In MRI, when more than one RF pulse is used, the acquired echo may contain multiple components with different contrasts. For example, in two-pulse experiments, the second pulse not only refocuses the spin echo, but also produces a gradient echo (GE). In standard SE sequences, the GE signal is usually unwanted and suppressed by a gradient crusher. However, if the GE can be preserved and effectively differentiated from the SE, more information can be obtained by producing two images. In this study, we employ controlled aliasing technique [1], i.e. RF phase cycling to shift the SE image away from the GE image, and thus the two images can be reconstructed by parallel imaging reconstruction.

The proposed SE-GE sequence (Figure 1) is modified from a standard SE sequence. The excitation pulse and refocusing pulse are generalized and denoted as pulse 1 (with flip angle α1) and pulse 2 (with flip angle α2). To produce GE signal, the slice selection crusher is replaced by balanced rephasing gradient. Pulse 1 is phase-cycled by [0, π] such that the SE image is shifted by half FOV. Pulse 2 is not phase-cycled such that the GE image is still in the center of FOV. To perform simultaneous spatial encoding for both signals, the dephasing gradients for phase encoding and readout are positioned after pulse 2. For reconstruction, the SE and GE images are separated using SENSE [1].

Experiments were performed at 7T (Bruker) using a four-channel surface coil. The fixed ex vivo rat brain was imaged with parameters TR/TE=5000/12.7 ms, α1= α2=90, slice thickness=2mm, FOV=3×3 cm, and matrix size=256×256. For comparison, standard SE images were acquired with the same TR and TE. The coil sensitivity maps were computed from the standard SE images.

Figure 2 shows the imaging results. In the raw image, the SE image was shifted away from the GE image by half FOV, while the GE image remained in the center of FOV. By SENSE, the GE and SE images were successfully separated, with minimal residual aliasing. The GE image and SE image had distinct contrasts: the former showed T1 and T2* contrast, while latter was strongly T2 weighted and similar to that acquired from a standard SE sequence.

(1) We demonstrate that GE and SE images can be simultaneously obtained using controlled aliasing technique and parallel imaging reconstruction. Unlike previous dual-contrast methods using sequential readout [2, 3], the GE and SE signals are mixed during acquisition in our new method. This will allow shorter TEs and enable more flexible contrasts.

(2) In our method, the contrast of the SE image is similar to that of a standard SE sequence yet the contrast of the GE image is complicated and dependent on a few parameters. Generally, the GE signal is affected by both TR_SE and TE_SE. Specifically, the actual TE_GE is half of TE_SE. Altering α1 and α2 also affects the contrasts and the relative signal intensities of the SE and GE images. Overall, the contrast of GE image is weighted by both T1 and T2*.

(3) In our method, pulse 2 has two functions: exciting the GE signal and refocusing the SE signal. Currently, a system-default Hermite excitation pulse is used, but in future studies, the pulse shape can be further optimized for both excitation and refocusing.

(4) Our current implementation also leaves a few problems. The GE image has limited SNR; the spin phase memory may interfere with phase cycling; the image quality is relatively sensitive to field inhomogeneity. We are addressing these problems, e.g. optimizing the flip angles to increase the signal intensity of GE.

(5) More importantly, the present study demonstrates as a proof-of-concept that it is possible to extract different contrasts from multiple echo components. Future studies may expand this idea to dual echo steady-state imaging [2, 3].

A novel method is proposed to separate spin echo and gradient echo using controlled aliasing and parallel imaging reconstruction. Two images with distinct contrasts can be simultaneously obtained from one scan. It is possible to extract contrasts from multiple echo components.