RF encoding using a multielement parallel transmit system

Abstract

Radiofrequency (RF) encoding using spatially variant RF transmission fields represents an alternative to the conventional signal-encoding techniques applied in MRI, which are based on main field gradients. Thus, RF encoding might allow omitting the use of all main field gradients, alleviating acoustic noise and other main field gradient-related problems. This study investigates the potential of RF transmit encoding using spatially nonlinear RF fields generated by an eight-channel parallel transmit system equipped with the corresponding transmit array. Appropriate spatial encoding functions are determined iteratively by randomized superposition of the array element sensitivities and judging their performance. Besides RF amplitude-based signal encoding, the possibility of RF phase-dominated encoding is investigated to allow future fast imaging applications. The theoretical background is given and experimental phantom results are compared with predictions from corresponding simulations to prove the basic feasibility of the approach for low-resolution MRI. Deviations of roughly 10-15% between main field and RF encoded images were found for an in-plane resolution of 5 3 5 mm 2 . The approach of RF encode MRI does not seem to be able to replace standard main field gradient encoding completely, but a suitable combination of both concepts could find promising applications.

High‐resolution MRI encoding using radiofrequency phase gradients

“…Other RF encoding imaging mechanisms have been proposed, but we believe that none offers the flexibility and high resolution demonstrated here. A key distinction of the presented method is that encoding is neither based on a single excitation event at the beginning of the sequence , nor on a single reception event at the sequence end , but rather on a process that can be repeated during the sequence – namely refocusing. This allows the progressive encoding action to, in effect, amplify the strength of the RF phase gradient and leads to high resolution.…”

“…This access to high spatial resolution is very surprising as it has been an axiom of MRI that imaging is derived from the B 0 gradient and, indeed, the Nobel Prize for Physiology or Medicine in 2003 was awarded for this discovery. Of fundamental importance is that, unlike B 1 amplitude gradients , B 1 phase gradients can be produced effectively over a volume, resulting in whole‐volume high‐resolution imaging. In addition, in contrast with B 0 encoding, RF encoding is independent of γ , the gyromagnetic ratio, meaning that identical high‐resolution encoding can be achieved for multi‐nuclear MRI, even for important low γ species, such as 13 C, 31 P and 129 Xe.…”

“…There is some redundancy between trains in the early part of most trains, but the multiply sampled points can be averaged to increase the signal‐to‐noise ratio. Because data are evenly spaced on a two‐dimensional Cartesian grid in Fourier space, image reconstruction is very straightforward and simply requires standard two‐dimensional Fourier transformation, unlike some other approaches .…”

“…Although not demonstrated, this latter proposal involves spatial encoding at very low field strength (made possible by the use of pre‐polarization), and so potentially could avoid the RF power deposition problem. Other encoding approaches using transmit arrays have been proposed, but are limited by low resolution and a lack of volume coverage .…”

Free‐breathing inner‐volume black‐blood imaging of the human heart using two‐dimensionally selective local excitation at 3 T

“…However, the duration of such 2D‐selective pulses is currently in the order of 5–10 ms, and therefore, they may not be well‐suited as part of segmented k ‐space gradient echo imaging sequences or steady‐state with free precession approaches. However, with advances in scanner hardware and particularly multitransmit capabilities (24, 36), such 2D‐selective pulses may be abbreviated significantly, which may enable the use of 2D‐selective pulses as part of segmented k ‐space gradient echo imaging sequences as well.…”