Short echo time CSI without outer-volume suppression at 7 T

Introduction:

At high field, localisation methods such as PRESS suffer from chemical-shift artefacts and T2-dephasing. An alternative is a pulse-acquire sequence with CSI. However, poorly shimmed regions can cause artefacts through the pointspread-function. It has been shown that this sequence can be combined with SAR-demanding OVS[1] to suppress these signals. However, with high spatial resolution and a voxel of interest far from poorly shimmed regions, OVS may not be necessary. Low-SAR water and lipid suppression may be sufficient, enabling a substantially shorter TR.

In this study we propose a low-SAR adiabatic water and lipid suppression technique for suppression over the slice.

Fig.1 In-vivo CSI spectrum (TR=1s, TE=6.4ms, 32×32 matrix) from a 5×5x10mm voxel away from the skull

Methods:

Spectra with were acquired on a phantom and in-vivo on a Philips 7T-MRI. Water and fat suppression was investigated with a modification of the SWAMP method [2]. Flanks of adiabatic inversion pulses generate transverse magnetization, by tuning the pulse it is possible to excite and spoil water and fat. By using two pulses and shifting the frequency the suppression-band is broadened thus creating a more complete suppression over the slice.

Fig.2a Simulated excitation profile of a single inversion pulse

Fig.2b. Experimental frequency response of two inversion pulses

Fig.2c. Experimental frequency response of two shifted inversion pulses. (50Hz shift). Note the broader suppressed region

Results & Discussion:

In-vivo results (Fig.1) show the possibility to obtain high quality spectra without OVS in a single voxel.

Fig.2 shows the response of the adiabatic pulses. Results from a phantom show a factor 38 water-suppression with 2 conventional pulses and a factor of 65 suppression with 2 shifted pulses. Use of this water and lipid suppression method can be included in the sequence to decrease artifacts from poorly shimmed regions and obtain high quality spectra at high spatial resolution.

Literature:

[1] A. Henning, proc. 16th ISMRM; 594 (2008).

[2] R.A. de Graaf, MRM 40:5 p690–696 (2005).