Fast-Spin-Echo (FSE) T1 multislice multishot Inversion-Recovery (IR) sequences provide valuable information about the brain and are routinely run in the clinic. However, standard implementations are often prone to inefficient sampling, especially when the Inversion-Time (TI) is long. In this study, a FSE-IR sequence was modified to make it as efficient as possible. The modified sequence, that we name distributed IR, was firstly proposed by Oh¹. However, to the best of our knowledge, it is not generally available in standard installations.

The distributed IR sequence was tested in an adult and in a neonate. Furthermore, since the latter population is characterized by longer relaxation times², contrast optimized IR sequences often requite a longer TI, thus there is major scope for efficiency improvement.

We will refer to the original and distributed IR sequences as A and B respectively. These are represented in Figure1 for an acquisition with a fixed TI and with different variants of sequence B that conserve either slice number or TR. Despite having the same TI, A and B’ are arranged in a different way: in sequence A, all the inversions are played within a TR. However, and especially when the prescribed TI is long, the sequence is inefficient as many other inversions and FSE modules could fit in. Sequence B’ alternates inversions to FSE modules throughout, and the sequence achieves maximal efficiency. With the fully interleaved structure in B’, TI can be set in increments of the combined Inversion+FSE modules, by simply adjusting the frequency offsets applied to each inversion pulse. This creates a completely cyclic structure with a consequence that it no longer makes sense to divide the sequence into discrete blocks. This could have consequences for the approach to steady state and for ensuring even time structures across all slices. The former is addressed by a single dummy cycle provided the excitation pulse is 90^o, which is no different than for a conventional IR sequence. The use of uniform time delays between the elements of the sequence allows fine tuning of TI, and adding additional slices can be used to adjust the TR.

All data for this study were acquired on a 3T Philips Achieva scanner, using a 32 channel head coil for the adult experiment, and a dedicated 32 channel head coil³ for the neonatal subject. The adult data was obtained by using the manufacturer provided Philips IR-FSE sequence that had a resolution of 0.57x0.73x5 mm³, TR of 2000 ms and TI of 800 ms. A second run with the distributed IR sequence B was acquired with identical resolution and a TR and a TI of 2034 ms and 837 ms.

A late premature baby (gestational age at scan 36+1 weeks) was scanned with the acquisition parameters shown in Table1. The TR and the TI of sequence were chosen to optimise gray-white-CSF contrast and all datasets were reconstructed accounting for 3D rigid motion among shots and slices⁴. To match sequences B to A, 3 different criteria were adopted and this resulted in 3 different scans (B1, B2 and B3 of Table1). B1 was designed so that its TR and the TI were as close as possible to A. The second sequence, B2, was obtained by minimizing the signal differences with A, using literature² T1 values for CSF, WM and GM of 4100 ms 2500 ms and 1800 ms. Finally, a third scan with minimal scan duration for approximately the same total number of slices was created, but this resulted in a shorter TR. To re-obtain the desired contrast, its TI was retuned. Note that in all cases the total number of slices required causes the sequence to be executed in packages, which are acquired sequentially with equally spaced interleaved slices that are finally put together to create a homogeneous stack. Finally, all scans were within the regulatory SAR limits.

The duration of the adult FSE-IR sequence reduced from 5 min 12 sec to 2 min 34 sec while maintaining a similar contrast. Figure2 shows a comparison between sequences A, B1, B2 and B3 in the neonatal experiment. Contrast is preserved in all cases. This is despite a decrease in scan duration by approximately 2 minutes for sequences B1 and B2, and 2 and a half minutes for sequence B3. This study demonstrates how simple modifications to conventional IR-FSE multislice sequences can have a significant impact in reducing the total acquisition time in adult and in neonatal subjects without changing either contrast or resolution. The resulting shorter scans have obvious benefits.