Time-efficient interleaved human²³Na and¹H data acquisition at 7 T

Abstract

The aim of this study was to implement and evaluate a flexible and time-efficient interleaved imaging approach for the acquisition of proton and sodium images of the human knee at 7 T within a clinically relevant timescale. A flexible software framework was established which allowed the interleaving of multiple, different, fully specific absorption ratio (SAR)-validated scans. The system was able to switch between these different scans at flexible time points. The practical example presented consists of interleaved proton (Dixon imaging and T2* mapping) and sodium (mapping the sodium content and fluid-suppressed component separately) sequences with the key idea to perform proton MRI whilst the sodium nuclei relax towards thermal equilibrium, and vice versa. Comparisons were made between these four scans being acquired sequentially in the normal mode of scanner operation and those acquired in an interleaved fashion. Images acquired in the interleaved mode were very similar to those acquired in sequential scans with no image artifacts produced by the slight intra-sequence variation in steady-state magnetization. A reduction in scanning time of almost a factor of two was established using the interleaved scans, allowing such a protocol to be completed within 30 min. Phantom experiments and in vivo scans performed in healthy volunteers and in one patient proved the basic feasibility of this approach. This approach for the interleaving of multiple proton and sodium scans, each with different contrasts, is an efficient method for the design of new practical clinical protocols for sodium MRI.

Simultaneous proton magnetic resonance fingerprinting and sodium MRI

“…To enable simultaneous sodium imaging during 1 H MRF, a 23 Na excitation with constant flip angle (1-ms Gaussian pulse, 30° flip angle) is placed after each proton pulse ( Figure 1). A gradient blip is inserted between the proton and sodium slice selection gradients, such that both isochromats are refocused at the end of the sodium slice rephasing gradient.…”

“…12 Consequently, intra-and extracellular sodium concentrations reflect important metabolic information that could help diagnose and understand many different pathologies. 13 The naturally abundant 23 Na nucleus has a spin 3/2 and is therefore MR visible. 14 However, sodium has a gyromagnetic ratio lower than that of protons ( 23 Na γ/2π ≈ 11.3 MHz/T versus 1 H γ/2π ≈ 42.6 MHz/T), and its concentration is approximately 2000 times lower than that of protons in brain tissue.…”

“…16 In addition, the quadrupolar nature of the sodium nuclear spin is responsible for its relatively short and biexponential T 2 decay (T 2,short = 0.5-5.0 ms, T 2,long = 15-60 ms) attributable to dominant quadrupolar relaxation processes. 17,18 In practice, these challenges lead to a relatively coarse image resolution (3-5 mm isotropic for 23 Na versus 0.75-1.00 mm for 1 H) and relatively long scan times (5-30 minutes, depending on the resolution and contrast of interest). 19 Sodium MRI is usually applied in conjunction with proton MRI, such that the low-resolution sodium image can be coregistered to one or more high-resolution proton images.…”

Synchronous radial 1H and 23Na dual‐nuclear MRI on a clinical MRI system, equipped with a broadband transmit channel

“…The increased complexities in both hardware and pulse sequence have limited the application of SMI on clinical MRI systems, as typical clinical systems are capable of irradiating or receiving only at 1 H frequency. Any system that is equipped with a broadband amplifier and that can demodulate both sodium and proton frequencies has the basic hardware capability to do SMI through interleaving proton and non‐proton pulses and acquisition . Even when a system has dual resonant 1 H/ 23 Na coils and multi‐nuclear MR capability, SMI is still uncommon because: (1) non‐proton imaging or spectroscopy on most human research systems is enabled by a switch that limits transmission to a single RF amplifier at any given time; and, (2) human research systems lack appropriate software and/or demodulation hardware that enables reception of two frequencies in a sequence.…”

“…In vivo 1 H/ 23 Na‐SMI has been reported in the brain, breast, and knee, with Cartesian gradient echo (GRE) imaging for both nuclei, which is inefficient for obtaining 23 Na images that have a short T 2 * (~15 ms). de Bruin has also shown SMI with radial sampling using a software‐based interleaved imaging approach . In this manuscript, we present a novel technique for SMI to obtain synchronous 1 H/ 23 Na‐GRE images with Cartesian sampling.…”