Steer-PROP: a GRASE-PROPELLER sequence with interecho steering gradient pulses

Abstract

Purpose: This study demonstrates a novel PROPELLER (periodically rotated overlapping parallel lines with enhanced reconstruction) pulse sequence, termed Steer-PROP, based on gradient and spin echo (GRASE), to reduce the imaging times and address phase errors inherent to GRASE. The study also illustrates the feasibility of using Steer-PROP as an alternative to single-shot echo planar imaging (SS-EPI) to produce distortion-free diffusion images in all imaging planes.

Methods: Steer-PROP uses a series of blip gradient pulses to produce N (N = 3-5) adjacent k-space blades in each repetition time, where N is the number of gradient echoes in a GRASE sequence. This sampling strategy enables a phase correction algorithm to systematically address the GRASE phase errors as well as the motion-induced phase inconsistency. Steer-PROP was evaluated on phantoms and healthy human subjects at both 1.5T and 3.0T for T₂ - and diffusion-weighted imaging.

Results: Steer-PROP produced similar image quality as conventional PROPELLER based on fast spin echo (FSE), while taking only a fraction (e.g., 1/3) of the scan time. The robustness against motion in Steer-PROP was comparable to that of FSE-based PROPELLER. Using Steer-PROP, high quality and distortion-free diffusion images were obtained from human subjects in all imaging planes, demonstrating a considerable advantage over SS-EPI.

Conclusion: The proposed Steer-PROP sequence can substantially reduce the scan times compared with FSE-based PROPELLER while achieving adequate image quality. The novel k-space sampling strategy in Steer-PROP not only enables an integrated phase correction method that addresses various sources of phase errors, but also minimizes the echo spacing compared with alternative sampling strategies. Steer-PROP can also be a viable alternative to SS-EPI to decrease image distortion in all imaging planes. Magn Reson Med 79:2533-2541, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Keywords: GRASE; PROPELLER; Steer-PROP; diffusion imaging; k-space trajectory; phase correction.

Figure 1

Different k-space sampling strategies in FSE-PROPELLER (a), Turboprop (b), X-PROP (c), and Steer-PROP (d). Only a segment of the pulse sequence between two consecutive refocusing RF pulses is shown. In FSE-PROPELLER (a), each spin echo is used to sample a k-space line in a blade. All spin echoes in a TR are used to sample a single PROPELLER blade. In Turboprop (b), X-PROP (c), and Steer-Prop (d), each spin echo is split into three gradient echoes. Turboprop uses all three gradient echoes to sample the same widened PROPELLER blade. X-PROP assigns the odd (solid lines) and even spin echoes (dash lines) into orthogonal blades and distributes all six blades evenly over an angular range of π, whereas Steer-PROP distributes the three gradient echoes to adjacent blades with a narrow angular range.

Figure 2

(a): A segment of a Steer-PROP sequence illustrating steering gradient design for the special case of N = 3. (b): k-Space trajectory of the sequence segment in (a). G_x and G_y are used to denote readout (or X) and phase-encoding (or Y) gradients in a conventional sequence. G_xd1, G_xd2, G_yd1 and G_yd2 are the diffusion gradients on the X and Y axes as indicated. G_pe is a phase-encoding pulse for the spin echo, G_xro1 is the readout gradient pulse corresponding to the 1^st gradient echo, G_xro2 and G_yro2 are the X and Y components corresponding to the 2^nd gradient echo, G_xro3 and G_yro3 are the X and Y components corresponding to the 3^rd gradient echo, G_xθ, G_yθ, G_x2θ, and G_y2θ are the steering gradient pulses, and G_xr and G_yr are the phase-rewinding pulses. The three k-space lines sampled by the three gradient echoes are illustrated as the white lines (denoted as b1, b2, and b3, respectively) in their respective color-coded blades. The curved arrow lines illustrate the effect of the steering gradient pulses or the rewinding gradient pulse on the k-space trajectory with the color of the lines corresponding to the same color of the gradient pulses in (a).

Figure 3

Steering gradient design using a gradient-echo train of 3 (N = 3). k-Space lines sampled by the three gradient echoes are denoted as b1, b2, and b3 in (a), (b), and (c), respectively. θ is the angle between two subsequent blades. φ_b1, φ_b2 and φ_b3 are the ending locations of k-space lines b1, b2, and b3, respectively. G_xθ, G_yθ, G_x2θ, and G_y2θ are the steering gradients as explained in the text. Rewinding gradient pulses (G_xr and G_yr ) return the k-space trajectory to the k_x -axis as if steering had not happened. The G_yr rewinding pulse performs k-space traversal from point φ_b3 to its projection on the k_x -axis (φ_yr) and the G_xr rewinding pulse performs k-space traversal from φ_yr to the starting location in the first blade (φ_xr).

Figure 4

A segment of Steer-PROP consisting of two repetition times (TRs or shots) denoted as 1^st TR and 2^nd TR. Three kinds of phase errors are shown: intra-blade (k-space lines within a single blade acquired by different spin echoes in the same echo train), inter-blade (among the blades acquired by the gradient echoes within a single spin echo), and inter-shot (between TRs) phase errors. Note that although the refocuses pulses are labeled as 180°, the actual flip angle was adjusted between 160° and 180° to stabilize the echo amplitudes in the spin-echo train.

Figure 5

Comparison of phantom images obtained at 3.0T using (a) FSE-PROPELLER and (b) Steer-PROP pulse sequences. The acquisition parameters were TR = 4s, effective TE = 72ms, M = 8, N = 3 (for Steer-PROP), number of shots = 16, FOV = 24cm, slice thickness = 5mm, bandwidth (BW) = ±125 kHz, matrix size = 256×256, and NEX = 2. The scan times for the two images were 6 mins 27 secs in (a) and 2 mins 9 secs in (b). The green and orange arrows on the Steer-PROP image indicate regions where signal and noise were measured, respectively, for SNR calculations.

Figure 6

Two slices of T₂-weighted (a1, b1, a2, and b2) and diffusion-weighted (b = 750 s/mm²; c1, d1, c2, and d2) images from a human volunteer obtained at 1.5T using FSE-PROPELLER (first and third columns) and Steer-PROP (second and fourth columns) with TR = 4s, effective TE = 72ms, M = 8, N = 3 (for Steer-PROP), FOV = 24cm, slice thickness = 5mm, BW = ±62.5kHz, matrix size = 256×256, and NEX = 2.

Figure 7

T₂-weighted images (TR = 4s, effective TE = 128ms, M = 8, N = 3 for Steer-PROP, matrix size = 256×256, FOV = 26cm, slice thickness = 5mm, and NEX = 2) of a healthy human volunteer obtained on a 3.0T scanner using (a) conventional Cartesian FSE, (b) FSE-PROPELLER, and (c) Steer-PROP. The subject’s head was moving randomly during all three scans.

Figure 8

Diffusion-weighted images (TR = 4s, effective TE = 72ms, M = 8, N = 3 for Steer-PROP, FOV = 24cm, slice thickness = 5mm, BW = ±125 KHz, matrix size = 256×256, NEX = 2, and b = 750 s/mm²) acquired on a 3.0T scanner using Steer-PROP (a–d) and SS-EPI (e–h) on axial (a, e), sagittal (b, f), coronal (c, g), and oblique (d, h) planes. The oblique slice orientation was selected to be in parallel to the cerebellar tentorium, ~ 40° from the axial plane. The sub-optimal image quality in (e) was a reflection of poor B₀-field homogeneity. Even in the presence of B₀-field homogeneity, a high quality Steer-PROP image in (a) was obtained.