Fast-spin-echo imaging of inner fields-of-view with 2D-selective RF excitations.

PURPOSE

To demonstrate the feasibility of two-dimensional selective radio frequency (2DRF) excitations for fast-spin-echo imaging of inner fields-of-view (FOVs) in order to shorten acquisitions times, decrease RF energy deposition, and reduce image blurring.

MATERIALS AND METHODS

Fast-spin-echo images (in-plane resolution 1.0 x 1.0 mm(2) or 0.5 x 1.0 mm(2)) of inner FOVs (40 mm, 16 mm oversampling) were obtained in phantoms and healthy volunteers on a 3 T whole-body MR system using blipped-planar 2DRF excitations.

RESULTS

Positioning the unwanted side excitations in the blind spot between the image section and the slice stack to measure yields minimum 2DRF pulse durations (about 6 msec) that are compatible with typical echo spacings of fast-spin-echo acquisitions. For the inner FOVs, the number of echoes and refocusing RF pulses is considerably reduced which compared to a full FOV (182 mm) reduces the RF energy deposition by about a factor of three and shortens the acquisition time, e.g., from 39 seconds to 12 seconds for a turbo factor of 15 or from 900 msec to 280 msec for a single-shot acquisition, respectively. Furthermore, image blurring occurring for high turbo factors as in single-shot acquisitions is considerably reduced yielding effectively higher in-plane resolutions.

CONCLUSION

Inner-FOV acquisitions using 2DRF excitations may help to shorten acquisitions times, ameliorate image blurring, and reduce specific absorption rate (SAR) limitations of fast-spin-echo (FSE) imaging, in particular at higher static magnetic fields.