Correction of physiologically induced global off-resonance effects in dynamic echo-planar and spiral functional imaging.

In functional magnetic resonance imaging, a rapid method such as echo-planar (EPI) or spiral is used to collect a dynamic series of images. These techniques are sensitive to changes in resonance frequency which can arise from respiration and are more significant at high magnetic fields. To decrease the noise from respiration-induced phase and frequency fluctuations, a simple correction of the "dynamic off-resonance in k-space" (DORK) was developed. The correction uses phase information from the center of k-space and a navigator echo and is illustrated with dynamic scans of single-shot and segmented EPI and, for the first time, spiral imaging of the human brain at 7 T. Image noise in the respiratory spectrum was measured with an edge operator. The DORK correction significantly reduced respiration-induced noise (image shift for EPI, blurring for spiral, ghosting for segmented acquisition). While spiral imaging was found to exhibit less noise than EPI before correction, the residual noise after the DORK correction was comparable. The correction is simple to apply and can correct for other sources of frequency drift and fluctuations in dynamic imaging.     

Resolution enhancement in single-shot imaging using simultaneous acquisition of spatial harmonics (SMASH).

Spatial resolution in single-shot imaging is limited by signal attenuation due to relaxation of transverse magnetization. This effect can be reduced by minimizing acquisition times through the use of short interecho spacings. However, the minimum interecho spacing is constrained by limits on gradient switching rates, radiofrequency (RF) power deposition and RF pulse length. Recently, simultaneous acquisition of spatial harmonics (SMASH) has been introduced as a method to acquire magnetic resonance images at increased speeds using a reduced number of phase-encoding gradient steps by extracting spatial information contained in an RF coil array. In this study, it is shown that SMASH can be used to reduce the effects of relaxation, resulting in single-shot images with increased spatial resolution without increasing imaging time. After a brief theoretical discussion, two strategies to reduce signal attenuation and increase spatial resolution in single-shot imaging are introduced and their performance is evaluated in phantom studies. In vivo single-shot echoplanar imaging (EPI), BURST, and half-Fourier single-shot turbo spin-echo (HASTE) images are then presented demonstrating the practical implementation of these resolution enhancement strategies. Images acquired with SMASH show increased spatial resolution and improved image quality when compared with images obtained with the conventional acquisitions. The general principles presented for imaging with SMASH can also be applied to other partially parallel imaging techniques.     

On the synchronization of transcranial magnetic stimulation and functional echo-planar imaging

PURPOSE: To minimize artifacts in echo-planar imaging (EPI) of human brain function introduced by simultaneous transcranial magnetic stimulation (TMS). MATERIALS AND METHODS: Distortions due to TMS pulses (0.25 msec, 2.0 T) were studied at 2.0 T before and during EPI. RESULTS: Best results were obtained if both the EPI section orientation and the frequency-encoding gradient were parallel to the plane of the TMS coil. Under these conditions, a TMS pulse caused image distortions when preceding the EPI sequence by less than 100 msec. Recordings with a magnetic field gradient pick-up coil revealed transient magnetic fields after TMS, which are generated by eddy currents in the TMS coil. TMS during image acquisition completely spoiled all transverse magnetizations and induced disturbances ranging from image corruption to mild image blurring, depending on the affected low and high spatial frequencies. Simultaneous TMS and radio-frequency (RF) excitation gave rise to T1-dependent signal changes that lasted for several seconds and yielded pronounced false-positive activations during functional brain mapping. CONCLUSION: To ensure reliable and robust combinations, TMS should be applied at least 100 msec before EPI while completely avoiding any pulses during imaging.     

Reduction of transient signal oscillations in true-FISP using a linear flip angle series magnetization preparation.

An electrocardiogram (ECG)-triggered, magnetization-prepared, segmented, 3D true fast imaging with steady-state precession (true-FISP) sequence with fat saturation was recently proposed for coronary artery imaging. A magnetization preparation scheme consisting of an alpha/2 radiofrequency (RF) pulse followed by 20 constant flip angle dummy RF cycles was used to reduce signal oscillations in the approach to steady state. However, if large resonance offsets on the order of 70-100 Hz are present, significant magnetization oscillations will still occur during data acquisition, which will result in image ghosting and blurring. The goal of this work was to validate that a linear flip angle (LFA) series can be used during magnetization preparation to reduce these image artifacts. Computer simulations, phantom studies, and coronary artery imaging in healthy volunteers were performed to compare this magnetization preparation scheme with that of an alpha/2 pulse followed by constant flip angle dummy RF cycles. The results demonstrated substantial reduction in the apparent image artifacts when using linearly increasing flip angles during magnetization preparation.     

Assessment of motion gating strategies for mouse magnetic resonance at high magnetic fields

PURPOSE: To assess the performance of motion gating strategies for mouse cardiac magnetic resonance (MR) at high magnetic fields by quantifying the levels of motion artifact observed in images and spectra in vivo. MATERIALS AND METHODS: MR imaging (MRI) of the heart, diaphragm, and liver; MR angiography of the aortic arch; and slice-selective 1H-spectroscopy of the heart were performed on anesthetized C57Bl/6 mice at 11.75 T. Gating signals were derived using a custom-built physiological motion gating device, and the gating strategies considered were no gating, cardiac gating, conventional gating (i.e., blanking during respiration), automatic gating, and user-defined gating. Both automatic and user-defined modes used cardiac and respiratory gating with steady-state maintenance during respiration. Gating performance was assessed by quantifying the levels of motion artifact observed in images and the degree of amplitude and phase stability in spectra. RESULTS: User-defined gating with steady-state maintenance during respiration gave the best performance for mouse cardiac imaging, angiography, and spectroscopy, with a threefold increase in signal intensity and a sixfold reduction in noise intensity compared to cardiac gating only. CONCLUSION: Physiological gating with steady-state maintenance during respiration is essential for mouse cardiac MR at high magnetic fields.     

Susceptibility compensated fMRI study using a tailored RF echo planar imaging sequence

Purpose

To implement a method using a tailored radiofrequency (TRF) pulse with a quadratic phase profile to recover susceptibility‐induced signal losses in gradient‐recalled echo‐planar images (EPI).

Materials and Methods

A functional magnetic resonance imaging (fMRI) experiment for compensation of susceptibility artifacts, known as the TRF pulse EPI sequence (TRF‐EPI), was used. TRF pulse compensates the susceptibility effect with a reduced signal‐to‐noise ratio (SNR) to one‐half when the maximum phase distribution is 2π. We demonstrate theoretically that the maximum phase distribution can also be reduced to π rather than 2π, improving the SNR accordingly. An analysis was conducted comparing this newly proposed strategy using a standard RF excitation with a linear phase distribution and a quadratic TRF excitation with a π phase distribution.

Results

Thorough experimental comparisons were also made between the TRF quadratic excitation with a π phase profile and conventional EPI with a standard excitation in human subjects during ventral brain activation.

Conclusion

With reduced maximum phase distribution in the TRF pulse, signals in the susceptibility‐affected areas, such as the orbitofrontal and inferior temporal cortex, were increased, suggesting that the technique could be a useful adjunct to fMRI. J. Magn. Reson. Imaging 2009;29:221–228. © 2008 Wiley‐Liss, Inc.

     

Eddy‐current compensated diffusion weighting with a single refocusing RF pulse

A modification of the Stejskal‐Tanner diffusion‐weighting preparation with a single refocusing RF pulse is presented which involves three gradient lobes that can be adjusted to null eddy currents with any given decay rate to reduce geometric distortions in diffusion‐weighted echo‐planar imaging (EPI). It has a very similar compensation performance as the commonly used double‐spin‐echo preparation but (i) is less sensitive to flip angle imperfections, e.g. along the slice profile, and B₁ inhomogeneities and (ii) can yield shorter echo times for moderate b values, notably for longer echo trains as required for higher spatial resolution. It therefore can provide an increased signal‐to‐noise ratio as is simulated numerically and demonstrated experimentally in water phantoms and the human brain for standard EPI (2.0 × 2.0 mm²) and high‐resolution EPI of inner field‐of‐views using 2D‐selective RF excitations (0.5 × 1.0 mm²). Thus, the presented preparation may help to overcome current limitations of diffusion‐weighted EPI, in particular at high static magnetic fields. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Real-time shimming to compensate for respiration-induced B0 fluctuations.

In MRI of human brain, the respiratory cycle can induce B0-field fluctuations through motion of the chest and fluctuations in local oxygen concentration. The associated NMR frequency changes can affect the MRI data in various ways and lead to temporal signal fluctuations, and image artifacts such as ghosting and blurring. Since the size of the effect scales with magnetic field strength, artifacts become particularly problematic at fields above 3.0T. Furthermore, the spatial dependence of the B0-field fluctuations complicates their correction. In this work, a new method is presented that allows compensation of field fluctuations by modulating the B0 shims in real time. In this method, a reference scan is acquired to measure the spatial distribution of the B0 effect related to chest motion. During the actual scan, this information is then used, together with chest motion data, to apply compensating B0 shims in real time. The method can be combined with any type of scan without modifications to the pulse sequence. Real-time B0 shimming is demonstrated to substantially improve the phase stability of EPI data and the image quality of multishot gradient-echo (GRE) MRI at 7T.     

Designing adiabatic radio frequency pulses using the Shinnar–Le Roux algorithm

Adiabatic pulses are a special class of radio frequency (RF) pulses that may be used to achieve uniform flip angles in the presence of a nonuniform B₁ field. In this work, we present a new, systematic method for designing high‐bandwidth (BW), low‐peak‐amplitude adiabatic RF pulses that utilizes the Shinnar–Le Roux (SLR) algorithm for pulse design. Currently, the SLR algorithm is extensively employed to design nonadiabatic pulses for use in magnetic resonance imaging and spectroscopy. We have adapted the SLR algorithm to create RF pulses that also satisfy the adiabatic condition. By overlaying sufficient quadratic phase across the spectral profile before the inverse SLR transform, we generate RF pulses that exhibit the required spectral characteristics and adiabatic behavior. Application of quadratic phase also distributes the RF energy more uniformly, making it possible to obtain the same spectral BW with lower RF peak amplitude. The method enables the pulse designer to specify spectral profile parameters and the degree of quadratic phase before pulse generation. Simulations and phantom experiments demonstrate that RF pulses designed using this new method behave adiabatically. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

K-space in the clinic

Magnetic resonance imaging (MRI) sequences are characterized by both radio frequency (RF) pulses and time-varying gradient magnetic fields. The RF pulses manipulate the alignment of the resonant nuclei and thereby generate a measurable signal. The gradient fields spatially encode the signals so that those arising from one location in an excited slice of tissue may be distinguished from those arising in another location. These signals are collected and mapped into an array called k-space that represents the spatial frequency content of the imaged object. Spatial frequencies indicate how rapidly an image feature changes over a given distance. It is the action of the gradient fields that determines where in the k-space array each data point is located, with the order in which k-space points are acquired being described by the k-space trajectory. How signals are mapped into k-space determines much of the spatial, temporal, and contrast resolution of the resulting images and scan duration. The objective of this article is to provide an understanding of k-space as is needed to better understand basic research in MRI and to make well-informed decisions about clinical protocols. Four major classes of trajectories-echo planar imaging (EPI), standard (non-EPI) rectilinear, radial, and spiral-are explained. Parallel imaging techniques SMASH (simultaneous acquisition of spatial harmonics) and SENSE (sensitivity encoding) are also described.     

Frequency stabilization using infinite impulse response filtering for SSFP fMRI at 3T.

The steady-state free precession (SSFP) method has been shown to exhibit strong potential for distortion-free functional magnetic resonance imaging (fMRI). One major challenge of SSFP fMRI is that the frequency band corresponding to the highest functional sensitivity is extremely narrow, leading to substantial loss of functional contrast in the presence of magnetic field drifts. In this study we propose a frequency stabilization scheme whereby an RF pulse with small flip angle is applied before each image scan, and the initial phase of the free induction decay (FID) signals is extracted to reflect temporal field drifts. A simple infinite impulse response (IIR) filter is further employed to obtain a low-pass-filtered estimate of the central reference frequency for the upcoming scan. Experimental results suggest that the proposed scheme can stabilize the frequency settings in accordance with field drifts, with oscillation amplitudes of <0.5 Hz. Phantom studies showed that both slow drifts and fast fluctuations were prominently reduced, resulting in less than 5% signal variations. Visual fMRI at submillimeter in-plane resolution further demonstrated 15% activation signals that were nicely registered in the microvessels within the sulci. It is concluded that the IIR-filtered frequency stabilization is an effective technique for achieving reliable SSFP fMR images at high field strengths.     

Short communication: assessment of environmental disturbances to the static magnetic field in magnetic resonance installations

The static magnetic field of MRI scanners can be affected by environmental factors. Magnetic resonance spectroscopy and functional imaging with single-shot echo-planar imaging (EPI) are particularly vulnerable to the movement of lifts, vehicles, trains and other large metallic masses in the vicinity. This work investigates the sensitivity of two different imaging techniques to assess disturbances of the static magnetic field: (i) phase changes in gradient-echo images of a uniform test object; and (ii) image displacement along the phase encoding direction in single-shot EPI images. For the latter a hexane sample was used, and the separation between CH2 and CH3 signals was taken as a reference. Both techniques were evaluated in a site known to be free of any significant environmental disturbances and validated by inducing a magnetic field disturbance. Both techniques provide valuable information in acceptance tests, allowing MRI users to evaluate and manage the environmental conditions surrounding a scanner. The single-shot EPI technique was found to be highly sensitive, being expected to detect magnetic field fluctuations down to 0.005 parts per million (ppm). The phase images method was found to be less sensitive (0.02 ppm) but is more easily available. The single-shot EPI technique was used in acceptance tests and environmental disturbances to the magnetic field of the order of 0.04 ppm were measured at the isocentre on two separate occasions.     

A pulse sequence for rapid in vivo spin-locked MRI

PURPOSE: To develop a novel pulse sequence called spin-locked echo planar imaging (EPI), or (SLEPI), to perform rapid T1rho-weighted MRI. MATERIALS AND METHODS: SLEPI images were used to calculate T1rho maps in two healthy volunteers imaged on a 1.5-T Sonata Siemens MRI scanner. The head and extremity coils were used for imaging the brain and blood in the popliteal artery, respectively. RESULTS: SLEPI-measured T1rho was 83 msec and 103 msec in white (WM) and gray matter (GM), respectively, 584 msec in cerebrospinal fluid (CSF), and was similar to values obtained with the less time-efficient sequence based on a turbo spin-echo readout. T1rho was 183 msec in arterial blood at a spin-lock (SL) amplitude of 500 Hz. CONCLUSION: We demonstrate the feasibility of the SLEPI pulse sequence to perform rapid T1rho MRI. The sequence produced images of higher quality than a gradient-echo EPI sequence for the same contrast evolution times. We also discuss applications and limitations of the pulse sequence.     

k-space detection and correction of physiological artifacts in fMRI

Signal phase variations caused by physiology are a major source of instability in fMRI images produced by multiple RF pulses. k-Space phase variation maps show cyclic phase variations at the frequency of respiration combined with a cardiac variation of lower amplitude. The amplitude of the variation increases with gradient echo time and proximity to the chest, suggesting that the dominant cause of the phase variation is a B₀ shift (～0.01 ppm) produced by movement of organs in the chest. A simple k-space phase correction method is proposed and demonstrated for FLASH fMRI. The retrospective method requires no pulse sequence modification, and is more effective than navigator echo correction. Physiological noise is dramatically reduced, especially at inferior slice locations.     

Retrospective estimation and correction of physiological artifacts in fMRI by direct extraction of physiological activity from MR data

A physiological artifact reduction method based on extracting respiratory motion and cardiac pulsation directly from functional MR data is described. In fast low angle shot (FLASH), respiratory cycles are derived utilizing the phase of the center of a navigator echo, in echo-planar imaging (EPI) from the phase of the center κ-space point. Cardiac cycles are determined from projections obtained from the navigator echo (FLASH) and the center κ-space line (EPI). Because direct extraction of physiological parameters eliminates the need for external monitoring, the method can be more readily implemented. Experimental results illustrate that the technique provides effective compensation for physiology-related signal fluctuations in functional MRI and performs as well as the retrospective technique using external physiological monitoring. Key words: fMRI; motion artifacts; physiological motion; image processing.     

Radiofrequency power deposition utilizing thermal imaging.

Wavelength effects influence radiofrequency (RF) power deposition distributions and limit magnetic resonance (MR) medical applications at very high magnetic fields. The power depositions in spherical saline gel phantoms were deduced from proton resonance shift thermal maps at both 1.5 T and 3.0 T over a range of conductivities. Phase differences before and after RF heating were measured for both a quadrature head coil and a circular surface coil. A long echo time (TE) pulse sequence with a 3D phase unwrap algorithm provided increased thermal sensitivity. The measured thermal maps agreed with a model of eddy-current heating by circularly polarized oscillating RF fields in a conducting dielectric sphere. At 3.0 T, thermal maps were acquired with a <0.32 degrees C temperature rise at 4 W. Proton resonance shift thermal maps provided a measure of hot spots in very-high-field MR imaging (MRI), in which both the phase sensitivity and signal-to-noise ratio (SNR) were increased. The method provides a means of studying the heat distribution generated by RF coils excited by clinical pulse sequences.     

High‐resolution functional MRI at 3 T: 3D/2D echo‐planar imaging with optimized physiological noise correction

High‐resolution functional MRI (fMRI) offers unique possibilities for studying human functional neuroanatomy. Although high‐resolution fMRI has proven its potential at 7 T, most fMRI studies are still performed at rather low spatial resolution at 3 T. We optimized and compared single‐shot two‐dimensional echo‐planar imaging (EPI) and multishot three‐dimensional EPI high‐resolution fMRI protocols. We extended image‐based physiological noise correction from two‐dimensional EPI to multishot three‐dimensional EPI. The functional sensitivity of both acquisition schemes was assessed in a visual fMRI experiment. The physiological noise correction increased the sensitivity significantly, can be easily applied, and requires simple recordings of pulse and respiration only. The combination of three‐dimensional EPI with physiological noise correction provides exceptional sensitivity for 1.5 mm high‐resolution fMRI at 3 T, increasing the temporal signal‐to‐noise ratio by more than 25% compared to two‐dimensional EPI. Magn Reson Med, 2013. © 2012 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance.     

Gradient field switching as a source for artifacts in MR imaging of metallic stents.

Metallic implants, such as stents, have long been a concern in magnetic resonance imaging (MRI). In addition to safety issues, they are commonly associated with image artifacts. The mechanisms of radiofrequency- (RF) and susceptibility-induced artifacts have been thoroughly investigated. However, gradient-switching-induced artifacts have not been analyzed. In this study it was demonstrated that gradient switching may be a source of artifacts in metallic stent MR imaging. These artifacts differ from those caused by the RF pulse. A theoretical explanation is provided as well.     

Single-point (constant-time) imaging in radiofrequency Fourier transform electron paramagnetic resonance.

This study describes the use of the single-point imaging (SPI) modality, also known as constant-time imaging (CTI), in radiofrequency (RF) Fourier transform (FT) electron paramagnetic resonance (EPR). The SPI technique, commonly used for high-resolution solid-state nuclear magnetic resonance (NMR) imaging, has been successfully applied to 2D and 3D RF-FT-EPR imaging of phantoms containing narrow-line EPR spin probes. The SPI scheme is essentially a phase-encoding technique that operates by acquiring a single data point in the free induction decay (FID) after a fixed delay (phase-encoding time), following the pulsed RF excitation, in the presence of static magnetic field gradients. Since the phase-encoding time remains constant for a given image data set, the spectral information is automatically deconvolved, providing well-resolved pure spatial images. Therefore, images obtained using SPI are artifact-free and the resolution is not significantly limited by the line width, compared to the images obtained using the conventional filtered back-projection (FBP) scheme, suggesting that the SPI modality may have advantages for EPR imaging of large objects. In this work the advantages and limitations of SPI as compared to FBP are investigated by imaging suitable phantom objects. Although SPI takes longer to perform than the FBP method, optimization of the data collection scheme may increase the temporal resolution, rendering this technique suitable for in vivo studies. Spectral information can also be extracted from a series of SPI images that are generated as a function of the delay from the excitation pulse.