NMR functional imaging using a tailored RF gradient echo sequence: A true susceptibility measurement technique

Measurement of the resonances of glucose between 3.2 and 3.9 ppm in ¹H NMR spectra from the human brain is difficult due to spectral overlap with peaks from more concentrated metabolites. The H1 resonance of α-D-glucose at 5.23 ppm is resolved from other metabolite peaks, but potentially overlaps with the intense water signal at 4.72 ppm. This paper demonstrates that the increased resolution at 4 Tesla permits to suppress the water signal sufficiently to reliably detect glucose directly at 5.23 ppm by ¹H MRS and the estimated peak intensity is consistent with previous ¹³C NMR quantification.     

A novel flow-suppression technique using tailored RF pulses

The pulsatile nature of blood flow makes zipper-like artifacts along the coding direction in the two-dimensional Fourier transform NMR image. So far, spatial presaturation, one of the correction methods, is known to be effective in eliminating flow artifacts when the Fourier spin echo acquisition is employed. However, this method requires an additional RF pulse and a spoiling gradient for presaturation. Described in this paper is a new flow suppression technique, based on spin dephasing, using a set of tailored RF pulses. The proposed method does not require additional saturation RF pulses or spoiling gradient pulses, making it advantageous over other methods. In addition, the method is relatively robust to flow velocity. The proposed technique is equivalent to the existing flow saturation technique except that the elimination of the flow component is achieved by a pair of tailored 90–180° RF pulses in the spin echo sequence. The principle of the proposed method is the creation of a linear phase gradient within the slice along the slice selection direction for the moving material by use of two opposing quadratic phase RF pulses, i.e., 90° and 180° RF pulses with opposing quadratic phase distributions. That is to say, all the spins of the moving materials along the slice selection direction become dephased. Therefore, no observable signal is generated. Computer simulations and experimental results obtained using a 2.0-T whole-body imaging system on both a phantom and a human volunteer are also presented.     

Advanced three-dimensional tailored RF pulse for signal recovery in T2*-weighted functional magnetic resonance imaging.

T(2) (*)-weighted functional MR images are plagued by signal loss artifacts caused by susceptibility-induced through-plane dephasing. We present major advances to the original three-dimensional tailored RF (3DTRF) pulse method that pre-compensates the dephasing using three-dimensional selective excitation. The proposed 3DTRF pulses are designed iteratively with off-resonance incorporation and with a novel echo-volumar trajectory that frequency-encodes in z and phase-encodes in x,y. We also propose a computational scheme to accelerate the pulse design process. We demonstrate effective signal recovery in a 5-mm slice in both phantom and inferior brain, using 3DTRF pulses that are only 15.4 ms long. Compared to the original method, the new approach leads to significantly reduced pulse length and enhancement in slice selectivity. 3D images of the slice volume confirm fidelity of the excited phase pattern and slice profile.     

Reducing susceptibility artifacts in fMRI using volume-selective z-shim compensation.

Susceptibility-induced magnetic field gradients (SFGs) can result in severe signal loss in the orbitofrontal cortex (OFC) in gradient-echo-based functional MRI (fMRI) studies. Although conventional z-shim techniques can effectively recover the MRI signal in this region, the substantial penalty in imaging time hampers their use in routine fMRI studies. A modified z-shim technique with high imaging efficiency is presented in this study. In this technique, z-shim compensations are applied only to a selective volume where the susceptibility artifact is severe. The results of an fMRI study (N=6) demonstrate the feasibility of detecting the OFC activation with z-shim in whole-brain fMRI studies at a temporal resolution of 2 s.     

Multiband accelerated spin‐echo echo planar imaging with reduced peak RF power using time‐shifted RF pulses

Purpose:

To evaluate an alternative method for generating multibanded radiofrequency (RF) pulses for use in multiband slice‐accelerated imaging with slice‐GRAPPA unaliasing, substantially reducing the required peak power without bandwidth compromises. This allows much higher accelerations for spin‐echo methods such as SE‐fMRI and diffusion‐weighted MRI where multibanded slice acceleration has been limited by available peak power.

Theory and Methods:

Multibanded “time‐shifted” RF pulses were generated by inserting temporal shifts between the applications of RF energy for individual bands, avoiding worst‐case constructive interferences. Slice profiles and images in phantoms and human subjects were acquired at 3 T.

Results:

For typical sinc pulses, time‐shifted multibanded RF pulses were generated with little increase in required peak power compared to single‐banded pulses. Slice profile quality was improved by allowing for higher pulse bandwidths, and image quality was improved by allowing for optimum flip angles to be achieved.

Conclusion:

A simple approach has been demonstrated that significantly alleviates the restrictions imposed on achievable slice acceleration factors in multiband spin‐echo imaging due to the power requirements of multibanded RF pulses. This solution will allow for increased accelerations in diffusion‐weighted MRI applications where data acquisition times are normally very long and the ability to accelerate is extremely valuable. Magn Reson Med 69:1261–1267, 2013 Wiley Periodicals, Inc.     

Improvements in parallel imaging accelerated functional MRI using multiecho echo‐planar imaging

Multiecho echo‐planar imaging (EPI) was implemented for blood‐oxygenation‐level‐dependent functional MRI at 1.5 T and compared to single‐echo EPI with and without parallel imaging acceleration. A time‐normalized breath‐hold task using a block design functional MRI protocol was carried out in combination with up to four echo trains per excitation and parallel imaging acceleration factors R = 1–3. Experiments were conducted in five human subjects, each scanned in three sessions. Across all reduction factors, both signal‐to‐fluctuation‐noise ratio and the total number of activated voxels were significantly lower using a single‐echo EPI pulse sequence compared with the multiecho approach. Signal‐to‐fluctuation‐noise ratio and total number of activated voxels were also considerably reduced for nonaccelerated conventional single‐echo EPI when compared to three‐echo measurements with R = 2. Parallel imaging accelerated multiecho EPI reduced geometric distortions and signal dropout, while it increased blood‐oxygenation‐level‐dependent signal sensitivity all over the brain, particularly in regions with short underlying T*₂. Thus, the presented method showed multiple advantages over conventional single‐echo EPI for standard blood‐oxygenation‐level‐dependent functional MRI experiments. Magn Reson Med 63:959–969, 2010. © 2010 Wiley‐Liss, Inc.     

Diffusion tensor echo planar imaging using surface coil transceiver with a semiadiabatic RF pulse sequence at 14.1T

Diffusion magnetic resonance studies of the brain are typically performed using volume coils. Although in human brain this leads to a near optimal filling factor, studies of rodent brain must contend with the fact that only a fraction of the head volume can be ascribed to the brain. The use of surface coil as transceiver increases Signal‐to‐Noise Ratio (SNR), reduces radiofrequency power requirements and opens the possibility of parallel transmit schemes, likely to allow efficient acquisition schemes, of critical importance for reducing the long scan times implicated in diffusion tensor imaging. This study demonstrates the implementation of a semiadiabatic echo planar imaging sequence (echo time = 40 ms, four interleaves) at 14.1T using a quadrature surface coil as transceiver. It resulted in artifact free images with excellent SNR throughout the brain. Diffusion tensor derived parameters obtained within the rat brain were in excellent agreement with reported values. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Signal recovery in free induction decay imaging using a stimulated spin echo.

A novel pulse sequence for acquiring a stimulated spin echo (SSE) is devised and implemented to recover the initial portion of the signal that is usually blanked or distorted in MRI using free induction decays (FIDs). The receiver-phase offset, center of the k-space, and time frame coordinates of the FID data points can be accurately determined from the SSE. A simple numerical method of signal recovery is formulated which can merge the FID and the SSE data for higher extrapolation accuracy. The application of the signal recovery method is demonstrated using the rotating ultrafast imaging sequence (RUFIS). The result shows the importance of experimentally finding the correct time origin of the FID signal.     

Comparison of quantitative perfusion imaging using arterial spin labeling at 1.5 and 4.0 Tesla.

High-field arterial spin labeling (ASL) perfusion MRI is appealing because it provides not only increased signal-to-noise ratio (SNR), but also advantages in terms of labeling due to the increased relaxation time T(1) of labeled blood. In the present study, we provide a theoretical framework for the dependence of the ASL signal on the static field strength, followed by experimental validation in which a multislice pulsed ASL (PASL) technique was carried out at 4T and compared with PASL and continuous ASL (CASL) techniques at 1.5T, both in the resting state and during motor activation. The resting-state data showed an SNR ratio of 2.3:1.4:1 in the gray matter and a contrast-to-noise ratio (CNR) of 2.7:1.1:1 between the gray and white matter for the difference perfusion images acquired using 4T PASL, 1.5T CASL, and 1.5T PASL, respectively. However, the functional data acquired using 4T PASL did not show significantly improved sensitivity to motor cortex activation compared with the 1.5T functional data, with reduced fractional perfusion signal change and increased intersubject variability. Possible reasons for these experimental results, including susceptibility effects and physiological noise, are discussed.     

Quantification of blood velocity and flow rates in the uterine vessels using echo planar imaging at 0.5 Tesla

Purpose:

To measure uterine artery and vein blood velocity and flow rate profiles using MRI during normal pregnancy.

Materials and Methods:

A two‐shot velocity magnitude‐encoded echo planar imaging (EPI) sequence is used at a magnetic field 0.5T. Data analysis procedures, necessary to overcome problems associated with low signal to noise ratio (SNR), and a spatial resolution comparable to the vessel size were used.

Results:

The measured blood flow values averaged over nine volunteers for the mean velocity are 5.33 and 3.97 cm/s and for the unilateral flow rate are 203 and 274 mL/min (for the arteries and veins respectively). Values for the flow rate are consistent with ultrasound Doppler studies. Arterial velocity measurements are more pulsatile than venous ones and validation calculations performed on average velocity values would suggest that the nature of blood flow in the uterine vessels is laminar.

Conclusion:

This study presents the first report of noninvasive quantitative measurements of uterine artery and vein blood velocity and flow rate profiles using MRI during normal pregnancy. Consistent and reproducible measurements have been obtained by subject specific sequence optimization and data analysis procedures. J. Magn. Reson. Imaging 2010;31:921–927. ©2010 Wiley‐Liss, Inc.     

Optimization of blood oxygenation level-dependent sensitivity in magnetic resonance imaging using intermolecular double-quantum coherence

PURPOSE: To optimize timing parameters in an intermolecular double-quantum coherence (iDQC) imaging pulse sequence for overall image signal-to-noise ratio (SNR) and blood oxygenation level-dependent (BOLD) sensitivity for brain functional imaging. MATERIAL AND METHODS: Fresh human blood was measured under different oxygenation conditions, and human brain functional magnetic resonance (fMR) images in three normal volunteers were obtained, using iDQC techniques at 1.5 T. The dependence of SNR and BOLD sensitivity was measured as a function of time delays after the iDQC evolution period. RESULTS: A time delay after the iDQC evolution period tau can be adjusted either to refocus the dephasing accumulated during tau, thus increasing SNR, with full rephasing occurring at delay = +/-2tau (for iDQC order n = +/-2), or to enhance BOLD effects with consequent reduced image SNR at delay = 0. CONCLUSION: Image SNR and BOLD sensitivity often impose different requirements for iDQC image sequence design and timing parameter selections. It is therefore important to select properly relevant parameters for different applications.