Generalized encoding through the use of selective excitation in accelerated parallel MRI

Sparse parallel transmission on randomly perturbed spiral k-space trajectory

Combination of parallel transmission and sparse pulse is able to shorten the excitation by using both the coil sensitivity and sparse k-space, showing improved fast excitation capability over the use of parallel transmission alone. However, to design an optimal k-space trajectory for sparse parallel transmission is a challenging task. In this work, a randomly perturbed sparse k-space trajectory is designed by modifying the path of a spiral trajectory along the sparse k-space data, and the sparse parallel transmission RF pulses are subsequently designed based on this optimal trajectory. This method combines the parallel transmission and sparse spiral k-space trajectory, potentially to further reduce the RF transmission time. Bloch simulation of 90° excitation by using a four channel coil array is performed to demonstrate its feasibility. Excitation performance of the sparse parallel transmission technique at different reduction factors of 1, 2, and 4 is evaluated. For comparison, parallel excitation using regular spiral trajectory is performed. The passband errors of the excitation profiles of each transmission are calculated for quantitative assessment of the proposed excitation method.

Two-axis acceleration of functional connectivity magnetic resonance imaging by parallel excitation of phase-tagged slices and half k-space acceleration

Whole brain functional connectivity magnetic resonance imaging requires acquisition of a time course of gradient-recalled (GR) volumetric images. A method is developed to accelerate this acquisition using GR echo-planar imaging and radio frequency (RF) slice phase tagging. For N-fold acceleration, a tailored RF pulse excites N slices using a uniform-field transmit coil. This pulse is the Fourier transform of the profile for the N slices with a predetermined RF phase tag on each slice. A multichannel RF receive coil is used for detection. For n slices, there are n/N groups of slices. Signal-averaged reference images are created for each slice within each slice group for each member of the coil array and used to separate overlapping images that are simultaneously received. The time-overhead for collection of reference images is small relative to the acquisition time of a complete volumetric time course. A least-squares singular value decomposition method allows image separation on a pixel-by-pixel basis. Twofold slice acceleration is demonstrated using an eight-channel RF receive coil, with application to resting-state functional magnetic resonance imaging in the human brain. Data from six subjects at 3 T are reported. The method has been extended to half k-space acquisition, which not only provides additional acceleration, but also facilitates slice separation because of increased signal intensity of the central lines of k-space coupled with reduced susceptibility effects.

MR fluoroscopy in vascular and cardiac interventions (review)

Vascular and cardiac disease remains a leading cause of morbidity and mortality in developed and emerging countries. Vascular and cardiac interventions require extensive fluoroscopic guidance to navigate endovascular catheters. X-ray fluoroscopy is considered the current modality for real time imaging. It provides excellent spatial and temporal resolution, but is limited by exposure of patients and staff to ionizing radiation, poor soft tissue characterization and lack of quantitative physiologic information. MR fluoroscopy has been introduced with substantial progress during the last decade. Clinical and experimental studies performed under MR fluoroscopy have indicated the suitability of this modality for: delivery of ASD closure, aortic valves, and endovascular stents (aortic, carotid, iliac, renal arteries, inferior vena cava). It aids in performing ablation, creation of hepatic shunts and local delivery of therapies. Development of more MR compatible equipment and devices will widen the applications of MR-guided procedures. At post-intervention, MR imaging aids in assessing the efficacy of therapies, success of interventions. It also provides information on vascular flow and cardiac morphology, function, perfusion and viability. MR fluoroscopy has the potential to form the basis for minimally invasive image-guided surgeries that offer improved patient management and cost effectiveness.

Minimum envelope roughness pulse design for reduced amplifier distortion in parallel excitation

Parallel excitation uses multiple transmit channels and coils, each driven by independent waveforms, to afford the pulse designer an additional spatial encoding mechanism that complements gradient encoding. In contrast to parallel reception, parallel excitation requires individual power amplifiers for each transmit channel, which can be cost prohibitive. Several groups have explored the use of low-cost power amplifiers for parallel excitation; however, such amplifiers commonly exhibit nonlinear memory effects that distort radio frequency pulses. This is especially true for pulses with rapidly varying envelopes, which are common in parallel excitation. To overcome this problem, we introduce a technique for parallel excitation pulse design that yields pulses with smoother envelopes. We demonstrate experimentally that pulses designed with the new technique suffer less amplifier distortion than unregularized pulses and pulses designed with conventional regularization.

MRI-Guided Electrophysiology Intervention

Catheter ablation is a first-line treatment for many cardiac arrhythmias and is generally performed under X-ray fluoroscopy guidance. However, current techniques for ablating complex arrhythmias such as atrial fibrillation and ventricular tachycardia are associated with sub-optimal success rates and prolonged radiation exposure. Pre-procedure 3-D magnetic resonance imaging (MRI) has improved understanding of the anatomic basis of complex arrhythmias and is being used for planning and guidance of ablation procedures. A particular strength of MRI compared to other imaging modalities is the ability to visualize ablation lesions. Post-procedure MRI is now being applied to assess ablation lesion location and permanence with the goal of identifying factors leading to procedure success and failure. In the future, intra-procedure real-time MRI, together with the ability to image complex 3-D arrhythmogenic anatomy and target additional ablation to regions of incomplete lesion formation, may allow for more successful treatment of even complex arrhythmias without exposure to ionizing radiation. Development of clinical grade MRI-compatible electrophysiology devices is required to transition intra-procedure MRI from preclinical studies to more routine use in patients.

Combining RF encoding with parallel imaging: a simulation study

OBJECT

The aim of this work was to investigate combining spatial encoding by radio frequency (RF) excitation with conventional parallel imaging (PI) methods to determine whether this could improve overall imaging performance.

MATERIALS AND METHODS

A simulation framework was developed to predict imaging performance for regular, central and random under-sampled parallel imaging methods augmented by RF spatial signal modulation. Optimisation methods were used to find the RF modulation patterns that produce optimal image reconstruction using the condition number of the PI encoding matrix as a quality metric. The diverse patterns of raw data sampling produced were compared using a measure of data uniformity across k-space.

RESULTS

Regular under-sampling of k-space provided the best reconstruction quality. When other under-sampling schemes were employed then RF modulation could be used to improve reconstruction, with the optimum achieved by redistributing the signal in k-space to return to regular sub-sampling. For all tested under-sampling patterns, no further improvements in image quality were attained.

CONCLUSION

Using the simulation framework and metrics described the interaction of different spatial encoding approaches could be investigated. Regular sub-sampling provided optimal reconstruction, independent of whether the spatial encoding was achieved by gradients only or a combination of gradient and RF.

Cardiovascular magnetic resonance guided electrophysiology studies

Catheter ablation is a first line treatment for many cardiac arrhythmias and is generally performed under x-ray fluoroscopy guidance. However, current techniques for ablating complex arrhythmias such as atrial fibrillation and ventricular tachycardia are associated with suboptimal success rates and prolonged radiation exposure. Pre-procedure 3D CMR has improved understanding of the anatomic basis of complex arrhythmias and is being used for planning and guidance of ablation procedures. A particular strength of CMR compared to other imaging modalities is the ability to visualize ablation lesions. Post-procedure CMR is now being applied to assess ablation lesion location and permanence with the goal of indentifying factors leading to procedure success and failure. In the future, intra-procedure real-time CMR, together with the ability to image complex 3-D arrhythmogenic anatomy and target additional ablation to regions of incomplete lesion formation, may allow for more successful treatment of even complex arrhythmias without exposure to ionizing radiation. Development of clinical grade CMR compatible electrophysiology devices is required to transition intra-procedure CMR from pre-clinical studies to more routine use in patients.

Parallel magnetic resonance imaging

Parallel imaging has been the single biggest innovation in magnetic resonance imaging in the last decade. The use of multiple receiver coils to augment the time consuming Fourier encoding has reduced acquisition times significantly. This increase in speed comes at a time when other approaches to acquisition time reduction were reaching engineering and human limits. A brief summary of spatial encoding in MRI is followed by an introduction to the problem parallel imaging is designed to solve. There are a large number of parallel reconstruction algorithms; this article reviews a cross-section, SENSE, SMASH, g-SMASH and GRAPPA, selected to demonstrate the different approaches. Theoretical (the g-factor) and practical (coil design) limits to acquisition speed are reviewed. The practical implementation of parallel imaging is also discussed, in particular coil calibration. How to recognize potential failure modes and their associated artefacts are shown. Well-established applications including angiography, cardiac imaging and applications using echo planar imaging are reviewed and we discuss what makes a good application for parallel imaging. Finally, active research areas where parallel imaging is being used to improve data quality by repairing artefacted images are also reviewed.