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.

     

Rapid fat suppression in MRI of the breast with short binomial pulses

PURPOSE: To develop a faster method of fat suppression for use in dynamic contrast enhanced MRI of the breast. MATERIALS AND METHODS: A method of fast fat suppression is presented using spatially nonselective rapid binomial pulses. In contrast to conventional binomial frequency-selective pulses, these short pulses are applied without interpulse delay, allowing for very rapid spectrally selective excitation. RESULTS: Effective water excitation and fat suppression were achieved in breast MRI at 3.0 Tesla with total excitation time as low as 160 microsec, which is several times shorter than the excitation time of currently used fat suppression techniques. Rapid fat suppression comes at the expense of increased specific absorption rate (SAR) and mildly degraded quality of suppression. A flexible tradeoff of short imaging time vs. SAR can be made to optimize imaging speed for fat-suppressed breast MRI. CONCLUSION: Rapid binomial pulses can be used for dynamic contrast enhanced breast MRI with excitation times significantly shorter than currently used fat suppression pulses. Shorter excitation time allows more rapid imaging, allowing greater temporal and spatial resolution for characterization of breast lesions.     

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.     

Embedded MR fluoroscopy: high temporal resolution real-time imaging during high spatial resolution 3D MRA acquisition.

A method termed "embedded fluoroscopy" for simultaneously acquiring a real-time sequence of 2D images during acquisition of a 3D image is presented. The 2D images are formed by periodically sampling the central phase encodes of the slab-select direction during the 3D acquisition. The tradeoffs in spatial and temporal resolution are quantified by two parameters: the "redundancy" (R), the fraction of the 3D acquisition sampled more than once; and the "effective temporal resolution" (T), the time between temporal updates of the central views. The method is applied to contrast-enhanced MR angiography (CE-MRA). The contrast bolus dynamics are portrayed in real time in the 2D image sequence while a high-resolution 3D image is being acquired. The capability of the 2D acquisition to measure contrast enhancement with only a 5% degradation of the spatial resolution of the 3D CE-MR angiogram is shown theoretically. The method is tested clinically in 15 CE-MRA patient studies of the carotid and renal arteries.     

Rapid NMR imaging of dynamic processes using the FLASH technique

FLASH (Fast Low-Angle SHot) imaging is a new method for rapid NMR imaging which has been demonstrated to provide abdominal images without artifacts due to respiratory or peristaltic motions. The sequence typically employs 15 degrees radiofrequency excitation pulses and acquires a free induction decay signal in the form of a gradient echo. Here FLASH images are recorded in the presence of dynamic processes with time constants even smaller than the measuring time of about 2 s for an image with a 128 X 128-pixel resolution. Experiments are carried out on flow phantoms and on rabbits yielding heart images without gating of the cardiac motion.     

Signal targeting with alternating radiofrequency (STAR) sequences: Application to MR angiography

We describe a time of flight subtraction method for cine MR angiography that provides nearly perfect suppression of background signal intensity with excellent flow contrast. The method consists of a preparation phase, during which the longitudinal magnetization of the target tissue is inverted on alternate acquisitions and the background tissue is presaturated, followed by a readout phase using a cine segmented turboFLASH sequence with a shared echo modification to improve temporal resolution. With appropriate alternation of the phases of the radiofrequency excitation pulses, there is cancellation of the background signal intensity but flow signal is optimized. By using a thick section (up to 25 mm), substantial portions of the vascular territory are encompassed in a single plane. This permits rapid, dynamic assessment of flow patterns in areas such as the circle of Willis, carotid bifurcation, or renal arteries. Applications of the method for bright and dark blood cine MR angiography are demonstrated.     

SSFP and GRE phase contrast imaging using a three-echo readout.

A technique for rapid in-plane phase-contrast imaging with high signal-to-noise ratio (SNR) is described. Velocity-encoding is achieved by oscillating the readout gradient, such that each 2DFT phase-encode is acquired three times following a single RF slice-selective excitation. Three images are reconstructed, from which both flow velocity and local resonance offset are calculated. This technique is compatible with both gradient-recalled echo (GRE) and balanced steady-state free precession (SSFP) imaging using a single steady-state. The proposed technique enables 1D velocity mapping with 40% higher temporal resolution and 80% higher SNR, compared to conventional PC-MRI using bipolar velocity-encoding gradient pulses.     

Improved gradient-echo 3D magnetic resonance imaging using pseudo-echoes created by frequency-swept pulses.

Frequency-swept pulses are not typically employed to excite spins in NMR. When used for selective excitation in MRI, such pulses do not produce a proper echo because the phase of the transverse magnetization varies in a quadratic manner across the slice or slab. Previously, frequency-swept pulses such as the chirp pulse have been shown to offer an approach to reduce the peak radiofrequency power required for excitation. It has also been shown that chirp excitation produces a unique type of echo (dubbed "pseudo-echo" here) and images can be generated from the resultant pseudo-echoes using a quadratic reconstruction method (J.G. Pipe, Magn Reson Med 1995;33:24-33). The present work describes a general theory and methods for exciting spins with other types of frequency-swept pulses (HSn pulses), which offer the advantage of delivering better excitation profiles than the chirp pulse. Here, pseudo-echoes are produced with HSn pulses in conventional gradient-echo 3D MRI, and high-quality images are reconstructed using standard fast Fourier transformation. An optional apodization procedure using a sliding window function is also introduced. When the dynamic range of the analog-to-digital converter is limiting, signal-to-noise ratio of pseudo-echo imaging is superior to that obtained with standard excitations.     

Fast dynamic 3D MR spectroscopic imaging with compressed sensing and multiband excitation pulses for hyperpolarized 13C studies

Hyperpolarized ¹³C MR spectroscopic imaging can detect not only the uptake of the pre‐polarized molecule but also its metabolic products in vivo, thus providing a powerful new method to study cellular metabolism. Imaging the dynamic perfusion and conversion of these metabolites provides additional tissue information but requires methods for efficient hyperpolarization usage and rapid acquisitions. In this work, we have developed a time‐resolved 3D MR spectroscopic imaging method for acquiring hyperpolarized ¹³C data by combining compressed sensing methods for acceleration and multiband excitation pulses to efficiently use the magnetization. This method achieved a 2 sec temporal resolution with full volumetric coverage of a mouse, and metabolites were observed for up to 60 sec following injection of hyperpolarized [1‐¹³C]‐pyruvate. The compressed sensing acquisition used random phase encode gradient blips to create a novel random undersampling pattern tailored to dynamic MR spectroscopic imaging with sampling incoherency in four (time, frequency, and two spatial) dimensions. The reconstruction was also tailored to dynamic MR spectroscopic imaging by applying a temporal wavelet sparsifying transform to exploit the inherent temporal sparsity. Customized multiband excitation pulses were designed with a lower flip angle for the [1‐¹³C]‐pyruvate substrate given its higher concentration than its metabolic products ([1‐¹³C]‐lactate and [1‐¹³C]‐alanine), thus using less hyperpolarization per excitation. This approach has enabled the monitoring of perfusion and uptake of the pyruvate, and the conversion dynamics to lactate and alanine throughout a volume with high spatial and temporal resolution. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Max CAPR: High‐resolution 3D contrast‐enhanced MR angiography with acquisition times under 5 seconds

High temporal and spatial resolution is desired in imaging of vascular abnormalities having short arterial‐to‐venous transit times. Methods that exploit temporal correlation to reduce the observed frame time demonstrate temporal blurring, obfuscating bolus dynamics. Previously, a Cartesian acquisition with projection reconstruction‐like (CAPR) sampling method has been demonstrated for three‐dimensional contrast‐enhanced angiographic imaging of the lower legs using two‐dimensional sensitivity‐encoding acceleration and partial Fourier acceleration, providing 1mm isotropic resolution of the calves, with 4.9‐sec frame time and 17.6‐sec temporal footprint. In this work, the CAPR acquisition is further undersampled to provide a net acceleration approaching 40 by eliminating all view sharing. The tradeoff of frame time and temporal footprint in view sharing is presented and characterized in phantom experiments. It is shown that the resultant 4.9‐sec acquisition time, three‐dimensional images sets have sufficient spatial and temporal resolution to clearly portray arterial and venous phases of contrast passage. It is further hypothesized that these short temporal footprint sequences provide diagnostic quality images. This is tested and shown in a series of nine contrast‐enhanced MR angiography patient studies performed with the new method. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Magnetization transfer contrast in fat-suppressed steady-state three-dimensional MR images.

We demonstrate that magnetization transfer contrast can be used to improve the diagnostic utility of fat-suppressed steady-state three-dimensional gradient-recalled images. Fat suppression is achieved using a "jump-return" pair of contiguous shaped pulses. No time interval exists between the pulses, and no RF echo is generated. The sequence normally produces images with "density" weighting. Preparation of the spin magnetization with off-resonance frequency-selective excitation creates magnetization transfer contrast which attenuates signal intensity in proportion to the exchange rate of magnetization from free water with magnetization from water bound to macromolecules or protons that have restricted mobility. The resulting images have excellent fat suppression with low sensitivity to motion since no subtraction is used. In addition, the mechanism of signal attenuation is independent of paramagnetic effects, and addition of Gd-DTPA produces signal enhancement from vascularized regions of tissue. Examples are presented for the knee and breast, where the observation of pathology with signal enhancement from Gd-DTPA is improved over conventional 3D fat-suppressed images.     

Locally focused MRI of interventions

Certain interventional MR procedures would benefit from T2-weighted imaging because of the sensitivity of T2-weighted images to tissue damage and target lesion contrast. To acquire such images with reasonable temporal resolution, a single-shot acquisition should be used because of the inherently long TR needed for T2 weighting. Unfortunately, most scanners require long readout times (eg, greater than 150 msec) and high bandwidths (eg, greater than 120 kHz) to perform conventional single-shot imaging with high spatial resolution. The resulting images are thus degraded by unacceptable artifacts and noise levels. This study illustrates how to create locally focused MR images that have high spatial resolution in a region of interest and lower spatial resolution elsewhere in the image. Because these images can be created from sparse k-space data, a scanner with modest gradients (eg, 10 mT/m maximal amplitude, 500 μsec minimal rise time) can acquire them after a single excitation with relatively short readout time and low bandwidth. This technique may make it practical to monitor interventions with T2-weighted imaging. The method was illustrated by reconstructing dynamic changes, which were simulated experimentally by moving objects in the vicinity of a normal human head.     

One-shot spatially resolved velocity imaging

For quantitative velocity measurement, we have developed a technique that acquires full velocity spectra without cardiac gating. After a cylindrical excitation restricts imaging to one spatial dimension, data are acquired while an oscillating gradient is played out. After each excitation, an image of velocity versus spatial location is obtained. For a given spatial location, a series of these images can be used to form an image of velocity versus time. Acquisition times are much shorter than for phase-contrast imaging or Fourier-encoded velocity imaging, obviating the need for cardiac gating. Although a two-shot version of this technique has been presented previously, we have developed a one-shot version that offers higher temporal resolution for a given velocity resolution and superior off-resonance properties.     

Instant images of the body by magnetic resonance

Proton magnetic resonance (MR) body images of the normal, adult human which have total scan times of typically only 40 ms per image are presented. There is no loss of spatial or contrast resolution due to motional blurring or ghosting; rather, movie loops of multiple 40-ms images directly demonstrate normal respiratory and peristaltic motion. Manifestation of "traditional" relaxation time contrast is demonstrated for a variety of spin echo (TE) and image repetition (TR) times. The images, obtained at 2.0 T on a new high-speed MR system, have a signal-to-noise ratio for muscle of approximately 30:1 (TE = 30 ms) for a 4.7-mm slice thickness (voxel size = 0.08 cm3). In a study presented as an example, 140 images covering the body from diaphragm to pelvis were all obtained within approximately 10 min. This method may help improve the efficacy of MR body imaging in general, and may play a role in applications which require high temporal resolution.     

Evaluation of temporal and spatial characteristics of 2D HYPR processing using simulations.

Highly constrained back-projection (HYPR) is a data acquisition and reconstruction method that provides very rapid frame update rates and very high spatial resolution for a time series of images while maintaining a good signal-to-noise ratio and high image quality. In this study we used simulations to evaluate the temporal and spatial characteristics of images produced using the HYPR algorithm. The simulations demonstrate that spatial accuracy is well maintained in the images and the temporal changes in signal intensity are represented with high fidelity. The waveforms representing signal intensity as a function of time obtained from regions-of-interest placed in simulated objects track the true curves very well, with variations from the truth occurring only when objects with very different temporal behavior are very close to each other. However, even when objects with different temporal characteristics are touching, their influences on each other are small.     

Phase-offset multiplanar (POMP) volume imaging: a new technique.

Phase-offset multiplanar (POMP) imaging is a technique that excites several sections simultaneously for improved imaging efficiency. The centers of the reconstructed images from each of the POMP sections are offset from each other in the phase-encoding direction by means of view-dependent phase modulation of the radio-frequency (RF) excitation pulses and are placed adjacent to each other in the reconstruction. With a suitable reconstruction matrix size, the images can be made nonoverlapping and stored separately. At constant imaging time, signal-to-noise ratio (S/N), and resolution, POMP imaging produces a factor NP more sections than a conventional sequence but with a reduced field of view. Alternatively, imaging time may be increased by the factor NP to retain the same field of view but with the expected S/N advantage. The average RF power deposited by the 90 degrees composite RF pulse is greater by the factor Np, but the power for the 180 degrees pulse is unchanged. The POMP method is discussed and compared with three-dimensional and Hadamard techniques.     

Fast MR cardiac profiling with two-dimensional selective pulses

A rapid-profiling NMR pulse sequence has been designed to provide an interactive, real-time cardiac probe analogous to M-mode ultrasound. The pulse sequence employs a two-dimensional (2D) selective NMR pulse to excite a narrow (nominally 1-cm-diameter) cylinder of magnetization intersecting the heart. This procedure is followed by a readout gradient applied along the length of the cylinder, or "beam," to yield an M-mode type profile with a one-dimensional Fourier transform reconstruction. k-space techniques were used to design 2D pulses which excite cylinders characterized by either Gaussian or square radial excitation profiles. Images of phantoms acquired at 1.5 T confirm the predictions of the k-space analysis. The cylinder can be displaced interactively by modulating the rf excitation and the beam axis can be reoriented to any oblique direction by changing the relative mixing of the gradient waveforms. Flow compensation using bipolar gradient waveforms inverts the contrast of flowing blood and suppresses flow artifacts. A gated cardiac image is acquired as a reference to locate the excitation axis. A series of cardiac experiments was performed on several healthy volunteers. As the beam is moved and rotated to probe the myocardium, the profile plots resemble an M-mode echocardiogram. Unlike in M-mode echocardiography, however, the axis of interrogation is not limited to specific windows, and there is distinct flexibility of contrast. However, the temporal resolution is currently less than that achieved by ultrasound. NMR M-mode profiling provides a direct, fast method of measuring heart motion to assess cardiac function as part of an MR cardiac exam.     

Fast spin-echo high-resolution MR imaging of the inner ear.

Advances in MR imaging continue to improve our ability to evaluate temporal bone anatomy and disease. CT remains the procedure of choice for fine-detail imaging of bone structures such as ossicular anatomy, but it is not the ideal imaging technique for soft-tissue structures (e.g., the membranous labyrinth and neural structures). Conventional spin-echo MR techniques used to image these structures cannot yield excellent contrast and spatial resolution in clinically acceptable time frames. Conventional spin-echo T1-weighted images lack tissue contrast between fluid (e.g., CSF, endolymph, perilymph), neural tissue, otic capsule septa, and surrounding temporal bone. Conventional T2-weighted imaging of the inner ear is needed to reveal the natural contrast between fluid, neural structures, and bone; unfortunately, the use of conventional T2-weighted images is limited by time constraints when large-matrix, thin-section techniques with more than one excitation are used. Fast spin-echo imaging is a recently developed technique that can provide T2-weighted, thin-section (2-mm) high-resolution images with excellent contrast in a fraction of the time needed for conventional spin-echo techniques. This speed advantage allows us to obtain high-resolution images in clinically acceptable time frames. Images produced by this technique are a useful addition, in conjunction with routine T1- and T2-weighted spin-echo images, in the diagnosis of disorders of the inner ear.     

Multishot 3D slice-select tailored RF pulses for MRI.

A multishot 3D slice-select tailored RF pulse method is presented for the excitation of slice profiles with arbitrary resolution. This method is derived from the linearity of the small tip angle approximation, allowing for the decomposition of small tip angle tailored RF pulses into separate excitations. The final image is created by complex summation of the images acquired from the individual excitations. This technique overcomes the limitation of requiring a long pulse to excite thin slices with adequate resolution. This has implications in applications including T*(2)-weighted functional MRI in brain regions corrupted by intravoxel dephasing artifacts due to susceptibility variations. Simulations, phantom experiments, and human brain images are presented. It is demonstrated that at most four shots of 40 ms pulse length are needed to excite a 5 mm-thick slice in the brain with reduced susceptibility artifacts at 3T.     

A simulation‐based analysis of the potential of compressed sensing for accelerating passive mr catheter visualization in endovascular therapy

Passive MRI is a promising approach to visualize catheters in guiding and monitoring endovascular intervention and may offer several clinical advantages over the current x‐ray fluoroscopy “gold standard.” Endovascular MRI has limitations, however, such as difficulty in visualizing catheters and insufficient temporal resolution. The multicycle projection dephaser method is a background signal suppression technique that improves the conspicuity of passive catheters by generating a sparse (i.e., catheter only) image. One approach to improve the temporal resolution is to undersample the k‐space and then apply nonlinear methods, such as compressed sensing, to reconstruct the MR images. This feasibility study investigates the potential synergies between multicycle projection dephaser and compressed sensing reconstruction for real‐time passive catheter tracking. The multicycle projection dephaser method efficiently suppressed the background signal, and compressed sensing allowed MR images to be reconstructed with superior catheter conspicuity and spatial resolution when compared to the more conventional zero‐filling reconstruction approach. Moreover, compressed sensing allowed the shortening of total acquisition time (by up to 32 times) by vastly undersampling the k‐space while simultaneously preserving spatial resolution and catheter conspicuity. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.