Research Publications

New from ISMRM 2008 in Toronto, Canada

Title: 7 Tesla Sodium MRI of Patients with Multiple Sclerosis: A Preliminary Study PDF
Introduction: Multiple Sclerosis (MS) is a multi-focal, inflammatory, demyelinating disease with pathological involvement of neurons and axons1. Pathological and imaging studies have shown that neuro-axonal loss occurs progressively from the onset of the disease, and that it correlates with disability. These findings have prompted the search for mechanisms of neurodegeneration and for surrogate markers to monitor in vivo degeneration and neuroprotection. There is increasing evidence that the intra-axonal accumulation of sodium ions may contribute to degeneration2,3. In addition, partial blockade of sodium channels protects axons from degeneration in experimental models of MS, and it is currently under nvestigation in clinical trials4. Sodium MR Imaging (23Na MRI) provides an indicator of cellular and metabolic integrity and ion homeostasis and has been applied to the study of patients with brain tumors and stroke5. The low sensitivity and spatial resolution of 23Na MRI make higher field strengths desirable to improve this imaging modality. The aim of this study was to optimize 23Na MRI at 7T and investigate its feasibility in the study of patients with MS.

Title: Assessment of Renal Function using MR Renography without Aortic Input Information PDF
Introduction: Dynamic contrast-enhanced MR renography (MRR) has been shown to be a valuable approach for the non-invasive assessment of renal function. Most current tracer-kinetic models to analyze MRR data require an input concentration versus time curve [1,2], which is usually measured from an aortic region of interest (ROI). However, aortic concentration versus time curve obtained from MR images suffers from several measurement errors, such as partial volume problem, flow effects, intravoxel dephasing, T2* effects, and others. Such errors in aortic curve may be propagated into the estimated renal parameters, such as glomerular filtration rate (GFR). In this study, we explored a new modeling approach which obviates input function measurements and estimates GFR based only on renal cortex and medulla enhancement curves.

Title: Electrodynamic constraints on homogeneity and RF power deposition in multiple coil excitations PDF
Introduction: Correction of B1 inhomogeneities and management of SAR are among the most difficult challenges faced by in-vivo ultra high field MR applications. The use of multiple independently driven transmit elements, which enables fine control over the distribution of electromagnetic fields, has been explored as a possible solution and various approaches have been developed. In parallel transmission techniques [1,2] the composite B1 field can be modulated in space and time by adjusting the independent RF waveforms transmitted by each coil, in order to generate a target excitation profile while minimizing SAR [2]. There are inherent limitations to this dual optimization process, which were described at a recent conference [3] for a uniform spherical sample. In this work we propose a different electrodynamic formulation for the calculation of ultimate intrinsic SAR, which enables derivation of the corresponding optimal current patterns on the surface of the sphere. The trade-off between transmit homogeneity and SAR reduction was investigated using this formulation, and the behavior of ideal current patterns was studied.

Title: A Novel Non-Contrast MR Angiography Technique Using Triggered Non-Selective Refocused SPACE for Improved Spatial Resolution PDF
Introduction: : A non-contrast-enhanced MRA technique using gated 3D Turbo Spin Echo (TSE) has been reported for various clinical applications [1] [2], whereby systolic images are subtracted from diastolic images to provide high resolution MRA without exogenous contrast agents. Half-Fourier single-shot TSE results in a relatively long readout duration which causes vessel blurring due to T2 filtration effects and susceptibility to motion artifacts and arrhythmias. To reduce the long acquisition window for this cardiac imaging, one can use parallel imaging to reduce the acquired lines [3] and higher receiver bandwidth to reduce echo spacing time (ETS) [4],another potential solution to this limitation include multi-shot approach with the trade-off of increased scan time is less clinically acceptable especially as the technique is subtraction based and long scan times result in higher potential for motion between the corresponding systolic and diastolic scans.

As an approach to alleviating the spatial resolution limitations and to improve the reliability of this technique in fast heart rates we have investigated the use of a variable flip angle method for non contrast MRA using triggered 3D SPACE (Sampling Perfection with Application optimized Contrasts by using different flip angle Evolutions), which uses selective RF excitation, non selective RF refocusing pulses and variable flip angles for contrast manipulation [5]. To determine times of peak and minimal flow within the cardiac cycle, triggered (pulse or ECG) phase contrast flow measurements were performed. SPACE helps achieve lower SAR and shorter ETS of 2 ms, resulting in total acquisition windows of around 100ms, a slice turbo factor in 3D direction can be used to speed up the acquisition, because two or more partitions can be acquired in one cardiac cycle. Figure 1 shows the intrinsic contrasts mechanisms between the two techniques, TSE based approach uses C-A, SPACE based approach use B-A. We performed a comparison between the SPACE and the TSE based approaches in terms of spatial resolution and reduced blurring and small vessel conspicuity.

Title: An MRI System with 128 Seamlessly Integrated Receive Channels for Multi-Nuclear Operation PDF
Introduction: Many groups are investigating the feasibility and advantages of array coils with an increasing number of coil elements beyond the industry standard of 16 to 32 channel systems. Several prototypes have been described with 64 channels [1], 96 channels [2], and with 128 channels [3, 4]. This paper describes the second 128 receive channel MR system installed at a customer site, based on a commercial 3T MAGNETOM Tim Trio, which allows the simultaneous acquisition and online reconstruction of images from 128 receivers. Unlike the system described in [3], this system offers multi-nuclear capability. Amongst many other options, this enables exploration of highly accelerated parallel imaging of hyperpolarized media. This combination of hyperpolarization with a very high number of receive elements represents an attractive proposition for a number of reasons: Firstly hyperpolarized media provide an initial boost of SNR based on the higher polarization compared to thermal polarization of protons. Secondly, the SNR in conventional thermally polarized parallel imaging is inevitably reduced by increasing acceleration factors, whereas hyperpolarized imaging works with a fixed pool of polarization that does not recover, and thus the flip angle for each excitation can be proportionally increased in the presence of acceleration, so that the SNR loss traditionally associated with parallel imaging may be mitigated or even eliminated [5,6]. Furthermore, applications can be realized that critically depend on short scan intervals. As an example, we show 128 channel lung images with hyperpolarized helium (3He).

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