Exploring the bandwidth limits of ZTE imaging: Spatial response, out-of-band signals, and noise propagation.

PURPOSE: Zero echo time (ZTE) imaging with single-pulse excitation is a fast, robust, and silent three-dimensional (3D) method for MRI of short T2 tissues. In this technique, algebraic reconstruction serves to fill gaps in the center of k-space due to finite acquisition dead time. The purpose of this study was to investigate the effect of this operation on depiction characteristics, noise behavior, and achievable bandwidth. METHODS: The spatial response function (SRF) and noise covariance resulting from ZTE reconstruction were studied using formal analysis, simulations, and phantom experiments. RESULTS: Three prominent limiting phenomena were identified: SRF behavior within the field of view, heightened sensitivity to out-of-band signal sources, and noise amplification. The related errors all appear as image distortions of low spatial frequency and are strongly attenuated upon the transition from one-dimensional projections to 3D image data. Relying on these observations, ZTE imaging was accomplished with a previously unreached gap size, permitting the depiction of a solid sample with T2 ≈ 25 µs at a bandwidth of 500 kHz. CONCLUSION: The tightest bandwidth limits in ZTE arise from background signal and radiofrequency (RF) switching transients. Significant advances in ZTE performance will be afforded by faster transmit-receive (T/R) switching with negligible transients and RF coils free of background signal.

A double-tuned (1)H/(23)Na dual resonator system for tissue sodium concentration measurements in the rat brain via Na-MRI.

A method for quantifying the tissue sodium concentration (TSC) in the rat brain from ²³Na-MR images was developed. TSC is known to change in a variety of common human diseases and holds considerable potential to contribute to their study; however, its accurate measurement in small laboratory animals has been hindered by the extremely low signal to noise ratio (SNR) in ²³Na images. To address this, the design, construction and characterization of a double-tuned ¹H/²³Na dual resonator system for ¹H-guided quantitative ²³Na-MRI are described. This system comprises an SNR-optimized surface detector coil for ²³Na image acquisition, and a volume resonator producing a highly homogeneous B1 field (<5% inhomogeneity) for the Na channel across the rat head. The resonators incorporated channel-independent balanced matching and tuning capabilities with active decoupling circuitry at the ²³Na resonance frequency. A quantification accuracy of TSC of <10 mM was achieved in Na-images with 1.2 µl voxel resolution acquired in 10 min. The potential of the quantification technique was demonstrated in an in vivo experiment of a rat model of cerebral stroke, where the evolution of the TSC was successfully monitored for 8 h after the stroke was induced.