Ramped hybrid encoding for improved ultrashort echo time imaging

Abstract

Purpose We propose a new acquisition to minimize the per-excitation encoding duration and improve the imaging capability for short T2* species. Method In the proposed ramped hybrid encoding (RHE) technique, gradients are applied before the RF pulse as in pointwise encoding time reduction with radial acquisition (PETRA) and Zero TE (ZTE) imaging. However, in RHE, gradients are rapidly ramped after RF excitation to the maximum amplitude to minimize encoding duration. To acquire central k-space data not measured during RF deadtime, RHE utilizes a hybrid encoding scheme similar to PETRA. A new gradient calibration method based on single-point imaging was developed to estimate the k-space trajectory and enable robust and high quality reconstruction. Result RHE enables a shorter per-excitation encoding time and provides the highest spatial resolution among ultrashort T2* imaging methods. In phantom and in vivo experiments, RHE exhibited robust imaging with negligible chemical shift or blurriness caused by T2* decay and unwanted slice selection. Conclusion RHE allows the shortest per-excitation encoding time for ultrashort T2* imaging, which alleviates the impact of fast T2* decay occurring during encoding, and enables improved spatial resolution.

Zero‐gradient‐excitation ramped hybrid encoding (zGRF‐RHE) sodium MRI

“…For the measurements presented in this work, only up to 27 single points needed to be acquired. Based on the The single point measurements in zG RF -RHE can be exploited to analyze eddy current-related distortions and correct gradient imperfections as suggested by Jang et al 23 Although this correction has not been applied in this work, it can provide further image quality improvements. Given the moderate gradient strengths ramped during the dead-time, eddy current effects are, however, expected to be minor in this study.…”

“…Imaging was performed on a research 7T MRI scanner (Siemens Healthcare, Erlangen, Germany) with a transmit/receive dual-tuned 1 H- 23 Na head coil (QED, Mayfield Village, OH).…”

A rapid and robust gradient measurement technique using dynamic single‐point imaging

“…Theoretically, the FOV in a 1D SPI image is a direct and independent measure of k‐space position, as implied in Equation []. This allows flexibility to measure k‐space trajectory in any type of pulse sequences, even when the central region of k‐space is not acquired, as in hybrid encoding and zero echo time (TE) imaging . Furthermore, in the SPI‐based method, noise or mis‐estimation at earlier encoding times is not propagated to the later estimation because no integration or cumulative summation is required to obtain k‐space position.…”

Technical Note: Deep learning based MRAC using rapid ultrashort echo time imaging

“…(a). Both k‐spaces in UTE and out‐of‐phase echo are hybrid‐encoded to improve image quality in which central k‐space is encoded by single point imaging (SPI), and outer k‐space is acquired by frequency encoding as illustrated in Fig. (b).…”