by putting a tissue sample in a spherical shell filled with materials with tissue-like magnetic susceptibility. We achieved this on an intact monkey brain by (a) holding the brain with a three-dimensional (3D)-printed holder with tissue-like (within 0.5ppm) susceptibility, and (b) enclosing the brain and the holder in an acrylic spherical shell filled with diamagnetic liquid. Furthermore, the sphere and the radio-frequency coil for MRI were mounted on a 3D-printed frame designed to reduce B0 inhomogeneity contributions. The sphere could be rotated freely without disturbing the RF coil to facilitate multi-orientation imaging. We verified our setup by mapping B0 in the monkey brain at 13 different orientations in a human 7T scanner, and measuring orientation-dependent relaxation rates in the white and gray matters of the brain. The results were then compared with a setup where the brain was held inside a cylindrical container. In all orientations, the B0 standard deviation in the brain in the spherical setup (converted to Larmor frequency offset) was less than about 10Hz. This corresponds to two-sigma deviation of B0 of <0.07ppm. The B0 gradient was <9Hz/mm in 95 % of the brain voxels in all orientations. In high-resolution imaging with e.g. voxel size <0.4mm, this corresponds to voxel line broadening of <4Hz (0.013ppm). in the corpus callosum showed distinctly different orientation dependence compared to the gray matter. The B0 uniformity and reliability were much reduced in the cylindrical container setup. We have demonstrated an experimental method to effectively minimize bulk susceptibility-induced B0 perturbation in multi-orientation ex vivo MRI. The method promises to benefit a range of tissue orientation-dependent MR property studies.

Evaluation of a Portable Doppler Ultrasound Gating Device for Fetal Cardiac MR Imaging: Initial Results at 1.5T and 3T.

Purpose:Fetal cardiac MRI has the potential to play an important role in the assessment of fetal cardiac pathologies, but it is up to now not feasible due to a missing gating method. The purpose of this work was the evaluation of Doppler ultrasound (DUS) for external fetal cardiac gating with regard to compatibility, functionality, and reliability. Preliminary results were assessed performing fetal cardiac MRI.Methods:An MRI conditional DUS device was developed to obtain a gating signal from the fetal heart. The MRI compatibility was evaluated at 1.5T and 3T using B1 field maps and gradient echo images. The quality and sensitivity of the DUS device to detect the fetal heart motion for cardiac gating were evaluated outside the MRI room in 15 fetuses. A dynamic fetal cardiac phantom was employed to evaluate distortions of the DUS device and gating signal due to electromagnetic interferences at 1.5T and 3T. In the first in vivo experience, dynamic fetal cardiac images were acquired in four-chamber view at 1.5T and 3T in two fetuses.Results:The maximum change in the B1 field and signal intensity with and without the DUS device was <6.5% for 1.5T and 3T. The sensitivity of the DUS device to detect the fetal heartbeat was 99.1%. Validation of the DUS device using the fetal cardiac phantom revealed no electromagnetic interferences at 1.5T or 3T and a high correlation to the simulated heart frequencies. Fetal cardiac cine images were successfully applied and showed good image quality.Conclusion:An MR conditional DUS gating device was developed and evaluated revealing safety, compatibility, and reliability for different field strengths. In a preliminary experience, the DUS device was successfully applied for in vivo fetal cardiac imaging at 1.5T and 3T.

Formula omitted] gradient coherence selection using a tapered stripline

Pulsed-field gradients are common in modern liquid state NMR pulse sequences. They are often used instead of phase cycles for the selection of coherence pathways, thereby decreasing the time required for the NMR experiment. Soft off-resonance pulses with a B1 gradient result in a spatial encoding similar to that created by pulsed-field (B0) gradients. In this manuscript we show that pulse sequences with pulsed-field gradients can easily be converted to one which uses off-resonance B1 field gradient (OFFBEAT) pulses. The advantage of B1 gradient pulses for coherence selection is that the chemical shift evolution during the pulses is (partially) suppressed. Therefore no refocusing echos are required to correct for evolution during the gradient pulses. A tapered stripline is shown to be a convenient tool for creating a well-defined gradient in the B1 field strength. B1 gradient coherence selection using a tapered stripline is a simple and cheap alternative to B0 pulsed-field gradients.

Radiofrequency fields in MAS solid state NMR probes

We present a detailed analysis of the radiofrequency (RF) field over full volume of a rotor that is generated in a solenoid coil. On top of the usually considered static distribution of amplitudes along the coil axis we describe dynamic radial RF inhomogeneities induced by sample rotation. During magic angle spinning (MAS), the mechanical rotation of the sample about the magic angle, a spin packet travels through areas of different RF fields and experiences periodical modulations of both the RF amplitude and the phase. These modulations become particularly severe at the end regions of the coil where the relative RF amplitude varies up to ±25% and the RF phase changes within ±30°. Using extensive numerical simulations we demonstrate effects of RF inhomogeneity on pulse calibration and for the ramped CP experiment performed at a wide range of MAS rates. In addition, we review various methods to map RF fields using a B0 gradient along the sample (rotor axis) for imaging purposes. Under such a gradient, a nutation experiment provides directly the RF amplitude distribution, a cross polarization experiment images the correlation of the RF fields on the two channels according to the Hartmann-Hahn matching condition, while a spin-lock experiment allows to calibrate the RF amplitude employing the rotary resonance recoupling condition.Knowledge of the RF field distribution in a coil provides key to understand its effects on performance of a pulse sequence at the spectrometer and enables to set robustness requirements in the experimental design.

Improving the robustness of pseudo-continuous arterial spin labeling to off-resonance and pulsatile flow velocity.

To improve pseudo-continuous arterial spin labeling (PCASL) robustness to off-resonance and pulsatile blood flow velocity. Bloch equations were solved to evaluate the effect of labeling parameters in a pulsatile flow model for a range of off-resonance. Experimental confirmation was achieved in volunteers using linear phase increase between labeling pulses to approximate off-resonance errors. We first assessed the location of the labeling plane in four volunteers. Next, we explored a range of parameters-including balanced and unbalanced gradients-in five more volunteers at an optimal labeling plane location. Simulations demonstrated that 1) high velocities are vulnerable to off-resonance, 2) unbalanced PCASL outperforms balanced PCASL, 3) increased B1 and low average gradient improve the labeling efficiency for high-velocity flow, and 4) a low ratio of selective to average gradient improves off-resonance robustness. A good agreement between theory and experiment was observed. The robustness of PCASL can be increased by selecting an unbalanced scheme with a low average gradient (0.5 mT/m), a low ratio (7×) of selective to average gradients, and the highest feasible B1 (1.8 μT). Placing the labeling plane above the carotid bifurcation and below the V3 segment, usually between the second and third vertebrae, yielded robust results. Magn Reson Med 78:1342-1351, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Spatially resolved spectroscopy using tapered stripline NMR

Magnetic field B0 gradients are essential in modern Nuclear Magnetic Resonance spectroscopy and imaging. Although RF/B1 gradients can be used to fulfill a similar role, this is not used in common practice because of practical limitations in the design of B1 gradient coils. Here we present a new method to create B1 gradients using stripline RF coils. The conductor-width of a stripline NMR chip and the strength of its radiofrequency field are correlated, so a stripline chip can be tapered to produce any arbitrary shaped B1 field gradient. Here we show the characterization of this tapered stripline configuration and demonstrate three applications: magnetic resonance imaging on samples with nL–μL volumes, reaction monitoring of fast chemical reactions (10−2–101s) and the compensation of B0 field gradients to obtain high-resolution spectra in inhomogeneous magnetic fields.

On the suppression of background signals originating from NMR hardware components. Application to zero echo time imaging and relaxation time analysis

Modern NMR imaging systems used for biomedical research are equipped with B0 gradient systems with strong maximum gradient strength and short switching time enabling 1H NMR measurements of samples with very short transverse relaxation times. However, background signal originating from non-optimized RF coils may hamper experiments with ultrashort delays between RF excitation and signal reception. We demonstrate that two simple means, outer volume suppression and the use of shaped B0 fields produced by higher-order shim coils, allow a considerable suppression of disturbing background signals. Thus, the quality of NMR images acquired at ultrashort or zero echo time is improved and systematic errors in quantitative data evaluation are avoided. Fields of application comprise MRI with ultrashort echo time or relaxation time analysis, for both biomedical research and characterizing porous media filled with liquids or gases.

SU-D-303-04: A Survey of MRI Distortion Measurements

Purpose: Magnetic Resonance Imaging (MRI) is hampered by geometrical distortions that arise from inhomogeneity in the B0 field, gradient non-linearity, and patient-induced susceptibility artifacts. We provide a survey of precise and accurate distortion measurements made over eight years on a number of different scanner/protocol arrangements. Methods: A phantom was originally developed for the Alzheimer’s Disease Neuroimaging Initiative (ADNI Phantom) to provide detailed mapping of MRI image distortion. The phantom contains an array of polycarbonate spheres each filled with a copper sulfate solution to provide image contrast. Included are 158 1.0 cm diameter and 2 1.5 cm diameter fiducial spheres enclosed in a 20 cm diameter spherical urethane shell. The center of gravity of the spheres can be determined to sub-voxel precision through segmentation of the phantom images. A software analysis determines the actual locations of the spheres independent of the position and orientation of the phantom. The imaged locations of spheres are compared to the known locations yielding up to fourth-order measurements of scanner distortion. We have surveyed MR distortion measurements acquired by groups doing routine QA and longitudinal studies over an eight year period. Results: Distortion fields vary in magnitude and form from between scanners and protocols. Corrections applied by the scanner typically reduce distortion to 1 mm or less throughout a 20 cm diameter volume centered on isocenter. Corrections are typically less successful in one direction depending on the protocol. Distortion increases with distance from the isocenter and can be highly non-linear. Conclusion: Distortion of MR images is inherent to current scanners. Distortion is a function of many factors and although often less than 1mm can be much greater depending on protocol, distance from isocenter, and other factors. Scanners are typically stable with respect to their distortion profile but variations are not uncommon. Research supported by Image Owl, Inc., The Phantom Laboratory, Inc. and Raforninn ehf. Authors Mallozzi and Healy employed by The Phantom Laboratory, Inc. Author Goodenough is a consultant to The Phantom Laboratory, Inc. Authors Fredriksson, Kristbjornsson, and Olafsdottir are consultants to Image Owl, Inc.