Turbo spin-echo (TSE) has replaced spin-echo (SE) for T2-weighted imaging in routine clinical neuroimaging due to its fast speed. However, the T2 contrast from a TSE scan is altered compared to the conventional SE sequence, primarily due to the repeated refocusing RF pulses. Another contrast, proton density (PD), was used in the past but has yielded to the FLAIR contrast (fluid attenuated inversion recovery) in part due to scan time constraints. Nevertheless, the FLAIR images are sometimes less robust than having PD/T2-weighted images, in the posterior fossa¹. Recently a spiral-in/out readout was incorporated into the SE sequence for improved T2 contrast with the capability of fat-water imaging in a short scan time². In this work we develop a dual SE technique with hybrid spiral readouts to generate both PD- and T2-weighted fat and water images, without increasing the scan time compared to the previous spiral SE sequence.

The schematic diagram of the proposed dual SE with hybrid spiral readouts is illustrated in Fig. 1. To minimize the TE of the first echo (TE1), a conventional spiral-out readout is used for the first echo. To increase the scan efficiency and consequently improve the signal-to-noise ratio (SNR) of the second echo, a spiral-in/out readout is used for the second echo. This is analogous to the variable bandwidth used in some techniques to increase the SNR of later echoes. Since the echo spacing is different, the Carr-Purcell-Meiboom-Gill condition is violated. To mitigate this issue, the stimulated echo is suppressed by adjusting the crusher gradients surrounding the first and second refocusing RF pulses. Gradient flow compensation is used for the second refocusing RF pulse, while stronger crusher gradients are used to suppress flow signals from the first echo. Data are acquired at multiple TE shifts to enable fat-water imaging.

Volunteer data were acquired on a 3T Philips Ingenia scanner. The spiral dual SE was compared to Cartesian dual SE and Cartesian mDixon TSE. All scans were acquired with 20 slices (4mm thick with 2mm gap), resolution = 1.0x1.0 mm², TR = 2350 ms. Other scan parameters are listed in Table 1. Note that fat-water imaging was not performed with the reference Cartesian SE due to the long scan time, which was already reduced by using partial Fourier acquisition. In this comparison, the reference Cartesian mDixon TSE was scanned twice, one for T2 contrast that is normally acquired in clinical routine scans, and one for PD contrast. The T2 and PD contrast can also be acquired using dual echo TSE (with a similar total scan time due to the constraint in ETL and TE), which was not chosen in this study.

Fig. 2 shows T2-weighted images. Fat-water imaging was not performed with Cartesian SE in this comparison due to the long scan time. Compared to Cartesian SE, Cartesian mDixon TSE provides fast scan speed with the capability of fat-water separation for T2-weighted imaging (1:43), at the cost of image sharpness in the phase encoding direction due to the T2 decay. The T2 contrast is also slightly altered with TSE. In particular, TSE is less sensitive to iron deposition, as pointed to by the arrows in areas such as the dentate nuclei, red nuclei, globes pallidus, putamen, etc. Due to the efficient spiral acquisition, spiral SE provides fast speed comparable to Cartesian mDixon TSE for T2-weighted imaging (1:36 vs 1:43). Compared to TSE, spiral SE also improves the T2 contrast, which is comparable to that from conventional Cartesian SE.

PD-weighted images are shown in Fig. 3. The PD contrast can provide complementary information. It is worth emphasizing that the acquisition of the PD-weighted images does not cost additional scan time with either Cartesian or spiral dual SE. However, it does require additional long scan time with Cartesian mDixon TSE (3:41, which is longer than for T2-weighted TSE due to the shorter ETL required). In this sense, spiral SE provides faster speed than Cartesian mDixon TSE when both T2 and PD contrast are acquired (1:36 vs 5:24).

Dual SE hybrid spiral readout imaging is a promising tool for simultaneous PD- and T2-weighted neuroimaging, providing image contrast similar to traditional Cartesian dual echo SE imaging with improved scan speed.