Sodium MRI using a density‐adapted 3D radial acquisition technique

Abstract

A density-adapted three-dimensional radial projection reconstruction pulse sequence is presented which provides a more efficient k-space sampling than conventional three-dimensional projection reconstruction sequences. The gradients of the density-adapted three-dimensional radial projection reconstruction pulse sequence are designed such that the averaged sampling density in each spherical shell of k-space is constant. Due to hardware restrictions, an inner sphere of k-space is sampled without density adaption. This approach benefits from both the straightforward handling of conventional three-dimensional projection reconstruction sequence trajectories and an enhanced signal-to-noise ratio (SNR) efficiency akin to the commonly used three-dimensional twisted projection imaging trajectories. Benefits for low SNR applications, when compared to conventional three-dimensional projection reconstruction sequences, are demonstrated with the example of sodium imaging. In simulations of the point-spread function, the SNR of small objects is increased by a factor 1.66 for the densityadapted three-dimensional radial projection reconstruction pulse sequence sequence. Using analytical and experimental phantoms, it is shown that the density-adapted three-dimensional radial projection reconstruction pulse sequence allows higher resolutions and is more robust in the presence of field inhomogeneities. High-quality in vivo images of the healthy human leg muscle and the healthy human brain are acquired. For equivalent scan times, the SNR is up to a factor of 1.8 higher and anatomic details are better resolved using density-adapted three-dimensional radial projection reconstruction pulse sequence. Key words: sodium magnetic resonance imaging; densityadapted sampling; radial imaging; projection reconstruction; sampling density; field inhomogeneities Sodium ( 23 Na) ions play an important role in cellular homeostasis and cell viability. In healthy tissue, the extracellular sodium concentration ([Na ϩ ] ex ϭ 145 mM) is about 10 times higher than the intracellular concentration ([Na ϩ ] in ϭ 10-15 mM) (1). Using sodium MRI, volume-and relaxation-weighted signal of these compartments can be measured. Thus, sodium MRI is a promising diagnostic tool since pathologic processes can alter this ion gradient.Many studies investigating the usefulness of sodium MRI in human pathologies have been performed recently. Brain neoplasia and sustained cell depolarization, a precursor of cell division, lead to an increase of the intracellular sodium concentration and to a rise in the average tissue sodium concentration (2). Furthermore, the application of sodium MRI has been shown to be valuable for muscular channelopathies (3,4), brain tumors (5), the human kidney (6), myocardial infarction (7), and cerebral ischemia (8,9) diagnostics.However, sodium MRI remains a challenging technique for several reasons. The sodium nucleus exhibits a fast biexponential transversal relaxation in the extreme narrowing limit, i.e., if the correlation time is much shorter...

The design of a double-tuned two-port surface resonator and its application to in vivo Hydrogen- and Sodium-MRI

“…In this study, it is likely that a compromise was reached between the SNR efficiencies for the 23 Na and 39 K channels, perhaps focusing on the latter, less sensitive nuclei at the expense of the 23 Na nuclei's sensitivity. The SNR can be further optimized through the use of an ultra-short TE sequence, such as the recently published density adapted radial technique [34].…”

How can sodium MRI techniques help us understand acute stroke?

“…examples include single-tuned resonators for 23 Na frequencies only, necessitating a swop of the RF coils to acquire 1 H images (6,3) , or dual-tuned to allow for 1 H-and 23 Na imaging without disturbing the patient in the scanner and losing valuable time (30,31) . However, such dual coil configurations invariably compromise the SNR on one channel, usually selected to be the 1 H channel, and the effect this may have on the quality of the images typically acquired as part of a clinical stroke protocol, for example involving time-of-flight angiography or echo planar imaging-based diffusion and perfusion measurements, have not been reported.…”

“…However, a further complication for Na-MRI arises from the quadrupolar nature of the sodium nucleus due to its spin 3/2 property, which due to its interaction with local electric field gradients from neighbouring nuclei in the tissue leads to a biexponential transversal T 2 relaxation, with fast and slow components (T 2 ~ 1-2 ms and 15-20 ms in the brain, respectively) with relative proportions of 60% and 40% respectively. To acquire signal from the fast component, one must use a short echo time (TE) imaging sequence; examples of such sequences which have been used for sodium imaging in stroke include 3D projectionreconstruction (3DPR) (35) , Twisted Projection Imaging (TPI) (36) , and density-adapted 3DPR (31) .…”

“…The first such UTE sequences developed were twisted projection imaging (TPI) (36) , followed in later years by 3D Cones (51) , 3D projection-reconstruction (3DPR) (35) and most recently density-adapted 3DPR (31) and its improved 2D variant (52) . Other techniques have been successfully adapted for use in sodium imaging, such as single point ramped imaging with T 1 enhancement (SPRITE), where a Centric SPRITE variant was recently used to quantify the TSC in brain tumours (53) , while the Sweep Imaging with Fourier…”

Rapid computation of sodium bioscales using gpu‐accelerated image reconstruction

“…The resultant quantitative map of TSC, termed a bioscale (Atkinson et al, 2010;Lu et al, 2010b), reflects sodium ion homeostasis that is tightly regulated in healthy tissue (Somjen, 2004). Although the clinical potential of 23 Na neuroimaging has been recognized for decades (Hilal et al, 1985), high-and ultra-high field MR scanners and optimized ultra-short echo time pulse sequences (Boada et al, 1997;Gurney et al, 2006;Qian et al, 2008;Nagel et al, 2009;Lu et al, 2010) have only recently made quantitative sodium MR imaging of the human brain possible with acceptable acquisition times of 8-10 min. TSC can be combined with a biochemical tissue model to compute the interstitial volume fraction (IVF) or tissue cell fraction (TCF) bioscales (Atkinson et al, accepted for publication).…”