Optimized phase scrambling for RF phase encoding

Ultra-fast MRI of the human brain with simultaneous multi-slice imaging

The recent advancement of simultaneous multi-slice imaging using multiband excitation has dramatically reduced the scan time of the brain. The evolution of this parallel imaging technique began over a decade ago and through recent sequence improvements has reduced the acquisition time of multi-slice EPI by over ten fold. This technique has recently become extremely useful for (i) functional MRI studies improving the statistical definition of neuronal networks, and (ii) diffusion based fiber tractography to visualize structural connections in the human brain. Several applications and evaluations are underway which show promise for this family of fast imaging sequences.

Compressed-sensing MRI with random encoding

Compressed sensing (CS) has the potential to reduce magnetic resonance (MR) data acquisition time. In order for CS-based imaging schemes to be effective, the signal of interest should be sparse or compressible in a known representation, and the measurement scheme should have good mathematical properties with respect to this representation. While MR images are often compressible, the second requirement is often only weakly satisfied with respect to commonly used Fourier encoding schemes. This paper investigates the use of random encoding for CS-MRI, in an effort to emulate the "universal" encoding schemes suggested by the theoretical CS literature. This random encoding is achieved experimentally with tailored spatially-selective radio-frequency (RF) pulses. Both simulation and experimental studies were conducted to investigate the imaging properties of this new scheme with respect to Fourier schemes. Results indicate that random encoding has the potential to outperform conventional encoding in certain scenarios. However, our study also indicates that random encoding fails to satisfy theoretical sufficient conditions for stable and accurate CS reconstruction in many scenarios of interest. Therefore, there is still no general theoretical performance guarantee for CS-MRI, with or without random encoding, and CS-based methods should be developed and validated carefully in the context of specific applications.

Spectroscopic imaging with volume selection by unpaired adiabatic pi pulses: theory and application

In NMR spectroscopy, volume selection can be advantageously achieved using adiabatic pi pulses, which enable high bandwidth and B(1) insensitivity. In order to avoid the generation of non-linear phase profiles and the subsequent signal loss caused by incoherent averaging, adiabatic pi pulses are usually used in pairs for volume selection in each spatial dimension. Alternatively, when performing spectroscopic imaging (SI), a high enough spatial resolution results in negligible phase dispersion within each pixel. This allows using only one pulse per selected spatial dimension, resulting in a reduced echo-time and reduced power deposition. In this work, the feasibility of such an approach is explored theoretically and numerically, allowing the derivation of explicit conditions to obtain SI images without artifact. Adequate spatial and spectral post-processing procedures are described to compensate for the effect of non-linear phase profiles. These developments are applied to SI in the rat brain at 9.4 T, using a new adiabatic sequence named Pseudo-LASER.

Improved resolution and signal-to-noise ratio in MRI via enhanced signal digitization

The high frequency k-space data in magnetic resonance imaging is often poorly reproduced due to the finite dynamic range of an analog-to-digital converter. The magnitude of this digitization error can equal and even exceed the magnitude of the thermal noise. Under such conditions, attempts to increase image signal-to-noise ratio via signal averaging meet with diminishing success. Because the relative size of the digitization error increases at higher spatial frequencies, a reduction in image resolution is incurred as well. By adjusting the level of the analog signal sampled by the analog-to-digital converter during the course of an imaging experiment, the magnitude of the digitization artifact can be greatly reduced. The results of simulations and imaging experiments are presented which demonstrate that this strategy improves both the signal-to-noise ratio and resolution of magnetic resonance images.

Steady state effects in fast gradient echo magnetic resonance imaging

Fourier methodology is applied to analyze steady state effects in fast gradient echo imaging. Simulations and MR imaging experiments on phantoms demonstrate the effectiveness of existing schemes under different experimental conditions, and modifications are introduced which result in reduced sensitivity to slow object motion as compared to conventional phase modulation schemes. In addition, phase modulation schemes are introduced which more than double temporal signal stability in human brain scans, at flip angles well above the Ernst angle and TR << T1, T2. Application of phase modulation to the measurement of higher order signals shows increased sensitivity, particularly for the highest order signals.

Analysis of localized quadratic encoding and reconstruction

Localized quadratic (LQ) encoding is a method of collecting multislice MRI data sets with greatly overlapping excited slices in such a way that the slice overlap can be deconvolved. This results in reconstructed data with resolution equal to the center-to-center slice spacing, regardless of the amount of excited slice overlap. LQ encoding is analyzed using the modulation transfer function of the encoding and reconstruction process. This allows analysis of many aspects of the technique in a well established theoretical framework. Many different characteristics of the method are explored, including excited slice profiles, required RF magnitudes and specific absorption rate, signal to noise ratio, signal dynamic range, reconstruction artifacts, sensitivity to motion, saturation and inflow effects, modulation transfer function shifting, and off-resonance artifacts. It is suggested that this technique is best suited for applications currently using multiple thin-slab three-dimensional encoding.

Spatial encoding and reconstruction in MRI with quadratic phase profiles

Application of a slice-selective frequency modulated radiofrequency excitation pulse in a magnetic resonance (MR) imaging experiment creates a quadratic phase profile along the direction of slice selection. This quadratic phase profile inherently localizes the signal contribution to its vertex, which can be shifted with the application of a linear magnetic field gradient. Different methods for reconstruction of the spatial profile are discussed. The point spread functions of these reconstructions are similar to that of conventional Fourier transforms; one important difference is the elimination of aliasing at the expense of MR signal amplitude when performing 'local,' or limited, reconstruction. Limited reconstruction together with spatially limited excitation combines some of the advantages of both three-dimensional phase-encoded and two-dimensional multislice techniques in appropriate situations. Applications discussed are three-dimensional gradient echo experiments applied to time of flight MR angiography and T2(+)-weighted data collection.