Synchronous Radial 1H and 23Na Dual-Nuclear MRI on a Clinical MRI System, Equipped With a Broadband Transmit Channel

Abstract

The purpose of this work was to synchronously acquire proton (1H) and sodium (23Na) image data on a 3T clinical MRI system within the same sequence, without internal modification of the clinical hardware, and to demonstrate synchronous acquisition with 1H/23Na-GRE imaging with Cartesian and radial k-space sampling. Synchronous dual-nuclear imaging was implemented by: mixing down the 1H signal so that both the 23Na and 1H signal were acquired at 23Na frequency by the conventional MRI system; interleaving 1H/23Na transmit pulses in both Cartesian and radial sequences; and using phase stabilization on the 1H signal to remove mixing effects. The synchronous 1H/23Na setup obtained images in half the time necessary to sequentially acquire the same 1H and 23Na images with the given setup and parameters. Dual-nuclear hardware and sequence modifications were used to acquire 23Na images within the same sequence as 1H images, without increases to the 1H acquisition time. This work demonstrates a viable technique to acquire 23Na image data without increasing 1H acquisition time using minor additional custom hardware, without requiring modification of a commercial scanner with multinuclear capability.

Keywords: dual-nuclear mri; metabolic imaging; multinuclear imaging; radiofrequency coils; sodium.

Fig. 1.

(a) Transmit/receive hardware for both 1H and 23Na. Arrows indicate transmit / receive pathways. The host system provided both 1H and 23Na transmit pulses through a single channel, which passed through a commercial 1H/23Na TR switch. The 23Na coil connected directly to the 1H/23Na TR switch; the 1H channel runs through a second 1H TR switch before connecting to the coil. The second 1H TR switch was necessary to redirect the 1H RF for mixing and down-conversion to 32MHz for simultaneous 1H/23Na reception. An attenuator is necessary due to the significant signal difference between 23Na and 1H. (b) TR switch schematic for 1H. The 1H signal is amplified at 123MHz before down-conversion to 32MHz.

Fig. 2.

(a) Linear sodium coil and linear proton coil concentric with each other and decoupled using traps. (b) Quadrature sodium coil (central circular loop and butterfly loop) and linear proton coil, each overlapped to minimize mutual inductive coupling.

Fig. 3.

Pulse sequence diagram for synchronous GRE imaging of 23Na and1H. Although the two frequencies share gradients, they are received on separate channels. RF and gradient pulses for 1H imaging are applied prior to the 23Na excitation RF pulse. All phase-encoding gradients are rewound (Grw) just before the acquisition of the phase reference echo. The 1H gradients before the 23Na pulse do not affect the 23Na signal, so that the 1H slice and phase encodes can be independent from 23Na slice and phase encodes. The method shown was used for both Cartesian and radial sampling. The phase reference pulse did not occur during GRE imaging, as there was sufficient 1H signal to enable phase correction without a reference pulse.

Fig. 4.

(a,b) Single-nuclear and (c,d) synchronously acquired proton images using Cartesian GRE reconstructed (a,c) without and (b,d) with phase correction from the reference echo. The images in (c,d) were obtained synchronously with sodium images in Fig. 5. (e) The SNR difference is shown between a registered single-nuclear image and phase-corrected synchronous image shows less than 10% SNR difference in the phantoms.

Fig. 5.

(a) Single-nuclear and (b) synchronously acquired sodium images using Cartesian GRE. The sodium image in (b) was obtained synchronously with proton images in Fig. 4. (c) The SNR difference is shown between the shifted, registered single-nuclear image and the phase-corrected synchronous data shows less than 6% SNR difference in the phantoms. The full reconstructed sodium FOV is shown here, which is 3.8 times smaller than the proton FOV.

Fig. 6.

(a-c) Proton and (d-f) sodium SMI images acquired during a single synchronous 21-minute 3D GRE Cartesian acquisition. Three orthogonal slices from a full 3D data set are shown for both (a-c) proton and (d-f) sodium images. Images are shown at the same physical scale showing the entire 1H FOV and ¼ of the 23Na FOV.

Fig. 7.

Single-nuclear (a,b) and synchronous multinuclear (c,d) knee images of a healthy volunteer. (a) and (b) were obtained in two 7 minute acquisitions for a total of a 14 minute scan time to acquire both nuclear images. (c) and (d) were obtained in a single 7 minute acquisition to obtain both nuclear images. The structure at the right of the proton images (a,c) is a plastic coil element that is visible because of the ultra-short TE acquisition.