Parallel imaging of hyperpolarized helium‐3 with simultaneous slice excitation

Abstract

Hyperpolarized (HP) gas imaging of the lungs is an ideal potential application for parallel imaging. This is due to the fact that there is limited scan time (breath hold of 20 s) and limited non-renewable polarization. Reduced phase encode parallel imaging is demanding on hardware in that it requires multiple receivers. In this work, simultaneous parallel acquisition of hyperpolarized (HP) 3 He images from multiple slices was demonstrated in phantoms and in vivo using a simultaneous slice excitation method, at a field strength of 1.5 T. The pulse sequence allows simultaneous acquisition of n slices per RF excitation, thus reducing the number of RF pulses needed to fully cover a given volume with multi-slicing. Unlike conventional parallel imaging, this method does not require prior reference scan information, which would consume some of the finite longitudinal polarization in lung ventilation studies with HP gas. Recent advances in laser optical pumping techniques allow mass production of hyperpolarized (HP) noble gases (He-3, Xe-129). Noble gas atoms have a 1 ⁄2 nuclear spin property and low gyro-magnetic ratio (␥ 3He ϭ 0.75 ␥ 1H and ␥ 129Xe ϭ 0.27 ␥ 1H ) (1). These gases are also stable and non-radioactive and thus enable for the first time, MR imaging of ventilation in the airways and airspaces with high SNR, spatial and temporal resolution. Ever since successful preliminary in vivo studies of hyperpolarized gas lung imaging on live animals (2,3) were demonstrated, there has been a large interest in human in vivo clinical studies. Despite growing clinical interest, there are multiple technical constraints and challenges remaining in HP gas MRI of the lungs. One of the main constraints is the choice of an optimum imaging strategy due to the nonrecoverable polarization of the spin population. We can approximate the longitudinal magnetization decay of the HP 3 He transverse signal within a single breath-hold as, M y (4):In Eq.[1] M 0 is the magnetization before first RF pulse, n is the n th RF pulse with flip angle ␣ and repetition time T R,

Simultaneous multislice acquisition without trajectory modification for hyperpolarized 13C experiments

“…Another SMS approach was demonstrated for hyperpolarized 3 He spins. In this approach, gradients along the slice‐select direction are used to rephase distinct linear phase profiles corresponding to each excited slice . This trajectory modification is difficult to adapt for relatively long 13 C single‐shot imaging trajectories (approximately 30 ms) because of potential ‐related signal modulation between slices.…”

Hyperpolarized 129 Xe imaging of the rat lung using spiral IDEAL

“…Furthermore, at lower field strength (e.g., 1.5T) the spatial resolution could be improved, even at longer read‐out times due to the increase in . In future, 2D imaging can also be extended to multislice imaging using parallel imaging approaches and a short duration slice‐selective pulse . Although, five TR values would be achievable for 2D human imaging, the total scan time required for obtaining the gas exchange curves would be challenging for the case of multi‐slice imaging.…”

Combined simulated annealing and Shinnar–Le Roux pulse design of slice-multiplexed RF pulses for multi-slice imaging

“…To date, only a two-slice multiplex pulse has been used, constructed by vector summation of an asymmetrically truncated sinc-pulse with its modulated time-reversed equivalent [14,15]. Although vector summation has been used successfully with large flip angles by others [16], it assumes a linear approximation for the Bloch equations, and in theory is not ideal for higher flip angles, or for the higher RF amplitudes expected as more component pulses are added for more slices.…”

“…Consequently, our method has improved simultaneity compared with sequential slice excitation, but without needing extra views or hardware of truly simultaneous slice methods. The method has recently been used to perform parallel imaging with hyperpolarized 3 He [15].…”