After its invention more than two decades ago [1,2,3], Zero TE has experienced a recent revival [4,5] with many interesting clinical applications [6,7,8,9]. Here, we describe a novel Zero TE method that allows multi-gradient-echo refocusing and its implementation and testing in phantom experiments.

Zero TE is a 3D pulse sequence consisting of 1) short non-selective RF excitation (typically below 5deg) and 2) a constant readout gradient with only directional updates in between repetitions. Image encoding happens in a 3D radial manner and starts immediately at the moment of RF excitation (TE=0). The Zero TE pulse sequence native contrast is proton density; however, different contrast can be added contrast preparation modules before the readout using e.g. inversion-recovery, or T2 preparation modules.

A novel, self-refocusing zero TE imaging pulse sequence (termed Looping star) is implemented as illustrated in Fig. Without loss of generality, we consider a single segment of 2*32=64 3D radial spokes (upper subplot) arranged in a 2D circle (lower left subplot) with a constant phase increment of 2*pi/32 (black arrows). Hence after 32 repetitions each spoke gets refocused and forms a gradient echo of the original FID signal (red connected arrows). In this way gradient refocusing can be achieved with minimal gradient switching and hence negligible acoustic noise; an important characteristic of zero TE imaging especially for silent imaging applications. In the pulse sequence (top plot) 32 spokes are first excited and encoded along the 3D spokes. Afterwards each of the early FID echoes gets sequentially refocused and forms a gradient echo. Hence for the latter 32 spokes, RF excitation is turned off. The refocusing can be repeated to form more echoes. For the following segments, the 3D spokes are rotated along the z-axis to obtain 3D coverage as illustrated in the lower right plot.

The Looping star pulse sequence was implemented on a GE MR750w 3T scanner (GE Healthcare, Waukesha, WI) and tested in phantom experiments. Each segment consisted of 128 spokes with the first 32 spokes encoding FID echoes and the latter ones encoding three gradient echoes. The ZTE parameters were chosen to be FA=3deg, TRspoke=1ms, FoV=19.2cm, 3x3x3 mm resolution, total_spokes=8192, acquisition BW=±15.6kHz. Accordingly, the echo time amounted to TE=32*TR=32ms and the total scan time was 12 seconds.

Figure 2 shows 3 orthogonal planes of the spherical phantom for the FID image (top) and the first gradient echo image (middle). The obtained quantitative T2* = 45ms was uniform and consistent with the value obtained from a conventional 3D Cartesian spoiled gradient echo measurement.

The presented Looping star method describes a novel scheme for gradient refocusing which differs from the conventional multi gradient echo readout following each excitation. Instead, the refocusing happens over multiple excitations; in a way similar to the PRESTO concept [10]. That way, the high efficiency of the Zero TE sequence and its silent imaging characteristic can be maintained. Also, its multiecho capability shows the potential of this imaging strategy for T2* quantification methods. As a next step, this method will be investigated in vivo for quantitative susceptibility mapping (QSM) and fMRI.